String theory, Ekpyrotic theory, and thermodynamics

[luvluv]

Hows that for a thread title?

Seriously, I've been wondering: do the laws of thermodynamics apply in string theory and, by extension, ekpyrotic theory?

I've always wondered why, in string theory, the strings don't slow down and eventually halt.

Also, I wonder why, in ekpyrotic theory, the energy the branes expend in their movement along the hidden dimension doesn't also wind down. From whence do they get the energy to move forever? Do they use energy? Will the process wind down at some point? Does anybody have a clue?

Thanks.

[Jesse]

luvluv:
Seriously, I've been wondering: do the laws of thermodynamics apply in string theory and, by extension, ekpyrotic theory?

I've always wondered why, in string theory, the strings don't slow down and eventually halt.

I can't tell you much about string theory, but I can tell you thermodynamics doesn't say a collection of moving particles will eventually halt, since that would violate conservation of energy. It basically just says that the energy is likely to become more and more spread out through the different components of the system over time, instead of having all the energy concentrated in a few components. You may find either http://www.2ndlaw.com or http://www.secondlaw.com (sister sites by the same author) helpful in explaining a little more about what this means.

Also, I wonder why, in ekpyrotic theory, the energy the branes expend in their movement along the hidden dimension doesn't also wind down. From whence do they get the energy to move forever? Do they use energy? Will the process wind down at some point? Does anybody have a clue?

Moving at a constant velocity doesn't use up energy, only acceleration does. If I throw a ball in space at 30 mph (in a given reference frame), then in the absence of external forces acting on the ball, it will keep moving at 30 mph in that reference frame forever.

[luvluv]

So if there are no other forces acting on branes then the branes could go on moving forever? Do we know that there are no other forces involved in ekpyrotic theory? I guess not. Would there need to be some influx of energy at some point in the past to get the whole thing going?

Moving at a constant velocity doesn't use up energy, only acceleration does. If I throw a ball in space at 30 mph (in a given reference frame), then in the absence of external forces acting on the ball, it will keep moving at 30 mph in that reference frame forever.

So what you're saying is a perpertual motion machine is possible, so long as it doesn't involve accelleration and is constructed somewhere where there are no external forces?

[Jesse]

luvluv:
So if there are no other forces acting on branes then the branes could go on moving forever? Do we know that there are no other forces involved in ekpyrotic theory? I guess not. Would there need to be some influx of energy at some point in the past to get the whole thing going?

I know nothing about the details of brane theory--I don't even know if energy is conserved in this theory, for example (my understanding is that it isn't necessarily conserved in general relativity). But again, if we go back to classical mechanics, let's say we have an eternal universe containing a bunch of objects moving around--in that case the objects would keep moving relative to one another forever, even if they were exerting forces on each other (a random external force is just as likely to make an object speed up in a given reference frame as it is to make it slow down).

So what you're saying is a perpertual motion machine is possible, so long as it doesn't involve accelleration and is constructed somewhere where there are no external forces?

Yes, when people talk about perpetual motion machines they really mean something more like "perpetual acceleration machine" or "perpetual useful-energy-producing machine", perpetual movement at constant velocity is nothing out of the ordinary. The thing you have to understand is that there's no absolute definition of being "at rest" or "moving" in physics, it's all relative to your choice of reference frame. If you see me fly by you moving at 50 mph to your right, it's equally valid to look at this situation in a reference frame where I'm at rest and you're moving 50 mph to my left.

[Tim Thompson]

luvluv: Seriously, I've been wondering: do the laws of thermodynamics apply in string theory and, by extension, ekpyrotic theory?

Yes & no. String theory obeys the laws of string thermodynamics, which are not necessarily the same as those of classical thermodynamics. For instance, in classical physics there is no such thing as a maximum possible temperature, but there is in string theory (the Hagedorn temperature (http://www.cerncourier.com/main/article/43/7/18)). The strings of string theory can come in the form of closed loops, or strings with the ends loose & flapping. As the temperature of the string universe increases, at the Hagedorn temperature the strings of the universe switch from one to the other (I can't recall which), reversing the temperature trend, and reversing the meaning of direction, such that a universe which appears to be contracting wiill reverse into expansion, and vice-versa. If it sounds confusing, don't worry, it really is confusing. I don't know of any discussion of this that is targeted for general readers, and I don't know how to convert the mathematics into sensible English better than this.

