Does time exist? [Archive I]

fast

January 12, 2005, 03:19 PM

Have any of you considered if that time does not exist. There is no spacetime and we live in a atemporal universe. If there was no motion at all and nobody counting time with a clock. How can we measure it? What arguments are there that spacetime DOES exist. For more info read:

http://www.ws5.com/spacetime/

If you read all of it, there may be the chance that spacetime does not exist and is an illision of motion. Bit weird eh, But possible. :confused:

Time exists independent of motion. Time exists as a concept of continuation.
Motion helps measure time. Lack of motion means inability to measure.
Lack of ability to measure does not mean no time (unless of course you defined it as such). I wouldn't define it as such because that's not how it's defined.

If we kept a clock suspended from the remainder of the universe and stopped all else in it's track, then with the exception of the clock, we would see only motionlessness, but from the clock you will see that the said motionlessness remains so for a specific calculable amount of time; hence, time is a measure of the continuation of existence and not the continuation of motion.

Seeing as that I have read absolutely nothing about time, I offer this explanation as a novice only.

Dark Knight Bob

January 12, 2005, 04:34 PM

I would say that the laymen concept of time doesn't exist as proven by relativity.

If we kept a clock suspended from the remainder of the universe and stopped all else in it's track, then with the exception of the clock, we would see only motionlessness, but from the clock you will see that the said motionlessness remains so for a specific calculable amount of time; hence, time is a measure of the continuation of existence and not the continuation of motion.

This is in simplistic terms what special relativity is saying. Every particle (well i'm not an expert on how relativty works at the quantum level so let's use that as the lowest reducable component) is in it's own time frame seperate for everything else. It's a weird conept because in that sense time really is just how objects move in relation to each other and not how they move to any universal time frame. So any time frame might as well be reduced to nothing but some form of vector component of space. I don't know much beyond special relativty i'm afraid. Someone else will have to explain how it works under gravitaitonal fields. Suffice to say that despite everything being relative, everythign is universally bound to everything else (So they're not completely seperated so as to get to a point where they don't interact at all. This would require faster than light travel). So as two objects reach the same space-time frame, they will come back to an "apparent" universal time frame that we confuse as the laymens view of time.

You can visualise it in several ways from time acting like elastic bands between objects stretching the further they get away from each other (where distance in this instance is their relative velocity to each other). When the objects come back to a relative velocity of zero the elastic bands unstretch and look "normal" once more.

Or you can view it in the pseudo-quantum visualisation of packets of data being sent via light. As the relative velocity gets higher, the frequency of packets is recieved gets lower. This being percieved as a slowing down of "time". If the two objects went to a relative velocity of 0 once more then all the packets would catch up with each other and go back to a steady frequency of transmission that we percieve as "normal" time.

Also i'm a bit confused as to why your link goes to "create new post" I suggest you fix that :D

Basically time exists but is itself as susceptable to change as velocity. (i.e not in some concrete ever constant form we are familiar with) Mathematically it can be easily to deal with but visually it can be well .... confusing.

And don't be afraid to find it VERY confusing. After all. Our brain is no more equipped to deal with relative time frames than it is to deal with a non-continous quantum one. :D

Edit: That explanation sucks btw. Hopefully someone better versed can explain it better :D

Richard of Chelmsfor

January 12, 2005, 04:52 PM

I've often thought that time does not eist.

It is just the movement of matter.

Neither does space exist.

Until you have two objects with a measurable distance between them.

That's why I suspect that 'spacetime' is a misleading concept.

epepke

January 12, 2005, 04:53 PM

Schneibster

January 12, 2005, 05:06 PM

According to SRT, time and space have no independent existence apart from one another. It is all one thing: spacetime.

Everything in the universe travels through spacetime at the speed of light, all the time. There is no explanation given for this; it is an assumption, which follows from one of the postulates upon which relativity is based. Objects that have mass can be "motionless" (i.e. have zero speed) in some frame of reference; if they are, then all of their motion is in time, at the speed of light. On the other hand, if they are not motionless in some frame of reference, then some of their inherent motion through time is "stolen" in order for them to move through space. This effect is called "time dilation." The precise equations for how much change there is in the movement through time are known as the "Lorentz Transform," because they describe how to transform the object's motion in one frame of reference into motion in another frame of reference. As an object approaches the speed of light, its mass increases exponentially; thus, a constant force would produce less and less acceleration, and the object could never reach the speed of light. Because the mass approaches the limit of infinity as the speed approaches the limit of the speed of light, no massive object can travel at the speed of light.

Objects that have no mass can and always do travel at the speed of light. This is the postulate referred to above; it is that the speed of light is always measured the same from any inertial frame of reference. An inertial frame of reference is one that is moving at a constant velocity less than that of light. Because they expend all their movement on space, such objects do not move through time.

Another of the postulates of relativity is that spacetime exists and forms a continuum. The first part is well accepted; but the contention that space and time are continuous at all scales has been challenged under some interpretations of quantum theory; there may be a minimum amount of spacetime, a minimum "step," beneath which it is meaningless to talk about spacetime. But this is merely conjectural, and no-one has challenged whether space and time exist (at least not seriously).

Because of the experimental success of the Special Theory of Relativity, it is accepted by nearly all physicists. It is not the most rigorously tested theory of science, but it is one of the three most rigorously tested. Many astronomical and laboratory tests have been done, and no one has ever found a repeatable experiment that violates SRT. And since the reality of spacetime, and therefore the reality of time (although not as an entity separate from spacetime) is a postulate of SRT, it also is equally supported.

Because of this, extraordinary evidence, in the form of a repeatable experiment, would be required before most physicists would allow that time might be non-existent. And that is your answer.

Thundun

January 12, 2005, 05:17 PM

Have any of you considered if that time does not exist.

Obviously you think time exists. Otherwise you wouldnt use words like "was" near words like "is." If you think time doesn't exist, please stop using tenses other than present tense; or even better, develop a tenseless language.

By the way, that link is awful. He makes deep philosophical mistakes (such as being unwilling to define space-time) and procedes to misinterpret philosophical methodology of science such as suggesting we impose the necessity that GR represents physical reality, instead of the more true statement that GR is a theory that is shown to predict physical observables.

Clivedurdle

January 12, 2005, 06:19 PM

premjan

January 12, 2005, 06:38 PM

What would be the consequence of timelessness? It is actually easier to conceptualize timelessness than time. Just imagine each point in the universe evolving and changing (or not changing) independently. There is no "state" corresponding to the past or the future, only the present exists. Only the present is capable of interacting with anything since nothing else exists. All you need is a sophisticated understanding of the changes occuring at a point in space in order to predict the future state. In the absence of being able to correlate the past of a point with its present (and no theory can do this) why postulate "space-time" as a concept. The only other concept you need is "information" which is also just a material thing that propagates at some speed between points. Then there is of course the quantum level at which sometimes things that are widely separated in space are really correlated. But this correlation is prior to actual observation Once things are observed, they go back to being normal spatially separated things.

epepke

January 12, 2005, 06:38 PM

Barbour end of time (http://www.edge.org/3rd_culture/barbour/barbour_index.html)

Excellent post Schneibster, but isn't the issue not that the experiments may not hold up, but more that we do not understand the meaning of the results properly?

A spacetime continuum, in which "things" without mass have a maximum velocity, and "things" with mass exhibit other features like gravity - almost as if gravity emerges when energy is somehow slowed down and at the same "time" become "material", in which this spacetime continuum is not just the classic four, with dark matter and dark energy.

Well, speculation about the meaning is fine.

However, you have to get things right to talk about things like gravity. Gravity has to do with a concept called energy/momentum. With this concept, energy and momentum are linked up the same way (in a 4-vector or a quaternion, but almost everybody uses the 4-vector) that time and space are. They transform just the same way. So the whole idea of gravity under GR only makes sense in terms of spacetime.

Going slow doesn't have any special magic; it just makes the GR field equation easier to figure out.

Richard of Chelmsfor

January 13, 2005, 05:00 PM

There's another way to think about this.

