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


"Curious" <anthonyroseuk-curious@xxxxxxxxxxx> wrote in message news:e803aff.0505030401.418177e7@xxxxxxxxxxxxxxxxxxxxx
> Trying to understand SR and GR at a basic level, I've read around on the web
> and now have questions I can't find answers for. Being new to this all, and
> having only physics 1, I may be inadvertently resurrecting dumb questions, but
> if someone has the time and inclination to work with me on this I'd appreciate
> some answers to these questions.
>
> What I seem to have found so far:
>
> 1. SR is invalid.

SR is valid in sufficiently confined regions of space and time.
SR is the local limit of GR.

> Inertia, motion, aging etc. depend on the locations of masses
> around us in the universe. There is an absolute frame of reference, the
> universe. (Whether it is infinite or not we don't know.)
> 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.

Some are stupid and don't know what they are talking about.

>
> Some questions I have now are:
>
> 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?

I don't agree that SR is dead.
It is very much alive.

>
> b1. GR also states, like SR, that time dilates
> How can time dilation explain the measurement of light speed as c

Time dilation does not *explain* the measurement of light speed as c.
Time dilation is one of the *consequences of* the way we measure
times and distances of remote events.

> in both
> the cases of a spaceship approaching and departing from the light source at
> near light speed? Surely in one of the cases time dilation is an exacerbating
> rather than mitigating factor?
>
> b2. Imagine two space-stations ST1,ST2 exactly one light-minute apart in their
> frame of reference, and two space-ships SP1,SP2 travelling at near-light-speed
> also the same distance apart,

same distance as seen in which frame?
You must be precise about that.

> approaching the two stations along the same axis
> on which they lie:
> ST1. . . . . . .ST2 <-SP1. . . . . . .<-SP2
> When spaceship SP1 is alongside ST1, and SP2 alongside ST2,

This can only happen simultaneously in the ST-frame
if the SP-ships are separated by one light minute as
seen in the ST-frame. So I will assume that.

> spaceship SP2
> flashes a beam of light towards ST1. The light reaches ST1 in one minute
> ('static'-time). Spaceship SP2 reaches ST2 a second or two later than the light.

I don't see a question, but, yes, this is right.

> b3.1 Does SP2 perceive the distance between ST1 and ST2 to be as far as we do?

If "we" are in the ST-frame, no SP2 preceives the distance
to be contracted by the gamma factor, giving 1/g = sqrt(1-v^2),
provided we use units where c=1, so to speak.

> b3.2 How many of SP2's seconds does it take the light to reach ST2?

None, since, like you said, the light was emitted
when SP2 was alongside ST2.
Exercise: try to calculate in how much time it reaches ST1.

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

Yes, we do it all the time.
In small enough regions of space and time, SR works perfectly.

>
> 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?

GR and Quantum theory seem not to be compatible with each
other in their current forms. Some are working on that...

Dirk Vdm


.

Relevant Pages

  • Re: SR time dilation on remote objects ?
    ... the relevant formula for time dilation is: ... observer A situated at a distance d from observer B ... "The Hubble law is true for all values of D, ...
    (sci.astro)
  • Re: Beam me up - trying to get a basic understanding of GR
    ... There is an absolute frame of reference, ... If you agree that SR is dead, can you explain how GR avoids the ... > Time dilation does not *explain* the measurement of light speed as c. ... > same distance as seen in which frame? ...
    (sci.physics.relativity)
  • Re: GPS CLOCK PARADOX
    ... The formula of the length contraction can be found at ... The equation for the time dilation can be found at ... Distance is always positive. ... your equations for calculating the distances and times were ...
    (sci.physics.relativity)
  • Re: (SR) Lorentz t, x = Intervals
    ... Express them in terms of time differences ... and distance differences between precisely defined events... ... There is only one instance of standard time dilation: ... between A and B in the train frame. ...
    (sci.physics.relativity)