Gravity and length

[Jacques]

I havwe a simple question. I tried to search on the forum but it gave to many results so I decided to ask the question:

General relativity tell that gravity slowdown time and that is proven by the GPS.

Does gravity have an effect on length (contraction or expansion) ???

Thank you for your responses

[wormholeman]

I know of some scientists that are trying to prove that theory. They shot up

a satalite around earths orbit that had a gyroscope to measure if Einstiens theory

is true. The results of it are not finished from what I know of.

[ydoaPs]

that is not what gravity probe b is for

[timo]

"Time slows down" is one of those quick and easy statement that tell you absolutely nothing if you don´t know what´s meant with it but somehow make you (erroneously) feel you understood something even if you don´t know what´s meant.

 

So what could it mean? Perhaps it means that if you synchronize two clocks, bring one to another place, leave it there long enough so that the time dilatation due to the transfer process becomes irrelevant and finally bring them back together, the two clocks will have measured different times. That´s in fact (sorry for the long sentence) what I´d think of the "time slows down"-statement in the context of gravity.

And here comes my point: I wouldn´t spontaneously know how to construct a similar experiment for space so I don´t really know what could be meant with "does gravity contract/expand space".

 

If that was too abstract I also have one of those non-saying statements that´s absolutely correct and might do the job in pretending to be an answer: "Yes it does. Mass affects spacetime which is both: Time and space."

 

Your question isn´t bad but it starts from a very vague point.

 

@Wormholeman: What´s the name of these guys you know ?

EDIT: Thx for the link. The HP looks nicely made and some of the articles there actually look interesting.

[wormholeman]

You are right yourdadonapogos...My mistake..(I cant rely 100% on my own brainpower always, seemed like it was correct one.) "They will measure how space and time are warped by the presence of the Earth, and, more profoundly, how the Earth's rotation drags space-time around with it"

As for Mr. Jacques question..I now admit I dont have an answer..but only this response!

 

:I

[wormholeman]

I dont know them personally but you can check out their website at http://einstein.stanford.edu/

 

Your welcome.. Atheist!

[franz_liszt]

Of course, all of this presupposes a B-theory of time which is quite questionable at best. The argument for a B-theory is less than appealing.

[Jacques]

Gravity slowdown time: We can do an experiment with two clocks and mesure the difference in reading. That was predicted by GR and verified by experiment.

Me too I don't know how to design an experiment to show if there is a space contraction or dilatation. But does GR predict a contraction or a dilation or no effect of gravity on space ?

[franz_liszt]

Again, please explain you all seem to hold to a B-theory of time.

[timo]

GR predicts spacetime to bend (space and time, as I allready said). But without the math behind it (and therefore knowing what "bending" means) this statement doesn´t tell you very much.

And my point was not only a "what experiment would you do to check it?" but more a "what do you mean by space being contracted?".

 

Perhaps I´ve got an idea how to construct a statement you might like: We have agreed that due to gravity the time measured can differ at different points in space and call it "gravitational time dilatation". What we might want now is an example that the distance between two points in space can change with time. An example for this would be the cosmic expansion. In our current model of the universe the distance between two fixed points increases with time (the picture with the baloon you might have already seen somewhere).

Like I said, that analogy is pretty much made up. But as I already tried to say, statements like "time slows down due to gravity" are mainly ones that are made up for laymen to fool them into believing they understood something, anyways.

[wormholeman]

Yes! I believe that only the beholder of the question can only necessitate on this.

[wormholeman]

Ok! I read somewhere that Einstien also had a theory that he concluded that a beam of light would bend at massive objects (like the sun). Some astrophysicist 's(pardon the spelling!) had a chance to prove his theory and the results where correct! So your question! "Does gravity have an effect on length (contraction or expansion)" ..So thinking that a massive object like our sun that has massive gravity property..If it can bend a beam of light, then the gravity of the sun (or space around it) is contracting or expanding in some form or another.

 

Very interesting! Jacques :^)

[□h=-16πT]

Gravity warps space-time=>different metric to that of flat-space=>different length to that of flat-space.

[Jacques]

Thanks for you answers

I will comeback on it after diner or later on

[losfomot]

I havwe a simple question. I tried to search on the forum but it gave to many results so I decided to ask the question:

General relativity tell that gravity slowdown time and that is proven by the GPS.

Does gravity have an effect on length (contraction or expansion) ???

