Why Time Travel Is Impossible

[Wasted Sapience]

Time exists only in the human mind as a means of measuring change. The physical universe only knows the present; it has no idea what the past and the future are. The physical universe knows only change and not time. There is no way to travel to the past because there is no past to travel to; the past does not exist. Same offer applies to the future.

Discuss, support, refute, ect...

[rjf]

I agree that time is merely a construct of the human mind, but only to a certain degree. The concept of minutes, hours, days, even millenia, are simply a means for us to deal with changes around us, just as centimeters, liters, and grams are means for us to deal with distance, volume, and mass. 'Time' does exist, but only as one dimension of the universe. I see no reason (and I am not an expert) why we should be able to travel freely in three dimensions and not through the fourth (with sufficiently advanced technology, which I will freely admit (and I am not an expert) will not exist for centuries, if it ever exists at all).

[Wasted Sapience]

I agree that time is merely a construct of the human mind, but only to a certain degree. The concept of minutes, hours, days, even millenia, are simply a means for us to deal with changes around us, just as centimeters, liters, and grams are means for us to deal with distance, volume, and mass. 'Time' does exist, but only as one dimension of the universe. I see no reason (and I am not an expert) why we should be able to travel freely in three dimensions and not through the fourth (with sufficiently advanced technology, which I will freely admit (and I am not an expert) will not exist for centuries, if it ever exists at all).

But where is this "past" to travel to? Where is the "future?"

[Octokun]

I tend to lean in your direction regarding the subject, but, the more I learn about what we call 'reality' the more screwed up it seems... so I don't actually know...

[Jesse]

Time exists only in the human mind as a means of measuring change. The physical universe only knows the present; it has no idea what the past and the future are. Well, not according to the theory of relativity. Are you familiar with the relativity of simultaneity (http://en.wikipedia.org/wiki/Relativity_of_simultaneity)? Two events that happen at the same time in one frame can happen at different times in another frame, and no frame is physically "preferred" over any other. And in the theory of general relativity which deals with curved spacetime, it is theoretically possible to have spacetimes which contain closed timelike curves (http://en.wikipedia.org/wiki/Closed_timelike_curve), paths through spacetime which loop around and return to their beginning point (though it is quite possible CTCs will be ruled out by quantum theory).

Philosophically, you could also look at the B theory of time (http://en.wikipedia.org/wiki/B-Theory_of_time), which aside from physics issues seems like a logically possible alternative to presentism (http://en.wikipedia.org/wiki/Presentism_%28philosophy_of_time%29), which you seem to be advocating.

[99Percent]

You can always travel to the future by suspended animation. In fact when you sleep or fall into a comma you are technically travelling to the future.

I also believe travelling to the past is impossible. But what is definitely possible is to see the past, say by putting a giant mirror in space, or recording everything.

[Derec]

You can always travel to the future by suspended animation.

Or by time dilation. If you spend a year in a spaceship flying at v --> c and there passed 100 years on Earth you have effectively traveled to future during your trip.

[Sultanist]

We could fill reams of thread pages with all the speculation which has been written about time travel.
I haven't spent much time in this forum but I would be very surprised if you haven't already devoted past threads to this.

Aside from all the paradoxes, what I find to be the best evidence that time travel into the past will likely not prove to be possible, is the fact that we are unaware of the presence of time travelers visiting us during all of our recorded history.
I realize some theorize that we will not be able to explore the past before the point at which time travel is invented. But I find the reasoning for that to be
purely speculative. As is the answer to it which was provided by a friend of mine in a marathon debate we once participated in...
Once time travel becomes possible, then almost 'immediately' (in the sense of immediate in a world where time travel can occur), all of time would be filled with travelers. Anyone who goes back and who knows the mechanism can then offer people further and further back in 'time' the ability to travel as well. It would effectively almost immediately (from any one person's real 'time' perspective) self destruct due to a massive feedback loop. Because, if time is already there in a linear path, then the entire future was there from the start, which means that time travel would have been possible from the very start as well.
And he continues...
The argument is, if time isn't just 'now', if it exists linearly, then time travel 'already' exists and has from the beginning. Therefore, from the very first, time travelers would have been all over the place, causing all kinds of effects, and would immediately self destruct the whole system in a huge feedback loop. Its an argument that it cannot be possible, because if it was, it would already be happening on a massive scale, and would have been happening on a massive scale since the beginning of 'time', since the end of time already existed at the beginning. Therefore, anyone who wanted to time travel, from the point on that line at which it was invented, until the end of that line, would already be traveling through time, and would have been for billions of years. The result would be feedback loop of truely epic proportions, that would have ripped any civilization apart before it could even begin.

[IanC]

I realize some theorize that we will not be able to explore the past before the point at which time travel is invented. But I find the reasoning for that to be
purely speculative.
It's called the Cauchy horizon, I think, it's not just a get-out clause. If you actually work it out for the theoretical methods of time travel, none let you go back further than the point where the machine started operation.

if it [time] exists linearly,
It doesn't though. See GR, as mentioned by Jesse.

[Sultanist]

It's called the Cauchy horizon, I think, it's not just a get-out clause. If you actually work it out for the theoretical methods of time travel, none let you go back further than the point where the machine started operation.


The Cauchy Horizon and Closed Timelike Curves (CTC) and The "Chronology Protection Agency" are explained on this page, as are a great many other concepts related to time travel...

http://www.iep.utm.edu/t/timetrav.htm


According to Hawking, causal loop stories that employ CTCs are like grandfather paradox stories. While backwards causation might be logically possible, it is not physically possible. The "Chronology Protection Agency" actively prevents them from occurring. The laws of physics conspire such that natural time travel into the past thwarts backwards or reverse causation. In closed spacetime, the Cauchy horizon of a CTC acts as an impenetrable barrier to a timelike worldline for objects. If a time traveler could travel to the past, whether or not that past included their younger self, they are prevented from interacting with the events of the past.

If causal loops are possible, then the objects may interact with the events of the past, but only in a consistent way, that is, only in a way that preserves the already established events of the past. Perhaps we could call it the CTC prime directive (see Ray Bradbury's short story "A Sound of Thunder"). Causal loops, like any other aporia of uncaused causes, occupy the inexplicable perimeter of philosophical thought about causation. Nevertheless, causal loop stories like that of Jennifer raise another issue: personal identity.

I mentioned in another thread recently that there is now a Caltech physicist who has the capability of communicating all this stuff as well as Sagan used to explain Cosmology. His name is Sean Carroll.

I didn't know it at the time, but Sean Carroll is on the "Honorary Board" of Internet Infidels.
http://www.infidels.org/infidels/honorary.html

I think it would prove to be very enlightening if we could get Dr. Carroll into some of these discussions.

[ashaktur]

In fact when you sleep or fall into a comma you are technically travelling to the future.


If you fall into a comma, do you get a semi-colon;

[NZSkep]

But where is this "past" to travel to? Where is the "future?"

depends if you follow Terminator rules or Back to the Future rules.

[Majestyk]

Time exists only in the human mind as a means of measuring change. I submit that the "mind" cannot exist without the property of time (or length, width, and depth). Neurologic activity is the result of matter changing states, being influenced and influencing other matter in its proximity, to form a coherent pattern of activity. Change/activity requires time.

[Majestyk]

But where is this "past" to travel to? Where is the "future?" Places where we aren't at that moment. Think of it this way. We don't really see other physical locations. We intercept light that traveled from those locations. We only experience what is in our proximity.

[c davis]

Time exists only in the human mind as a means of measuring change. The physical universe only knows the present; it has no idea what the past and the future are. The physical universe knows only change and not time. There is no way to travel to the past because there is no past to travel to; the past does not exist. Same offer applies to the future.

Discuss, support, refute, ect...

I use to think that time travel was possible. Why because I saw that it was possible on the TV, then I learned that Time does not exist. Time is just another construct of the mind. It is a measurement of space. We cannot change those events that have already occurred (Past), but we can change our subjective reality and thus change the future.

Which makes sense, went you think about it... because the present moment always contains the past, the present moment (the now) and the future. So in a way we can change the past, and the future by how we act in the present moment.

[IanC]

Did you read the comment from Jesse? Time is more complicated than you think.

[premjan]

B theory speaks only of relative position in the temporal stream so that is consistent with relativity where there is neither absolute space nor absolute time. We could perhaps think of space-time as a bundle of evolving here-and-nows. A more radical approach is the one of Julian Barbour which is more akin to "nowism" which I suppose is less radical than presentism.

[Majestyk]

B theory speaks only of relative position in the temporal stream so that is consistent with relativity where there is neither absolute space nor absolute time. Kinda begs the question, has anybody ever traveled back in space?

[35Kas]

To travel to the past is impossible because it would require the inversion of entropy, redirection of momentum, etc, of the entire universe and this would require infinite energy?

However, if the traveller is able to isolate himself from space into some sort of bubble, he could be able to travel freely in the time dimension, and the energy requirements for that would not be infinite.

Personally, I believe that time travel is only possible into the future. If it were possible to travel to the past, it would be very unlikely that in the entire age of the universe from big bang to fizle, no civilization would have been able to go back into the past and teach others how to do the same. If time travel occurs once, it must happen anytime, because all times will witness time travel. Since we do not witness time travel, it doesnt exist.

[Canard DuJour]

If time travel occurs once, it must happen anytime, because all times will witness time travel. Since we do not witness time travel, it doesnt exist.

..yet.

[Preno]

Personally, I believe that time travel is only possible into the future. If it were possible to travel to the past, it would be very unlikely that in the entire age of the universe from big bang to fizle, no civilization would have been able to go back into the past and teach others how to do the same. If time travel occurs once, it must happen anytime, because all times will witness time travel. Since we do not witness time travel, it doesnt exist.Not necessarily. If a civilization using a time machine discovers that it has unpredictable consequences on the present, it may decide not to use it. Also, it may turn out that it's possible, but not practically feasible for larger distances (which is a fairly reasonable assumption, I think) - perhaps all the resources in a galaxy combined together are not enough to support a time jump of more than, say, one day (second, month, year) into the past?

[windsofchange]

We're already traveling through time. Forward. One second at a time. Every minute that goes by was in the future just a minute ago.