With the notion of spacetime, a lot of things in the universe make sense, and you can make a lot of predictions. Considering spacetime as basic explains and predicts these things better a lot easier than considering space and time separately. Considering motion as a relationship, timeless in and of itself, amongst space and time connected works really well and is easy to understand.

Considering it some other way, you either get a lot of really ugly mathematics and hairs growing out of theories, or you get theories that just don't work.

Which is "right" or "true" is a philosophical question and probably unanswerable. As long as all theories work, it doesn't make sense to me to say that one is wrong and the other right. It may make sense to a philosopher. To me, if they all work as well, I'll pick the one that is the cleanest and is the easiest to think about. For me, that's Special and General Relativity.

A lot of people agree with that view, epepke, but I advance the other view so that, although the concept of 'spacetime' works well, it does not give you a 'false positive' anywhere.

For example, I would want to see a little more proof of Einstein's contention that gravity does not exist as a 'wavelength' or anything physical as such. he said it was just 'an effect of spacetime.'

Yes, but if the Earth is cruising through space, say, and a large asteroid wanders close, although nothing physical connects them, how exactly is 'spacetime' working in order to pull the asteroid in and down on our heads?

epepke

January 13, 2005, 08:22 PM

A lot of people agree with that view, epepke, but I advance the other view so that, although the concept of 'spacetime' works well, it does not give you a 'false positive' anywhere.

Fair enough.

For example, I would want to see a little more proof of Einstein's contention that gravity does not exist as a 'wavelength' or anything physical as such. he said it was just 'an effect of spacetime.'

OK. The issue is still controversial. There are some people working on quantum gravity. They haven't had much success, yet. There may be something about it, maybe in string theory, but it hasn't worked yet. GR, so far, works great.

Yes, but if the Earth is cruising through space, say, and a large asteroid wanders close, although nothing physical connects them, how exactly is 'spacetime' working in order to pull the asteroid in and down on our heads?

Ah, a fundamental misconception. This is good, and there is no dishonor in it. In GR, the asteroid isn't pulled. There is no force acting upon it. It is just following a straight line in spacetime.

Gravity is a fictitious force in GR. It isn't a real force. It's a bit like the centrifugal force. If you're on a carousel, you will feel a force pulling you outward. But really, that's just inertia, the tendency to travel in a straight line. Inertia has some good quantum stuff behind it. The real force is the force of the floor of the carousel accelerating our feet inwards. However, we experience it as an outward force because we're used to the floor not going anywhere, much.

We see gravitation as a force because we're used to living on a planet, and we've evolved to see things that way. A being that evolved in freefall would probably see things a bit differently.

Richard of Chelmsfor

January 14, 2005, 04:19 AM

So would it be true to say that the Earth is travelling a straight line round the sun?

OK, perhaps some way it is.

Five minutes ago I just posted on an astronomy site

http://www.badastronomy.com

that scientists often come out with philosophical answers when they don't have the hard facts unearthed as yet and i think there's an element of that here.

Difficulties with words makes it hard for me to make my point clearly (also I did a night shift last night!! :snooze: ) but I'll try to take this further later.

LSHAFC2004

January 14, 2005, 04:52 AM

Well overall I think clearlt time exists. It has been calculated that if you enter a black hole you could come out the other side in a different area of the universe and at a different time. Einstein calculated that travelling faster than the speed of light would mean that time travel was possible. No time meant Einstein the greatest mind was wrong.Nearly all cosmoligists, Stephen Hawking all belive in the concept of space time. If there is no time then this gives a blow to the post that I sent. The man who wrote it is a theist. If there was no time then the universe could most likely be infinte. The cause and effect argument(there strongest argument) would fail. Finally if there is no time, we can re-run history back to infinte. This would mean that we could never get to the present(our time). The only answer can be that before the big bang. Time did not exist. The big bang was the start of time. This is the only logical conclusion. :)

Clivedurdle

January 14, 2005, 07:45 AM

If gravity does not really exist - it depends on context - who says straight lines exist? It feels like we are not at the "ground" concepts.

The same types of arguments can be made about the existence of time as about the existence of gravity.

Maybe spacetime is an emergent property of something else? Are the possible ten dimensions emergent? Newton works at a certain level, Einstein at a different level, something else at another level- like russian dolls. Quantum mechanics is a possible further level, why should there not be more? Chaos and complexity theories - the universe is a fractal idea - seem to show repeating but slightly different is endemic.

What if it is all circular somehow and the ends join up, the connections may be through black holes and big black holes like the big bang.

Is the total energy of the universe zero? Maybe straight lines are emergent results of certain levels of complexity within this fractal universe. (by universe I mean the set of all possible universes considered as a whole). Maybe we see straight lines as a result of our four dimensional existence?

Our technologies and experiments really only prove things within the understood framework within which the questions are asked. We get pretty decent results that are sound, but as classical mechanics has now been joined by other types, who says we are at the limit of possible types of mechanics that will continually refine further our understanding.

Maybe we should be talking about earth centred physics - what would physics be like to species that habitually lived in six dimensions instead of our four?

wonkothesane

January 14, 2005, 09:21 AM

"Time is an illusion. Lunchtime doubly so."
- Douglas Adams

lenrek

January 15, 2005, 12:49 AM

...There is no "state" corresponding to the past or the future, only the present exists. Only the present is capable of interacting with anything since nothing else exists. All you need is a sophisticated understanding of the changes occuring at a point in space in order to predict the future state. In the absence of being able to correlate the past of a point with its present (and no theory can do this) why postulate "space-time" as a concept. The only other concept you need is "information" which is also just a material thing that propagates at some speed between points...

Of all the post, this one strike a core in my heart... :)

I never really think of time in the manner as expressed in the discussion here. Really an interesting read. :thumbs:

Schneibster

January 15, 2005, 01:21 AM

Barbour end of time (http://www.edge.org/3rd_culture/barbour/barbour_index.html) Interesting. I have come across the gent before now; I have not, however, read his book. I'll have to.

Excellent post Schneibster, but isn't the issue not that the experiments may not hold up, but more that we do not understand the meaning of the results properly?Thank you. No, the Special and General Theories of relativity go into the subject with a great deal of depth. It's pretty straightforward, at least until you get down into the guts of GRT, where nothing whatsoever is straightforward.

A spacetime continuum, in which "things" without mass have a maximum velocity, and "things" with mass exhibit other features like gravity - almost as if gravity emerges when energy is somehow slowed down and at the same "time" become "material", in which this spacetime continuum is not just the classic four, with dark matter and dark energy.Actually, there is a deep connection between energy and time. The fact that experiments in physics are symmetrical in time (that is, if I do this experiment, and then do it again tomorrow, I will get the same results) is associated by Noether's Theorem with the Law of the Conservation of Energy. Interestingly, this also appears to be associated with the fact that energy and position in time are conjugate under Heisenberg uncertainty.

Is e=mc squared actually the GUT? What if all the various forces and dimensions are various solutions to possible ways energy can exist?Nope, been there, done that. Didn't work out.

Gravity is associated with the warping of spacetime; thus, it is associated with symmetry of translation (I can do the experiment over here and get the same results as over there), which is associated with conservation of momentum (and position and momentum are conjugate); and with symmetry of rotation (I can do the experiment facing this way, and then turn it around and do it facing that way, and get the same results), which is associated with conservation of rotational momentum (and rotational orientation and rotational momentum are conjugate); and with symmetry of time, which is associated with conservation of energy. Thus, three of our most powerful conservation laws, and three of the most important symmetries, and three of the most important sets of conjugate variables, are all associated with gravity, and the three-dimensionality of space, and the four-dimensionality of spacetime, and with time. So no, I think the whole damn thing falls apart if time doesn't exist; but notice this and note it most carefully: we have not yet spoken of any of the other three forces: EM, weak, or color. None of them is included in this. So no, we have no GUT here.

mirage

January 15, 2005, 01:35 AM

What would be the consequence of timelessness? It is actually easier to conceptualize timelessness than time. Just imagine each point in the universe evolving and changing (or not changing) independently. There is no "state" corresponding to the past or the future, only the present exists. Only the present is capable of interacting with anything since nothing else exists.Mmm, but this isn't compatible with special relativity which discards the concept of simultaneity. The present only has meaning relative to a given inertial reference frame.

