relativity time dilation questions

You might want to check on the responses Alan gets in the relativity newsgroup before attaching much credence to what he says.

Sylvia.

Reply to
Sylvia Else
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You have to be rather strict about "see" vs "observe" too.

See implies a light travel time to the observer - and at relativistic speeds things get to appear quite distorted and rotated as a result. Looking at a relativistic jet pointed towards the Earth we see apparent superluminal motion. Nothing is moving FTL but it looks to us that way.

Observers have to be strung out in advance along the trajectory having moved nice and slowly so that their clocks are still synched to the master clock (and at the same gravitational potential).

Events are only truly simultaneous in special relativity if they happen at the same spacetime coordinates.

You can derive special relativity rather beautifully by the careful consideration of the mutual events of two metre rulers A-B and C-D passing each other at a relative speed v and requiring that the laws of physics must be the same for all observers in an inertial frame.

AD event A----B C--D ->v

AC event A----B C--D

BD event A----B C--D

BC event A----B C--D

From the timings of these mutual events and a bit of high school algebra it is possible to derive the Lorentz transformations from this alone. Or

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The garage "paradox" is merely an example of compressing the length of the garage to allow the Lorenz contracted car to just fit inside it.

The latest generation of atomic clocks are sensitive to a 10cm height difference and stable to 10^-17

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Makes the radioastronomy H-masers look rather passe at a few x 10^-15

It is old hat to fly portable atomic clocks around the world and show that the one which travels ages less when it returns home (although the gravitational potential effect also has to be taken into account)

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Regards, 
Martin Brown
Reply to
Martin Brown

Hi,

If judging if he is correct based on responses of others was valid, they would have to have a counter argument, care to mention one?

I also noticed Phil Hobbs mentioned the link between acceleration and time dilation in this thread already too:

"That's a form of the 'twin paradox'. If I take a relativistic spacecraft and fly a year out from Earth and then back again, I'll have aged less than you. But that isn't a paradox at all, because our histories are not comparable--I've accelerated to and from relativistic speed three times and you haven't. "

cheers, Jamie

Reply to
Jamie M

How about the experiments that demonstrate time dilation for a start?

That is more to do with taking a relativistic trip and returning back to the same location. "Common sense" and Galilean transforms will get you into trouble when you try to reason about relativistic motion.

BTW these effects are measurable on Earth with portable atomic clocks and at speeds comparable with jet airliners. It has been done many times and even once for the Royal Society Xmas lectures.

A botched launch of a pair of Galileo satellites provided a serendipitous experimental test of the predictions of GR recently:

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In slightly more technical detail - the first really big improvement in testing GR in almost half a century.

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The predictions of GR are so good that when the first binary pulsar was found and some timings seemed awry the fault was ultimately found to be in the symbolic algebra package conversion of trig expansions to FORTRAN continuation cards for position of Jupiter (after an exhaustive search).

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Regards, 
Martin Brown
Reply to
Martin Brown

Hi,

So if the clock requires say 1picojoule? for each tick of the clock (I'm sure it is less) then are the missing ticks accounted for as gravity waves emitted by the satellite? If so why isn't LIGO harvesting this? :D

cheers, Jamie

Reply to
Jamie M

Hi,

I don't see why returning to the same location would have any effect on the time dilation. Here is the thought experiment I was considering with two added examples not returning to the start location:

  1. For a journey from Earth, accelerating to 0.999C over a 0.5 light year distance, then decelerating over a 0.5 light year distance, and heading back towards earth: accelerating to 0.999C over a 0.5 light year distance, then decelerating over a 0.5 light year distance. (total trip distance 2 light years)

  1. For a journey from Earth, accelerating to 0.999C over a 0.5 light year distance, then coasting for a 1 light year distance, then decelerating over a 0.5 light year distance, and heading back towards earth: accelerating to 0.999C over a 0.5 light year distance, then coasting for a 1 light year distance, then decelerating over a 0.5 light year distance. (total trip distance 4 light years)

two added examples, not returning to start location:

  1. For a journey from Earth, accelerating to 0.999C over a 0.5 light year distance, then decelerating over a 0.5 light year distance, and continuing away from Earth: accelerating to 0.999C over a 0.5 light year distance, then decelerating over a 0.5 light year distance. (total trip distance 2 light years)

  1. For a journey from Earth, accelerating to 0.999C over a 0.5 light year distance, then coasting for a 1 light year distance, then decelerating over a 0.5 light year distance, and continuing away from Earth: accelerating to 0.999C over a 0.5 light year distance, then coasting for a 1 light year distance, then decelerating over a 0.5 light year distance. (total trip distance 4 light years)

Those four examples all have the same rates of acceleration for the same overall time dilation if time dilation only occurs during their acceleration time. The final distance to Earth is known (2ly or 4ly), so to check the clocks for 3 and 4, the communication time can be added to the measured clock time on the distant clock to see what the time dilation was.

cheers, Jamie

Reply to
Jamie M

That is incorrect I know, the change in gravitational potential field strength is what determines the change in time dilation in that example.

But gravitational waves could also create a change in time dilation, and bodies in asymmetrical orbit emit gravitational waves, however the delta time dilation due to that effect would be much^much smaller than the gravitational potential field proportional to the orbital heights.

cheers, Jamie

Reply to
Jamie M

Hi,

I think an easy way to visualize why there is time dilation due to gravity potential, is the increased spacetime path length caused by the gravitational field. That gravitational field stretches equally at all scales, atomic, or a satellite, so I don't think it is just a

3rd dimensional warping of spacetime, I think it would have to be a 4th dimension, in order for the gravity well to explain time dilation via increased spacetime path length at all scales causing slower time.

cheers, Jamie

Reply to
Jamie M

IIRC the speed of light is postulated to be the same for any observer in a non accelerating frame of reference.

It is directly from that coupled with a "light beam in a spacecraft" thought experiment that you can derive the formula for Lorentz contraction (which is also the formula for time dialation and mass change).

I could probably work that out again, given long enough.

I cannot remember how E=mc2 is derived (being vaguely related), but I know I did work that out when I was about 17 using 2 sides of A4 paper and no more than British A Level maths (which was very slightly more advanced than today's highschool pre-university stuff).

The thought experiments are hard, but the maths is not terrible. Unlike general Relativity that I never understood.

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Reply to
Tim Watts

There is a somewhat novel way to derive the form of E=MC2 based on conservation of energy.

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-- Kevin Aylward

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- SuperSpice
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Reply to
Kevin Aylward

I also have a more accessible description of mass-curving space as well....:-)

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Well, the 1st paragraph anyway...

-- Kevin Aylward

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- SuperSpice
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Reply to
Kevin Aylward

One should note though that "space-time" is only a *MODEL*. It allows a description of *how * physical objects move, but not *why*.

It assumes that time and space actually exists independently of the objects in the universe. That is, it is a pure mathematical construct.

Clearly, this is false in physical reality. The *only* way to define distance is to have physical objects to mark out locations. The *only* way to define time, is to have objects change their position. There has to be a real, physical process to create a clock, i.e. time.

-- Kevin Aylward

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- SuperSpice
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Reply to
Kevin Aylward

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