Special relativity - frame of reference

  • #1
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This must be a basic question. :)
Bob and Alice have the same age.
So in special relativity Bob leaves Alice and travels at very high speed and when it returns is younger than Alice. Bob's time is dilated and his space is contracted from Alice frame of reference.

But now, if I take Bob's frame as the frame of reference I can say the same: Alice leaves Bob and travels at very high speed and when it returns is younger than Bob.

So what is the truth: is Bob younger or is Alice younger?
Question is: how do you decide witch of these 2 are younger when they meet again?

Thak you.
 

Answers and Replies

  • #3
russ_watters
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So in special relativity Bob leaves Alice and travels at very high speed and when it returns is younger than Alice....

But now, if I take Bob's frame as the frame of reference I can say the same: Alice leaves Bob and travels at very high speed and when it returns is younger than Bob.

So what is the truth: is Bob younger or is Alice younger?
Question is: how do you decide witch of these 2 are younger when they meet again?
Only one of them actually "leaves" - the one that fired their rocket engines. The other one remains stationary.
 
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  • #4
Ibix
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The short version of the Insight article Vanadium50 linked: it turns out that your watch measures "distance" through spacetime in much the same way your car's odometer measures distance through space. The twins took different routes, which happen to have different "lengths", so they experience different amounts of time.
 
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  • #5
Dale
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if I take Bob's frame as the frame of reference
Bob’s frame is non inertial. It is not equivalent to Alice’s.
 
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  • #6
bobob
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But now, if I take Bob's frame as the frame of reference I can say the same: Alice leaves Bob and travels at very high speed and when it returns is younger than Bob.

So what is the truth: is Bob younger or is Alice younger?
Question is: how do you decide witch of these 2 are younger when they meet again?

Thak you.
You decide by determining which of the twins experiences a force by accelerating to make the journey. The two frames are not equivalent.
 
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  • #7
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Can someone provide me with actual experimental evidence that biological aging is affected by acceleration in this manner? II can certainly understand that mechanical clocks can be affected by a gravitational-like force, but biological aging is not a mechanical clock, far from it. To me, it sounds like ontological, philosophical speculation equating human cells to mechanical clocks. Is there experimental verification?
 
  • #8
Ibix
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but biological aging is not a mechanical clock,
This is not correct. It's merely a fairly inaccurate clock.
To me, it sounds like ontological, philosophical speculation equating human cells to mechanical clocks. Is there experimental verification?
You are asking for evidence that people don't get younger?
 
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  • #9
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This is not correct. It's merely a fairly inaccurate clock.
You are asking for evidence that people don't get younger?
In what manner would you suggest that biological entities of any sort can be equated to a clock. A clock, by my understanding, most present itelf as a regular rythmic movement/pulse in space. I just want to make sure we have strong experimental evidence for this and not philosophical speculation.
 
  • #10
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Can someone provide me with actual experimental evidence that biological aging is affected by acceleration in this manner? II can certainly understand that mechanical clocks can be affected by a gravitational-like force, but biological aging is not a mechanical clock, far from it. To me, it sounds like ontological, philosophical speculation equating human cells to mechanical clocks. Is there experimental verification?
The issue is not about the type of clock, it is about the actual passage of time. Acceleration/force is not important here, except in as much as it determines relative speed, which is important.
 
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  • #11
Ibix
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To be slightly more helpful, imagine someone with a very steady heartbeat. Wire them to an ECG and use the beat as a timer to open and shut a gate. Arrange a pendulum clock so that the pendulum passes through the gate if it is open and crashes if it is closed. It must be synchronised to the person's heartbeat, of course.

If time dilation does not apply equally to the heartbeat as the clock, an observer in motion cannot explain why the clock continues to run, and would be forced to reject relativity. See the sticky at the top of this forum for why we don't reject relativity.
 
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  • #12
Ibix
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In what manner would you suggest that biological entities of any sort can be equated to a clock. A clock, by my understanding, most present itelf as a regular rythmic movement/pulse in space. I just want to make sure we have strong experimental evidence for this and not philosophical speculation.
You might wish to look up the cosmic ray muons. They do not have a clock, yet when in motion relative to us their half lives change exactly as relativity predicts.
 
