Time Dilation & Frame of Reference: Who's Time Slows Down?

[kawikdx225]

Well folks start throwing around words like accelerometer, of which I have no direct knowledge, but I've got to assume that such a device merely measures the G-force present and from that infers acceleration. There is no other possible configuration of such a device that comes to mind: it measures the G-force. So, my clock B would feel such a force, yet my clock A would most definitely NOT.


Those were rhetorical questions addressed to geometer to get him to back off his latest claims implying that unequivocal statements can be asserted about "moving" and "state of motion", based on accelerometer readings.

You'd need to review the back-and-fro between geometer and me, this thread, to appreciate exactly the context.

Yes, sorry didn't mean to jump in the middle of you two.

Anyway, Clock A will measure acceleration until it reaches terminal velocity.
Clock B will NOT measure any acceleration if it is truly hovering.

An accelerometer measures acceleration not gravitational fields

 

[geometer]

Those were rhetorical questions addressed to geometer to get him to back off his latest claims implying that unequivocal statements can be asserted about "moving" and "state of motion", based on accelerometer readings.

I disagree. Since "An observer in an isolated laboratory can detect acclerations." (1) This observer can make an unequivocal statement that his state of motion has changed. That's the definition of acceleration! He may not be able to say anything about what his new state of motion is depending on what other data he has access to, but he can unequivocally say his state of motion has changed.

 

[geometer]

oops - forgot to include the reference for my quote - "Concepts of Modern Physics, Fourth Edition, Arthur Beiser, page 2.

 

[ostren]

Yes, sorry didn't mean to jump in the middle of you two.

Please don't apologize! You are avidly WELCOME to jump in!

Anyway, Clock A will measure acceleration until it reaches terminal velocity.
Clock B will NOT measure any acceleration if it is truly hovering.

An accelerometer measures acceleration not gravitational fields

I AM sorry, but you are one hundred percent in error! You miss the entire point of Relativity if you think that an instrument aboard a freefalling object can ever determine if the object is "accelerating". Read this quote from Brittanica Online:

Acceleration cannot be measured directly. An accelerometer, therefore, measures the force exerted by restraints that are placed on a reference mass to hold its position fixed in an accelerating body.

So will you NOW concede my point?? an accelerometer merely measures the G-force. The accelerometer aboard the hovering rocket will measure 1g of such force; while an accelerometer strapped to the freefalling clock (A) will indicate zero!

So it is as I've stated (and geometer take note), that in the scenario of the two clocks, one hovering and one freefalling, accelerometers show positive ONLY for the hovering one -- yet is it "moving"?? is its "state of motion changing"? This goes to the heart.

 

[ostren]

... but he can unequivocally say his state of motion has changed.

No he cannot. He remains motionless in his frame. See my post #51 for proof of that. I'm sure you noticed my post #51, didn'cha?? When you can rebut my post #51... then you will have learned, grasshopper (but don't count on it).

 

[geometer]

SCENARIO. One clock (A) is plummeting to Earth from the heavens above; while another clock (B) is some distance below it, on board a rocket ship that has left the launch pad and is now using controlled burns to stay hovering motionless just above the ground. The accelerometer says that B is the one undergoing acceleration. Is clock B moving? is its "state of motion" changing, hmmm??

If B is truly hovering, it is not accelerating so it's acclerometer reads 0.

 

[ostren]

Geometer's assertion in his post #66 is laughable, especially since kawikdx225 just raised the very same objection in post #61 and I refuted him handily in my post #64. Please to open eyes.

 

[kawikdx225]

Please don't apologize! You are avidly WELCOME to jump in!

I AM sorry, but you are one hundred percent in error! You miss the entire point of Relativity if you think that an instrument aboard a freefalling object can ever determine if the object is "accelerating". Read this quote from Brittanica Online:So will you NOW concede my point?? an accelerometer merely measures the G-force. The accelerometer aboard the hovering rocket will measure 1g of such force; while an accelerometer strapped to the freefalling clock (A) will indicate zero!

Lets just say we agree to disagree.

By your definition, a clock dropped off a 10 story building will not accelerate? You can't possible believe this!

I think you are confused about how an accelerometer works.

 

[ostren]

And kawikdx225: as for your mention of "terminal velocity", this discussion is about Relativity, NOT about coefficients of air resistance. The freefalling clock will never reach terminal velocity because this is a purist discussion and all scenarios are presumed to be absent of material media, to wit air.

.. I think you are confused about how an accelerometer works.

Well I found the definition of the device at http://www.britannica.com/eb/article?tocId=9003472&query=accelerometer&ct= and you saw the relevant passage from it I quoted. Accelerometer is a device to measure G-forces, like those felt by an occupant of a lurching car, or like those felt by astronauts during lift-off. Read it again for the first time, that definition.

 

[kawikdx225]

And kawikdx225: as for your mention of "terminal velocity", this discussion is about Relativity, NOT about coefficients of air resistance. The freefalling clock will never reach terminal velocity because this is a purist discussion and all scenarios are presumed to be absent of material media, to wit air.

