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SRH
2012-Jul-28, 07:53 PM
I propose a new test of SR where there are 3 atomic clocks.

1. In a plane flying very fast, for 12 hours, along the Earth's equator (in either direction - doesn't matter), at a low altitude.

2. In a helium balloon, for 12 hours, at the Earth's equator, travelling relatively much slower than the plane, upwards high high high up into the Earth's atmosphere (and staying up there for a while) and then back down to the Earth's surface.

3. An atomic clock stationary on the Earth's surface, at the equator, for 12 hours.


SR would say that the faster motion would cause the atomic clock on the plane to run the slowest, followed by the balloon's clock which had motion as well.
I think the atomic clock on the balloon would actually run slowest.
This is because I think atomic clocks run slower at higher altitudes, as a function of relatively less gravity.


Any thoughts?

caveman1917
2012-Jul-28, 08:09 PM
SR would say that the faster motion would cause the atomic clock on the plane to run the slowest, followed by the balloon's clock which had motion as well.
I think the atomic clock on the balloon would actually run slowest.
This is because I think atomic clocks run slower at higher altitudes, as a function of relatively less gravity.


Any thoughts?

First of all it would only be meaningful to compare elapsed times between shared events, ie we start all the clocks at the same location and then at the end everybody meets up again at the same location and we compare.

Everything else equal higher altitudes show more elapsed time (ie "faster clocks"). So comparing the ground based and balloon based clocks is easy, the balloon clock goes faster. Now it still depends on what exactly the plane is going to do, is its route going to be of the type where it would be like orbiting the earth at low altitude or will it be accelerating the entire time... Note that the ground-based clock is constantly accelerating at g.

SRH
2012-Jul-28, 08:32 PM
Thank you!

So what you are saying is that SR has to do with the acceleration of motion and not the speed of motion. Is that correct?


btw, do you have links to experiments where vertically traversing clocks showed faster elapsed times with higher altitudes?

primummobile
2012-Jul-28, 08:43 PM
Time dilation is about how fast you are going. Say, for example, you were going to travel at 1/3 light speed. The amount of time it took you to get to 1/3 light speed wouldn't have anything to do with how fast your clock was ticking once you reached that speed.

You can Google all kinds of experiments on this gravitational time dilation

http://en.wikipedia.org/wiki/Gravitational_time_dilation

primummobile
2012-Jul-28, 08:51 PM
But I don't really understand why you are asking this. We have plenty of experimental evidence for time dilation, and the data matches theory very well. What are you hoping to accomplish that hasn't already been accomplished?

I would also suggest that you ask a moderator to move this thread, or maybe your other. Both seem to be questions, but when they see you have two ATM threads open they're going to close one of them.

caveman1917
2012-Jul-28, 08:54 PM
Thank you!

So what you are saying is that SR has to do with the acceleration of motion and not the speed of motion. Is that correct?

Not exactly. Perhaps compare the twin paradox. One twin stays at home and one flies off to a distant star to return there. They both see eachother's clocks as slow during the journey. However a meaningful comparison involves them meeting up again so they can compare, and when they do that one will have needed to accelerate (there is a moment of acceleration when the journey-twin stops and reverses course), and it is this that will say whose clock "really" went slow.

A bit more technical, the "slowness" of a clock is related to how much it diverges from a geodesic which is the path without acceleration.


btw, do you have links to experiments where vertically traversing clocks showed faster elapsed times with higher altitudes?

I don't have links to experiments handy (not much into that side of things :)), though i'm sure someone will come along to provide some.

But an easy way to see how this works is considering redshift-blueshift. I suppose you already know that deep in the gravity well of a black hole things get redshifted as viewed from far away. Saying that some place looks redshifted is the same as saying that clocks seem to run slow there, likewise saying that some place looks blueshifted is the same as saying that clocks seem to run fast there. You can substitute one perspective for the other (redshift/blueshift is a statement as to the frequency of some process (at least in vacuo), likewise a clock that is ticking slow or fast is also a statement as to the frequency of a process, in this case the clock's ticking). So it follows that deeper in a gravity well clocks tick slower.

SRH
2012-Jul-28, 08:58 PM
Thanks for the help.

Mods, please close this thread.

caveman1917
2012-Jul-28, 09:06 PM
Time dilation is about how fast you are going.

Not exactly. Since inertial motion is relative, so is time dilation. I think i am standing still but you're going at 1/3 c away from me, and i say your clock is running slow. But you think that you're standing still and i'm going at 1/3 c away from you, and you'd say that my clock is running slow. Time dilation between inertial frames is a map between coordinates and depends on a chosen synchronization scheme, which is a free choice. Unambiguous comparison between clocks to say which one is going slow requires shared start and end events for both paths, and thus involves acceleration (which becomes the deciding factor).

