Einstein equations don't work at large scale?

[Ibix]

As Bill Clinton once testified, “That depends what your definition of ‘is’, is.” Other than “exists at my point in spacetime” the other definitions seem fungible.

The first line in deriving the FLRW cosmological model is to assume that everything is the same everywhere at any given time. This inevitably leads to the universe being the same age everywhere because we insisted that the universe be the same everywhere, and following through the Einstein Field Equations adds that this age is finite.

However, GR is a theory of spacetime, not space and time. There is nothing in the physics requiring you to use the definition of time used in the initial derivation, not in any spacetime, and other definitions of time don't necessarily share the same definition of "now".

That said, anybody talking about "the" age of the universe must be using a definition of time where there is a unique age. And in cosmology, AFAIK everybody uses either cosmological or conformal time, and both share the same definition of "now" that has the universe being the same age everywhere. Cosmological time corresponds directly to clock time.

 

[Sagittarius A-Star]

here Claudia de Rham talk about some problems:

Her related paper is described in the Wikipedia article under "Ghost-free massive gravity":
https://en.wikipedia.org/wiki/Massive_gravity#Ghost-free_massive_gravity

 

[gen x]

What theory are you referring to? Can you give a reference?

Bear in mind that there is no way with our current technology to even test predictions about gravitons.

Sagittarius A-Star post some material at post #84

 

[martinbn]

Sagittarius A-Star post some material at post #84

In the other thread yous said you were not interested in theories that will not be around in 100+ years. Well, this one has not been around, as an experimentaly tested theory, for 0 years. Chances are it will never be a part of accepted science. Why are you interested in it?

 

[Dale]

I am interested in Einstein Cartan theory, but for different reasons. To me the issue is that it needs to be experimentally tested and the only places where it differs from GR are inside matter. So, I anticipate that it will be some time before we can test it, probably not in my lifetime.

 

[PeterDonis]

The first line in deriving the FLRW cosmological model is to assume that everything is the same everywhere at any given time.

To rephrase this in a way that might make the point clearer: we assume that there exists a set of spacelike hypersurfaces that foliate the spacetime (i.e., every event is on one and only one such hypersurface) and which are homogeneous and isotropic. We then construct a coordinate chart on the spacetime that uses those hypersurfaces as its surfaces of constant time, and we scale the time coordinate so that it directly represents the proper time of observers at rest in the spatial coordinates, who always directly observe the homogeneity and isotropy. In other words, we assume a particular geometric property of the spacetime that ensures that there are coordinates in which "everything is the same everywhere at any given time".

 

[pervect]

@Dale
@Ibix

I would like that some day it turns out that time is absolute, it better suit human intuition.

If time is relative as we learn today, universe is old 13.8 billions years, looking from witch frame if absolute frame at rest dont exist?

So universe age is different from different places in space?

That's going to be very difficult (if it is possible at all) to reconcile with existing evidence. Specifically, the constancy of the speed of light is well established at this point, and there is no known way to reconcile that with absolute time. My understanding is it's already been proved impossible, actually, but I'll settle for the weaker statement since I don't have a quotable reference in front of me. The argument might need additional assumptions that I haven't mentioned, like isotropy, as well.

 

[gen x]

That's going to be very difficult (if it is possible at all) to reconcile with existing evidence. Specifically, the constancy of the speed of light is well established at this point, and there is no known way to reconcile that with absolute time. My understanding is it's already been proved impossible, actually, but I'll settle for the weaker statement since I don't have a quotable reference in front of me. The argument might need additional assumptions that I haven't mentioned, like isotropy, as well.

But Newton physics was also well established 200years until Einstein comes and shocked the world.

Isn't today quantum physic in not god relation with relative time?

 

[PeterDonis]

Isn't today quantum physic in not god relation with relative time?

Not at all. Quantum field theory, to which ordinary non-relativistic QM is an approximation, takes relativity into account and is perfectly consistent with it.

 

[Dale]

Isn't today quantum physic in not god relation with relative time?

No. Today’s quantum physics (quantum field theory) is fully compatible with relative time.

Edit: @PeterDonis for the win!

 

[Ibix]

Isn't today quantum physic in not god relation with relative time?

Quantum field theory is fully consistent with special relativity and works in curved spacetime too.

What we don't have a good model for is the case where spacetime curvature is extreme enough for quantum effects to become apparent (i.e. very very very strong fields) or the case where quantum effects are important in describing sources of gravity (i.e. the gravity of very small numbers of particles).

