# Special relativity - frame of reference

• B
• victorqed

#### victorqed

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.

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.

Alfredo Tifi and m4r35n357
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.

if I take Bob's frame as the frame of reference
Bob’s frame is non inertial. It is not equivalent to Alice’s.

Alfredo Tifi and m4r35n357
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.

Alfredo Tifi
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?

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?

russ_watters
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.

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.

Ibix
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.

m4r35n357
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.

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.

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.

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?

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.

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.

Ibix
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.

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?

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.

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?

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.

Dale
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.

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?

you are specifically saying that real, biological aging is observer dependent?

No. Go read my post #23.

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.

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?

Are you sure you know what "acceleration" and "inertial" mean? That might be the source of confusion here.

So, you are specifically saying that real, biological aging is observer dependent?
The problem is that you do not seem to understand the relativity of simultaneity.

So, you are specifically saying that real, biological aging is observer dependent?
No. I'm saying that my measurements of your aging rate will be the same as my measurements of your clock rate: slow. My own measurements of my own aging rate will be the same as my measurements of my clock rate: normal. And you would say the same, the other way round.

No. I'm saying that my measurements of your aging rate will be the same as my measurements of your clock rate: slow. My own measurements of my own aging rate will be the same as my measurements of my clock rate: normal. And you would say the same, the other way round.

Are you saying that real biological aging is dependent upon clock measurements? It seems that this is a running theme. Had this idea been tested? I understand that observers can disagree on their measurements but that is different than saying that biological aging is dependent upon these measurements.

Are you saying that real biological aging is dependent upon clock measurements?

Real biological aging is governed by the same thing that governs clocks--proper time along your path through spacetime. So are all other processes involving change.

Grinkle and Dale
Are you saying that real biological aging is dependent upon clock measurements?
They are the same thing! Have you ever seen hair get shorter? Fewer wrinkles? Your nails shrink? Does your heart undo a few beats? If not, all these processes are useable as rough and ready clocks.

Edit: it's all just chemistry when you get down to it.

We can't measure biological aging precisely enough to confirm, for example, that astronauts who spend extended periods on the International Space Station age slightly less than people who remain on Earth.

However, there are plenty of experiments to show that all processes involving change that we can measure precisely enough are affected by their paths through spacetime in precisely the way that SR and GR predict. The GPS system is the most commonly known example of such an experiment; if SR and GR were not correct in their predictions, GPS would not work.

russ_watters
When is a human or any biological entity actually been used as a clock?

You mean like a birthday party? It's the child's 5th birthday today, so if you wanted to know what year it is on the day of the party you can just add the child's age to his birth date.

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.

If that's your objection then remove the humans from the example and create an example that doesn't involve humans. The physics is the same, either way.

When you state a person's age what you are really doing is declaring the amount of time that has elapsed since their birth. Are you claiming that mechanical clocks can't be used to measure that amount of time?

Dale and victorqed
Real biological aging is governed by the same thing that governs clocks--proper time along your path through spacetime. So are all other processes involving change.

Would you say this is scientifically proven? Can you point me to a paper on this subject with experimental evidence. I am trying to separate ontological, philosophical speculation from scientifically established evidence.

We can't measure biological aging precisely enough to confirm, for example, that astronauts who spend extended periods on the International Space Station age slightly less than people who remain on Earth.

This is very interesting and addresses my question, which is solely about biological aging. Can you explain or direct me to a paper that describes how it was determined that someone biologically aged less in space than they would have if they were on Earth? This would be an extremely interesting read. Thank you.