Can time dialation really just be the slowdown of physical processes?

[kmarinas86]

If we an accelerometer we would know. If the accelerometer reads zero, then we are in an inertial frame, whether we are rest far away from any gravitational field, or moving relative to something else or free falling in a gravitational field.
Zero reading on accelerometer = zero proper acceleration = inertial motion.

This would mean that an object in free fall doesn't gain any additional energy, right? Is it also true that, during coordinate acceleration (with no proper acceleration simultaneous to it), what may simply be happening is that the paths of internal momenta in that object converge toward the world line of the path of the overall object? Would that reduce the rate of events involving perpendicular motions with respect to that world line and thereby explain gravitational time dilation? Or is that not looked upon? If that is a different concept than what is used to explain gravitational time dilation in General Relativity, wouldn't it be a redundant explanation for gravitational time dilation?

 

[PAllen]

This would mean that an object in free fall doesn't gain any additional energy, right? Is it also true that, during coordinate acceleration (with no proper acceleration simultaneous to it), what may simply be happening is that the paths of internal momenta in that object converge toward the world line of the path of the overall object? Would that reduce the rate of events involving perpendicular motions with respect to that world line and thereby explain gravitational time dilation? Or is that not looked upon? If that is a different concept than what is used to explain gravitational time dilation in General Relativity, wouldn't it be a redundant explanation for gravitational time dilation?

This thread was all about pure SR effects (including non-inertial motion, but *not* including gravity effects). If you want to discuss energy change during free fall in a gravitational field, that deserves a separate thread. It is complex and unrelated to this thread.

 

[cosmik debris]

There are three types of acceleration in GR.

1). Co-ordinate acceleration - your usual 3 acceleration i.e. the second derivative of position.

2). 4-acceleration - a vector quantity which is invariant.

3). proper acceleration - 4-acceleration projected on to co-moving co-ordinates.

In free fall there is co-ordinate acceleration but 4-acceleration is zero.

 

[PAllen]

There are three types of acceleration in GR.

1). Co-ordinate acceleration - your usual 3 acceleration i.e. the second derivative of position.

2). 4-acceleration - a vector quantity which is invariant.

3). proper acceleration - 4-acceleration projected on to co-moving co-ordinates.

In free fall there is co-ordinate acceleration but 4-acceleration is zero.

Of course you mean the may be coordinate acceleration in free fall. It depends on the coordinates.

 

[DaveC426913]

So you think that the acceleration would be felt by the whole water simultaneously? We are talking about a rocket here. Water is not a rigid body. How does having different forces on each of the little H20 molecules, as peaks and valleys of sound waves pass through it, violate the equivalence principle?

I am simply saying that there is no test you can do in a closed room that will distinguish between uniform acceleration and gravity. That is the very essence of EP.

Under uniform acceleration, the water will be compressed normally, and will stay that way throughout the acceleration, just as if the beaker of water were sitting on Earth.

For ease of imagining, picture the water molecules as bunch of large soft plush balls stacked 6ft high vertically. While at rest, the balls exert no pressure on each other. You begin accelerating to 1g, causing all the balls to push on each other (yes, at the speed of sound), which compresses them.

But you have not yet reached full acceleration.

By the time you reach you 1g acceleration gthe balls are fully compressed to 3ft. They will remain that way, stacked on top of each other, 3 ft high and motionless as long as your acceleration remains 1g. The uniform force results in uniform compression, and does not require any "transmission of force from ball to ball at the speed of sound".

 

[ItsDaveDude]

I am simply saying that there is no test you can do in a closed room that will distinguish between uniform acceleration and gravity. That is the very essence of EP.

What about tidal forces? Although they could be incredibly small (and I don't think anyone denies they exist) you could conceive of a test in a closed room that detects tidal forces (if it is gravity) and when it doesn't detect them it is uniform acceleration. Isn't this an effect that makes it misleading to say gravity and uniform acceleration are equivalent, because tidal forces are a detectable force that exist in one case (gravity) but not in the other (uniform acceleration), therefore making them not equivalent effects?

 

[DaveC426913]

What about tidal forces? Although they could be incredibly small (and I don't think anyone denies they exist) you could conceive of a test in a closed room that detects tidal forces (if it is gravity) and when it doesn't detect them it is uniform acceleration. Isn't this an effect that makes it misleading to say gravity and uniform acceleration are equivalent, because it is a detectable force that exists in one case (gravity) but not in the other (uniform acceleration)?

Yes. Which is why the closed system is defined as small. Small enough to not include tidal forces.

 

[ItsDaveDude]

Yes. Which is why the closed system is defined as small. Small enough to not include tidal forces.

Really? I hate being correct. To me, this means uniform acceleration and gravity are not equivalent then, and the equivalence principle is just some happenstance of physics that happens to work if we don't look too closely, but in reality these are totally different physical laws/effects from different fundamental processes, and we shouldn't be going on about how they are they same effect or equivalent. Is this a fair statement? Is the equivalence principle just a convenience/artifice for thinking about physics but doesn't hold in reality if you want to think about physics fundamentally?

I know this is getting off topic so I put it in a new post: https://www.physicsforums.com/showthread.php?p=3344706#post3344706

 

[Dale]

What do you think the equivalence principle says? There is nothing wrong with the equivalence principle, you can't blame it if you thought it applied to tidal gravity.

 

[harrylin]

I've seen so many descriptions of the twin paradox that mention the equivalence principle (and thus GR), but I never was told, until you stated the above, that the legit twin paradox totally ignores it.

Sorry I should have said it differently. What I meant was that the twin paradox ignores the gravitational effect of mass on clock rate.

But no single sound bite can catch the whole history. In a nutshell:

The original setting (by Langevin) was a straightforward SR problem of two people, one staying on Earth and the other making a space travel.
However, the twin problem can also be solved with GR, using the equivalence principle: then the traveler has the POV to be "in rest" and so "induced gravitational fields" appear. That solution (by Einstein, about two clocks) is much less obvious, or "paradoxical".

 

[harrylin]

Wouldn't there be a difference in the deflection seen from the accelerometer's frame when the accelerometer is in free fall versus when the accelerometer is just sitting on the ground? [...]

Yes in free fall the two effects together are zero: one can interpret it as -9.8+9.8=0 (no resulting force on the beam due to free fall) or as 0+0=0 (no resulting force due to "inertial" motion). "Inertial motion" has a different meaning in GR than in SR. As PAllen also noticed, this thread is being muddled up by the mixing of GR with SR while this thread is about SR.