Schneibster, you mention that: Another of the postulates of relativity is that spacetime exists and forms a continuum. Are you suggesting substantive spacetime or just that the concept of spacetime as opposed to space through time is well established?

premjan

January 15, 2005, 01:44 AM

mirage

January 15, 2005, 02:12 AM

What does SR say about interaction of the present and the past? Is it possible?

There does not have to be any concept of simultaneity in what I said, just propagation of information (e.g. photons) at the speed of light (or slower).
SR has the speed of light as the upper bound for information transfer. I must have misunderstood your post but you were talking about only the present existing, when the present is relative to the observer.

premjan

January 15, 2005, 02:34 AM

SR has the speed of light as the upper bound for information transfer. I must have misunderstood your post but you were talking about only the present existing, when the present is relative to the observer.
I was just saying that we don't even need the concept of space-time to think about SR or probably GR.

kaleidescope

January 17, 2005, 02:18 PM

"Time is an illusion. Lunchtime doubly so."
- Douglas Adams

Agree, time is an illusion.

llanitedave

January 17, 2005, 02:51 PM

A man had been in the habit of praying silently each night, for many years. Finally God appeared to him in answer to his prayer.

"What is it that you would ask of me?" asked God.

"Lord," said the man, "Is it true that to you a million years is as but a day?"

"Yes, my son," said the Lord. "It is."

"Then does it also follow that to you, a million dollars is as but a penny?"

"That," said God, "Is also true."

The man hesitated. "Well, God -- if it's not a sin to ask -- could I beg a penny from you?"

"Certainly!" God smiled. "It'll just take a minute."

(Wish I knew the author of this!!)

Schneibster

January 17, 2005, 04:26 PM

Mmm, but this isn't compatible with special relativity which discards the concept of simultaneity. The present only has meaning relative to a given inertial reference frame.

Schneibster, you mention that: Another of the postulates of relativity is that spacetime exists and forms a continuum. Are you suggesting substantive spacetime or just that the concept of spacetime as opposed to space through time is well established?I'm stating nothing but that there exists a postulate and that its content is the assumption that spacetime exists and forms a continuum.

In regard to your question, the Lorentz transform changes space into time, and time into space. The meaning of this is taken to be that if they can be transformed into one another, and in fact must be in order to describe motion, then they must all be part of the same underlying entity, not separate the way that we assign meaning to our perceptions. Thus I would say that the concept of spacetime as opposed to separate space and time is well established, since the experimental proof for the Lorentz-Fitzgerald contraction is extensive.

I'm not sure what you mean by "substantive spacetime."

Anglican

January 17, 2005, 07:45 PM

The 'existance' of spacetime isn't one of the canonical postulates as of course the original theory pre-dates the concept. However you can formulate relativty where "the laws of physics can be expressed in terms of spacetime" is a postulate for example and the continuity of space an time is an unwritten postulate in most, though not all, physical theories.

mirage

January 17, 2005, 11:19 PM

I'm not sure what you mean by "substantive spacetime."The idea that spacetime itself has existence, beyond a concept we use to order the relationship between things in it, rather like the ether idea. It's a realist metaphysical position where the concepts of a theory are considered to really exist rather than simply conveniently model the phenomena.

Newton was a substantivalist, Leibniz was a relativist, giving his name to the Leibniz transformation, i.e. would swapping East and West result in an identical universe or not? Clearly objects would be in different positions in a substantive spacetime, but they would maintain all their relations unchanged. My understanding was that the debate has raged ever since but that the Hole argument that Einstein came up with whilst wrestling with General Relativity has been recently reinterpreted to put a serious dent in the substantivalists position.

"The Hole" is supposed to be a local transformation of a bit of the metric field of the event manifold. More of a kink than a hole. (the event manifold being the 4D ordered set of event locations of spacetime, and the metric field describing the time direction, distance between event locations, all the stuff that gets measured.) So if a galaxy passes through this kink or "hole" it will occupy a different "path" of event locations, but be observationally identical to a universe without the hole, as the invariant measurements (as well as the galaxy) follow the metric field.

So if you grant the event manifold reality, you have two separate universes that are observationally identical and no theoretical means of telling which one you're in.

If you want to include the metric field as constituting part of this spacetime the argument doesn't work so well, but since the metric field carries properties more associated with things in spacetime than of it, like the energy/momentum of the gravitational field, some people don't like this "solution" for the substantivalists.

I get the impression that modern physicists just assume Leibniz invariance, ie are relativist.

Aesthetically, I prefer granting spacetime an independent reality. I don't have a problem with including the metric field with gravity waves and mass/energy. I would like it even better if everything could be reduced to spacetime, perhaps as strings of folded spacetime, the supersubstantivalist program, but I really don't know what I'm talking about here.

I'm no expert at all in physics so please correct any blatant errors if you understand what I've been on about.

epepke

January 17, 2005, 11:34 PM

If gravity does not really exist - it depends on context - who says straight lines exist? It feels like we are not at the "ground" concepts.

For GR, at least, the ground concept is a geodesic which maximizes proper time. This is one of the few happy instances where GR fits neatly with quantum behavior. If I may be permitted to use the word "why" in an approximate sense, QED explains why things move along geodesics.

"Straight line" is a metaphor. In SR, in flat Minkowski space, it is a straight line, and people have a sense of what straight lines are. But if you throw a rock on Earth, then it follows a geodesic which is the analogue of a straight line in "curved" spacetime.

Maybe spacetime is an emergent property of something else? Are the possible ten dimensions emergent?

Who knows? I don't.

Newton works at a certain level, Einstein at a different level, something else at another level- like russian dolls.

Just to clear up a popular misconception: Newtonian mechanics works just fine with Special and General Relativity. At least it does the way Newton himself wrote it down. Later on, people made assumptions above and beyond what Newton himself wrote. I don't know; maybe he just got lucky, but Newton, despite his myriad faults, didn't phrase his three laws of motion in such a way that there is any problem with Relativity. The big problem was with the Galilean transformation, which are wrong.

Quantum mechanics is a possible further level, why should there not be more?

Already noted: QED and GR fit together nicely. There is a problem because GR is a classical theory, in that it assumes a continuous universe. I seldom disagree with Schneibster, but I will have to disagree (and maybe this is a minor disagreement or a matter of terms) that it assumes that this continuous universe is spacetime. Spacetime just happens to be something that falls out of SR. SR makes a lot of sense with spacetime, and so it's reasonable to assume for GR, but spacetime isn't an assumption that goes into SR. SR can be derived by treating space and time separately.

Schneibster

January 18, 2005, 12:12 AM

The 'existance' of spacetime isn't one of the canonical postulates as of course the original theory pre-dates the concept. A, my understanding is that it is one of the postulates, and I checked it several different places. It turns out that you cannot define "speed" or "frame of reference" (necessary to Einstein's original two postulates- that of relativity, and that of the constancy of the speed of light) without first defining defining spacetime as a continuum, and defining and asserting the existence of inertial frames of reference. As a result, many people state that there are four postulates instead of two.

However you can formulate relativty where "the laws of physics can be expressed in terms of spacetime" is a postulate for example and the continuity of space an time is an unwritten postulate in most, though not all, physical theories.I think this was the point. Certainly, it has to have been recognized immediately by all who understood Lorentz' transform that this must be so; Minkowski's comment about the fading away of separate space and time could have been uttered anytime after Lorentz published his transform without the need to wait for special or general relativity.

