Re: Beam me up - trying to get a basic understanding of GR

Curious wrote:
1. SR is invalid. Inertia, motion, aging etc. depend on the locations of masses around us in the universe.

Nobody expects SR to be valid globally. That's a major reason why Einstein searched for GR.


There is an absolute frame of reference, the universe. (Whether it is infinite or not we don't know.)

This is supposed to be physics. Just saying "the universe" does not help -- you need to give a _measurable_ method of determining this supposed "absolute frame of reference".


2. GR replaces SR. Some say that it works because it takes gravity into account, but some say it is also invalid, either logically inconsistent or they point to a math flaw.

No actual mathematical flaws have ever been shown in GR. Numerous cranks around here may claim such, but their claims have never stood up under scrutiny.


a. If you agree that SR is dead, can you explain how GR avoids the bullet that killed SR? I thought that GR ADDs the space-curving effect of gravity to SR?

SR is the local limit of GR. GR has no need for global inertial frames, and it explicitly handles the curvature observed in the world we inhabit.


b1. GR also states, like SR, that time dilates
How can time dilation explain the measurement of light speed as c in both the cases of a spaceship approaching and departing from the light source at near light speed?

Time dilation alone does not "explain" this. The fact that in GR light always follows null geodesics does. In particular, null geodesics always have speed c relative to any locally-inertial frame -- each spaceship (approaching or departing from the source) uses its individual locally-inertial frame to measure the speed of light from the source, and so they both obtain c.


c. Is it possible that we can use SR as a useful approximation, like Newtonian physis, under limited circumstances? 

Yes indeed.


d. Are there situations in which GR is known to be experimentally dead wrong, and if so, do we use GR as a useful approximation, under limited circumstances?

There are no such situations at present. But dark matter and energy and the anomalous acceleration of spacecraft like Pioneer may become such situations in the future. Or not.


1. It seems to me that one can philosophically prove the existence of
an absolute frame of reference. Does this bother SR/GR?

It seems to me your "philosophical proof" is completely unrelated to physics. This does not bother physicists.


2. If SR/GR rule that time dilates, in actual fact, according to speed,
then this leads immediately to a direct logical contradiction.

This depends on your meanings of words, and your "logical contradiction" is almost surely due to unacknowledged puns in your argument.

Note that real experiments show that two clocks can follow different trajectories and display different elapsed proper times when they rejoin. That is an "actual fact" that refutes your claim of a "logical contradiction".


If SR/GR rule that time only *appears* to dilate according to speed,
(i.e. the
travelling twin in the twin paradox is NOT younger upon return) then
this leads to a logical impossibility.

Real experiments show the traveling twin experiences less elapsed proper time than the inertial twin. This is not "appears". But note I am discussing elapsed proper time, and not "time dilation".


Does the traveller's time REALLY dilate the faster they go?

The traveling twin's clock most definitely displays less elapsed time between meetings than the inertial twin's clock displays. Relating that fact to your question is a linguistic exercise, not physics.


Tom Roberts	tjroberts@xxxxxxxxxx
.

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