In Limiting Temperature, Limiting Curvature and the Cyclic Universe (http://cul.arxiv.org/abs/hep-th/0307061) (23 July 2003), the authors present a theory of string thermodynamics which, if valid, allows them to deduce that the universe has already gone through the "big bounce" 40 times. So, yes, in some form, the laws of thermodynamics apply to the ekpyrotic scenario.

luvluv: I've always wondered why, in string theory, the strings don't slow down and eventually halt.

One does not need string theory to field such questions. Why don't the electrons around a nucleus slow down & stop? it was this question which forced physicists into quantum mechanics. Rather, you should ask, why should the strings stop anyway?

luvluv: Also, I wonder why, in ekpyrotic theory, the energy the branes expend in their movement along the hidden dimension doesn't also wind down. From whence do they get the energy to move forever? Do they use energy? Will the process wind down at some point? Does anybody have a clue?

I'm guessing that the last bit about clues is the most likely. But in general, if they get energy from anywhere, they get it from the bulk, which is the higher dimension space that the brane is embedded in (i.e., a 5-dimensional brane embedded in an 11-dimensional bulk, or some such contrivance). In all liklihood, I don't think they use energy, so much as they are energy.

The best you can hope for is to read Brian Greene's Elegant Universe, which is really quite readable (and 1000 times better than the TV program was). It's the only book on strings that non-mathematicians might bother with, I think.

[luvluv]

Thanks Tim Thompson. Are you a physicist, if I may ask?

I actually read Brian Greene's book but it was something like 2 years ago. I do remember him mentioning string thermodynamics but I don't recall him going into much detail on it.

One does not need string theory to field such questions. Why don't the electrons around a nucleus slow down & stop? it was this question which forced physicists into quantum mechanics. Rather, you should ask, why should the strings stop anyway?

I guess what I'm asking is, will the general "heat death" that awaits the universe effect things on the Planck level? Will strings and branes and such also run out of useful energy at some point? Or do the laws of thermodynamics only apply in that particular way to the macro-verse?

[Tim Thompson]

luvluv: Are you a physicist, if I may ask?

Yep. Just click on the profile button under any message I post (M.S., physics, 1985, CSULA). Most of my experience is in planetary atmospheres, but I teach myself fundamental physics.

It's Planck level physics that will determine the heat death of the universe, not the other way around. The apparently accelerated expansion (http://snap.lbl.gov/brochure/redshift.html) of the universe, whether it is seen as a cosmological constant (http://www.astro.ucla.edu/~wright/cosmo_constant.html), or a "quintessence (http://www.sbf1.if.usp.br/eventos/enfpc/xx/procs/res302/res302.html)" field, most likely derives its energy from the "zero point" or vacuum energy (http://en.wikipedia.org/wiki/Vacuum_energy). And that's completely determined by physics at the Planck scale (http://csep10.phys.utk.edu/astr162/lect/cosmology/planck.html). The classical laws of thermodynamics (http://hyperphysics.phy-astr.gsu.edu/hbase/heacon.html#heacon) do not necessarily apply, or at least it is not clear that they do, in scenarios dominated by quantum mechanics (http://hyperphysics.phy-astr.gsu.edu/hbase/quacon.html#quacon).

[luvluv]

Thanks for your help, Tim! (And cool home page! You still sporting the beard?)

I guess to get down to brass tacks my question is whether or not the scenario described in ekpyrotic theory could have really been going on forever. If I understand you correctly, so long as acceleration isn't fundamental to the process, there's no reason this can't be the case. Is that correct? Or are there other processes that are determined by string theory which could also limit the interminability of the process?

(Last question, I promise.)

[Tim Thompson]

Indeed I have not shaved a hair since the summer of '76, though I confess to occasionally trimming my moustache to avoid eating it.

The short answer is that nobody knows enough about the ekpyrotic or cyclic cosmology, or string theory, to provide a realistic response. There is no reason to presume either way, that the cycles could be infinite or otherwise, either choice would be quite arbitrary. That may well change with the state of learning, but that's where the matter sits today.

[luvluv]

Thanks for your help, my friend.

I'm sure it will provide you no comfort at all to know that some of the hairs on your face are older than I am. ;)