Schneibster

January 18, 2005, 12:26 AM

The idea that spacetime itself has existence, beyond a concept we use to order the relationship between things in it, rather like the ether idea. It's a realist metaphysical position where the concepts of a theory are considered to really exist rather than simply conveniently model the phenomena.OK, then yes, I believe that spacetime has separate existence, as opposed to being simply the concept of a measurement between two events. In support, I offer Newton's Bucket:

Fill a bucket with water. Suspend it from a rope. Twist the bucket so that the rope gets wound up. Release the bucket. What happens to the water?

OK, now do the same experiment in empty space with a rock. Say it's hollow; will anything inside it and moving with it "feel" the acceleration of the spin? Of course it will; it's centrifugal force. OK, what causes that force?

OK, now spin the rock in an empty spacetime with nothing in it. No galaxies, no stars, nothing but the rock. Will things inside it still feel the force?

An imaginary Ernst Mach that we conjured up for this occasion says, "No, they will not. The entire universe creates the gravity field that creates the metric of spacetime; without the rest of the universe, there would be no space or time, and thus no frame of reference to differentiate between the spinning rock and the still rock."

An imaginary Isaac Newton also brought to us by blacke magick saith: "Yes, of course they will. The metric has separate physical existence; you merely illustrate and measure it by the spin."

I hope you understand why I used the term "believe." To the best of my knowledge, there is no way to distinguish between these points of view, since they require an experiment in an empty universe. ;)

I was not familiar with the "hole in space" thing you used, nor do I pretend to fully understand what was intended. Do you have a link to a reference, or can you tell me enough information that I can get my hands on the book it is in?

Schneibster

January 18, 2005, 12:36 AM

I seldom disagree with Schneibster, but I will have to disagree (and maybe this is a minor disagreement or a matter of terms) that it assumes that this continuous universe is spacetime. Hee hee, actually I got it somewhere else. I'm not convinced that spacetime is continuous. But in reading Albert's book, Relativity, I can see from the discussion of Gaussian co-ordinate systems that he certainly thought it was, and assumed it in the logic that led up to the equations of GRT. I'm not sure its a matter of terms or not. I do know that it is an interesting topic of discussion, and a fairly important clarification due to the nature of the topic, so let's hash it out.

Spacetime just happens to be something that falls out of SR. SR makes a lot of sense with spacetime, and so it's reasonable to assume for GR, but spacetime isn't an assumption that goes into SR. SR can be derived by treating space and time separately.I'm not sure about the derivation part, but Einstein's two postulates apparently in the opinion of some people included two more that rode along as "hidden baggage:" the assumption of the spacetime continuum, and the assumption and definition of "inertial frames of reference."

I think, as I stated elsewhere, that the notion of the association of space and time into spacetime started with Lorentz' transform, and that of course grew out of Fitzgerald's contraction solution to the conundrum of the failed Michelson-Morely experiment.

epepke

January 18, 2005, 01:05 AM

I'm not sure about the derivation part, but Einstein's two postulates apparently in the opinion of some people included two more that rode along as "hidden baggage:" the assumption of the spacetime continuum, and the assumption and definition of "inertial frames of reference."

I think I'd like to hear about this "hidden baggage."

I wrote a lot more, but it's probably obvious. But still, I think that the notion of spacetime, as opposed to space and time separately, can be introduced after SR has been adequately explained for sufficient grokkage by most people.

mirage

January 18, 2005, 01:41 AM

Hee hee, actually I got it somewhere else. I'm not convinced that spacetime is continuous. But in reading Albert's book, Relativity, I can see from the discussion of Gaussian co-ordinate systems that he certainly thought it was, and assumed it in the logic that led up to the equations of GRT. I'm not sure its a matter of terms or not. I'm pretty sure that at least a locally continuous spacetime is assumed in the geometry of the pseudo-Riemannian manifold, isn't it?

I'm not sure about the derivation part, but Einstein's two postulates apparently in the opinion of some people included two more that rode along as "hidden baggage:" the assumption of the spacetime continuum, and the assumption and definition of "inertial frames of reference."Special Rel has been axiomatised several times in different ways. There are several assumptions to the theory (the constant speed of light being one, others unstated in the postulates) that differentiate it from the other possible theories that save the observations.

Axiomatisations make explicit assumptions like the following:

1. Light goes at the same speed in all directions for any observer.
2. Nothing goes faster.
3. You can tell if two events are identical (ie are the same, coincident event at the same point and time)
4. You can trace a causal chain to establish the history of an object like a moving particle or a reflected light ray
5. The spatial and temporal measurements of objects at rest in a frame define their true relations.
6. These spatial and temporal relations as they exist relative to one frame, are functions only of the same relations between events relative to another frame and of the relative motions between frames.

Then you choose arbitrarily any two from:

Rigid measuring rods
Clocks
Free moving particles
Light rays

To construct your spacetime.

The two left out will obviously be defined in terms of the others.

mirage

January 18, 2005, 01:47 AM

OK, then yes, I believe that spacetime has separate existence, as opposed to being simply the concept of a measurement between two events. In support, I offer Newton's Bucket
That argument kicks more ass than the Multiboot AzzKikor 9000 XLE. The use of acceleration to get around the absence of absolute motion. I'm like you, I just can't bring myself to believe the bucket (or two buckets connected by a rope) wouldn't behave as it does in our universe.

And I like the idea of matter as little knots of vibrating spacetime rippling about interacting with each other.

Edit - oh, The Hole Argument (http://plato.stanford.edu/entries/spacetime-holearg/#6) - just googled the encyl philosophy site, but I think was I read a while back was based on it.

Anglican

January 18, 2005, 07:03 AM

A, my understanding is that it is one of the postulates, and I checked it several different places. It turns out that you cannot define "speed" or "frame of reference" (necessary to Einstein's original two postulates- that of relativity, and that of the constancy of the speed of light) without first defining defining spacetime as a continuum, and defining and asserting the existence of inertial frames of reference. As a result, many people state that there are four postulates instead of two.

Obviously you need to postulate the existance of space and time (though people tend to think that you get these notions and others for free in sopecial relativty from pre-relatvistic physics), howver thta they should be combined in anyway is not really a postulate (spacetime is simply a mathematical object so unless you do something specific like impose a metric your not actually postulating anything about the physical world). Velocity can be defined without specifically using the concept of spacetime (as of course it is in pre-reltavistic physics) and once you have velcoity then inertial frmaes too are easily definmed (again just like they are in pre-relatvistic physics)

I think this was the point. Certainly, it has to have been recognized immediately by all who understood Lorentz' transform that this must be so; Minkowski's comment about the fading away of separate space and time could have been uttered anytime after Lorentz published his transform without the need to wait for special or general relativity.

Well no not really, indeed as I believe the Lorentz transformations were originally part of theory that when formulated in terms of spacetime has a prefered basis, the usefulness of spacetime was not instantly obvious.

premjan

January 18, 2005, 07:44 AM

doesn't the bucket argument just indicate there is a flux of particles that causes inertia?

SLUGFly

January 18, 2005, 09:37 AM

Please forgive me for jumping off the mood of the current debate. I don't know physics well, and most of what you all are saying goes well over my head. I simply don't have the patience or desire for learning advanced physics. However this is a debate I'm interested in and for what it's worth I'd like to throw in my own two cents.

Time does exist and it's undeniable. If we get into physics and relativity then we're overcomplicating an issue that has only two main factors. Memory and aspiration.

I remember smoking a cigarette a few minutes ago. It's gone, I did it and I can't undo it. I can't unsmoke that cigarette. This is something that is now only memory. If we get into physics, perhaps that action no longer exists and its "reality" is disputable, but the memory is real, it's there. We need a word for it, let's call it past.

I want to have a cookie. I may have one, I may not have one, it's an unknown. (certainly the fact that I have none in the house may be a hindrance). This is a hope for an action that I wish to do... if not right this moment. Let's call this future. It might not exist, and because I have no cookies it's true that it may never ever exist, but the aspiration is there. The expectation that after this moment there will be another.

Everything is linear and regardless of any theories on spacetime or even regardless of how anything moves outside of our small planet, our minds are linear and composed of memory, perception and aspiration, or past, present and future. We need a word to group these together since they're clearly related, so we call it time. We could have called it honkdonkey (I might have enjoyed that more.)