 

[gen x]

No. Today’s quantum physics (quantum field theory) is fully compatible with relative time.

Edit: @PeterDonis for the win!

Isnt true that we have problems to connect quantum p. with einstein relativity?

 

[Ibix]

Isnt true that we have problems to connect quantum p. with einstein relativity?

Only with some aspects of GR, as I noted in my previous post. It works just fine with SR and most of GR.

 

[PeterDonis]

Isnt true that we have problems to connect quantum p. with einstein relativity?

In the sense that we don't have a generally accepted theory of quantum gravity, yes. But we also have no experimental evidence for any quantum aspects of gravity, and most physicists believe we will not be able to get such evidence any time soon, because the Planck scale, which is the scale where most physicists expect such effects to show up, is about twenty orders of magnitude away from what we can currently probe experimentally.

We do, however, have methods for doing quantum field theory in curved spacetime, as @Ibix mentioned. So we can model quantum aspects of matter in the presence of gravity. This is the framework, for example, that is used to predict that black holes emit Hawking radiation (although we have no prospect of testing that prediction any time soon either).

 

[Dale]

Isnt true that we have problems to connect quantum p. with einstein relativity?

Indeed, but they relate to extreme curvature dynamics, not relative time.

 

[Sagittarius A-Star]

Isn't today quantum physic in not god relation with relative time?

QFT requires relative time.
For example, QFT describes an electron/positron pair creation, which requires Einstein's $E_0=mc^2$.
The equivalence of mass and energy follows from time-dilation.

If the photon is near an atomic nucleus, the energy of a photon can be converted into an electron–positron pair
...
The photon's energy is converted to particle mass in accordance with Einstein's equation, $E = mc^2$; where $E$ is energy, $m$ is mass and $c$ is the speed of light. The photon must have higher energy than the sum of the rest mass energies of an electron and positron

Source:
https://en.wikipedia.org/wiki/Pair_production#Photon_to_electron_and_positron

 

[PeterDonis]

The equivalence of mass and energy follows from time-dilation.

Your link here is to an earlier post by you in this thread, which doesn't say what you're saying here. Nor do I think that what you're saying here is correct.

 

[Sagittarius A-Star]

Your link here is to an earlier post by you in this thread, which doesn't say what you're saying here. Nor do I think that what you're saying here is correct.

This earlier post uses the time-dilation-factor in $\alpha=\gamma^3 \frac{dv}{dt}$ for deriving $E = K + m c^2 = \gamma m c^2$. Then in the momentary inertial rest frame of the object: $\gamma=1$, so $E_0 = m c^2$.

 

[PeterDonis]

This earlier post uses the time-dilation-factor

$\gamma$ plays many other roles besides the time dilation factor.

$E = K + m c^2 = \gamma m c^2$.

That "derivation" assumed that $E_0 = m c^2$, since you wrote $mc^2 \left( \gamma -1 \right) = E - E_0$. You can't argue in a circle.

 

[Sagittarius A-Star]

$\gamma$ plays many other roles besides the time dilation factor.


That "derivation" assumed that $E_0 = m c^2$, since you wrote $mc^2 \left( \gamma -1 \right) = E - E_0$. You can't argue in a circle.

Yes. Einstein derived it in his 1905 paper
"DOES THE INERTIA OF A BODY DEPEND UPON ITS ENERGY-CONTENT?"

https://www.fourmilab.ch/etexts/einstein/E_mc2/www/

 

[PeterDonis]

Einstein derived it in his 1905 paper
"DOES THE INERTIA OF A BODY DEPEND UPON ITS ENERGY-CONTENT?"

He derived the result that if a body emits radiation of energy $L$, its invariant mass (in modern terminology) is reduced by $L / c^2$. That is not the same as deriving mass-energy equivalence from time dilation.

Please do not go any further on this tangent in this thread. If you want to discuss it further, let me know and I'll spin it off into a separate thread.

 

[bhobba]

What we don't have a good model for is the case where spacetime curvature is extreme enough for quantum effects to become apparent (i.e. very very very strong fields) or the case where quantum effects are important in describing sources of gravity (i.e. the gravity of very small numbers of particles).

This is an important point that is often not made clear even in professional literature:
https://websites.umass.edu/donoghue/research/quantum-gravity-and-effective-field-theory/

Thanks
Bill

 

[bhobba]

I also know that Einstein theory dont fit in quantum world.

As explained above, it does fit, but only at low energy scales. In fact, it is suspected that all the so-called Standard Model is like that.

Thanks
Bill