Thank you for your responses

 

I believe many interferometers are in operation today, that were built to detect gravitational waves, where the idea behind them is to measure the distance between 2 points. If a gravitational wave comes by, space will contract or expand, changing the distance between these two points, and setting off a bunch of bells and whistles that will mark the beginning of a huge party.

[Zeth]

L is proper length, L0 is mesaured length.

L = L0 / sqrt(1-v^2/c^2).

 

So how do you use latex again?

[DQW]

So how do you use latex again?

Like this :

 

[math]L = \frac{L_0} {\sqrt{(1-v^2/c^2)}} [/math]

 

Click the text to pop-up a box containing the latex code (or simply hit the QUOTE button.

[□h=-16πT]

L is proper length' date=' L0 is mesaured length.

L = L0 / sqrt(1-v^2/c^2).

 

So how do you use latex again?[/quote']

 

It's [math]L=L_0\sqrt{1-\tfrac{v^2}{c^2}}[/math] not [math]\frac{L_0}{\sqrt{1-\tfrac{v^2}{c^2}}}[/math]

 

This formula has nothing to do with gravity's effect on the metric of space-time. It's the formula for the contraction of length in the direction of motion of an observer travelling with a velocity v relative to some other frame.

[Zeth]

You're wrong on the first count. But you're right about the equation being useless in this context, I don't know what I was thinking.

 

Testing:

[math]L = \frac{L_0} {\sqrt{(1-v^2/c^2)}} [/math]

[DQW]

deleted...(didn't read a previous post carefully)

[Jacques]

OK I think the position of GR is that space/time curve.

The reason for that question was:

The space is expanding. If we go back in time 1 million years after the bigbang the density of matter was superior. All the mass of the visible universe would be concentrated 14000^3=2.7x10^9 (Assuming 14 billions age).

The gravity will be more intense and time will go slower. What apper to us now as 1 million year looked for our ancestor 14 billion years (Assuming there was some one to observe).

Also the expansion seems to accelerate. Can it be that the gravity field is less intense and time goes faster ???

I was asking if space was expanding our contracting to see if it hold the road.

I know I used the layman answer, but I need help to understand exactly what "space time curve" means.

I am learning Special Relativity now. I know the boost of the spacetime diagram. Does a 'boost' transformation exist for gravity?

[Halucigenia]

Gosh, this thread is confusing.

 

I will try and paraphrase Jacques' question and see if I can answer that.

 

GPS shows that the GR effect of gravity slowing time is correct (here's a reference for this http://relativity.livingreviews.org/Articles/lrr-2003-1/index.html so let's assume that it is true).

 

The real question I suspect is:-

Is there a means of testing that the GR effect of gravity contracting length is correct?

 

In theory you could design an experiment thus:-

 

Take a standard measuring rod, place it vertically at the Earths surface, (at a fixed temperature to exclude thermal effects) measure its length from somewhere in space (let's imagine a GPS sat system could measure it accurately enough). Take this measuring rod out into space (again at that fixed temperature), where there is little effect from the Earth's gravity and measure it's length with the same GPS sat system. Would the measurement in space be greater than the measurement on Earth?

 

My understanding is that the answer would be yes.

 

Does anyone know of any such experiment?

[□h=-16πT]

You're wrong on the first count. But you're right about the equation being useless in this context' date=' I don't know what I was thinking.

 

Testing:

[math']L = \frac{L_0} {\sqrt{(1-v^2/c^2)}} [/math]

 

If that equation were true then the length [math]L_0[/math] would dilate, not contract, with increase in velocity.

 

It's the forumula for time dilation that involves multiplication by [math]\gamma[/math], not that for length contraction ([math]L=\frac{L_0}{\gamma}[/math]).

 

Length contraction from Wolfram

[DQW]

If that equation were true then the length [math]L_0[/math] would dilate' date=' not contract, with increase in velocity.

 

It's the forumula for time dilation that involves multiplication by [math']\gamma[/math], not that for length contraction ([math]L=\frac{L_0}{\gamma}[/math]).

 

Length contraction from Wolfram

Re-read the original post for the convention used (which is, to say the least, non-standard). Lo is not used to designate the proper length (the length in the rest frame of the body).

[Halucigenia]

After extensive googleing I found the length contraction in a gravitational feild formula for GR here:-

Length Contraction Formula

 

Here it is in LaTeX -

[math]ds=\left(1-\frac{2GM}{c^2r}\right)^{-1/2} dr[/math]

 

Not that I understand most of the math used for GR myself, but I think this looks simple enough so that if you plugged the figures in it would give the difference in length of a standard measuring rod under the influence of a gravitational field.