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2019-09-23, 10:41   #12
xilman
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Quote:
 Originally Posted by retina We have no data to support such a conclusion AFAICT.
It isn't expanding into anything by definition. The universe is defined as the entirity of space-time and its contents. There is nothing else into which it could expand.

Of course, you may define the universe however you wish, but don't be surprised if the conclusions you draw differ from the standard definitions and, possibly, disagree with experiment and observation.

I can try to clarify the GR point of view of expanding space-time in greater depth but I need to think carefully about how to phrase it without going too deeply into differential geometry and tensor calculus. Later.

2019-09-23, 11:06   #13
retina
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Quote:
 Originally Posted by xilman It isn't expanding into anything by definition. The universe is defined as the entirity of space-time and its contents. There is nothing else into which it could expand.
Then I think we have a reference problem. We could be shrinking ... OR ... the universe is expanding (or both together). Where is the reference point for which scale is the "correct" one?

2019-09-23, 15:43   #14
xilman
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Quote:
 Originally Posted by retina Then I think we have a reference problem. We could be shrinking ... OR ... the universe is expanding (or both together). Where is the reference point for which scale is the "correct" one?
In relativity theory, the speed of light is defined to be constant.

Set up a classical Einstein thought experiment. You have two observers in an otherwise empty universe. Each makes an arbitrary choice of co-ordinate system, the technical term for which is "reference frame". Note that their chosen co-ordinate systems may be quite different, in scale and/or orientation. Neither of them can detect any force acting, no matter how carefully they make their measurements; this means that both are in inertial reference frames.

At t=0, as measured in his own local coordinate system, the first observer sends off a signal to the other. Upon receipt, the second observer immediately replies with the observed time of arrival as measured in her own coordinate frame. All messages contain timing pulses which are separated by the duration of a number of disparate physical phenomena, such as a specific number of cycles of the light emitted in an agreed upon atomic transition, or the half-life of a specific radioactive decay. Upon receipt of the reply, they immediately reply themselves and do so for every message they receive.

After at least two of such exchanges, each observer can deduce their mutual separation and its rate of change, aka their relative velocity along the line joining them.

Now let them continue making these exchanges for a long time --- time as measured in their own co-ordinate systems. Suppose they discover that each successive exchange of messages takes slightly longer (in their own cordinate system, remember) and that the duration of each message becomes longer in exactly the same proportion.

Here's the critical bit: both observers are convinced that they are in their own inertial reference frame because they are not feeling any forces acting. Nonetheless, each observes that the other is moving away at a constant acceleration. They also observe that the messages they exchange are not only taking longer to arrive, the duration of each message is increasing as well. They conclude that the space-time between them is expanding specifically because neither feels any acceleration.

Added in edit: I forgot to address your "shrinking" point. It's as valid a view as any. It doesn't change the physics: that each message is taking more and more co-ordinate units as local time goes by. As measured in those shrinking units, the universe is still expanding.

Last fiddled with by xilman on 2019-09-23 at 16:02

 2019-09-23, 15:50 #15 LaurV Romulan Interpreter     Jun 2011 Thailand 2×3×31×47 Posts So, if all the space is expanding, then all the space is expanding. This includes the space between the atoms of any material, and the space between the protons in a nucleus, etc. Will we ever reach a point where the weak force is overtaken by the electric force, and the nuclei of all elements will disintegrate, because the protons are too far apart to "hold together" and will repeal each other due to "same charge" stuff? Or I am digging in the wrong direction?
2019-09-23, 16:15   #16
xilman
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Quote:
 Originally Posted by LaurV So, if all the space is expanding, then all the space is expanding. This includes the space between the atoms of any material, and the space between the protons in a nucleus, etc. Will we ever reach a point where the weak force is overtaken by the electric force, and the nuclei of all elements will disintegrate, because the protons are too far apart to "hold together" and will repeal each other due to "same charge" stuff? Or I am digging in the wrong direction?
Correct as far as it goes. The better version is "all the space-time is expanding".

Yup. If the cosmological constant increases sufficiently, there will be a Big Rip.

At the moment, the strong, electro-weak and gravitational forces are sufficiently strong to hold everything within the local group of galaxies together. The local group is gravitationally bound, so they don't need to worry about universal expansion. That pretty much describes the meaning of gravitationally bound.