[Answerer]

Oh, time travel...............my favourite topic since its something that will never happen. :D

Why Time Travel Is Impossible

Because entropy forbids it if you are referring to travelling back past. No one can reverse the arrow of time and although one might experience some types of illusionary time travel due to relativity principle and influence of gravity or unobservable quantum effects like sending a wave backwards in time. There is no chance for time travel as we seen in movies like one getting younger, going back before one is killed or to visit their ancestors, etc. Some fictional stuffs are totally bullshit. (And sorry, I still don't buy the parallel universes theorem)

Others might disagree with me but I still must say the above. Of course, if one want to see your future descendants and planet Earth, there is a method however, you will never see your wife and children of the present time again. :D

[Majestyk]

...perhaps all the resources in a galaxy combined together are not enough to support a time jump of more than, say, one day (second, month, year) into the past? Again the question of space is in play. I speculate that if, an object does not move from a point in absolute space during its travel through time and its originating environment (room, planet, star) is in motion, that object will move in relation to its original environment (excepting quantum non-locality).

May be there's a few time-probes floating out in space somewhere? :Cheeky:

[Preno]

What kind of absolute space do you have in mind? Modern physics is based on the assumption that there is no privileged coordinate system that could correspond to some kind of absolute space.

[Answerer]

We're already traveling through time. Forward. One second at a time. Every minute that goes by was in the future just a minute ago.

We are travelling at the speed of light every moment although not always in the same dimension.

[drewjmore]

what's your opdef of time?

[covers head]

[Lógos Sokratikós]

Could there exist a "time telescope" where you might not travel through time but recover images of the past? Maybe we can't do the travelling, but particles or waves may?

[Preno]

Could there exist a "time telescope" where you might not travel through time but recover images of the past? Maybe we can't do the travelling, but particles or waves may?Whether it's possible or not I don't know, but it is the basic premise of A.C.Clarke's The Light of Other Days.

[skepticalbip]

Could there exist a "time telescope" where you might not travel through time but recover images of the past? Maybe we can't do the travelling, but particles or waves may?All telescopes are "time telescopes". When you look at a star that is ten Light-Years distant, you are seeing it as it was ten years ago.

[Majestyk]

What kind of absolute space do you have in mind? Modern physics is based on the assumption that there is no privileged coordinate system that could correspond to some kind of absolute space. Poor choice of terms on my part. Just trying to convey the motion involved. Rotation, orbit, planetary, solar, galactic. I would think that targeting the location of an object being "displaced" in time, would be somewhat problematic. Then I'm assuming that most people think of time travel in the H.G. Wells sense; sit there in your house as the world changes around you in some geocentric fashion.

[Jesse]

Poor choice of terms on my part. Just trying to convey the motion involved. Rotation, orbit, planetary, solar, galactic. I would think that targeting the location of an object being "displaced" in time, would be somewhat problematic. Then I'm assuming that most people think of time travel in the H.G. Wells sense; sit there in your house as the world changes around you in some geocentric fashion. The types of time travel suggested by known laws of physics like general relativity don't involving "jumping" discontinuously from one time to another like it's usually portrayed in sci-fi, it's about taking a continuous path through some distorted region of spacetime, like a wormhole, that leaves you in your own past. So where you'd end up would depend on the details of the distorted region--in the case of the wormhole, if you entered one mouth, you'd exit the other mouth wherever it was at moment you exit.

[CreamFilledGiraffe]

Time exists only in the human mind as a means of measuring change.

So there was no time "before" humans?

The physical universe only knows the present;

Fascinating. I didn't realize the physical universe knew shit!
it has no idea what the past and the future are. The physical universe knows only change and not time. There is no way to travel to the past because there is no past to travel to; the past does not exist. Same offer applies to the future.

Discuss, support, refute, ect...

At this precise moment I am traveling through time at the rate of one second per second. No matter how I have tried I have been unable to stop travelling through time.

[steamer]

Time exists or rather it is an attribute of things that exists.

If existence exists then it must exist for some time. If a thing exists for zero time, then that thing never existed.

If I toss time out of the equation, then is there anything that never existed that did exist for zero time?

Which statement is false?

A purple magic dragon did live in my garage for zero time.
A purple magic dragon did not live in my garage for zero time.

A purple magic dragon can both live and not live in my garage without time.

[enoch007]

being completely uninformed I am only slightly less informed on this speculative subject than the greatest physicists who have theorized but have not yet cracked this dilemma, so, now after positing my ignorant and rambling disclaimer I offer this:


The key to time travel as I see it (and I am a sucker for all time travel sci-fi) is "teleportation". Crack teleportation, the ability to transmit matter from one stationary location to another virtually instantaneously through matter dispersal, and you have opened the door to time travel. How, you might ask (or not), well , say I can teleport myself at this moment to the other side of the world. At the moment I do this the other side of the world will be somewhere plus or minus 12 to 16 hours ahead or behind myself where I originate. If the teleportation is virtually instantaneous (ie approaching speed of light or a reasonable phone call) I have advanced myself forward of backward the roughly 1/2 days time (depending on direction I travel in). Now, let's say I can teleport into an indeterminate reigion in space, say the moon, why then sir, the time shift/dilation increases. It is virtually travelling at the relatavistic measurement of light speed with hopefully the corresponding time effect. Ha! Time travel. Just be sure not to let any pesky insects into the chamber with you and travel nude. Now, please excuse me, I have an urgent appointment on planet Earth.

[Biff the unclean]

If the Earth rotates at 1000 mph and orbits the sun at 67,000 mph, not to mention the sun orbiting the galaxy and the galaxy hurtling through infinite space, then if you took a time machine back or forward 5 years where the hell would you be if it stayed in the same spot?

[IanC]

what's your opdef of time?

[covers head]

BobK
BobK
BobK

NOOOOOOOOOO

[BigJim]

I also believe travelling to the past is impossible. But what is definitely possible is to see the past, say by putting a giant mirror in space, or recording everything.

Wow, that's a cool thought I'd never pondered before. We could (for instance) prove OJ murdered his wife if we could put a mirror in space 6.5 light years away and aim it at his house (given a cloudless sky, and good enough resolution). Of course it would take a while to get there, but if we could move at more than .5 light years/year we could theoretically look into any past event that would be visible under the conditions...

[Jesse]

If the Earth rotates at 1000 mph and orbits the sun at 67,000 mph, not to mention the sun orbiting the galaxy and the galaxy hurtling through infinite space, then if you took a time machine back or forward 5 years where the hell would you be if it stayed in the same spot? Like I said above to Majestyk, the "instantaneous jump" version of time travel that you see in sci-fi doesn't have any basis in science, time travel in general relativity would always involve a continuous path through space and time. And as Preno said in an earlier post, there's no "absolute space" in relativity, so there's no single point in space now that's "the same spot" that the Earth was 5 years ago, different reference frames would define "the same point in space at different times" in different ways (in every observer's own rest frame, objects which are at rest relative to themselves are remaining at the same space coordinate).

[Jesse]

Wow, that's a cool thought I'd never pondered before. We could (for instance) prove OJ murdered his wife if we could put a mirror in space 6.5 light years away and aim it at his house (given a cloudless sky, and good enough resolution). Of course it would take a while to get there, but if we could move at more than .5 light years/year we could theoretically look into any past event that would be visible under the conditions... No, you'd have to move the mirror faster than light to see events before you launched it, which is impossible. If you launch it in 2000 at 0.9 light years per year, then in 2010 years it's 9 light-years away, so it's reflecting light that came from Earth 9 years earlier in 2001. It'd be a lot more practical just to set up a network of cameras on Earth (or in orbit around it) and record everything that way!

[rjf]

Not necessarily. If a civilization using a time machine discovers that it has unpredictable consequences on the present, it may decide not to use it.

This has always been my response to the 'where are they' question of time travelling tourists, and is, I think, a very underrated argument. People are so fascinated with the idea of time travel that they seem to gloss over the possibility that it will do nothing but cause problems. It may very well be that one day, we will discover the technology to allow time travel, but will make a conscious decision as a people never to utilize that technology on historical or moral grounds (when I was a teenager, just before I became a Xian, I wrote a short story about a guy from the future who goes back in time to meet Jebus, his time machine is destroyed by the Romans, and he gets stranded with a few futuristic gadgets and some advanced knowledge--and becomes Jebus. How would you like to be the cause of Xianity?)

[Krosis]

......because all times will witness time travel. Since we do not witness time travel, it doesn't exist.


Since we do not witness the wind, it doesn't exist...

Lame perhaps.. I know xtians use the same line or reasoning for god(tm) but for me here.. it illustrates a possiblity. How do we know that we aren't living in a timeline that has been altered radically?

Fiction gives plenty of Examples..

My favorite: Some time travelers accidentally interfere and save JFK from being assassinated.. as a result that time line was altered.. to the point that J Edgar Hoover became President and the Cuban Missile Crisis escalated to the point of Almost WWIII.. JFK went to prison in disgrace over some sort of sex scandel... So the time travelers went to the prison in the altered time line.. picked him up and recruited him to save the original timeline by.. assassinating himself. (Gunman on the grassy knoll.. yeah. ;) ) So he does.. and as the time line shifts he disappears and all is right again.

Would we notice time lines shifting?

(Incidentally the above scenario is from Red Dwarf Series 8.. favorite line: "It'll drive the conspiracy nuts crazy but (shrug) they'll never figure it out.." )

-Adam

[Gracchus]

My favorite time travel story is Heinlein's "All You Zombies".

http://en.wikipedia.org/wiki/'All_You_Zombies%E2%80%94'

:wave:

[35Kas]

How do we know that there are other timelines at all? If I were to travel to 1 day ago, would you still exist now (when I went to the past)?

If you kept existing, and I (1 day into the past) destroy the machine that allows time travel in the past, would that alter the future (1 day after I destroyed the machine)? Would it alter the future only for me in the past or would it destroy the timeline from i came from the future?

Does time travel entails the reversal into the past for the entire universe?

In order to prevent the paradoxes illustrated above, I believe so. It would seem that the universe only exists in one point in the time dimension. We could travel into the future for example. This would create no paradoxes so it seems very possible. Think of relativistic travel or going close to a black hole and back...