So, does honkdonkey really exist? Can't touch it, can't see it by itself, but you can't touch or see anger and happiness either, but just like honkdonkey, its effects are clearly visible and if we look at what's happening inside our heads, we can see that anger, happiness and honkdonkey are indesputable survival mechanisms. Honkdonkey exists for animals too, they remember friend, food and foe, they remember locations, they anticipate danger, they anticipate food, they look for things in the hopes that they'll find it. Does honkdonkey exist for a clock? Well... it does use a battery which will eventually run out, but it's we who anticipate that, not the clock. Does honkdonkey exist for water inside a spinning bucket? No, it exists for us watching the bucket waiting for the spin to stop. (incidentally the water spins from friction not centrifuge. If centrifuge were the case then a bucket spinning fast enough could make a freestanding column of water.)

Honkdonkey exists for life and life alone because, quite frankly, nothing else gives a shit... nor is capable of giving such shits. If we remove all sentient life from the universe, will honkdonkey exist? Absolutely not, but nothing will exist because in order for something to exist there needs to be a perceiver to see (or to claim, as in the case of gods) that it exists. In order for anything to have some kind of name or label there has to be a namer.

Answerer

January 18, 2005, 11:10 AM

Ok, I too, am sick of using too many physics in the "time exist or not" discussions. Indeed, I had read some books by physicists on why time is just a mere illusion. But I probably won't get into that.

I personally don't believe time is anything more than illusion produced by a combination of our senses, memories and lack of full understanding of how the world works . If time do exist, there must exist a platform, just like our normal space, in which everything or "reality" could move to and fro freely between the worlds of the past and future. Yet as scientist had proved, our reality only move in one direction and the freedom of travelling in both dimensions of time isn't present at all. Worse still, the reasons given for our compulsary one dimension motion are still very vague.

Furthermore, in order for time to exist, the existence of its components; the past and future events must be proved (otherwise it will be pointless to talk about a time dimension that reality move upon). However.......

If past events do exist, then where are those events stored or kept since they no longer could be found in our "present reality"? And does our universe have a big memory storage system just like our mind?

If future events do exist, then where are those events hidden or kept out before they are revealed to our sight? Can they be found in some parallel universes or higher dimensions? If they do exist, does that imply our actions are all pre-deterministic?

If we have seen, the existence of time give rise to lots of difficult questions and imaginary answers. And that, in itself, is probably the best proof of why time is unlikely to be something more than our imaginary minds and a by-product of our science and mathematics.

Just my view.

Schneibster

January 18, 2005, 12:07 PM

doesn't the bucket argument just indicate there is a flux of particles that causes inertia?Hmmm, but is your universe truly empty if it contains a "flux of particles?"

premjan

January 18, 2005, 12:12 PM

Hmmm, but is your universe truly empty if it contains a "flux of particles?"
In a totally empty universe, there would be no bucket either. Or perhaps the water would show no reaction to being spun.

Schneibster

January 18, 2005, 12:58 PM

That argument kicks more ass than the Multiboot AzzKikor 9000 XLE. The use of acceleration to get around the absence of absolute motion. I'm like you, I just can't bring myself to believe the bucket (or two buckets connected by a rope) wouldn't behave as it does in our universe.It's the one that kicked my ass, for sure. I'd forgotten about the buckets connected by a rope. Some people grok it better. Thanks.

And I like the idea of matter as little knots of vibrating spacetime rippling about interacting with each other.Yeah, it's kind of why I like string theory as much as I do.

I'm still struggling with the Higgs particle ocean, though. Lorentz' transform connects space and time because of the formula for time (t' = ((t-vx/c^2)/(1-v^2/c^2)^(1/2)), and Einstein connects mass and energy through the relativistic mass formula (E=mc^2); Noether's theorem connects spacetime and mass/energy (conserved quantities <==> translational, rotational, and temporal symmetries); and to top it all off, Einstein connects spacetime to gravity (GRT), and gravity to inertia (equivalence principle). So where does the Higgs particle fit in, in the mass picture? Is it really necessary? What are the Big Bang/Inflationary Universe implications of nothing having inertia until after electroweak symmetry breaking? If those things don't have inertial mass, does gravity even exist yet? Is it only quantum gravity, and what is that like? Has anybody tried any of this stuff out in BBT/IUS and checked whether it's even a possible scenario that creates a universe that looks like ours?

Edit - oh, The Hole Argument (http://plato.stanford.edu/entries/spacetime-holearg/#6) - just googled the encyl philosophy site, but I think was I read a while back was based on it.Excellent, I got the Hole Argument from that; thank you. Here is something to think about.

For everybody else, here is the brief form of the Hole Argument:

If one has two distributions of metric and matter fields related by a hole transformation, manifold substantivalists must maintain that the two systems represent two distinct physical systems.
This physical distinctness transcends both observation and the determining power of the theory since:

The two distributions are observationally identical.
The laws of the theory cannot pick between the two developments of the fields into the hole.

Therefore the manifold substantivalist advocates an unwarranted bloating of our physical ontology and the doctrine should be discarded.

Earlier on, the article makes the argument for the hole, the premise for #1 above: "For example, imagine that a galaxy passes through some event E in the hole. After the hole transformation, this galaxy might not pass through that event. For the manifold substantivalist, this must be a matter of objective physical fact: either the galaxy passes through E or not. The two distributions represent two physically distinct possibilities."

I deny that E exists, or can exist, separate from the metric. So if the metric moves, then E moves with it and one cannot define any point E that does not move with the metric. In fact, it involves one in a contradiction to believe that E exists; because in order for E to have a position separate from the spacetime metric, there must be another spacetime metric for it to have a position in!

Thus, the Hole Argument leads to an infinite regress that is even more unpalatable than anything that can be forced onto the substantivalist real spacetime by the Hole. While the loss of determinism in spacetime is disconcerting, it is not a disaster; but the immediate proliferation of a required infinite number of spacetimes to define one another in so that E can have a location is a catastrophe.

Schneibster

January 18, 2005, 01:02 PM

I think I'd like to hear about this "hidden baggage."mirage made the point as well as I could.

I wrote a lot more, but it's probably obvious. But still, I think that the notion of spacetime, as opposed to space and time separately, can be introduced after SR has been adequately explained for sufficient grokkage by most people.It might be more natural that way, but as soon as you start to delve into first causes, it's way easier to see how to derive SRT from the Lorentz transform than it is to see how to derive the transform from SRT. I don't recall which way 'round I learned it originally, or I might have a different opinion :D

Schneibster

January 18, 2005, 01:26 PM

Obviously you need to postulate the existance of space and time (though people tend to think that you get these notions and others for free in sopecial relativty from pre-relatvistic physics), howver thta they should be combined in anyway is not really a postulate (spacetime is simply a mathematical object so unless you do something specific like impose a metric your not actually postulating anything about the physical world). Velocity can be defined without specifically using the concept of spacetime (as of course it is in pre-reltavistic physics) and once you have velcoity then inertial frmaes too are easily definmed (again just like they are in pre-relatvistic physics)True enough; but as soon as you start to use the Lorentz transform, it is immediately obvious that they cannot be considered separate. The formula for space in the Galilean transform contains time, which is natural considering the definitions of velocity and acceleration; but the formula for time in the Lorentz transform contains space, which it does not in the Galilean transform. The fact that this is asymmetric in the Galilean transform but symmetric in the Lorentz transform is what led Minkowski to say what he said, IMO.

Well no not really, indeed as I believe the Lorentz transformations were originally part of theory that when formulated in terms of spacetime has a prefered basis, the usefulness of spacetime was not instantly obvious.I'm not sure what you said "no not really" to. I was perhaps overly ambitious to state that it "has to have been recognized immediately by all who understood Lorentz' transform," but certainly Minkowski could have said what he said anytime after Lorentz made his transform.

