True vacuums & 0 Kelvin

[h.g.Whiz]

Is there really a such thing as a true vacuum ? :confused:

Is 0 kelvin an impossible temperature for matter to reach ?

Is the tempature of the dense matter that lies within black holes really hot or really cold ? I would guess really cold.

[jj]

An interesting question:

What is the entropy of a pure vacuum? :)

[Ziggurat]

Is there really a such thing as a true vacuum ? :confused:

What do you mean by a "true vacuum"? Is it a vacuum as long as it's devoid of atoms? Is it still a vacuum if you get some neutrinos in it? What about photons? What about virtual photons?

Is 0 kelvin an impossible temperature for matter to reach ?

Yes, it's impossible. That's the 3rd law of thermodynamics.

Is the tempature of the dense matter that lies within black holes really hot or really cold ?

I don't think there's any way to define a temperature for the singularity. The entire black hole has a temperature that is related to the Hawking radiation it gives off (small black holes are hot, large black holes are cold).

[Ziggurat]

What is the entropy of a pure vacuum? :)

Depends. Do you allow photons in your vacuum?

[lionking]

Depends. Do you allow photons in your vacuum?

I'm a novice here (and find your posts incredibly useful BTW), but isn't it possible to exclude photons? By some sort of solid barrier?

[rlr]

Is there really a such thing as a true vacuum ? :confused:

Is 0 kelvin an impossible temperature for matter to reach ?

Is the tempature of the dense matter that lies within black holes really hot or really cold ? I would guess really cold.

0 Kelvin is an impossible temperature for matter to reach. 3rd law of thermodynamics.

The way you mean "true vacuum" sort of has a Catch-22 attached to it. The only universe which could be at 0K would be a universe with nothing in it. But Einstein maintained that "space" is the space between objects, and that without referents that there can be space between, there is no meaning to a spatial dimension at all. So the only universe which could be at 0K is a universe which could not meaningfully exist in any way.

Although matter cannot ever be at absolute zero, it can actually, under very contrived situations, have a negative Kelvin temperature. Negative Kelvin is not "colder than absolute zero", though, but actually "hotter than infinity".

[lionking]

Although matter cannot ever be at absolute zero, it can actually, under very contrived situations, have a negative Kelvin temperature. Negative Kelvin is not "colder than absolute zero", though, but actually "hotter than infinity".

Wow! Do you have any links on this?

[Reality Check]

I'm a novice here (and find your posts incredibly useful BTW), but isn't it possible to exclude photons? By some sort of solid barrier?
IMHO, you cannot exclude photons. If you have a barrier that is not at 0 Kelvin then it gives off radiation (photons) corresponding to its temperature.

[Ziggurat]

Wow! Do you have any links on this?

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

[Reality Check]

Is the tempature of the dense matter that lies within black holes really hot or really cold ? I would guess really cold.
My guess would be really hot. You have matter falling in a gravitational field. It is losing gravitational energy and gaining kinetic energy. At the same time it is undergoing spaghettification (http://en.wikipedia.org/wiki/Spaghettification). I would expect that both processes would heat the matter up as it falls.
But if you are talking about the singularity then all bets are off. That is an regime that our current physics cannot describe.

[Ziggurat]

I'm a novice here (and find your posts incredibly useful BTW), but isn't it possible to exclude photons? By some sort of solid barrier?

Nope. The problem is that any solid barrier you make will actually emit photons as well. The only way to keep a material from emitting photons is to either cool it down to zero or make it perfectly reflective at all frequencies. But you can't actually do either of those things.

[lionking]

Nope. The problem is that any solid barrier you make will actually emit photons as well. The only way to keep a material from emitting photons is to either cool it down to zero or make it perfectly reflective at all frequencies. But you can't actually do either of those things.

Thanks. That makes sense. And to you too Reality Check.

[rlr]

Wow! Do you have any links on this?

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/neg_temperature.html

[sol invictus]

The way you mean "true vacuum" sort of has a Catch-22 attached to it. The only universe which could be at 0K would be a universe with nothing in it. But Einstein maintained that "space" is the space between objects, and that without referents that there can be space between, there is no meaning to a spatial dimension at all. So the only universe which could be at 0K is a universe which could not meaningfully exist in any way.

That's incorrect. Perhaps you've confused Einstein with Mach? According to Einstein, and more importantly according to general relativity, there is absolutely nothing wrong with a universe with nothing in it. Space and time exist just fine.

As for the the question about entropy: the vacuum is by definition a single state - it's the state with minimum energy. Any single state has zero entropy automatically, no matter what it is. So yes, the vacuum has precisely zero entropy. Whether one could ever reach it starting from a state with non-zero entropy is another matter.

[rlr]

That's incorrect. Perhaps you've confused Einstein with Mach? According to Einstein, and more importantly according to general relativity, there is absolutely nothing wrong with a universe with nothing in it. Space and time exist just fine.