[Jesse]

How do we know that there are other timelines at all? If I were to travel to 1 day ago, would you still exist now (when I went to the past)?

If you kept existing, and I (1 day into the past) destroy the machine that allows time travel in the past, would that alter the future (1 day after I destroyed the machine)? Would it alter the future only for me in the past or would it destroy the timeline from i came from the future?

Does time travel entails the reversal into the past for the entire universe?

In order to prevent the paradoxes illustrated above, I believe so. It would seem that the universe only exists in one point in the time dimension. We could travel into the future for example. This would create no paradoxes so it seems very possible. Think of relativistic travel or going close to a black hole and back... The resolution usually favored by physicists who take the possibility of backwards time travel seriously is just that the timeline is totally fixed, and only self-consistent sets of events are permitted. So you wouldn't be able to change the past--you would be able to influence the past, but whatever influences you had would have been part of history all along, even before you took the trip back in time.

At least one physicist I know of (David Deutsch (http://en.wikipedia.org/wiki/David_Deutsch)) has also suggested that the science fiction idea where each trip back in time takes you to a parallel timeline, while your original timeline continues to exist, might actually be correct if the many-worlds interpretation (http://en.wikipedia.org/wiki/Many-worlds_interpretation) of quantum mechanics is correct. But other physicists think that even if the MWI is correct and time travel is possible, each time traveler would be stuck in a single self-consistent "world", just like in the fixed timeline theory above.

As far as I know, no physicists or philosophers take seriously the idea of "changing" the past in a single timeline a la Back to the Future and most other time travel movies, probably because the idea doesn't really make much sense (what does it mean to talk of 'changing' an entire timeline, when change itself seems to assume a difference between one time and another? Would there be a second time dimension to keep track of these 'changes'?) and doesn't really resolve the problem of paradoxes as the two views above do.

[Chuck Rightmire]

The comic strip non sequitor had two of its characters talking about time and one said to the other, I'll prove to you that time travel exists, then went away and came back five minutes later and said, "See you're in the future."

But, I agree. Despite the mathematics that use time quite effectively, I think that it is only a mathematical concept that makes formulas work out. But time is not a separate dimension. It is, instead, as I've said on some other threads working on this idea, a human measurement of the universal changes that have been occurring since they were set into motion by the Big Bank. That started the steam roller of a changing universe. Things happened, one after the other, the expansion, the cooling, the creation of various elements, the birth of stars that then exploded and led to other stars that also exploded and led to third generation stars like the earth. But that is just a consequence of change. Time is how we measure change and is strictly a human construct and will probably disappear with us although change will remain. We consider time a constant and thus we view changes as happening in an order that we postulate and name. But change actually occurs at different rates for different elements of the universe. And it is also something that will go on until the universe dies in one of the many ways physicists have postulated. The reason we cannot time travel except by moving ahead with change is that change never recurs. Once it's done it's done. In fact, the future is already happening as I have typed these words because there are being put in place long after, in real time, I actually conceptualized them. Or I might say I am typing these in the past.

[Jesse]

But, I agree. Despite the mathematics that use time quite effectively, I think that it is only a mathematical concept that makes formulas work out. But time is not a separate dimension. It is, instead, as I've said on some other threads working on this idea, a human measurement of the universal changes that have been occurring since they were set into motion by the Big Bank. Again, if you imagine there is a single universal "present" and thus an objective truth about whether two events at different locations happen at the "same time" or "different times", this would contradict the relativity of simultaneity, and thus relativity itself (unless you introduce this present as a purely metaphysical notion and agree with relativity that there is absolutely no way to experimentally determine which frame's definition of simultaneity is the 'correct' one).

[Lógos Sokratikós]

Originally Posted by Lógos Sokratikós
Could there exist a "time telescope" where you might not travel through time but recover images of the past? Maybe we can't do the travelling, but particles or waves may?


All telescopes are "time telescopes". When you look at a star that is ten Light-Years distant, you are seeing it as it was ten years ago.

Oh, I know that, but I was thinking it would be nice if we could view historical events on earth, a kind of particle or wave search engine (a la internet) that could crawl through the centuries. Put one in Jerusalem and see if there really was a Christ riding on a donkey... put one at a fossilized dinosaur nesting place and see what they were like millions of years ago... etc.

That would be awesome. I'm wondering if any current theory allows for that possibility.

[coloradoatheist]

Oh, I know that, but I was thinking it would be nice if we could view historical events on earth, a kind of particle or wave search engine (a la internet) that could crawl through the centuries. Put one in Jerusalem and see if there really was a Christ riding on a donkey... put one at a fossilized dinosaur nesting place and see what they were like millions of years ago... etc.

That would be awesome. I'm wondering if any current theory allows for that possibility.

I don't even think it's theoritically possible because to get the mirror in place you would have to get the telescope farther out then the light source from that time so you would have to send the telescope out faster than the speed of light. But theoritically another civilization could have one and like a satellite in earth that can see people and buildings now they could see 2,000 years ago.

Mike

[premjan]

According to relativity, a wormhole may permit travel to the past of another location in space, at least a past that does not precede the time taken for light to travel between the two locations. Technically you could then take two steps to go into the past of your own spacetime location. Though I wonder if this is just an outcome of equations with no physical basis. According to the Barbour's theory there is no state of the past maintained anywhere and the whole universe just switches state from one moment to the next. Maybe it does so in a local manner so different parts of the universe may be at different times.

[c davis]

There is no time history to the universe. Therefore, there is no fixed past or future to be observed. I don't see how time travel is possible. I think that everything looks good on paper as an equation.

[untermensche]

Traveling backward in time would not require that time were anything more than change, but it would require a past to go back to.

The specific arrangement of matter and energy that was some past moment must exist infinitely so that it can be returned to.

But of course once some person from the future appeared, it would not be the same moment from the past.

And then there is the problem of the conservation of matter.

The person from the future is made up of matter that was making up other things in the past. To introduce a person to the past is to add matter to the universe, unless the matter that makes up the person instantly disappears from the past as the person appears in the past.

[Answerer]

According to relativity, a wormhole may permit travel to the past of another location in space, at least a past that does not precede the time taken for light to travel between the two locations.

A wormhole travel is not possible because:

1) The entrance of the hole is damn small. (Size of a lepton or close)

2) Any spacecraft need to survive the gravitational force of a black hole which is impossible

3) The hole opens and closes in an instant

4) You need a great energy equivalent to that of a galaxy in order to make a hole

Conclusion: Another star war fantasy..............

[Chuck Rightmire]

Again, if you imagine there is a single universal "present" and thus an objective truth about whether two events at different locations happen at the "same time" or "different times", this would contradict the relativity of simultaneity, and thus relativity itself (unless you introduce this present as a purely metaphysical notion and agree with relativity that there is absolutely no way to experimentally determine which frame's definition of simultaneity is the 'correct' one).

:huh:

I think that change happens at different rates to different things, although we measure them with our clocks and say they occur at the same time. The live span of the butterfly, the turtle, the Sequoia and the Joshua Tree change at different rats although we all go through similar changes. If we could look at the world through their perceptions, we would have a different view for each one. Time is also different for each of us. When I was a child and a year was one-tenth of my life, it passed much more slowly than it does now (but I'm not going to say how much slower).:o

[steamer]

But, I agree. Despite the mathematics that use time quite effectively, I think that it is only a mathematical concept that makes formulas work out.

Time existing seems to explain quite well why each of our posts are ordered as they are in this thread. We comment on things already written and sometime after I post this, someone else may comment on it. You cannot comment on this post until after I post it (Later in time). It seems to me any reply would be an admission that time exists.

[Jesse]

:huh:

I think that change happens at different rates to different things, although we measure them with our clocks and say they occur at the same time. The live span of the butterfly, the turtle, the Sequoia and the Joshua Tree change at different rats although we all go through similar changes. If we could look at the world through their perceptions, we would have a different view for each one. Time is also different for each of us. When I was a child and a year was one-tenth of my life, it passed much more slowly than it does now (but I'm not going to say how much slower).:o Sure, things change at different rates, but do you think there is a single universal "present" and a single truth about whether two events happen "at the same time" or "different times"? Say the two events are me opening a door in Rhode Island and you turning on a light in Montana. Do you think there must be a single truth about whether you turned on the light before I opened the door, after I opened the door, or at the same time as I opened the door?

[Jesse]

A wormhole travel is not possible because:

1) The entrance of the hole is damn small. (Size of a lepton or close) Why do you say that? Wormholes are not known to exist at all, they are just known as theoretical solutions to the equations of general relativity. And theoretically the wormhole opening can be as big as you like. 2) Any spacecraft need to survive the gravitational force of a black hole which is impossible Well, the larger the black hole, the closer you can get to the central singularity before being ripped apart by tidal forces. A traversable wormhole doesn't have a singularity, there's no region where the tidal forces would go to infinity as you pass through it, in theory I think they can be made small enough to be manageable. 3) The hole opens and closes in an instant This is true for the Schwarzschild wormhole (http://en.wikipedia.org/wiki/Wormhole#Schwarzschild_wormholes) or "Einstein-Rosen bridge", but when physicists talk about the possibility of using wormholes for time travel, they are talking about a different type discovered by Kip Thorne known as a Traversable wormhole (http://en.wikipedia.org/wiki/Wormhole#Traversable_wormholes) or "Morris-Thorne wormhole", which is held open by "exotic matter" with a negative energy density (which may be possible due to quantum effects such as the Casimir effect (http://en.wikipedia.org/wiki/Casimir_effect)). Thorne's book Black Holes and Time Warps is a good place to learn about how these wormholes are consistent with general relativity. 4) You need a great energy equivalent to that of a galaxy in order to make a hole No one really knows how it would be possible to create a wormhole in the first place, sometimes physicists speculate about blowing up a naturally-occurring planck-scale wormhole which arises in the hypothesized quantum foam (http://en.wikipedia.org/wiki/Quantum_foam) but the details of how this would be done are totally unknown and so the energy requirements are as well. It was once thought that the energy needed to hold open an existing traversable wormhole using exotic matter would be tremendous, it was later discovered that this energy could be made arbitrarily small (see http://arxiv.org/abs/gr-qc/0301003 ). Conclusion: Another star war fantasy.............. Well, even if it's allowed by the fundamental laws of physics--and most physicists would probably bet that a theory of quantum gravity would forbid time travel, though not necessarily all traversable wormholes--it's still not something that would be possible anytime soon, only a very "advanced civilization" (as physicists say) would have the resources to make it reality.

