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Gedanken
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If the Universe is expanding, wouldn't it be common sense to think everything inside it is expanding too?
Gedanken said:If the Universe is expanding, wouldn't it be common sense to think everything inside it is expanding too?
David Lewis said:If spacetime is expanding then new heres and nows are being created.
Albrecht said:I have not really understood this. If the space expands, do fields like the electrical field and the gravitational field expand by the same amount? - That would mean that any objects and also galaxies expand in the same way as the space and are so unchanged in relation to the space.
No, it would not mean that at all. Again, objects on the order of galactic clusters and smaller do not expand. EM radiation BETWEEN such bound systems does change. Light drops in frequency and loses energy when traveling between bound systems.Albrecht said:I have not really understood this. If the space expands, do fields like the electrical field and the gravitational field expand by the same amount? - That would mean that any objects and also galaxies expand in the same way as the space and are so unchanged in relation to the space.
Interesting discussion. Are you saying that electromagnetic fields inside atoms do not expand but such fields in between galaxy clusters, galaxies, stars perhaps or even planets , do ? At what scale does this transition from expansion to non-expansion occur ?phinds said:No, it would not mean that at all. Again, objects on the order of galactic clusters and smaller do not expand. EM radiation BETWEEN such bound systems does change. Light drops in frequency and loses energy when traveling between bound systems.
Albrecht said:If the space expands, do fields like the electrical field and the gravitational field expand by the same amount?
Albrecht said:That would mean that any objects and also galaxies expand in the same way
Albrecht said:If there is for instance relativistic contraction (in SRT) then there is contraction for all objects and for all fields and for everything.
Albrecht said:My question is whether expansion in case of the whole universe is the same
Albrecht said:the distances of objects expand but fields of any kind do not change
my2cts said:At what scale does this transition from expansion to non-expansion occur ?
Expansion is only measurable on large scales. If you do the calculation then the expansion between the Earth and Sun is about 10m permy2cts said:Interesting discussion. Are you saying that electromagnetic fields inside atoms do not expand but such fields in between galaxy clusters, galaxies, stars perhaps or even planets , do ? At what scale does this transition from expansion to non-expansion occur ?
Looks like you're using Hubble's law for these calculations. I.e. you took the Hubble constant, and calculated the Hubble flow for 1 AU, which over 1 year gives about 10 metres.PeroK said:Expansion is only measurable on large scales. If you do the calculation then the expansion between the Earth and Sun is about 10m per year.
The Earth, however, cannot keep drifting further away each year. Instead, gravity and any expansion settle into a stable equilibrium, with expansion slightly reducing the effect of gravity.
The Earth's orbit, therefore, is slightly larger than it would be if there were no expansion.
In this case, the solar system is gravitationally bound. Which means gravity is the dominant factor.
Bandersnatch said:Looks like you're using Hubble's law for these calculations. I.e. you took the Hubble constant, and calculated the Hubble flow for 1 AU, which over 1 year gives about 10 metres.
It's not the right way to go about, as you're mixing expansion with acceleration. Expansion by itself (w/o dark energy) is similar to inertial motion, rather than a force - i.e. it can't make Earth's orbit larger. The 10 m/year increase is what you'd get if only if the Earth and the Sun were moving with the Hubble flow. They aren't - they're gravitationally bound. They had long decoupled from the Hubble flow, which has as much bearing on the size of Earth's orbit today as the velocity of gas particles in the molecular cloud from which the Solar system coalesced. There's no equilibrium to talk about with expansion (without DE).
What does increase the orbit is dark energy, but that's many orders of magnitude less pronounced than what you calculated. At 1 AU it should result in acceleration of something like ##~10^{-25} m/s^2##. Compare with centripetal acceleration in Earth's orbit: ##~10^{-2} m/s^2##.
So, unless I borked the calculations, that means the orbit is one picometre larger than it'd be without it.
But again, that's the effect of DE. Without it, the expansion itself wouldn't have any effect.
PeroK said:The Earth's orbit, therefore, is slightly larger than it would be if there were no expansion.
PeroK said:In this case, the solar system is gravitationally bound. Which means gravity is the dominant factor.