2019-09-23, 16:36   #17
petrw1
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Nov 2006

104438 Posts

Well I can confirm that I am expanding ... theory proven!!!!

Quote:
 Originally Posted by LaurV So, if all the space is expanding, then all the space is expanding. This includes the space between the atoms of any material, and the space between the protons in a nucleus, etc. Will we ever reach a point where the weak force is overtaken by the electric force, and the nuclei of all elements will disintegrate, because the protons are too far apart to "hold together" and will repeal each other due to "same charge" stuff? Or I am digging in the wrong direction?

2019-09-23, 18:02   #18
LaurV
Romulan Interpreter

Jun 2011
Thailand

2·3·31·47 Posts

Quote:
 Originally Posted by petrw1 Well I can confirm that I am expanding ... theory proven!!!!
Well, from that point of view, the space around and inside my belly dilates much faster than the rest of the space, and I don't know how to stop it... But that is another subject, here in this topic, between jokes, we try to clarify some "serious" points.

 2019-09-23, 19:37 #19 jwaltos     Apr 2012 15A16 Posts Is there a granularity to space-time? I am not aware of any physical object whose material or non-material property (ie. wave-particle duality) can be considered composed of something "absolutely" and I cannot really state that space-time has a composition..mathematically it seems to exist but what keeps it in place? Does space-time "slosh about" if shaken like a Martini? Would there be a resonant frequency? If something were to "tunnel through" the interstices of space-time (if such "non-space time exists") could such things be described by GR or could they be inferred from GR (other than the inference of dark matter/energy)? As a non-practicing Amateur Radio person, I would like to do some pioneering DX'ng. SETI is not having much luck so they must be doing something (or everything) wrong. Since there some answers but many more questions and much more nonsense I'd like to reach out in some non-conventional manner to obtain some answers from someone/something somewhere. Pointers to insightful papers are always appreciated. Last fiddled with by jwaltos on 2019-09-23 at 19:40 Reason: added stuff
2019-09-24, 03:34   #20
retina
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Quote:
 Originally Posted by xilman It isn't expanding into anything by definition. The universe is defined as the entirity of space-time and its contents. There is nothing else into which it could expand.
Further to my previous response. I think that is a circular argument. The definition says there is nothing else, so therefore there is nothing else? No. There could be some other structure, unspace-untime, that our space-time universe is embedded in. We have no proof that there isn't such a structure, since we cannot measure unspace-untime with our space-time implements.

2019-09-24, 06:23   #21
xilman
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Quote:
 Originally Posted by retina Further to my previous response. I think that is a circular argument. The definition says there is nothing else, so therefore there is nothing else? No. There could be some other structure, unspace-untime, that our space-time universe is embedded in. We have no proof that there isn't such a structure, since we cannot measure unspace-untime with our space-time implements.
In that case, you are propounding metaphysics, not physics. I thought we were discussing the latter.

2019-09-24, 06:35   #22
xilman
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Quote:
 Originally Posted by jwaltos Is there a granularity to space-time?
We don't know. There is no observational evidence at present but attempts to merge gravity with quantum mechanics suggest that there should be granularity at the Planck scale.

Quote:
 Originally Posted by jwaltos Does space-time "slosh about" if shaken like a Martini? Would there be a resonant frequency?
Yes, it sloshes and there is observational evidence that it does so. The sloshing is called gravitational radiation. See https://en.wikipedia.org/wiki/Cosmic...d#Polarization for a treatment of resonant behaviour.
Quote:
 Originally Posted by jwaltos If something were to "tunnel through" the interstices of space-time (if such "non-space time exists") could such things be described by GR or could they be inferred from GR (other than the inference of dark matter/energy)?
GR can not describe them because GR assumes continuous space-time. They could be inferred by observations which differ from that predicted by GR. One such discrepancy is that the speed of light would not be constant but, rather, frequency dependent. Light of wavelengths comparable to the size of the interstices would be scattered more than those which are much longer. This effect imposes a non-zero refractive index on the vacuum and so high frequency photons travel more slowly. Searches for this effect have been made but, as far as I know, without success.

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