[Answerer]

Okay Jesse, I should take a serious look at traversable wormholes, to see if they really stands up to the scrutiny of able to produce a FLT phenomena within our human capability. If its true, it will probably begs the question of whether higher dimension exists or not.

[DietCoke]

Time exists only in the human mind as a means of measuring change. The physical universe only knows the present; it has no idea what the past and the future are. The physical universe knows only change and not time. There is no way to travel to the past because there is no past to travel to; the past does not exist. Same offer applies to the future.

Discuss, support, refute, ect...

It seems intuitive to me that my observations of the universe are merely constructs. However, I don't see how my observations being constructs leads to the conclusion that the thing I'm observing does not exist. They seem like seperate questions to me.

[Hawkingfan]

According to relativity, a wormhole may permit travel to the past of another location in space, at least a past that does not precede the time taken for light to travel between the two locations. Technically you could then take two steps to go into the past of your own spacetime location. Though I wonder if this is just an outcome of equations with no physical basis. According to the Barbour's theory there is no state of the past maintained anywhere and the whole universe just switches state from one moment to the next. Maybe it does so in a local manner so different parts of the universe may be at different times.
Yes, I think this is along the lines of what Jesse is saying, too.

A perfect example would be the "time" between the Earth's frame and the Sun's frame of reference. It takes the light from the Sun eight minutes to reach the Earth. So if the Sun exploded, we on Earth would not know it until eight minutes later (by the sun's frame) and by the Earth's frame the Sun had exploded eight minutes prior to us seeing it.

If someone travelled through a wormhole to the Sun, saw it explode, they would be in the future from our frame of reference (because the sun hasn't exploded for us, it hasn't happened yet).

[Jesse]

Yes, I think this is along the lines of what Jesse is saying, too.

A perfect example would be the "time" between the Earth's frame and the Sun's frame of reference. It takes the light from the Sun eight minutes to reach the Earth. So if the Sun exploded, we on Earth would not know it until eight minutes later (by the sun's frame) and by the Earth's frame the Sun had exploded eight minutes prior to us seeing it.

If someone travelled through a wormhole to the Sun, saw it explode, they would be in the future from our frame of reference (because the sun hasn't exploded for us, it hasn't happened yet). In this case the wormhole is not actually allowing you to enter your own past light cone, though, so it isn't genuine time travel (you couldn't go shake hands with your own younger self, for example), although it is a form of "apparent" FTL travel ('apparent' because you're only moving faster than light which travels outside the wormhole, any light that moves through the wormhole with you will still go faster than you, so you never 'locally' move faster than light). The key to using a wormhole for actual time travel is to take one of the two wormhole mouths on a trip at relativistic velocities. Here's a quick explanation from the wikipedia article on time travel (http://en.wikipedia.org/wiki/Time_travel): A proposed time-travel machine using a traversable wormhole would (hypothetically) work something like this. One end of the wormhole is accelerated to nearly the speed of light, perhaps with an advanced spaceship, and then brought back to the point of origin. Due to time dilation, the accelerated end of the wormhole has now aged less than the stationary end, as seen by an external observer; however, time connects differently through the wormhole than outside it, so that synchronized clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole, no matter how the two ends move around. This means that an observer entering the accelerated end would exit the stationary end when the stationary end was the same age that the accelerated end had been at the moment before entry; for example, if prior to entering the wormhole the observer noted that a clock at the accelerated end read a date of 2005 while a clock at the stationary end read 2010, then the observer would exit the stationary end when its clock also read 2005, a trip backwards in time as seen by other observers outside.

[IanC]

Anyone remember Malletts idea? Where you spin space with lasers and fire something around the moving space so as to send it back in time?

I know there are objections due to the requirements of spinning space with lasers (large energy, etc). How valid is the idea though? Say one were to spin space with something else, like a rotating black hole (if you have a large one handy), could you use this to go back in time?

[StillDreaming]

It may very well be that one day, we will discover the technology to allow time travel, but will make a conscious decision as a people never to utilize that technology on historical or moral grounds
Or because a time travel device might not be portable. If a time-travel machine requires a nuclear-plant-style or collider-style installation and hundreds of people to operate it, it will get hard to return if you travel to a time/location where no such machine is available.

[c davis]

Traveling backward in time would not require that time were anything more than change, but it would require a past to go back to.

The specific arrangement of matter and energy that was some past moment must exist infinitely so that it can be returned to.

But of course once some person from the future appeared, it would not be the same moment from the past.

And then there is the problem of the conservation of matter.

The person from the future is made up of matter that was making up other things in the past. To introduce a person to the past is to add matter to the universe, unless the matter that makes up the person instantly disappears from the past as the person appears in the past.

That is one of the many problems I have with the thought of "time" or "travel in time"....your example the person in the future is made up of matter. Again I cannot see the person in the past instantly disappear to make up the person in the future. However, the though of observing "time" as change stands. Time as change observed can be seen as events from the past. Change can be view as things or events that have occurred in the past.... even thou we may be viewing those (things) or events in the moment. I believe someone else used a good example of this......starlight traveling through space...it take hundreds of years to reach earth. Therefore we are not seeing the stars as they are within the moment.. but as they were in the past.

[c davis]

:huh:

I think that change happens at different rates to different things, although we measure them with our clocks and say they occur at the same time. The live span of the butterfly, the turtle, the Sequoia and the Joshua Tree change at different rats although we all go through similar changes. If we could look at the world through their perceptions, we would have a different view for each one. Time is also different for each of us. When I was a child and a year was one-tenth of my life, it passed much more slowly than it does now (but I'm not going to say how much slower).:o

Good example. Went we try to find the origin of time it vanishes, although we may have a feeling of time passing... moving fast or slow. I think everyone and every living thing on this planet carries their own sense of time. Matter is simply little time bombs waiting to entropy and convert back to energy. So therefore I can conclude that everyone carries with them their own past and future. It must be the same for the universe as a whole.

[Bob K]

When time is defined as the use of a chosen duration modeled from a physical cycle as the time-interval to be used for the unit of temporal measurement in the temporal measurement process which is the measurement of the number of time-intervals in the durations between the occurrences of events and the durations of single events and for the determination of the simultaneity of events then the Continuum of Time becomes thus:

Past Infinity <- ... <- T-2 <- T-1 <- T0 (Origin) -> T+1 -> T+2 -> ... -> Infinity Future

When the matter/energy of the universe is considered to by an isolated m/e system because ...

1. M/e cannot be added (where would the additional m/e come from?);
2. M/e cannot be removed (where would the removed m/e go?);
3. The sum total of the universal m/e is a constant;
4. The universal m/e is indestructible

... then at each timepoint on the Continuum of Time there is a configuration of the universal m/e for which the sum total is a constant.

Time travel is not possible because the arrow of time must measure the occurrences and durations of physical phenomena which are the m/e events which progress in causality and momentum only in the forward direction from previous m/e configurations at previous timepoints through the present m/e configuration in the present timepoint into future m/e configurations in future timepoints and thus there is no possibility of returning to a previous m/e configuration at a previous timepoint or moving forward to a future m/e configuration at a future timepoint because the m/e of an individual attempting time travel cannot be removed from the present m/e configuration sum total nor added to the m/e configuration sum total of a previous or future m/e configuration.

[Chuck Rightmire]

Time existing seems to explain quite well why each of our posts are ordered as they are in this thread. We comment on things already written and sometime after I post this, someone else may comment on it. You cannot comment on this post until after I post it (Later in time). It seems to me any reply would be an admission that time exists.

Not exactly. I can post this after you as a change in this thread. Because you're change hadn't happened at my last perusal, I couldn't respond to it until this perusal. But the sequence existed. The little element in the upper left corner will let us tell how much of our measurement of time has elapsed since you posted and I posted. But the sequence remains in the order because one has to follow the other. The time differential in the upper left means nothing for this reply as it meant nothing for your original reply to me. But the sequence of change means something and that is not our concept of time.

[Chuck Rightmire]

Sure, things change at different rates, but do you think there is a single universal "present" and a single truth about whether two events happen "at the same time" or "different times"? Say the two events are me opening a door in Rhode Island and you turning on a light in Montana. Do you think there must be a single truth about whether you turned on the light before I opened the door, after I opened the door, or at the same time as I opened the door?

I don't think it matters. The two elements of change are not in any connectable sequence. What element of our time they take place in has no bearing on anything. If they were to come together through a wormhold where your opening the wormhole and I turned on a light it might be consequential, but as it is, I don't think so. Let's say you open the door at 10 p.m. and I turn on the light at 10 p.m. Does it make any difference that we are two hours apart?

[Jesse]

I don't think it matters. The two elements of change are not in any connectable sequence. What element of our time they take place in has no bearing on anything. If they were to come together through a wormhold where your opening the wormhole and I turned on a light it might be consequential, but as it is, I don't think so. Let's say you open the door at 10 p.m. and I turn on the light at 10 p.m. Does it make any difference that we are two hours apart? Well, my point here is that if there's no objective present, then there's also no objective sense in which some events "haven't happened yet" or "have already happened", so that leads naturally to the spacetime view where what we call the "past" and "future" are both equally real, just different "locations" in a 4-dimensional continuum. If that's true, it would seem to contradict your earlier statement that "time is not a separate dimension. It is, instead, as I've said on some other threads working on this idea, a human measurement of the universal changes that have been occurring since they were set into motion by the Big Bank."

[Answerer]

But of course once some person from the future appeared, it would not be the same moment from the past.

And then there is the problem of the conservation of matter.

The person from the future is made up of matter that was making up other things in the past. To introduce a person to the past is to add matter to the universe, unless the matter that makes up the person instantly disappears from the past as the person appears in the past.

According to CPT symmetry and Feynman diagrams, antimatter are sometimes viewed as matter travelling back to the past. Personally, I like that concept for it gives a rather beautiful picture of the otherwise randomness and chaotic sub quantum world.


