Quantum Spacetime Points & General Covariance

stglyde

In LET, the aether frame is one particular inertial frame. So? Are you saying LET aether can never be physical?

atyy

OK, if you define LET as SR in a particular inertial frame, ie. the LET aether is a particular inertial frame, then by definition the theory is equivalent to SR.

It is not so much that the LET aether frame is not "physical" as that it is not unique. The moment you have one LET aether frame, LET itself predicts that you have an infinite number of LET aether frames, all equally "physical". The equivalent statement in SR are that there is infinite number of Lorentz inertial frames, all equally preferred.

Anyway, since you define LET as SR, let's just use the term SR to include LET.

stglyde

Is the main reason "LET itself predicts that you have an infinite number of LET aether frames, all equally "physical"" is because when one uses a length contracted and time dilated ruler and clock in any frame to measure your length and time, they would see you as similarly length and time contracted, hence one may just say as well that there is "infinite number of Lorentz inertial frames, all equally preferred?".

atyy

I'm not sure about all technical details, but that is the essential idea. Technically, we say the laws of physics (even written in a single inertial frame) have Poincare invariance.

stglyde

What do you make of this I read at google sci.physics? Please comment.

"V -> Velocity with respect to (wrt) the aetherial background (CMBR)
v -> Velocity of a second moving object (Frame), again wrt the aetherial frame
dv -> net differential speed

LET and SR "are not 'identical' Lorentz says that velocity is dv = V - v and SR says dv = v. In SR one arbitrarily assumes a rest frame and in Lorentz's theory they do not. In Lorentz's theory it is ALWAY dv and in SR v relative to one's choice. Since the transform used only dv as in Sqrt(1 - [dv/c]^2) the computed results are the same. Also, since dv is squared the sign (as in direction relative to V) is masked but actually important. There is NO symmetry in LET, the faster you move the more phyically time slowly and contracted you are, period! Finally, where in LET is relative simultaniety mentioned???"

marcus

I'm not arguing with you, I just don't completely understand the bolded sentence in your post and would like a little clarification as to what you are saying.

Are you referring to the socalled Hole Argument? Are you saying that according to the (often cited) Hole Argument diffeo invariance implies that spacetime points don't have physical existence?

stglyde

It's settled already. Wiki says:

"It [the hole argument] is incorrectly interpreted by some philosophers as an argument against manifold substantialism, a doctrine that the manifold of events in spacetime are a "substance" which exists independently of the matter within it. Physicists disagree with this interpretation, and view the argument as a confusion about gauge invariance and gauge fixing instead."

PeterDonis said: "In other words, the hole argument does not show that general covariance is inconsistent with spacetime being a "real thing". All it shows is that GR is a gauge theory."

So it doesn't have to do with the Hole Argument. Unless you want to say it is indeed?
What is your comment about "general covariance and diffeomorphism invariance require the spacetime points are not composed of any substance or something that can be tracked in time". Do you agree with it or disagree and why? Thanks.

atyy

You defined LET as SR in one inertial frame (aether, following your definition in post #18). However, dv is not defined relative to the aether. Thus if you define dv you automatically switch frames, and you unmask the hidden Lorentz symmetry in LET, and bring in relative simultaneity.