Proof of Warped Time

[ParanoiA]

I understand, to an extent, that the timespace warpage is gravity. Or rather gravity is the medium we feel timespace warpage (I'm not sure how we "feel" time, but anyway...). What I don't understand is how we know about time warpage. I'm sure an experiment of some kind has proven this.

 

The example used in the book is the rigidly rotating disk. I can see how an observer's clock on the edge of the rotating disk would run slower than the observer's clock in the center of the disk due to the speed and etc. And I can see how that is time warpage.

 

But that's using acceleration to produce the gravity effect. How about real gravity? How do we know that time literally warps around large masses?

[5614]

It's not only time that warps around large masses (and obviously small masses, although it warps a lot less in that case), spacetime itself warps around mass.

 

A simple way to see this is to follow the path of a photon (light) around a dense planet.

 

A photon takes the shortest route between any two points. Often referred to as a straight line. However when spacetime itself is bent, the shortest route means following the curvature of spacetime itself. That is, the photon would appear to curve through space, even though it is spacetime which is curving, and not the light.

 

So if you look at a dense planet then you can see light from things behind it. Not a long way behind it, because the effect is small on photons, but things which should be just hidden behind the planet can be seen as the photons 'bend' around the planet.

[ParanoiA]

Ok, now that part I understand. Einstein's general relativity was validated by that event.

 

What I'm talking about though is the time part. How do we know spacetime warps and not just space? I mean, is it just the math, or is there a way to validate that through experiment?

[[Tycho?]]

Ok, now that part I understand. Einstein's general relativity was validated by that event.

 

What I'm talking about though is the time part. How do we know spacetime warps and not just space? I mean, is it just the math, or is there a way to validate that through experiment?

 

Look up general relativity, I'm pretty sure there have been experiments to show time dilation results from a gravitational field.

[swansont]

Ok, now that part I understand. Einstein's general relativity was validated by that event.

 

What I'm talking about though is the time part. How do we know spacetime warps and not just space? I mean, is it just the math, or is there a way to validate that through experiment?

 

It's done continually with GPS satellites that run at different rates, in part, because of the gravitational term in the time dilation. It also was part of the "clocks on a plane" experiment of Hafele and Keating back in the early 70's, though kinematic terms affect both situations as well.

 

Atomic clock labs have to account for their elevation when comparing timing signal.

 

(FYI, I have an email telling me that Richard Keating passed away a few days ago)

[Ragib]

yea i heard that too..though i have no idea what he did other than that very famous experiment.

 

ParanoiA: Think about it this way. If space is curved then so must be time. This is because if c is constant, then light will take a different time to travel from one point to another, because the path is different. To another observer, this difference in time is the time dialation you are inquiring about.

[ParanoiA]

yea i heard that too..though i have no idea what he did other than that very famous experiment.

 

ParanoiA: Think about it this way. If space is curved then so must be time. This is because if c is constant, then light will take a different time to travel from one point to another, because the path is different. To another observer, this difference in time is the time dialation you are inquiring about.

 

That makes sense, actually. 5614 tried to explain that to me and it sailed right past me. Funny how two people can say basically the same thing, but only one manages to get through.

 

Although, it should still be testable through experiment of some kind. In reading about the airplane test with Keating, it seems more of a time dilation in reference to speed rather than in reference to gravity. But Swansont mentioned adjusting atomic clocks to account for distance from the earth - that, to me, sounds like the proof I was inquiring about.

 

That's absolutely fascinating. It's almost like mass is "squeezed" into spacetime.

[[Tycho?]]

I think they put atomic clocks at the top and bottom of a water tower, and measured the difference.

[Edtharan]

I think they put atomic clocks at the top and bottom of a water tower, and measured the difference.

Yes they did do this (can't remember where the reference is though).

 

Also the results of this experiemnt prove that you head is onlder than you body (as it is - usually - further from the Earth than the body). So the old saying that you can't put an old head on young shoulders is proven wrong...

[swansont]

;308352'']I think they put atomic clocks at the top and bottom of a water tower, and measured the difference.

 

There's the Pound and Rebka experiment, which was done at a tower at Harvard, using Mossbauer spectroscopy to measure the redshift. The fractional frequency change is [math]gh/c^2[/math], which is about a part in 10¹⁶ per meter; it has been only relatively recently that you'd expect to be able to measure the effects of several meters with atomic clocks.