Anyway Jesse, back to your proposed "traversable wormholes", it seems to be improbable that they can be anything more than a mathematical solution for GR. Our technological ability to create or find even a few negative and complex masses seems to be more improbable than finding monopoles or traces of dark energy spectrum. I don't know about you but even if "erotic" matter is to exist, surely due to no observable records of their existence, it must lie beyond our familiar spacetime and to obtain them will require a great feat that humans might most likely never reach.

Its more difficult than resurrecting the dead. :grin:

[premjan]

Would clumps of antimatter violate cosmological arrow of time, reduce entropy spontaneously etc? Don't QM spooky effects, delayed choice etc. imply that space and time are not real in QM? If the universe is holographic maybe we can pop ourselves into existence anywhere in time and space if we can go "back to the focal origin" of the holographic projector?

[Answerer]

Would clumps of antimatter violate cosmological arrow of time, reduce entropy spontaneously etc?

Law of Entropy applies for macro systems while CPT symmetry applies to Quantum particles, no contradiction.................

Don't QM spooky effects, delayed choice etc. imply that space and time are not real in QM?

QM violates all our normal notions and perceptions of spacetime, position, energy and magnetic spin. However, that doesn't apply that they don't exist or are imaginary.

If the universe is holographic maybe we can pop ourselves into existence anywhere in time and space if we can go "back to the focal origin" of the holographic projector?

You don't need QM to do that. If you are travelling at the speed of light, then in your point of observation, you are already everywhere simultaneously (Omnipresent) along the plane of direction that you are moving. That is if you still have the sense of time to feel that.

[Tartantyco]

-Timetravel in 1,2,3:

Get one of your friends to stand stationary while you go for a run. Congratulations, you've just travelled into the future(In relation to your friend that is), have a cookie.

[Bob K]

[1] According to CPT symmetry and Feynman diagrams, antimatter are sometimes viewed as matter travelling back to the past. [2] Personally, I like that concept for it gives a rather beautiful picture of the otherwise randomness and chaotic sub quantum world.

INRE 1: Regardless of theories, it is not possible for matter or antimatter or any object comprised of m/e of any kind to travel back into time simply because of the fact that time has an asymmetrical arrow and the universal m/e is an isolated m/e system from which m/e cannot be removed and to which m/e cannot be added and in which the universal m/e sum total is a constant and the universal m/e is indestructible; thus, for each timepoint on the Continuum of Time there is a one-and-only-one universal m/e configuration which cannot be returned to (time travel into the past) or leaped into (time travel into the future).

INRE 2: Determinism is alive and well at all physical scalar levels.

Determinism eliminates chaos and randomness.

Law of Inertia and Its Corollaries

The Law of Inertia: An object having an inertial state of being at rest or in motion remains at rest or in motion until acted upon by a force.

Corollaries:

1. Only a force, a form of matter/energy, a push or a pull, which causes accelerations and decelerations, can cause a change of the inertial state of an object comprised of matter and/or energy.
2. The observation of a change of inertial state implies its cause to be a force of some kind.

Corollary 2 reveals that any observation of a change of inertial state implies the cause to be a force of some kind and therefore determinism is occurring.

At QM and smaller scales where scientists cannot observe changes of inertia of individual particles or quanta they can nevertheless observe the percentages of changes of inertia in known quantities of particles/quanta in finite volumes to determine probabilistic mathematical laws inre predictable changes of inertial states, but these changes of inertial states, like all changes of inertial states, are caused by forces of some kind and therefore determinism is occurring at QM and smaller scales.

[rjf]

I have been thinking more about this over the last couple of days, and I have a question.

'Time' seems to be little more than a means to measure the change in spatial dimension of particles (or, somewhat more clearly, a change in where things are). This change is measured relatively; how much more distant or closer things are to each other in relation to one another. In other words, in a room where absolutely no particles ever moved relative to one another, time could not be said to be passing (yes, I am aware that such a room is impossible; bare (bear?) with me, I'm going somewhere with this).

We measure years by the relative location of the sun as it rises; days and hours by the movement of the sun across the sky, hours and minutes by our own movements and activities. When I say that I have been sitting here in a chair in a hotel room in Tulsa for an hour, I mean that the sun has moved 1/24th of the way across the sky, that John Smith driving down the turnpike is 75 miles further from me than he was when I sat down, and that my location relative to the table upon which I have set my computer has remained (more or less) unchanged. For me to travel in time, therefore, it seems that I must move not only myself, but all of these other things back to the state they were in when I sat down here. For me to travel one hour back in time, the table must remain where it is, John must drive in reverse, and the sun must return 15 degrees. Not only that, but the entire rest of the universe must reset itself to how it was when I sat down. This seems to me to be impossible, and, as I am a firm believer that the mind is a product of our physical states, means that even my mind would return to its previous state and I would simply type all this again.

As for the way we typically think of time travel, (moving a single body through time) the problem isn't reduced much--all we have done is change the state of the entire universe except that body. I would be put into a sort of 'anti-time' bubble while the rest of the universe was rewound.

Reading back through this, I hope I made myself clear (I understood what I meant, but then, I already did), and someone can address this idea (I am totally okay with someone calling me an idiot, too, if they can explain why).

[Tubby Lardmore]

'Time' seems to be little more than a means to measure the change in spatial dimension of particles (or, somewhat more clearly, a change in where things are). This change is measured relatively; how much more distant or closer things are to each other in relation to one another.

I wonder if an exception to this would be the decay of certain particles. The muon, for instance, is currently thought to be elementary. It has a (quite brief) half-life for decay into other particles.

[jgh3115]

Time travel is indeed possible, but not in the traditional hop in a machine and blast back to the past sense. Using Einsteins theory of relativity, one could travel into the future 400,000 years relative to the Earth, but only feel as though 4 days had passed relative to themselves. If this person were to return to Earth, the Earth would have aged 400,000 years, and they would have aged 4 days.

As for traveling back into the past, if a person could move faster than the speed of light (which is the natural speed barrier), then theoretically they would start moving backwards in time relative to Earth. Time travel is possible, but all your doing is playing on the relativity of time.

[Hawkingfan]

-Timetravel in 1,2,3:

Get one of your friends to stand stationary while you go for a run. Congratulations, you've just travelled into the future(In relation to your friend that is), have a cookie.
Is this supposed to be a joke? You do realize what you have said is true, yes? (Although the amount of time is exponentially miniscule.) What Jesse said refers to wormholes. But being in and out of light cones still applies here.

[Bob K]

Time travel is indeed possible, but not in the traditional hop in a machine and blast back to the past sense. Using Einsteins theory of relativity, one could travel into the future 400,000 years relative to the Earth, but only feel as though 4 days had passed relative to themselves. If this person were to return to Earth, the Earth would have aged 400,000 years, and they would have aged 4 days.

As for traveling back into the past, if a person could move faster than the speed of light (which is the natural speed barrier), then theoretically they would start moving backwards in time relative to Earth. Time travel is possible, but all your doing is playing on the relativity of time.

The fact that the universe = space, time and physics (matter/energy) reveals that at one timepoint there is one-and-only-one m/e configuration and the fact that the universal m/e configuration is an isolated m/e system from which m/e cannot be removed and into which m/e cannot be added, therefore there can be no time travel from one timepoint & m/e configuration to another.

Time is the use of a chosen duration to be the time-interval which is the unit of measurement of the durations between the occurrences of events and the durations of of events and the determination of the simultaneity of events.

A clock's time-interval generates the clock's rate of ticking, timepoints and timeline. The clock's timecount is the clock's recording of the chronological change it has measured.

The temporal process measures by the time-interval the changes of physical phenomena including biological changes; the temporal measurement process does not cause changes of physical phenomena.

If an organism's rate of metabolism/biological change is linked to the organism's chronological age, then, in theory, all you would need to know is the organism's chronological age to know how far its metabolism has progressed.

But if an organism's rate of metabolism/biological change can be changed while the temporal measurement of his rate of change of his chronological age remains unchanged, then his chronological age will increase at a different rate than his metabolic or biological age.

You are confusing the change of the rate of the metabolic processes of an organism (space traveler) which are relevant to the aging process with the organism's chronological age and with time travel; the chronological age of a space traveler has no bearing on his biological "age."

These considerations provide more physcal evidence proof of the impossibility of time travel.

[Caine]

These considerations provide more physcal evidence proof of the impossibility of time travel.

Physical evidence is not proof. There is no real proof in science.

[Mr. Logic]

I have been thinking more about this over the last couple of days, and I have a question.

'Time' seems to be little more than a means to measure the change in spatial dimension of particles (or, somewhat more clearly, a change in where things are). This change is measured relatively; how much more distant or closer things are to each other in relation to one another. In other words, in a room where absolutely no particles ever moved relative to one another, time could not be said to be passing (yes, I am aware that such a room is impossible; bare (bear?) with me, I'm going somewhere with this).

We measure years by the relative location of the sun as it rises; days and hours by the movement of the sun across the sky, hours and minutes by our own movements and activities. When I say that I have been sitting here in a chair in a hotel room in Tulsa for an hour, I mean that the sun has moved 1/24th of the way across the sky, that John Smith driving down the turnpike is 75 miles further from me than he was when I sat down, and that my location relative to the table upon which I have set my computer has remained (more or less) unchanged. For me to travel in time, therefore, it seems that I must move not only myself, but all of these other things back to the state they were in when I sat down here. For me to travel one hour back in time, the table must remain where it is, John must drive in reverse, and the sun must return 15 degrees. Not only that, but the entire rest of the universe must reset itself to how it was when I sat down. This seems to me to be impossible, and, as I am a firm believer that the mind is a product of our physical states, means that even my mind would return to its previous state and I would simply type all this again.

As for the way we typically think of time travel, (moving a single body through time) the problem isn't reduced much--all we have done is change the state of the entire universe except that body. I would be put into a sort of 'anti-time' bubble while the rest of the universe was rewound.

Reading back through this, I hope I made myself clear (I understood what I meant, but then, I already did), and someone can address this idea (I am totally okay with someone calling me an idiot, too, if they can explain why).