At an intermediate scale - The Milky Way and Andromeda galaxies say - although still gravitationally bound, the effects of expansion would be more noticeable.
On a larger scale, the gravity between distant galaxies is so small compared to the expansion that gravity becomes negligible and you have effectively only expansion.
PeroK said:at which point do you transition from a gravitationally bound system to one where expansion applies.
PeterDonis said:For objects, yes. Fields don't have a size so they can't "contract". The components of fields are affected by a Lorentz transformation, yes, just like the components of any vector or tensor.
Albrecht said:If you have a charge, then at a distance r from the charge you may have a field strength E. If now the field contracts (by whatever cause) by a factor of 2, then at the position r you will measure a field of E/4.
Albrecht said:A similar thing happens in a gravitational field. If space contracts which contains a gravitational field, also objects contract there because the binding fields inside contract.
Albrecht said:cosmology tells us that the space of the universe expands.
Albrecht said:That is the opposite to contraction but understood to be fundamentally the same phenomenon like relativistic contraction.
Small but nonzero. What is not measurable today, is tomorrow. See the LIGO case.PeroK said:Expansion is only measurable on large scales. If you do the calculation then the expansion between the Earth and Sun is about 10m per
year.
Indeed. Expansion of a hydrogen atom, or the orbit of the Earth, requires energy.The Earth, however, cannot keep drifting further away each year. Instead, gravity and any expansion settle into a stable equilibrium, with expansion slightly reducing the effect of gravity.
Do you have a reference for this ?The Earth's orbit, therefore, is slightly larger than it would be if there were no expansion.
my2cts said:Expansion of a hydrogen atom, or the orbit of the Earth, requires energy.
However that argument holds at any scale.
Do you have a reference for this ?
Define immeasurable. Besides immeasurable is not the same as nonexistent. We cannot measure planets in distant galaxies, but we know they are there.PeroK said:See the comments above. The difference, if there is one, would be immeasurably small.
"trying" to expand ? And how would "other factors [] override this"?I still think you are missing the point that - even if hypothetically the solar system or a hydrogen atom was "trying" to expand - the other factors would simply override this. I think you are looking for an ongoing unstoppable expansion at scales below which the expansion of the universe is not a relevant factor.
my2cts said:Expansion of a hydrogen atom, or the orbit of the Earth, requires energy.
my2cts said:Define immeasurable. Besides immeasurable is not the same as nonexistent. We cannot measure planets in distant galaxies, but we know they are there."trying" to expand ? And how would "other factors [] override this"?
I hope you mean conservation of energy and momentum.
That's my point.
Please specify the complexities and more advanced details, if necessary with references.PeterDonis said:You are sweeping a lot of complexities under the rug. ... there are more advanced details.
up !PeterDonis said:"B" level thread"
than for example the effect gravitational waves.PeroK said:There are changes and variations due to the gravity of everything in the solar system. These effects will be many orders of magnitude larger than ...
my2cts said:"Attempting to measure a change in arm length 1,000 times smaller than a proton means that LIGO has to be " etc.
https://www.ligo.caltech.edu/page/ligos-ifo
Yet it works.
I would need a definition of that as well.PeroK said:That's a "controlled" environment. The solar system is not.
my2cts said:Please specify the complexities and more advanced details, if necessary with references.
I am fully aware of the physical complexities and advanced features of atoms and the solar system, by the way.PeterDonis said:Not in a "B" level thread; that's why I pointed out the thread level. If you are genuinely curious about the complexities, please start a separate thread at the "I" or "A" level.
my2cts said:"trying" to expand ? And how would "other factors [] override this"?
In my knowledge there do not exist any real experiments, in which contraction was directly proven. But contraction has to be assumed in special relativity to avoid conflicts. So it follows indirectly from dilation and the constancy of c. As well the contraction of fields. - One can find this in every textbook about special relativity. My favorite is: A.P French, "Special Relativity".PeterDonis said:Please give a reference for the experimental results that demonstrate this.
The same situation here. Contraction follows indirectly.PeterDonis said:Same comment here: please give a reference for the specific experimental results that demonstrate this.