  • #13
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Bob’s frame is non inertial. It is not equivalent to Alice’s.
Can you explain to me how an outside observer would determine this? It would seem like either can be said to be accelerating away from each other and accelerating away from the observer, or the observer accelerating away from Bob and Alice.
 
  • #14
Ibix
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Can you explain to me how an outside observer would determine this? It would seem like either can be said to be accelerating away from each other and accelerating away from the observer, or the observer accelerating away from Bob and Alice.
Off the top of my head: Look at Alice and Bob's accelerometers. Doppler radar (in tandem with your own accelerometer). Spotting the rocket exhaust.
 
  • #15
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Off the top of my head: Look at Alice and Bob's accelerometers. Doppler radar. Spotting the rocket exhaust.
I am speaking of an outside observer who has no knowledge of any instrumentation or history. The only information are the objects that are moving away from each other from the observer's point of view?
 
  • #16
Ibix
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I am speaking of an outside observer who has no knowledge of any instrumentation or history. The only information are the objects that are moving away from each other from the observer's point of view?
Then there's no difference. Either or both could be moving. But they can't meet up again without turning around, so there's no unambiguous definition of which one is younger.
 
  • #17
Mister T
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In what manner would you suggest that biological entities of any sort can be equated to a clock.
By measuring the changes in clock readings between biological events. For example birth and death, conception and birth, successive heart beats, hair length measurements. People have been doing these things for as long as we've been building clocks.
 
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  • #18
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By measuring the changes in clock readings between biological events. For example birth and death, conception and birth, successive heart beats, hair length measurements. People have been doing these things for as long as we've been building clocks.
You are suggesting to use a mechanical clock of some sort to measure a person's age. Yes, the sun has been used for this purpose for eons. I am asking something different. When is a human or any biological entity actually been used as a clock? The entire example presented depends up experimental evidence that biological life is a clock. Lacking that, then humans have to be removed from the example and substituted with clocks for which there is evidence.
 
  • #19
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Then there's no difference. Either or both could be moving. But they can't meet up again without turning around, so there's no unambiguous definition of which one is younger.
It would appear that if an outside observer was attempting to determine who is aging faster, Bob, Alice, or the observer, then the observer needs to determine who is accelerating. How does the observer do this?
 
  • #20
Ibix
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It would appear that if an outside observer was attempting to determine who is aging faster, Bob, Alice, or the observer, then the observer needs to determine who is accelerating. How does the observer do this?
Neither is aging faster in any absolute sense. Different observers will have different opinions on which one they measure to be aging faster at any given time.

This isn't what's happening in the twin paradox (or, at least, it's not really relevant). The effect there is effectively that one twin took a shortcut through spacetime, a path with a lower interval, which turns out to be proportional to the time along the path.
 
  • #21
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Neither is aging faster in any absolute sense. Different observers will have different opinions on which one they measure to be aging faster at any given time.

This isn't what's happening in the twin paradox (or, at least, it's not really relevant). The effect there is effectively that one twin took a shortcut through spacetime, a path with a lower interval, which turns out to be proportional to the time along the path.
So as to understand you clearly, you are saying that no one is biologically aging faster. Correct?
 
  • #22
Ibix
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So as to understand you clearly, you are saying that no one is biologically aging faster. Correct?
No. I said that who is aging faster at any given moment is observer-dependant. Who ends up younger is invariant (everyone agrees on it), and that is because the two twins follow different paths with different durations.
 
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  • #23
PeterDonis
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you are saying that no one is biologically aging faster. Correct?
Everyone ages at one second per second along their own path through spacetime. The difference between the two twins is that Bob follows a shorter path through spacetime--one that has fewer total seconds along it--than Alice does. That is why Bob is younger when they meet up again.
 
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  • #24
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No. I said that who is aging faster at any given moment is observer-dependant. Who ends up younger is invariant (everyone agrees on it), and that is because the two twins follow different paths with different durations.
So, you are specifically saying that real, biological aging is observer dependent?
 
  • #25
PeterDonis
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you are specifically saying that real, biological aging is observer dependent?
No. Go read my post #23.
 

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