Sounds like your living on Tulsa Time! <s>
They told me when I agreed to come back from the Future that there would skeptics like you! <s>
O.K. on a serious note: I agree! I do believe however that we can travel back virtually! If we can rewind a Video Tape, etc. why not History?
This way History can't be changed, but we could see what truly happened, etc.
Ever hear of Time Traveler John Teter? I don't believe him for the reasons that you gave.

[Mr. Logic]

So there was no time "before" humans?

Fascinating. I didn't realize the physical universe knew shit!


At this precise moment I am traveling through time at the rate of one second per second. No matter how I have tried I have been unable to stop travelling through time.

CreamFilledGiraffe? Wow that's a mouthful! <s> Sorry, I couldn't resist!

[Brian_Boru]

BobK
BobK
BobK

NOOOOOOOOOO

Where's that smilie with the shaking fist when I need it. :wave:

[Bob K]

Physical evidence is not proof. There is no real proof in science.

Your comment is non-productive nonsense.

[aguy2]

Physical evidence is not proof. There is no real proof in science.

Your comment is productive and sensible.
aguy2(amen)

[Chuck Rightmire]

While thinking of this topic in the middle between going to bed and going to sleep, I thought of something that I would like to reactivate this thread to discuss. Let us say that time is an actual fourth dimension as we treat it in relativity. Do we then have a long trail, like the body of a worm, trailing behind us in space? Or is what we exist in now kind of like a pie plate being tossed through space before it dies out or hits the final victim? In other words is the present a slice moving through space, or is it the lead point of a long trail of events? If it is the latter, then it seems to me that time travel would be possible. If it is the former, then there is nothing left to travel back to. It has changed into the present. I'll admit that I tend to believe in the latter. But I would like some thoughts on this.:confused:

[Caine]

Your comment is non-productive nonsense.

:mad:

Boooooooooo.

Your comment is productive and sensible.
aguy2(amen)

:thumbs:

At least some people appreciate the precision of language when discussing scientific matters.

[Jesse]

Let us say that time is an actual fourth dimension as we treat it in relativity. Do we then have a long trail, like the body of a worm, trailing behind us in space? Well, it'd be trailing behind us in spacetime, in the time direction. And it'd also be trailing ahead of us until the point of our death--the "worm" is frozen in spacetime like a real worm trapped in amber. In other words is the present a slice moving through space, or is it the lead point of a long trail of events? In relativity "present" is not absolute but relative to a given moment of observation, just like the term "here". There is no notion of a "true present" which is moving forward, with everything beyond it in an indefinite state (a 'true present' would conflict with the relativity of simultaneity (http://en.wikipedia.org/wiki/Relativity_of_simultaneity), which basically means that different reference frames slice spacetime at different angles and so disagree about whether two events happed at the 'same moment' or at 'different moments)--in relativity all of spacetime, past present and future, is modeled as having a definite state with a definite geometry. Backward time travel would just mean the "worm" representing the time traveler's true 4-dimensional self is curled around in such a way that it can cross itself, with the point of crossing as the event of the time traveler encountering her younger self.

[Bob K]

Let us say that time is an actual fourth dimension as we treat it in relativity. [1] Do we then have a long trail, like the body of a worm, trailing behind us in space?

[2A] Well, it'd be trailing behind us in spacetime, in the time direction. And it'd also be trailing ahead of us until the point of our death--the "worm" is frozen in spacetime like a real worm trapped in amber.

[2B] In other words is the present a slice moving through space, or is it the lead point of a long trail of events?

[3] In relativity "present" is not absolute but relative to a given moment of observation, just like the term "here". There is no notion of a "true present" which is moving forward, with everything beyond it in an indefinite state (a 'true present' would conflict with the relativity of simultaneity, which basically means that different reference frames slice spacetime at different angles and so disagree about whether two events happened at the 'same moment' or at 'different moments)--in relativity all of spacetime, past, present and future, is modeled as having a definite state with a definite geometry. Backward time travel would just mean the "worm" representing the time traveler's true 4-dimensional self is curled around in such a way that it can cross itself, with the point of crossing as the event of the time traveler encountering her younger self.

INRE 1: When time is defined as the use of a duration called a time-interval for the unit of measurement of the occurrences and durations of events and for the determination of the simultaneity of events, then time is obviously a measurement process—the temporal measurement process—in contrast to the spatial measurement process conducted by the use of space-intervals.

NOTE: Simultaneity is defined as two or more events occurring at the same instant, or timepoint. This is in contrast to Einstein's specification that simultaneity of two or more events equidistant from a single observer be observed to strike the observer at the same instant/timepoint.

Simultaneity Definitions

http://en.wikipedia.org/wiki/Simultaneity

Simultaneity
From Wikipedia, the free encyclopedia

Simultaneity is the property of two events happening at the same time in at least one reference frame.

* Gramatically, it means happening at the same time.

The Oxford Dictionary of Physics
Alan Isaacs, ed.
Oxford University Press, Fourth Edition, 2000

simultaneity The condition in which two or more events occur at the same instant. In classical physics any two events found to be simultaneous by one observer will also appear to be simultaneous to any other observer. In the theory of relativity, however, this is not true.

The essence of time is the time-interval which is the unit of temporal measurement.

The time-interval generates a clock's rate of ticking and the clock's set of timepoints on the clock's timeline—a record of the accumulation of timepoints.

The essence of the temporal measurement process is the direction which is always the measurement of timepoints from the past through the present into the future.

At each timepoint there is a configuration, or arrangement, or organization, or pattern, of the universal matter/energy.

If time-intervals were invariable, which would occur if their host clocks were "independent of the state of motion of [their bodies] of reference" [Einstein in Relativity] because they—the clocks—were adjusted to compensate for changes in their kinetic mass-energies caused by accelerations and decelerations and which affect the clocks' rates of ticking, then the invariable time-intervals would measure universal time and generate universal timepoints for the universal timeline which is the Continuum of Time:

Past Infinity <- ... <- T-2 <- T-1 <- T0 (Origin/Now) -> T+1 -> T+2 -> ... -> Infinity Future

NOTE: The US GPS satellite clocks are "independent of the state of motion of [their bodies] of reference" (their host satellites) and are therefore measuring universal time so long as the Earth, which is the body of reference of the master clock which is controlling by radio signals the satellite clocks and thereby synching them with Earth time and thereby making the satellite clocks slave clocks which are "independent of the state of motion of [their bodies] of reference," is in a stable uniform motion (which it has been for millions or billions of years) inre its rotation about its axis and its orbit about the Sun.

The essence of space is the space-interval which is the unit of spatial measurement.

The space-interval generates spacepoints on a ruler which are used for measuring the lengths of spatial distances—spatial separations of space-points.

If space-intervals were invariable, which would occur if their rulers were "independent of the state of motion of [their bodies] of reference" because their lengths were adjusted to compensate for the changes in their kinetic mass-energies caused by accelerations and decelerations, then the invariable space-intervals would measure universal space and generate universal spacepoints for the universal m/e configuration present at each universal timepoint on the Continuum of Time.

The past timepoint and m/e configuration combinations no longer exist; they were in existence together but once separated by time can never, EVER, be reunited under any circumstances because it is the essence of temporal measurement that time is measured from the past through the present and into the future with the result that no past or present timepoint could ever be generated again.

The universal m/e system is an isolated m/e system to which m/e cannot be added and from which m/e cannot be removed and in which the sum total of m/e is a constant and all m/e is indestructible in accord with the conservation laws of matter, energy, momentum, and electric charge.

It will never, EVER, be possible to reverse the motions of all objects so as to return them to previous universal m/e configurations because to cause such reversals would require adding energy into the universal m/e system which is impossible because the universal m/e system is an isolated m/e system as described to which energy cannot be added.

Also, because of the essence of temporal measurement from the past through the present into the future, any attempts to reverse any motions of any objects would occur at future timepoints and any recurrences of previous universal m/e configurations would occur at future timepoints, and, thus, there could never, EVER, be a reunion of a past timepoint with a previous m/e configuration.

Thus, time travel is absolutely impossible.

INRE 2: At present I cannot imagine combinations of timepoints on the Continuum of Time and universal m/e configurations generating a trail or tail as hypothesized.

The spatial reality of the universe, which is space, has a 3D configuration at each timepoint.

If we could create holograms of each timepoint & m/e configuration, which would be essentially 3D photographs, then the holograms could be played backwards or forwards and would generate 3D images similar to motion pictures being generated by 2D photoframes, but at no level of imagination could these holograms BE the actual timepoint & m/e configuration combinations any more than a motion picture could ever BE the people/objects/events it visually and audibly generates. (You night see it and here it, but you could never touch it!)

INRE 3: From the considerations presented in INRE 1, it should be obvious that spacetime can indeed be separated into space inre the universal m/e configuration and time inre the universal Continuum of Time and regardless of whether or not a previous universal m/e configuration could ever be re-generated the previous universal timepoint could never, EVER, be re-generated, and thus the previous timepoint & m/e configuration combination could never, EVER, be re-generated regardless of m/e crossovers, etc.

Simultaneity, in the modern definition, is local when the timepoints at which local events occur are measured by local clocks—clocks whose time-interval durations are deformed by accelerations and decelerations and therefore the clocks are not "independent of the state of motion of [their bodies] of reference"—but can be universal when the timepoints at which events in different reference frames (on different reference bodies) are measured by universal clocks—clocks whose time-interval durations are adjusted to compensate for the deformation caused by accelerations and decelerations and therefore the clocks are "independent of the state of motion of [their bodies] of reference"; thus, there is a difference between local simultaneity, which Einstein stated could be determined by the use of synchronized local clocks, and universal simultaneity, which Einstein state could not be determined but yet can be determined when the modern definition of simultaneity is accepted and by the use of synchronized universal clocks.

By the use of synchronized universal clocks, whose time-intervals, rates of ticking, timepoints, and timelines are all identical, the present as a combination of the present/NOW timepoint and the present/NOW universal m/e configuration can be established as a physical reality.

[Caine]

OK Bob K, we get it. In Bobland where relativity is a load of old cobblers, time travel is impossible.

In the real world where relativity is correct, meaningful discussions can be had regarding the subject.

[Bob K]

OK Bob K, we get it. [1] In Bobland where relativity is a load of old cobblers, time travel is impossible.