OK, fine.

Riddle me this.
If I drop a clock off the Empire state building, will it accelerate on it's journey to the street below?
By your definition it will not.

 

[ostren]

You got me with that riddle! My answer would have to be an emphatic NO -- it doesn't accelerate, as gauged in the frame of onboard occupants and instruments. It sits motionless in space and the Earth accelerates up to meet it... and no meticulous study of the behavior of light would reveal differently.

 

[juju]

Hi DocAL,

I think I have found a way to reconcile our two points of view on the basic twin paradox as given without invoking any other similar experiment.

The fact that the distance to be traveled is defined on the rest frame of the earth, points unequivocably to the fact that it is the spaceship that has been accelerated.

However, it is also this fact of the distance being in a specific rest frame that allows one to determine which clock is actually running slower. (who counts less ticks in his own reference frame with his own counter.)

juju

 

[geometer]

Geometer's assertion in his post #66 is laughable, especially since kawikdx225 just raised the very same objection in post #61 and I refuted him handily in my post #64. Please to open eyes.

You didn't refute anything, handily or otherwise. You made some kind of comment that accelerometers measure g forces? Well, that's true, but it's also trivial. Any acceleration can be expressed as some multiple of Earth's gravitational acceleration, i.e. so many gs'. They're all accelerations.

kawikdx's response was right on. Assuming that by hovering, you mean the e rocket is stationary with respect to the suface of the Earth at some point above the surface of the earth, and the only forces acting are the rockets and the Earth's gravity, the rockets produce an acceleration exactly equal to but opposite in direction to the acceleration generated by Earth's gravity. These two accelerations add to 0 then and the hovering rocket is NOT accelerating.

 

[ostren]

...accelerometers measure g forces?

.. I think you are confused about how an accelerometer works.

Well I found the definition of the device at http://www.britannica.com/eb/article?tocId=9003472&query=accelerometer&ct= and you saw the relevant passage from it I quoted. Accelerometer is a device to measure G-forces, like those felt by an occupant of a lurching car, or like those felt by astronauts during lift-off. Read it again for the first time, that definition.

It's the same principle as a bathroom scale; how much you weigh on it is an indirect measure of how many G's of downward pseudo-force you're experiencing, gravity or acceleration.. they're equivalent. If you were in freefall, ie. my clock A, the bathroom scale would indicate zero. If you were hovering, ie. my clock B, or just standing on Earth, the scale would read your usual weight, indicating that you are experiencing ONE GEE of pseudo-force.

 

[kawikdx225]

You got me with that riddle! My answer would have to be an emphatic NO -- it doesn't accelerate, as gauged in the frame of onboard occupants and instruments. It sits motionless in space and the Earth accelerates up to meet it... and no meticulous study of the behavior of light would reveal differently.

lol
An accelerometer on the falling clock will measure 9.8m/s² while an accelerometer on the street below will measure 0m/s² therefore you can conclude it was the clock that accelerated. In theory the Earth and clock both accelerate toward each other but the mass of the Earth is so big that it's acceleration is way too small to be measured.

Anyway, we seem to be at a standstill so I will disappear for a while unless a third or fourth party joins in with their opinion.

HEY MENTOR'S, can someone clear this up please!

 

[ostren]

lol
An accelerometer on the falling clock will measure 9.8m/s² while an accelerometer on the street below will measure 0m/s²

No, just the reverse is true.

 

[kawikdx225]

OK, I'm back already.

It's the same principle as a bathroom scale; how much you weigh on it is an indirect measure of how many G's of downward pseudo-force you're experiencing, gravity or acceleration.. they're equivalent.

Here is the definition of acceleration:
Acceleration is a vector quantity which is defined as "the rate at which an object changes its velocity." An object is accelerating if it is changing its velocity.

When I stand on my bathroom scale my velocity doesn't change.

If you were in freefall, ie. my clock A, the bathroom scale would indicate zero. If you were hovering, ie. my clock B, or just standing on Earth, the scale would read your usual weight, indicating that you are experiencing ONE GEE of pseudo-force.

I agree that you are experiencing one g of pseudo-force when standing on Earth but since your velocity isn't changing then you are not accelerating.

 

[chroot]

There are a number of problems with this thread.

The most significant problem is ostren's tirade that "All physical motion, bar none, is relative, with zero favoritism attached."

This is quite simply not true in relativity theory. It's a strawman. This misconception is actually one of the most common I've seen -- people seem to think that everything is relative in relativity. That's just silly, of course. There are many invariants in relativity -- quantities which appear the same for all observers.

Acceleration is not relative.

Second, I firmly suggest that you not use the Encyclopedia Brittanica as a scientific reference. It is not rigorous. You would do better to actually learn relativity from a reputable, properly rigorous book before attempting to preach to others about it.

osten, if you wish to continue posting this sort of uneducated tripe, you will need to post it to a different website.

- Warren