Schneibster

January 18, 2005, 01:33 PM

Does honkdonkey exist for water inside a spinning bucket? No, it exists for us watching the bucket waiting for the spin to stop. (incidentally the water spins from friction not centrifuge. If centrifuge were the case then a bucket spinning fast enough could make a freestanding column of water.)A quick technical point; it is not that the water spins, and of course it is friction with the bucket that makes it spin, you are correct in that. The point is that the water pushes up the sides of the bucket and lowers down in the middle. As mirage pointed out earlier, the two buckets connected by a rope is more intuitive for some people. Perhaps thinking about that will help you understand the implications of Mach's Hypothesis better.

For the rest, your post of course expresses an opinion that I do not share, but which is as unfalsifiable as my own; to the "tree falling" question, you answer, "No, sound is what you hear," and I answer, "Yes, sound is vibrations in the air." It is all a matter of definitions.

Schneibster

January 18, 2005, 01:39 PM

In a totally empty universe, there would be no bucket either. Hmmm, MU, the answer that unasks the question. While this is a valid philosophical point, it is not a valid physical point; we were playing "what if," and you get no answer, and the answer is the point of the game. So you are not any better off than you were before you started.

Or perhaps the water would show no reaction to being spun.Entirely possible; but I don't buy it.

Anglican

January 18, 2005, 02:07 PM

[QUOTE=Schneibster]True enough; but as soon as you start to use the Lorentz transform, it is immediately obvious that they cannot be considered separate. The formula for space in the Galilean transform contains time, which is natural considering the definitions of velocity and acceleration; but the formula for time in the Lorentz transform contains space, which it does not in the Galilean transform. The fact that this is asymmetric in the Galilean transform but symmetric in the Lorentz transform is what led Minkowski to say what he said, IMO.[//quote]

That's true, mathematically a Lorentz boost is just a change of basis in spacetime and it can be recognized as such just by looking at the equations. However it was not recognized straight away due to the fact the metric has a funny signature and it took Minkowski to recognize this.

In Lorentz's orginal theory the fact that we can view the transformations as a change of basis in spacetime, becomes much obscured and doesn't really relate well to the actual theory which still contains the concept of the ether.

Anglican

January 18, 2005, 02:19 PM

I deny that E exists, or can exist, separate from the metric. So if the metric moves, then E moves with it and one cannot define any point E that does not move with the metric. In fact, it involves one in a contradiction to believe that E exists; because in order for E to have a position separate from the spacetime metric, there must be another spacetime metric for it to have a position in!

But thta's part of the problem though. A mainfold and the points on the mainfold have a seperate 'existance' from the metric (field) that is defined upon it (a manifold needn't have a metric defined upon it infact).

epepke

January 18, 2005, 02:55 PM

It might be more natural that way, but as soon as you start to delve into first causes, it's way easier to see how to derive SRT from the Lorentz transform than it is to see how to derive the transform from SRT. I don't recall which way 'round I learned it originally, or I might have a different opinion :D

I think the first time I understood SR fully was from a high school class, and they taught it using the Lorentz transformation first. Decades later, I heard Feynman's explanation, which is the geometrical one.

In any event, which is preferable is unimportant. The fact that all of the important things about SR can be described by treating space and time separately and using Euclid and the Pythagorean Theorem suggests to me that Minkowski spacetime is not part of a minimal set of postulates for SR.

V-Bird

January 18, 2005, 03:25 PM

Hi.

Been away for some time... :D

http://www.iidb.org/vbb/showpost.php?p=1515810&postcount=9

epepke

January 18, 2005, 04:42 PM

Ok, I too, am sick of using too many physics in the "time exist or not" discussions. Indeed, I had read some books by physicists on why time is just a mere illusion. But I probably won't get into that.

I personally don't believe time is anything more than illusion produced by a combination of our senses, memories and lack of full understanding of how the world works . If time do exist, there must exist a platform, just like our normal space, in which everything or "reality" could move to and fro freely between the worlds of the past and future. Yet as scientist had proved, our reality only move in one direction and the freedom of travelling in both dimensions of time isn't present at all. Worse still, the reasons given for our compulsary one dimension motion are still very vague.

I think I see the problem. There are two concepts that are getting treated as the same: time and the arrow of time.

Time exists, I'm pretty sure. You're on pretty shaky ground asserting that time as a dimension is just an illusion.

As for what scientists have shown about the arrow of time, that's not really correct. Nearly all of the universe, at a fundamental level, looks time-symmetric. Representing some aspects of quantum behavior as going backward in time is commonly done as a metaphor. Up until 1957, it seemed the entire universe was perfectly time symmetric, that it, that CP symmetry explained everything. Since then, decay of the k-meson and another meson I can't remember have been shown to require CPT symmetry. Yet this is still a symmetry that involves time.

With respect to the arrow of time, then you're on much more solid ground. I'd put it a bit backward. Our brains and all other information-processing systems (with the exception of reversible quantum computers, which are currently little more than toys) are heat engines. They require an increase in entropy to run. Since that's what is macroscopically related to the arrow of time, then of course we remember the past and not the future, because the act of remembering requires a transition from a state of lower entropy to a state of higher entropy. So does the act of figuring stuff out.

I'm a bit disheartened seeing people react against physics, though. Most physics isn't that difficult to understand qualitatively, and I think it has a lot of good informative stuff in it.

Answerer

January 18, 2005, 11:19 PM

Time exists, I'm pretty sure.

And what makes you so sure?

As for what scientists have shown about the arrow of time, that's not really correct. Nearly all of the universe, at a fundamental level, looks time-symmetric. Representing some aspects of quantum behavior as going backward in time is commonly done as a metaphor. Up until 1957, it seemed the entire universe was perfectly time symmetric, that it, that CP symmetry explained everything. Since then, decay of the k-meson and another meson I can't remember have been shown to require CPT symmetry. Yet this is still a symmetry that involves time.

Ok, I know about the CPT thing. There is no need for you to repeatedly remind me about its history and definitions.

I'm a bit disheartened seeing people react against physics, though. Most physics isn't that difficult to understand qualitatively, and I think it has a lot of good informative stuff in it.

What in the world makes you think I'm reacting against physics? :rolleyes: I just sick of using physics in the "time exist or not" discussions. Can't I take a break?

epepke

January 18, 2005, 11:33 PM

And what makes you so sure?

Ok, I know about the CPT thing. There is no need for you to repeatedly remind me about its history and definitions.

What in the world makes you think I'm reacting against physics? :rolleyes: I just sick of using physics in the "time exist or not" discussions. Can't I take a break?

Well, the only thing that I can glean from this response is that you seem mighty pissed off about something or other. I don't know what it is, but maybe a break would be a good idea after all.

SLUGFly

January 19, 2005, 12:02 AM

A quick technical point; it is not that the water spins, and of course it is friction with the bucket that makes it spin, you are correct in that. The point is that the water pushes up the sides of the bucket and lowers down in the middle. As mirage pointed out earlier, the two buckets connected by a rope is more intuitive for some people.

Ah...

As I had read it the point was only the water spinning and (I had assumed) bulging in the center (as in centrifugal gravity... I don't know physics well so I'm not sure that this is right) which immediately shows as wrong cuz of the fiction thing...

Never mind me, I'm swinging a rubber chicken when the rest of you are fencing.

premjan

January 19, 2005, 08:14 AM

I am a bit confused by this. Could someone explain it? It appears to be saying that if the bucket were stationary the water would still form a concave surface? :confused:

In 1918 Joseph Lense and Hans Thirring obtained approximate solutions of the equations of general relativity for rotating bodies. Their results show that a massive rotating body drags space-time round with it. This is now called 'frame dragging' or the 'Lense-Thirring effect'. In 1966 Dieter Brill and Jeffrey Cohen showed that frame dragging should occur in a hollow sphere. In 1985 further progress by H Pfister and K Braun showed that sufficient centrifugal forces would be induced at the centre of the hollow massive sphere to cause water to form a concave surface in a bucket which is not rotating with respect to the distant stars. Here at last was a form of the symmetry that Mach was seeking.
http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Newton_bucket.html

Schneibster

January 19, 2005, 04:32 PM

In Lorentz's orginal theory the fact that we can view the transformations as a change of basis in spacetime, becomes much obscured and doesn't really relate well to the actual theory which still contains the concept of the ether.Yes, that's true. I had forgotten that they were still fighting about the aether then; which is kind of absent-minded, considering that SRT was the solution for the question of the aether!