[2] In the real world where relativity is correct, meaningful discussions can be had regarding the subject.

INRE 1: Relativity is fine up to a point—local time, local rulers, measurement problems caused when forces which cause accelerations and decelerations cause changes of the kinetic mass-energies of clocks and rulers and thereby cause time and space dilations, etc.

But when the reality is that universal time and universal space can be measured by clocks and rulers whose time-itnervals and space-intervals can be adjusted to compensate for the changes of kinetic mass/energies caused by forces which cause accelerations and decelerations so the clocks and rulers measuring time and space are "independent of the state of motion of [their bodies] of reference" [rememvber, THAT is Einstein's specification for what would have for clocks to measure universal time and rulers to measure universal space] then universal time and universal space result/are proven to be realities and relativity has to be adjusted to accommodate the reality.

Thus, by universal time and universal space, time travel is nonsense.

INRE 2: Meaningful discussions inre time travel that ignore the reality of universal time and universal space are meaningless because they ignore the reality that time measurement is done in one direction only, from the past through the present and into the future, and, therefore, no previous universal timepoints will ever be revisited and without that revisiting there will never be a reuniting of the previous universal timepoints with the previous universal m/e configuration and therefore there can never, EVER, be time travel.

As long as you disregard Einstein's specification that clocks and rulers which are to measure universal time and space must be "independent of the state of motion of [their bodies] of reference" then you will fail to understand the reality of universal time and universal space which can be measured when clocks and rulers which are adjusted to compensate for the changes in their time-intervals and space-intervals are used as units of measurement of time and space.

[Caine]

These discussions are only meaningless in Bobland. You see what you are doing is taking a discussion out of the context of our universe where Einstein is correct, and transferring it into the context of Bobland, where all Einstein ever did was provide the quote which you repeat in every post.

[Ubercat]

Whether it's possible or not I don't know, but it is the basic premise of A.C.Clarke's The Light of Other Days.

Was that the one where some entrepreneur(?) set it up [the time viewing technology] where people could actually trace back through their female ancestors, generation by generation, for any number of years?

People in that story could get a snapshot of their direct female ancestors, and were able to remotely observe their own great, great, ............ great, great grandmothers so far back that they weren't even primates yet, let alone human.

What a concept! Loved that book.

-Ubercat

[Bob K]

[1] These discussions are only meaningless in Bobland. [2] You see what you are doing is taking a discussion out of the context of our universe where Einstein is correct, and transferring it into the context of Bobland, [3] where all Einstein ever did was provide the quote which you repeat in every post.

INRE 1: Discussions of time travel remain meaningless when universe = space, time and physics (m/e) and the space is understood to be the i-volume—the volume of infinite radius which surrounds any and all finite x-volumes, and the unit of measurement of space is the space-interval, which is variable if not adjusted and invariable if adjusted, time is the measurement of the occurrences and durations of events and the determination of the simultaneity of events by the use of a chosen duration for the unit of temporal measurement which is the time-interval which is variable if not adjusted and invariable if adjusted, and the direction of temporal measurement is infinite from the past through the present into the future, and physics is the universal matter/energy which is an isolated m/e system because m/e cannot be added to it, m/e cannot be removed from it, the sum total of the m/e is a constant, and the m/e is indestructible and therefore eternal/infinite in existence in space and time.

From this understanding of the universe it is clear that time is measured only in one direction and the reality of the temporal measurement of the occurrences and durations and determination of the simultaneity of physical events is that physical events occur in one direction from the past through the present to the future regardless of claims that in theory events could physically run in directions opposite to their current directions of motion, a principle that will never be a practical reality because the universal m/e system is 100% "invested" or "involved" or "consumed" in current operations and directions motions and to reverse ALL of THAT would require additional m/e to be added to the universal m/e system which is an impossibility because of the fact that the universal m/e system is an isolated m/e system to which m/e cannot be added (where would the additional m/e come from?) as well as the fact that space surrounds all the universal m/e that exists and therefore additional m/e simply does not exist outside the space, time and physics of the universe.

Moreover, the Continuum of Time, when set up using invariable (adjusted) time-intervals, is a timeline upon which timepoints are linked with spacepoints and m/e to form combinations of timepoints with m/e spatial and physical configurations so that at each timepoint there is a one-and-only spatial distribution of the m/e in an m/e configuration to which humans can never return because to do so would require their m/e to be removed from the present m/e configuration, which is an impossibility for reasons cited inre the fact that the universal m/e is an isolated m/e system from which m/e cannot be removed and in which the sum total of m/e is a constant, and added to the previous m/e configuration, which is an impossibility again for reasons cited inre the fact that the universal m/e system is an isolated m/e system to which m/e cannot be added and in which the sum total of m/e is a constant.

Note that the refutation of time travel actually fits known facts regardless of anyone's interpretation of Einstein's SR/GR to the contrary.

The present universal m/e configuration is the only universal m/e configuration that exists.

I note that no one here is ready, willing and able to refute the claim that simultaneity when defined as two or more events occurring at the same timepoint within a single reference frame or upon a single reference body cannot occur when timepoints are generated by time-intervals synched local clocks. Einstein admitted such was possible.

When the essence of temporal measurement is recognized to be the time-interval, the unit of measurement of time, which generates a clock's rate of ticking and its timepoints on its timeline, then simultaneity has to be linked to timepoints generated by time-intervals, and whereas variable time-intervals are recognized to be variable because their host clocks are NOT "independent of the state of motion of [their bodies] of reference" (because of the effects which are changes in their kinetic mass/energies which are caused by the forces which cause accelerations and decelerations and which cause changes in the durations of variable time-intervals which cause variations of the rates of ticking of their host clocks and variations of their host clock's sets of timepoints and timelines) they nevertheless can be synched by radio signals from a master clock within a single reference frame/upon a single reference body ("body of reference") and thereby the simultaneity of events within a single reference frame/upon a single reference body can be determined by the timepoints generated by the synched local clocks, then invariable time-intervals ought to be recognized to be invariable if adjusted to compensate for the effects on their kinetic mass/energies caused by the forces which cause accelerations and decelerations, because the adjustments would cause the invariable time-intervals' host clocks to be "independent of the state of motion of [their bodies] of reference" and result in uniform/identical rates of ticking, timepoints and timelines regardless of their reference frames, "independent of the states of motion of [their bodies] of reference," and therefore the simultaneity of events in different reference frames/upon different reference bodies can be determined by the synched universal clocks.

We thus can determine local simultaneity measurable by local clocks and universal simultaneity measurable by universal clocks.

SR/GR deals ONLY with local time as measured by local clocks; it should be modified to include universal time as measured by universal clocks.

INRE 2: Einstein's SR/GR continues to be correct inre the determination of the local simultaneity of events as measured by local time via local clocks; but now there is a new/different theory that describes the reality of universal time as measured by universal clocks which enables the determination of the universal simultaneity of events as measured by universal time via universal clocks.

If synched/identical universal clocks are used for the measurement of time, then space travelers in different reference frames can synch/co-ordinate their activities so they can meet at the same universal timepoint at close enough sets of spacepoints to be able to hold meetings and engage in other activities which require them to be in close spatial proximity (close enough sets of spacepoints) at the same timepoints.

If this new/different theory could not account for universal simultaneity as well as local simultaneity, then it could be dismissed as irrational and irrelevant; but since it can, and does, account for universal simultaneity as well as local simultaneity then it can never be dismissed as irrational and irrelevant.

INRE 3: Einstein did in fact set up the criteria for universal clocks when he described what would be necessary for clocks to be universal instead of just local clocks, and thereby to measure universal time and generate the universal rates of ticking, timepoints and timelines necessary for the determination of universal simultaneity of events in different reference frames/upon different "[bodies] of reference" when he said that universal clocks must be "independent of the state of motion of [their bodies] of reference."

Einstein actually said what was quoted. Because this definition/description/specification of universal clocks is generally accepted by physicists, and because the definition of simultaneity is, as shown in previous posts, generally accepted by physicists to be two or more events occurring at the same instant/timepoint, regardless of distances from observers, then any clock which is "independent of the state of motion of [its] body of reference" because its time-interval is adjusted to compensate for the known effects caused by the forces which cause accelerations and decelerations which cause changes of a clock's kinetic mass-energy which cause changes of a clock's rate of ticking and timepoints and timeline has a universal or invariable time-interval which generates a universal rate of ticking and universal timepoints and universal timeline which can measure universal time and determine universal simultaneity.

We have an example of the practical reality of this new/different theory of time in the US GPS satellite clocks which are controlled by radio signals from a master clock at Falcon AFB. These slave clocks are not measuring satellite local time; instead, they are measuring the time generated by the master clock. These slave clocks are therefore "independent of the state of motion of [their bodies] of reference"—their satellites. This fact is confirmed by the engineer who designed the GPS timing system.

As long as the Earth is stable in its rotation about its axis and in its orbit about the Sun the synched clocks in the GPS timing system will remain stable and will measure universal time and enable the determination of the simultaneity of events in different reference frames (in different satellites as well as on Earth).

Universal time-intervals which generate universal rates of ticking and universal timepoints and universal timelines for the measurement of universal time and the determination of universal simultaneity are a reality.

So be it.

[drewjmore]

BobK
BobK
BobK...

Eva Longoria
Eva Longoria
Eva Longoria!

-djm [asking for something he actually wants]

[Chuck Rightmire]

Actually, as I understand what Bob is saying is, in simpler words, that change, once it occurs is irreversible and therefore we cannot travel back in time. If we use his universal clock, then time is a plate, or a slice of something rushing forward and at any given instant can hold the universe before the next instant of change. That's my preferred version. But, if time travel was to be possible, would it not require the worm through space?

[Bob K]

Actually, as I understand what Bob is saying is, in simpler words, that change, once it occurs is irreversible and therefore we cannot travel back in time. If we use his universal clock, then time is a plate, or a slice of something rushing forward and at any given instant can hold the universe before the next instant of change. That's my preferred version. But, if time travel was to be possible, would it not require the worm through space?

