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Author Topic: Telling the difference between accelerating and being in gravitational field  (Read 3324 times)

Offline McKay

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I know and understand about the fact(?) that there is no experiment/ instrument that could tell if the instrument is in a accelerating box in free space or it is in a box that is sitting on the ground on some gravitating object.
.. but one instrument bothers me a bit - atomic clocks measuring time dilation at different heights above the ground. That is, I have red that some super precise clocks have been made that can measure the time dilation of just few meters.
Cant a atomic clock tell the difference of accelerating box or gravitating object? That is - there would be no such time dilation in the box?


 

Offline Ethos_

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I know and understand about the fact(?) that there is no experiment/ instrument that could tell if the instrument is in a accelerating box in free space or it is in a box that is sitting on the ground on some gravitating object.
.. but one instrument bothers me a bit - atomic clocks measuring time dilation at different heights above the ground. That is, I have red that some super precise clocks have been made that can measure the time dilation of just few meters.
Cant a atomic clock tell the difference of accelerating box or gravitating object? That is - there would be no such time dilation in the box?
Whether in a gravitating field or and accelerating one, time dilation remains evident. The one difference between an accelerating frame and a gravitating one is what science calls the tidal effects. While there aren't tidal effects in an accelerating frame, tidal forces are present in a gravitational field. Check this out on Wikipedia.
« Last Edit: 30/07/2014 22:29:53 by Ethos_ »
 

Offline McKay

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Hmm, yes, tidal forces, thats kinda the same thing - difference in strength of gravity = difference in flow of time.
Now im even more confused - why is there time dilation in accelerating frame but no tidal effect?
 

Offline Ethos_

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Hmm, yes, tidal forces, thats kinda the same thing - difference in strength of gravity = difference in flow of time.
Now im even more confused - why is there time dilation in accelerating frame but no tidal effect?
Consider the gravitational effects surrounding a black hole. Each line of approach converge as one draws nearer to the center of mass. This convergence is the cause for the tidal effects. In straight line acceleration, there are no tidal effects because the line of travel does not converge.
« Last Edit: 30/07/2014 23:26:27 by Ethos_ »
 

Offline McKay

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Yes, I understand that and understood what tidal forces are before I wrote this thread. So, when it is said that one cannot tell the difference, they, actually, can. Thats kinda what I was pointing out when I started this thread. But you wrote that time dilation is evident in accelerating frame.
 

Offline PmbPhy

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Quote from: McKay
Now im even more confused - why is there time dilation in accelerating frame but no tidal effect?
There isn't any difference in accelerating frames. You have an error in your understanding of the Equivalence Principle. It really says
Quote
A uniformly accelerating frame of reference is equivalent to a uniform gravitational field.
This means that if the field is not uniform such as that around the earth then this doesn't hold globally, only locally.
 

Offline jccc

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What if drop a basketball to the floor, count how many times it jumps till it stops?



 

Offline chiralSPO

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One would need fairly sensitive equipment to observe this (or measure over very large distances), but:

A gravitational field is curved in that the force pulls toward the center of the mass that generates the field. For an accelerating reference the force is just parallel to the direction of acceleration. (Imagine being in a very wide room, dropping a ball on each end of the room, and observing whether the trajectories are perfectly parallel, or whether they actually converge slightly)

This is slightly different than observing a tidal effect (I think) in that the gravitational acceleration is equal in magnitude at all locations measured, but differ in direction.
 

Offline PmbPhy

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Quote from: chiralSPO
One would need fairly sensitive equipment to observe this (or measure over very large distances), but:..
Thatís true. However it should be noted that such extremely sensitive instruments do exist and have been in use since at least 2006. The one I know of is called an Electrostatic Gravity Gradiometer] and itís roughly the size of a breadbox and can make a measurement in a fraction of a second. It can detect differences in acceleration as small as 10-10cm/s2 over a distance of 50 cm, that is a gradient of 2x10-12/s2.
A good account of all of this is in my article Einsteinís Gravitational Field which is online at
http://xxx.lanl.gov/abs/physics/0204044
 

Offline jeffreyH

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Quote from: chiralSPO
One would need fairly sensitive equipment to observe this (or measure over very large distances), but:..
Thatís true. However it should be noted that such extremely sensitive instruments do exist and have been in use since at least 2006. The one I know of is called an Electrostatic Gravity Gradiometer] and itís roughly the size of a breadbox and can make a measurement in a fraction of a second. It can detect differences in acceleration as small as 10-10cm/s2 over a distance of 50 cm, that is a gradient of 2x10-12/s2.
A good account of all of this is in my article Einsteinís Gravitational Field which is online at
http://xxx.lanl.gov/abs/physics/0204044

Is there any information on the gravitational field of a mass under acceleration. I am really thinking of the extra energy input required when accelerating. What is the effect on field strength during the acceleration? This is one area I have not really looked into.
 

Offline chiralSPO

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Quote from: chiralSPO
One would need fairly sensitive equipment to observe this (or measure over very large distances), but:..
Thatís true. However it should be noted that such extremely sensitive instruments do exist and have been in use since at least 2006. The one I know of is called an Electrostatic Gravity Gradiometer] and itís roughly the size of a breadbox and can make a measurement in a fraction of a second. It can detect differences in acceleration as small as 10-10cm/s2 over a distance of 50 cm, that is a gradient of 2x10-12/s2.
A good account of all of this is in my article Einsteinís Gravitational Field which is online at
http://xxx.lanl.gov/abs/physics/0204044

Nifty! Thanks for the info.
 

Offline evan_au

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The equivalence between accelerating and being in a gravitational field is often expressed as "an observer in a small box" (read: microscopic box).

Tidal effects are mostly visible over larger areas.
 

Offline PmbPhy

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The equivalence between accelerating and being in a gravitational field is often expressed as "an observer in a small box" (read: microscopic box).

Tidal effects are mostly visible over larger areas.
I don't understand. What do you mean by "mostly"? Tidal effects can be detected in areas as small as a breadbox.
 

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