Uh, no, I'm not confusing Einstein with anyone.

http://www.relativitybook.com/resources/Einstein_space.html

We are now in a position to see how far the transition to the general theory of relativity modifies the concept of space. In accordance with classical mechanics and according to the special theory of relativity, space (space-time) has an existence independent of matter or field.

In order to be able to describe at all that which fills up space and is dependent on the co-ordinates, space-time or the inertial system with its metrical properties must be thought of at once as existing, for otherwise the description of "that which fills up space" would have no meaning. On the basis of the general theory of relativity, on the other hand, space as opposed to "what fills space", which is dependent on the co-ordinates, has no separate existence. Thus a pure gravitational field might have been described in terms of the gik (as functions of the co-ordinates), by solution of the gravitational equations. If we imagine the gravitational field, i.e. the functions gik, to be removed, there does not remain a space of the type (1), but absolutely nothing, and also no "topological space". For the functions gik describe not only the field, but at the same time also the topological and metrical structural properties of the manifold.

A space of the type (1), judged from the standpoint of the general theory of relativity, is not a space without field, but a special case of the gik field, for which – for the co-ordinate system used, which in itself has no objective significance – the functions gik have values that do not depend on the co-ordinates. There is no such thing as an empty space, i.e. a space without field.

Space-time does not claim existence on its own, but only as a structural quality of the field.

Thus Descartes was not so far from the truth when he believed he must exclude the existence of an empty space. The notion indeed appears absurd, as long as physical reality is seen exclusively in ponderable bodies.

It requires the idea of the field as the representative of reality, in combination with the general principle of relativity, to show the true kernel of Descartes' idea; there exists no space "empty of field".

That's not actually the passage I was looking for, but it'll do.

Edit:

Ah, here it is. I was looking in the back of my copy when I should have been looking at the front.

Note to the Fifteenth Edition (of Relativity: The Special and the General Theory):

In this edition I have added, as a fifth appendix (ed: which I just quoted), a presentation of my views on the problem of space in general and on the gradual modifications of our ideas on space resulting from the influence of the relativistic view-point. I wished to show that space-time is not necessary something to which one can ascribe a separate existence, independently of the actual objects of physical reality. Physical objects are not in space, but these objects are spatially extended. In this way the concept "empty space" loses its meaning.

A. Einstein
June 9th, 1952

Edit: Also there's Kevin S. Brown's brilliant "Reflections on Relativity" website, which goes into more conceptual detail on relativity than anyone would ever want unless they had a real interest in physics.

http://www.mathpages.com/rr/rrtoc.htm

The relevant page for this is here:

http://www.mathpages.com/rr/s1-01/1-01.htm

It explains why it wouldn't even be conceptually meaningful to speak of truly empty space, except in contrast to non-empty space.

[Speedskater]

As my father would say:
"There is no such thing as a vacuum"

[sol invictus]

Uh, no, I'm not confusing Einstein with anyone.

Then you've failed to understand what he is saying. (The first quote is very clear - the second a little more ambiguous and philosophical.)

He is not saying that one needs "objects" - or even particles in any ordinary sense - to create or define space. He's saying one needs a metric tensor with a non-zero expectation value - in other words a classical metric that isn't zero. If you interpret that metric as a field, then it's true that there is always something even in empty space (or more accurately, empty space is something). But it's not an object or a particle - it's more like a condensate. Actually I would say we understand that now quite a bit better than Einstein could have in 1952.

To give an explicit example, the simplest solution to Einstein's equations in vacuum (i.e. with zero matter or energy) is simply flat, empty, Minkowski space. The metric isn't zero - it's more like 1 - but other than that there is nothing in the space.

General relativity is a kind of compromise between Newtonian ideas of absolute, fixed space, and Cartesian/Machian ideas that space is created by the matter in it. In GR space can exist with precisely zero matter (with only a metric tensor), but matter warps and deforms the metric when it's present.

[rlr]

He is not saying that one needs "objects" - or even particles in any ordinary sense - to create or define space. He's saying one needs a metric tensor with a non-zero expectation value - in other words a classical metric that isn't zero. If you interpret that metric as a field, then it's true that there is always something even in empty space (or more accurately, empty space is something). But it's not an object or a particle - it's more like a condensate. Actually I would say we understand that now quite a bit better than Einstein could have in 1952.

So you're saying that a) I (a physics teacher myself) am misinterpreting words in clear, plain English, and additionally, b) Einstein didn't understand general relativity, after previously saying that c) Einstein didn't say that, which he clearly did. No empty space means no empty space.

I think everyone has a great idea of the extent of your ego now, if they didn't before. At least I didn't.

[paximperium]

So you're saying that a) I (a physics teacher myself) am misinterpreting words in clear, plain English, and additionally, b) Einstein didn't understand general relativity, after previously saying that c) Einstein didn't say that, which he clearly did.

I think everyone has a great idea of the extent of your ego now, if they didn't before. At least I didn't.
So says the "physics teacher" to the physics professor.