Strange
2012-Jul-28, 09:13 PM
Wasn't something very like this done already:
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/airtim.html

And, of course, GPS receivers have to correct for the difference in gravity between the satellite and receiver, the acceleration of the satellites and the relative velocity of satellite and receiver.

Strange
2012-Jul-28, 09:15 PM
btw, do you have links to experiments where vertically traversing clocks showed faster elapsed times with higher altitudes?

Not exactly the same thing, but the Pound-Rebka experiment measured this: http://en.wikipedia.org/wiki/Pound%E2%80%93Rebka_experiment

primummobile
2012-Jul-28, 10:28 PM
Not exactly. Since inertial motion is relative, so is time dilation. I think i am standing still but you're going at 1/3 c away from me, and i say your clock is running slow. But you think that you're standing still and i'm going at 1/3 c away from you, and you'd say that my clock is running slow. Time dilation between inertial frames is a map between coordinates and depends on a chosen synchronization scheme, which is a free choice. Unambiguous comparison between clocks to say which one is going slow requires shared start and end events for both paths, and thus involves acceleration (which becomes the deciding factor).

I understand that. I was attempting to keep the explanation simple. Strictly speaking, the dilation occurs because you are accelrating in a non-time dimension. I was just saying that how much depends on your speed. I don't think that's what toefetish meant when he said acceleration, but I may have misunderstood.

slang
2012-Jul-28, 10:35 PM
Mods, please close this thread.

We do not close threads without reason. We might if this was an ATM proposal and you decided that you no longer wish to support it. Do you intend to support an ATM proposal here, or shall we move this thread to S&T?

speedfreek
2012-Jul-29, 11:44 AM
I would like to point out something that caveman1917 alluded to:


I propose a new test of SR where there are 3 atomic clocks.

1. In a plane flying very fast, for 12 hours, along the Earth's equator (in either direction - doesn't matter), at a low altitude.
The direction does matter. It matters a lot. If the plane is flying eastwards, with the Earth's rotation, it has a higher velocity than the clock on the ground, whereas if the plane is flying westwards, against the rotation of the Earth, it has a slower velocity than the clock on the ground.

In the Hafele-Keating experiment we flew atomic clocks around the world in both directions, and when we compared them with the clock on the ground we found the clock that flew eastwards lost time ("ran slow") in relation to the clock on the ground whereas the clock that flew westwards gained time ("ran fast").

This all makes sense for a frame at rest in relation to all three clocks - a frame at rest in relation to the centre of the Earth, where the Earth rotates around it.

In relation to such a frame, the clock flying east moves fastest, the clock on the ground moves slower and the clock flying westwards moves slower still.


2. In a helium balloon, for 12 hours, at the Earth's equator, travelling relatively much slower than the plane, upwards high high high up into the Earth's atmosphere (and staying up there for a while) and then back down to the Earth's surface.

3. An atomic clock stationary on the Earth's surface, at the equator, for 12 hours.

SR would say that the faster motion would cause the atomic clock on the plane to run the slowest, followed by the balloon's clock which had motion as well.
I think the atomic clock on the balloon would actually run slowest.
This is because I think atomic clocks run slower at higher altitudes, as a function of relatively less gravity.
(My bold)
In which case you are not testing SR any more, are you? If the results are affected by gravity, then you are testing GR.

And SR only says that a clock flying eastwards would "run the slowest", whilst a clock flying westwards would "run the fastest".

HenrikOlsen
2012-Jul-30, 10:22 AM
Thank you!

So what you are saying is that SR has to do with the acceleration of motion and not the speed of motion. Is that correct?
SR has to do with different velocities and how they'll be observed by people moving at those or other differing velocities given the experimentally confirmed fact that light speed (which is what's used for observing) is absolute rather than relative.
SR is fairly easy to get a reasonable grip on if you can keep your coordinates organized and use the right one for each observer.

The moment you involve acceleration you get over into GR territory, which, since all the clocks in your experiment are experiencing acceleration, was where you were from the start.

ctcoker
2012-Jul-30, 03:42 PM
That is not true. The moment you involve gravity you get into GR territory. SR is a complete system of mechanics; it deals with accelerated frames as easily as anything else. Think about it - if SR could not deal with accelerations, it would be fairly useless as a mechanical theory.

caveman1917
2012-Jul-31, 12:45 AM
That is not true. The moment you involve gravity you get into GR territory. SR is a complete system of mechanics; it deals with accelerated frames as easily as anything else. Think about it - if SR could not deal with accelerations, it would be fairly useless as a mechanical theory.

I'll refer you to this post (http://cosmoquest.org/forum/showthread.php/117613-FTL-in-General-Relativity?p=1909654#post1909654) in an earlier discussion regarding this distinction.