Schneibster

January 19, 2005, 04:41 PM

But thta's part of the problem though. A mainfold and the points on the mainfold have a seperate 'existance' from the metric (field) that is defined upon it (a manifold needn't have a metric defined upon it infact).Right, but without a metric, it is impossible to speak of the "position" of anything; thus, it is impossible to discuss a "distribution" of somethings. Without this ability, I do not see how it is possible to speak of the "distribution" being "different."

Schneibster

January 19, 2005, 04:51 PM

In any event, which is preferable is unimportant. The fact that all of the important things about SR can be described by treating space and time separately and using Euclid and the Pythagorean Theorem suggests to me that Minkowski spacetime is not part of a minimal set of postulates for SR.This point along with Anglican's is pretty conclusive against this argument. I still have one in reserve, though- see my prior post.

And in addition, I have to agree with your statement that the misunderstanding may rest upon the difference between "time" and "time's arrow." Certainly, we can experimentally demonstrate the existence of time.

In discussing the reality of time's arrow, it is worth noting that there are time-symmetry-violating interactions, which may establish a preferred "direction" in time. It is also worth noting that CPT symmetry means that the probabilities of interactions will not change if all of C, P, and T are reversed; not that events will proceed backward. The essential fact that quantum behavior is probabilistic rather than deterministic guarantees that as time flows "in reverse" from what was an "ending" state vector, it will become continually more and more unlikely that the state vector at any point will match the state vector at that same point when time was flowing "forward."

Schneibster

January 19, 2005, 04:53 PM

I just sick of using physics in the "time exist or not" discussions. Can't I take a break?What else are you going to use to discuss whether time exists or not?

Schneibster

January 19, 2005, 04:55 PM

Never mind me, I'm swinging a rubber chicken when the rest of you are fencing.Keep on swingin- if your technique improves, perhaps you can graduate to a stick!
:thumbs:

Anglican

January 19, 2005, 05:00 PM

Right, but without a metric, it is impossible to speak of the "position" of anything; thus, it is impossible to discuss a "distribution" of somethings. Without this ability, I do not see how it is possible to speak of the "distribution" being "different."

Mathematically, the points define the postion so postion is not a problem, infact even if a manifold doesn't have a metric field which is defined at every point there always still a local notion of distance. I guess this is why people prefer to think of spacetime describing the relationships between the object rather than out and out 'manifold realism'.

Schneibster

January 19, 2005, 05:00 PM

I am a bit confused by this. Could someone explain it? It appears to be saying that if the bucket were stationary the water would still form a concave surface? :confused: The point is, the hollow sphere rotating around the bucket results in frame dragging and the frame dragging results in the centripetal force that makes the concave surface. The bucket is at rest with respect to the distant stars, not with respect to the hollow sphere.

I disagree with that quote's assumption that this proves Mach's Hypothesis. In fact, I interpret this effect as proving the real existence of the spacetime metric; otherwise, what is the sphere dragging?

There is a satellite experiment underway right now to verify frame dragging. If the effect fails to be confirmed, it will mean a revision to GRT.

Anglican

January 19, 2005, 05:04 PM

I disagree with that quote's assumption that this proves Mach's Hypothesis. In fact, I interpret this effect as proving the real existence of the spacetime metric; otherwise, what is the sphere dragging?

There is a satellite experiment underway right now to verify frame dragging. If the effect fails to be confirmed, it will mean a revision to GRT.

Yep it's worth pointing out that the Lense-Thirirng effect is a specifc preiction of general relativity which was influenced by, but does not include, Mach's principle. A conformtaion of frame dragging is not a confirmation of Mach's principle.

Schneibster

January 19, 2005, 05:25 PM

Mathematically, the points define the postion so postion is not a problem, infact even if a manifold doesn't have a metric field which is defined at every point there always still a local notion of distance. I guess this is why people prefer to think of spacetime describing the relationships between the object rather than out and out 'manifold realism'.Ummmm, hmmm. If the points define a position, then we can derive all of physics from them, and therefore from the manifold. What necessity is there for a metric field, then? Perhaps we are talking at cross purposes.

Anglican

January 19, 2005, 06:35 PM

Ummmm, hmmm. If the points define a position, then we can derive all of physics from them, and therefore from the manifold. What necessity is there for a metric field, then? Perhaps we are talking at cross purposes.

Each point on the mainfold corresponds to a different postion, however this doesn't tell us anything about the physics that is goimng on we need much more information than this.

Schneibster

January 19, 2005, 06:49 PM

Each point on the mainfold corresponds to a different postion, however this doesn't tell us anything about the physics that is goimng on we need much more information than this.But how does this explain the necessity for a "metric field?" And how can the Hole Problem exist if the metric is not needed to define position on the manifold?

Anglican

January 19, 2005, 07:06 PM

But how does this explain the necessity for a "metric field?" And how can the Hole Problem exist if the metric is not needed to define position on the manifold?

I mean postion in the most basic sense, without necessarily having any means of relating different postions (though as I said on a mainfold you can always relate different postions locally).

Schneibster

January 19, 2005, 07:21 PM

I mean postion in the most basic sense, without necessarily having any means of relating different postions (though as I said on a mainfold you can always relate different postions locally).I have never been happy with the definition of such manifolds. Because you can distinguish the points from one another, and therefore order them, I have always held that that implicit order is equivalent to a frame of reference, because once you order things locally, then you can go to the edge of whatever you have defined as "local" and do another local ordering, thus relating things that are not "local" to one another.

The Hole Problem falls apart based on the premise that no measureable difference == no real difference.

mirage

January 19, 2005, 07:22 PM

mirage

January 19, 2005, 07:30 PM

I mean postion in the most basic sense, without necessarily having any means of relating different postions (though as I said on a mainfold you can always relate different postions locally).
Yep, this is undoubtedly true, but I'm with Schneibster in that it seems silly to argue that space has no physical reality beyond the relations of things in it, then insist that the manifold is an entirely separate thing to the metric. I would think that the two together describe spacetime, and it is the two together (clumsy though the geometric description is that needs two structures) that I would like to "really exist"!

Anglican

January 19, 2005, 07:52 PM

I have never been happy with the definition of such manifolds. Because you can distinguish the points from one another, and therefore order them, I have always held that that implicit order is equivalent to a frame of reference, because once you order things locally, then you can go to the edge of whatever you have defined as "local" and do another local ordering, thus relating things that are not "local" to one another.

Unfortunately it's nowhere near that simple as the local coordinate system is not the same as a general coordinate system. If you tried to do it they way you describe, depending on how you did it, you'd probably end up with a Euclidean metric every time or you'd end up with a 'metric' field that wasn't even symmetric (generally in the broadest sense whatever function is labelled as a 'metric' it is just about always symmetric, this is certianly true for Riemannian and pseudo-Riemannian metrics).

The Hole Problem falls apart based on the premise that no measureable difference == no real difference.
I'm not overly famlir with the hole problem so I can only go from wjhat is in the link, but from what I understand they have two mathematically different situations which describe physically equiavlent situations. By asserting the reality of the mathematics, you have the problem that when you come to descibe a certain physical situation you have more than one mathematical way of describing it

Anglican

January 19, 2005, 08:08 PM

Yep, this is undoubtedly true, but I'm with Schneibster in that it seems silly to argue that space has no physical reality beyond the relations of things in it, then insist that the manifold is an entirely separate thing to the metric. I would think that the two together describe spacetime, and it is the two together (clumsy though the geometric description is that needs two structures) that I would like to "really exist"!

Well okay a pseudo-Rimenanin manifold implies that we've already got our metric, but I'm just pointing out that we have our mainfold first and then we define a metric field over it and indeed we can say cretain things about a spacetime without having to assign it a speciifc (Lorentzian) metric field.