[rlr]

Add that, of course, to the fact that general relativity is a completely uncorroborated theory, aside from being congruent with observation. Nobody has actually ever measured spacetime curvature, and I doubt anyone ever will. One could literally posit an infinite number of alternatives to GR, which agree with all observables and which differ in unobservables. One could almost equivalently reformulate GR in terms of torsion instead of curvature, which Einstein himself attempted to do.

So I think you're attaching quite a lot more significance to general relativity than it warrants, by comparison to other science which actually is grounded in fact rather than interpretation. GR is only correct in the same sense that BCS superconductivity theory is correct. It adequately explains observation while possibly (maybe even likely) being totally wrong about how those observations come about. We refer to these as "effective" theories, because their predictions are "effectively" right even if their mechanics are not literally true. They are more or less logically equivalent to the right answer, even if they're not actually the right answer and even if we don't know the right answer.

Likewise, special relativity, and observation itself, can never actually prove to us that the speed of light is constant. Only that the round-trip speed of light is constant. That it is constant going from A, around a loop, back to A. Not that it's constant from A to B, and the same from B to A. There's a difference. It's quite possible that the velocity of light is different in one direction than in another. We'd have no way of knowing. This possibility is discounted because there's no good reason for a preferred direction to exist.

[rlr]

So says the "physics teacher" to the physics professor.

So says the, uh, oh right, nothing relevant to this discussion, to the... oh right, conversation you contributed nothing to. Again.

[sol invictus]

So you're saying that a) I (a physics teacher myself) am misinterpreting words in clear, plain English, and additionally, b) Einstein didn't understand general relativity, after previously saying that c) Einstein didn't say that, which he clearly did. No empty space means no empty space.

The first quote is very explicit - he's talking about the metric tensor, gik, being non-zero. The metric is not an object, it's a field expectation value (as he also says in clear, plain, English). Nowhere does he say "no empty space" - he says "no space empty of field", just as I did.

In the second quote he equivocates - "not necessarily" - and he doesn't make clear what he means by "object". The only interpretation consistent with the first quote, and with his own theory, is that he includes metric tensor expectation values in "object" - a rather strange thing, but it may make sense in the context you took that quote out of.

I think everyone has a great idea of the extent of your ego now, if they didn't before. At least I didn't.

My ego is irrelevant to the question of whether what you are saying is correct.

Add that, of course, to the fact that general relativity is a completely uncorroborated theory, aside from being congruent with observation.

What a totally nonsensical statement. How could GR possibly be corroborated other than by being congruent with observation (and by being mathematically consistent and essentially unique given one or two basic principles, as it also is)? That's what science is.


Nobody has actually ever measured spacetime curvature, and I doubt anyone ever will.

Nonsense - of course they have. I sincerely hope you're not a physics teacher, as you claimed.


One could literally posit an infinite number of alternatives to GR, which agree with all observables and which differ in unobservables.

That's true only in a totally stupid sense. One could say the same about everything.

One could almost equivalently reformulate GR in terms of torsion instead of curvature, which Einstein himself attempted to do.

Tried and failed. That's "almost equivalent"?

It adequately explains observation while possibly (maybe even likely) being totally wrong about how those observations come about. We refer to these as "effective" theories, because their predictions are "effectively" right even if their mechanics are not literally true.

I have no idea what you mean by "literally true".

Likewise, special relativity, and observation itself, can never actually prove to us that the speed of light is constant. Only that the round-trip speed of light is constant. That it is constant going from A, around a loop, back to A. Not that it's constant from A to B, and the same from B to A. There's a difference.

Umm, what about measurements that don't involve round trips?


It's quite possible that the velocity of light is different in one direction than in another. We'd have no way of knowing. This possibility is discounted because there's no good reason for a preferred direction to exist.

No, it's discounted because it's been tested to incredible accuracy and it doesn't happen. You're plainly not a physicist.

[quarky]

zero kelvin is the religious temp in the perfect vacuum of god's empty throne.

(sorry. still feeling philosophical.)

[paximperium]

zero kelvin is the religious temp in the perfect vacuum of god's empty throne.

(sorry. still feeling philosophical.)
Does god have enough suck to create a vacuum even he can't fill?

[sol invictus]

Likewise, special relativity, and observation itself, can never actually prove to us that the speed of light is constant. Only that the round-trip speed of light is constant. That it is constant going from A, around a loop, back to A. Not that it's constant from A to B, and the same from B to A. There's a difference. It's quite possible that the velocity of light is different in one direction than in another. We'd have no way of knowing. This possibility is discounted because there's no good reason for a preferred direction to exist.

Actually this is even more wrong than I noticed at first. Suppose all you know is that the round trip time for light is doesn't depend on direction. That suffices to prove that the speed in each direction must be equal.

[fuelair]

Does god have enough suck to create a vacuum even he can't fill?

Since it doesn't exist, of course it does!!

I are also a physics teacher, I are not a physicist, I fully understand the difference.:)

[Fnord]

What is the entropy of a pure vacuum? :)
.
The sound of one hand clapping.

[quarky]

Does god have enough suck to create a vacuum even he can't fill?

God hath most major suckage.