The best way for me to conceptualize the combination of the Continuum of Time and the universal m/e configuration at any timepoint is to not utilize the idea that something relevant to time is moving, such as an arrow of time, and to not utilize the idea that somehow the universal m/e is a slice, or any box-like or sphere-like volume, which too often is easily but wrongly conceivable as a volume having a finite boundary, but, instead, to consider the universal m/e to be a swirl of physical phenomena within the infinite i-volume of space, with the individual m/e stuffs in motion, but the entire swirl as kinda-sorta always in existence in one place—within space, and time to be the record of what the swirl was at each timepoint on the Continuum of Time.

Thus, the individual stuffs within the swirl are moving but the swirl stays within space the present or NOW timepoint on the Continuum of Time.

It is the nature/essence of temporal measurement that the timepoints add rather than subtract and therefore the future timepoints be the timepoints which accumulate, i.e., the face-readings or time-counts of clocks be the additive measurement or time-count of timepoints from the present/NOW into the future.

This nature/essence of temporal measurement is its own natural/essential direction of temporal measurement or arrow-of-time from the past through the present into the future and is therefore absolutely independent of the motions of the m/e stuffs of the universal m/e configuration/swirl. All those motions of physical stuffs which are supposedly described by mathematics as being time-reversal are nevertheless and regardless of the mathematics independent of the temporal measurement process which will always be from the past through the present and into the future and thus all time-counts are to be additive and never subtractive.

Whereas we can easily observe space and the universal m/e as photographed or videotaped in 2D at a specific timepoint, which can be time-stamped upon photos or videos, we are now becoming more familiar with the 3D phenomena of holograms, because holograms do in fact have a 3D dimensionality that would enable the coupling of the images of the 3D universal m/e within space with the timepoints on the Continuum of Time.

I therefore imagine the record of the existence of the universal m/e configuration/swirl at the timepoints on the Continuum of Time to be best represented by holograms—3D photographs/videotapes.

Whereas the holograms are at best 3D representations of the universal m/e configuration/swirl they are not the actual universal m/e configuration/swirl at each timepoint. Thus, whereas to me the idea of a worm extending through space implies the worm to somehow be the actual m/e configuration at each timepoint, or otherwise a continuous contiguity, which would be a physical impossibility, the idea of of holograms being 3D representations of the m/e configuration/swirl is far more understandable and therefore far more acceptable than the worm representation. If the holograms were frozen 3D images of the m/e configuration/swirl at each timepoint and the images were displayed in uniform succession using the universal time-intervals of the Continuum of Time then to uniformly moving observers the m/e stuffs would appear to be in motion in their natural trajectories and velocities, and we would get a deterministic vision of the universal m/e from timepoint to timepoint, and an awareness of the past could very well enable a rational and realistic prediction of the future.

[Jesse]

Actually, as I understand what Bob is saying is, in simpler words, that change, once it occurs is irreversible and therefore we cannot travel back in time. If we use his universal clock, then time is a plate, or a slice of something rushing forward and at any given instant can hold the universe before the next instant of change. That's my preferred version. But, if time travel was to be possible, would it not require the worm through space? The problem is that according to relativity it is impossible to find a way to construct the sort of universal clocks that Bob describes (and Bob, if you wish to discuss this statement, please do so in Absolute Time vs Relativity (http://www.iidb.org/vbb/showthread.php?t=182794) rather than here). In relativity there is no physical way to distingish different inertial (non-accelerating) reference frames, and different frames define "simultaneity" differently, meaning they disagree about whether two events happened at the same time or different times, so the "slice of something" representing each observer's present is actually slicing spacetime at a different angle. Of course we cannot rule out the idea that there is a "metaphysically preferred reference frame" whose definition of the present is the "correct" one, but no physical experiment could tell you which frame this is, so the idea seems pretty inelegant to me.

[Scott M 74]

Regarding the question of why we haven't seen time travelers if this is possible:

1) Maybe they have cloaking technology

2) Maybe the future doesn't exist yet

Let me explain the second one. Space is created as the universe expands. So time is created as well. As with space, there is time that has been created (the past), time that is being created (the present), and time that has yet to be created (the future). If we live in the present (as opposed to the past), then nothing exists ahead of us. Since there is no future, there is no one in the future to travel back in time.

That may change someday, but it will only affect the people who live in the past, which we do not.

I'm not saying this is correct, and I certainly don't have any math to back it up. It's just a thought.

[Jesse]

Let me explain the second one. Space is created as the universe expands. So time is created as well. As with space, there is time that has been created (the past), time that is being created (the present), and time that has yet to be created (the future). If we live in the present (as opposed to the past), then nothing exists ahead of us. Since there is no future, there is no one in the future to travel back in time. As I said above, this idea contradicts relativity, because in relativity there can be no unique notion of "the present", different frames disagree on whether two events happened at "the same time" or "different times" and each frame is equally valid.

[Scott M 74]

different frames disagree on whether two events happened at "the same time" or "different times" and each frame is equally valid.But that is in relation to us knowing we are in the present, which we do not. We could be living in the past (that is, sometime other than the ultimate present). The only way we could know for sure is to step outside of time and see what place we left behind. At this point, the theory of relativity no longer applies.

It is akin to the old scenario of being below deck on a ship riding a calm sea. You don't know if you're moving unless you leave the hold and look outside. But at that point you are no longer in the hold, so it doesn't matter what you know.

[drewjmore]

As I said above, this idea contradicts relativity, because in relativity there can be no unique notion of "the present", different frames disagree on whether two events happened at "the same time" or "different times" and each frame is equally valid.

Is there any limit on how much two observers may disagree on 'simultaneity'? Merely seconds, or up to the age of the universe?

[Jesse]

But that is in relation to us knowing we are in the present, which we do not. We could be living in the past (that is, sometime other than the ultimate present). The only way we could know for sure is to step outside of time and see what place we left behind. At this point, the theory of relativity no longer applies. It has nothing to do with whether we are "in the present" or not. (although I'm puzzled why you would even believe in the 'moving present' view if you don't think it's necessary to explain the subjective sense of the flow of time--if we would have exactly the same subjective sense if we were part of the frozen past, why not just consider all of spacetime as a 'frozen' 4 dimensional object?) The point is that if you believe that at some "earlier" point in the movement of the present (by the way, what can this even mean, unless we introduce a second time dimension to time the changing position of the present moment?), the current point in spacetime you're experiencing was the present, then you presumably believe there is some unique truth about whether other far away events were also in the present "at the same time". For example, whenever the event of my checking my watch and seeing 4:00 was happening "in the present", then I'd expect you'd also think that the event of an astronaut on Mars checking his watch and seeing 4:00 was either also in "the present" at that moment, or it was in "the past" or "the future". But according to relativity their is no unique truth about whether two events happen at the same moment or at different moments.

[Jesse]

Is there any limit on how much two observers may disagree on 'simultaneity'? Merely seconds, or up to the age of the universe? There's no limit, two events which are simultaneous in one frame can have an arbitrarily large separation in time in another frame if the two frames are moving apart at sufficiently close to the speed of light. But note that two events can only be simultaneous in any frame if all frames agree there is a "spacelike separation" between them, meaning that a signal moving at the speed of light or slower could not move from one event to the other, so neither event lies in the other event's future light cone (which means neither event can have a causal influence on the other according to relativity). If you have an event B that lies in the future light cone of another event A, then all frames will agree that B happened at a later time than A.

[Scott M 74]

why not just consider all of spacetime as a 'frozen' 4 dimensional object?You lost me. If the universe is expanding, how can you consider it "frozen"?

Understand, I'm no physicist, but all (I think) I'm suggesting is that if space and time are interconnected and space is expanding, then time is expanding as well. If we can say the universe used to be this way but is now this other way (for example, if we can say that certain stars used to be closer to each other than they are now)--in other words, if we can say that the universe has changed--they why can't we say that time has changed? Change implies movement and requires a past and a present. It does not, however, require a future.

Astronomers are quick to point out that the light we see through our telescopes is from stars that no longer exist. You are saying this is untrue, as relative to our vantage point the stars are still there. So you are saying that reality does not exist and is based purely on our perceptions. If such is the case, then smoking a powerful drug actually alters the universe. That has never been my understanding of Relativity (but, as I said, I'm not a physicist). My understanding has always been that Relativity explains our journey through the universe but does not affect the universe itself.

I must admit, this has enormous consequences to my proposition because by this logic we exist in another galaxy's future, and they exist in ours. But this would mean that the now defunct concept of an eternal universe is true, and I still don't see how our perception of events can define reality. A drunkard will perceive things more slowly than someone who is sober. If a they are sitting together and experience the same event, then the drunk will say it happened later. Reality certainly does not work this way.

:confused:

Anyway, I'll be leaving work soon, and my stupid Sprint Internet doesn't work right, so I may not be back on here until Tuesday.

[Jesse]

You lost me. If the universe is expanding, how can you consider it "frozen"? Spacetime is what's frozen in the usual picture of relativity, expanding space is not. But expanding space is like a series of cross-sections through the static spacetime, and you are free to slice the same spacetime at different angles. Have a look at the recent thread Is the universe expanding?? (http://www.iidb.org/vbb/showthread.php?p=4467485) which has a long discussion of what it means to say space is expanding, and check out my football analogy in post #53: I don't it actually makes sense to talk about spacetime expanding, or time expanding. The notion of space expanding is based on taking a "foliation" of spacetime, which basically involves slicing up the static 4D manifold into a stack of 3D "spacelike hypersurfaces" which evolve over time. In cosmology there would be a unique way of doing this slicing so that each surface was homogeneous, which corresponds to picking the rest frame of the cosmic microwave background radiation. If you divide up space and time this way, then the you can talk about space expanding over time, but spacetime as a whole is still a totally static entity. Imagine spacetime as the 2D surface of an American football, and then slice it along the axis from tip to tip into a series of cross-sections, each of which is a 1D line curved into a circle representing space at a particular moment in time--as time passes, the circles grow from a point to a maximum size, then shrink to a point again, as with a closed universe growing from a Big Bang and then collapsing to a Big Crunch. Astronomers are quick to point out that the light we see through our telescopes is from stars that no longer exist. You are saying this is untrue, as relative to our vantage point the stars are still there. Depends what you mean. If you take spacetime as a frozen entity as is traditional in relativity, all events, past, present, and future, are coexistent in the single spa