I'm just happy to say it models physical reality and avoid ontology, as asserting the truth of one ontology over another is ultimtaely philosphy rather than science and as I've tried to illustrate can also be problematic.

Schneibster

January 19, 2005, 08:30 PM

Unfortunately it's nowhere near that simple as the local coordinate system is not the same as a general coordinate system. If you tried to do it they way you describe, depending on how you did it, you'd probably end up with a Euclidean metric every time or you'd end up with a 'metric' field that wasn't even symmetric (generally in the broadest sense whatever function is labelled as a 'metric' it is just about always symmetric, this is certianly true for Riemannian and pseudo-Riemannian metrics).OK, but if you can't do it that way, then by defining local co-ordinate systems on points that are at the edges of other local co-ordinate systems, you get inconsistent co-ordinates!

I'd like to refer you to Wikipedia on manifolds (http://en.wikipedia.org/wiki/Manifold), where it says that a manifold is "a type of space, characterised in one of two equivalent ways: ... near every point, the environment is like that in Euclidean space of a given dimension." Then we look at the article on Euclidean space (http://en.wikipedia.org/wiki/Euclidean_space), where it says, "Formally, for any non-negative integer n, n-dimensional Euclidean space is the set Rn (where R is the set of real numbers) together with the distance function obtained by defining the distance between two points (x1, ..., xn) and (y1, ...,yn) to be the square root of Σ (xi-yi)2, where the sum is over i = 1, ..., n. This distance function is based on the Pythagorean theorem and is called the Euclidean metric. " And finally, the article on metric spaces (http://en.wikipedia.org/wiki/Metric_space), where it states that "a metric space is a set (or "space") where a distance between points is defined." So, a manifold is defined in terms of a Euclidean space, and a Euclidean space is a metric space; which is precisely what I said, a manifold has no meaning without a metric.

I'm not overly famlir with the hole problem so I can only go from wjhat is in the link, but from what I understand they have two mathematically different situations which describe physically equiavlent situations. By asserting the reality of the mathematics, you have the problem that when you come to descibe a certain physical situation you have more than one mathematical way of describing itBut as shown above, there need be only one way of describing it; thus, it falls apart on its premises, which is about typical of philosophical arguments confronted with real math, not to mention the real world.

premjan

January 19, 2005, 08:52 PM

I would guess that local means instantaneous or infinitesimal relative to global so it would not be possible to go from local to global.

Anglican

January 19, 2005, 08:58 PM

OK, but if you can't do it that way, then by defining local co-ordinate systems on points that are at the edges of other local co-ordinate systems, you get inconsistent co-ordinates!

They are different cooridnate systems, so that they are not part of the same global, well-behaved coordinate system is no problem. Infact it was Einstein's obsrevation that gravity preclides globally inertial coordinate systems in spacetime that led him to the connection with curved mainfolds.

I'd like to refer you to Wikipedia on manifolds (http://en.wikipedia.org/wiki/Manifold), where it says that a manifold is "a type of space, characterised in one of two equivalent ways: ... near every point, the environment is like that in Euclidean space of a given dimension." Then we look at the article on Euclidean space (http://en.wikipedia.org/wiki/Euclidean_space), where it says, "Formally, for any non-negative integer n, n-dimensional Euclidean space is the set Rn (where R is the set of real numbers) together with the distance function obtained by defining the distance between two points (x1, ..., xn) and (y1, ...,yn) to be the square root of Σ (xi-yi)2, where the sum is over i = 1, ..., n. This distance function is based on the Pythagorean theorem and is called the Euclidean metric. " And finally, the article on metric spaces (http://en.wikipedia.org/wiki/Metric_space), where it states that "a metric space is a set (or "space") where a distance between points is defined." So, a manifold is defined in terms of a Euclidean space, and a Euclidean space is a metric space; which is precisely what I said, a manifold has no meaning without a metric.

Yes but it's the symmetric (0,2) tensor field smoothly defined over the mainfold that we are inetrested in and we call the metric. That mainfolds are always locally metricizable is just a simple fact that follows from their definition

But as shown above, there need be only one way of describing it; thus, it falls apart on its premises, which is about typical of philosophical arguments confronted with real math, not to mention the real world.

I'm not sure what you mean, if you take what the link says as correct there is more than one way to describe the physical reality within the model, so the probelm with directly attatching realiity to the model is which is the right way to decsibe the physical situation?

Anglican

January 19, 2005, 09:06 PM

I would guess that local means instantaneous or infinitesimal relative to global so it would not be possible to go from local to global.

If a property is local it means that for every point there is a neighbourhood where it holds true. On a manifold this menas that we pick a point and we automatically get a set of points which form a Euclidean space.

premjan

January 19, 2005, 09:08 PM

If a property is local it means that for every point there is a neighbourhood where it holds true. On a manifold this menas that we pick a point and we automatically get a set of points which form a Euclidean space.
OK so no reference to epsilon or delta anywhere?

mirage

January 20, 2005, 05:17 AM

They I'm not sure what you mean, if you take what the link says as correct there is more than one way to describe the physical reality within the model, so the probelm with directly attatching realiity to the model is which is the right way to decsibe the physical situation?The way I see it, the manifold, which can be arbitrary is just an artifact of geometry that requires a tensor function to be applied over a set of event locations. What we really want to describe is a set of points described by both these mathematical structures, i.e. spacetime. If we could adequately describe it using a single math structure, we would. It seems a little like separating the scalar from a vector when considering velocity. Despite that description, we can think of motion as a single physical "thing".

I would say that a solitary manifold just lacks physical application, like an affine space or summink.

I would like to attach reality to this model (whilst accepting that the maths isn't the thing) in the same way that I would like to attach reality to the mental model I have of my lunch.

Sure, it's just a philosophical position. It all comes down to aesthetics.

mirage

January 20, 2005, 05:22 AM

Answerer,

I have a lot of sympathy with your metaphysical position on time.

It is quite true and not at all trivial that time certainly does exist as a phenomenon in our minds, and we need a word for it. It exists in the same way the "world outside our heads" exists, i.e. any other position is meaningless and/or an affront to Occam. "Time is just an illusion" type people are shallow fools trying to sound deep.

It is then up to physics and neuropsychology to relate this obvious time to observations beyond our everyday world and accurate time observations in our everyday world but perceived through our imprecise brains.

atrahasis

January 20, 2005, 06:31 AM

To start with..what is to exist? according to dictionary.com exist means 1.To have actual being; be real.
2. To be present under certain circumstances or in a specified place; occur:

You may go ahead and ask what do we mean by real? again in philosophy it is simply Existing objectively in the world regardless of subjectivity or conventions of thought or language

According to that, does time exist? or rather does a dozen exist?

simply we use time to measure decay,change. if nothing changed i.e if nothing moved and everything remains as it was since its beginning then what would be the need of having time? If you did't have 12 items, then why have a dozen?

The existence of time is simply relative and not absolute..

Although...

We humans 'experience' the existence of time because 'i think' we are the only organisms who are self aware.Does a dog know that there is a next year? a next week? NO so according to it, time is like santa.

Einstein proved that Time is not absolute and said something about how we experience time in different ways depending on how we are moving. But he hardly explained what Time is. And nobody else ever has.
British physicist Julian Barbour has a theory that Time does not exist, and that most of Physics' troubles arise from assuming that it does exist. We have no evidence of the past other than our memory of it. We have no evidence of the future other than our belief in it.
Barbour believes that it is all an illusion: there is no motion and no change. Instants and periods do not exist. What exists is only "time capsules", which are static containers of "records". Those records fool us into believing that things change and events happen.
There exists a "configuration space" that contains all possible instants, all possible "nows". This is "Platonia." These instants -- We experience a set of these instants, i.e. a subset of Platonia.
Barbour is inspired by Leibniz' theory that the universe is not a container of objects, but a collection of entities that are both space and matter. The universe does not contain things, it "is" things.