Why does time slow down as you approach C?

  • #26
ghwellsjr
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What other impression might he get from anything I've said?
His specific question was prompted by the statement that time dilation is reciprocal.
WHOA HERE ... unless I have this totally wrong, time does NOT slow down for you when you get near the speed of light. It APPEARS to other inertial frames of reference to have slowed down but to YOU it doesn't seem to slow down at all. To you it seems like things in the OTHER frames of reference have slowed down (and of course, they would disagree).
Then why does one person age more than the other?
Your posts state that it all is a result of one person having a speed while the other one is at rest. You seem to be stating the opposite of what phinds was saying. Can't you incorporate the reciprocal nature of time dilation into your discussion and explain why the "person at rest on earth" could be the one with the slowed down "biological oscillators" just as validly as the person moving at 0.5c in a spaceship?
 
  • #27
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@ghwellsjr
thanks for the response.

I still have doubts about the 'twin paradox'.
Lets take the example of stationary A and moving B (lets say at .9999c).
As you also agreed - "You are correct, each observer will read the other one's clock as ticking slower by exactly the same amount".

Lets say at some instant B sends message/tick to A, that he is 10 y/o as per B's clock. A would get this message lets say when A is 20 y/o (as per A's clock). and vice-versa.

Now at that instant (when at receives that message), would B also not have aged to 20? Even though A would not know about it.
So, essentially both should age at same rate (from neutral perspective), and it should just be the time gap in knowledge of their ages with each other.
 
  • #28
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Thanks for the feedback ghwellsjr. You and DaleSpam (primarily, but many others also) are helping me to learn how to think and talk about this stuff. I've gone from believing that differential aging is just a matter of 'perspective' (don't ask what I was thinking at the time), to understanding it as a well supported physical fact of nature, to incorrectly stating the role that acceleration plays, to hopefully refining my understanding (and any statements) regarding that, and any inferences that might follow from that.

Now to the comments:

His specific question was prompted by the statement that time dilation is reciprocal.
Ok, but his question was about differential aging not time dilation. Time dilation is symmetric, and is due to the SR conventions applied by observers in relative motion wrt each other in a relativistic universe (ie., apparently our universe, wherein the speed of light is constant and the same for all observers regardless of their state of motion).

On the other hand, differential aging is asymmetric, and is due to the real physical effects undergone by an oscillator as a function of its speed.

Your posts state that it all is a result of one person having a speed while the other one is at rest.
That's just one scenario, that is, where one person and his clock move about at relativistic speeds wrt the earth while the other person and his clock remain at rest wrt the earth.

You seem to be stating the opposite of what phinds was saying.
phinds said (I'm paraphrasing) that each observer would see the other's clock as slowing down, and that each observer would see his own clock as not slowing down. And I said that, "As others have noted, nobody will feel like they're aging any differently as their speed increases or decreases, and their own clocks won't appear to them to be slowing down or speeding up". Which doesn't contradict what phinds said.

Can't you incorporate the reciprocal nature of time dilation into your discussion and explain why the "person at rest on earth" could be the one with the slowed down "biological oscillators" just as validly as the person moving at 0.5c in a spaceship?
No, because SR predicts, and analogous experiments support the expectation, that the person moving at 0.5c in a spaceship would be the one who actually aged less, and with that the inference that the traveller's biological and reference clock oscillators' periods actually physically dilated.
 
  • #29
ghwellsjr
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@ghwellsjr
thanks for the response.

I still have doubts about the 'twin paradox'.
Lets take the example of stationary A and moving B (lets say at .9999c).
As you also agreed - "You are correct, each observer will read the other one's clock as ticking slower by exactly the same amount".

Lets say at some instant B sends message/tick to A, that he is 10 y/o as per B's clock. A would get this message lets say when A is 20 y/o (as per A's clock). and vice-versa.

Now at that instant (when at receives that message), would B also not have aged to 20? Even though A would not know about it.
So, essentially both should age at same rate (from neutral perspective), and it should just be the time gap in knowledge of their ages with each other.
You can say precisely what each twin will see of the other one's clock, but there is no neutral perspective from which you can identify an instant that applies to both of them.
 
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  • #30
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I still have doubts about the 'twin paradox'.
Lets take the example of stationary A and moving B (lets say at .9999c).

Lets say at some instant B sends message/tick to A, that he is 10 y/o as per B's clock. A would get this message lets say when A is 20 y/o (as per A's clock). and vice-versa.

Now at that instant (when at receives that message), would B also not have aged to 20?
You specified that A is stationary and B is moving at .9999c, which means that B is aging and his clock is accumulating time slower than A by a factor of 70.7.

So, essentially both should age at same rate (from neutral perspective) ...
By "neutral perspective" I assume you mean some common referent. Like say A is on Earth, so he's stationary wrt the Earth, and B is moving at .9999c wrt A and the Earth. Ok, so when the Earth-Sun system marks 10 years, then A will have aged 10 years and A's clock will have accumulated 10 years, but B will have aged (and B's clock will have accumulated) only about 51 days and 15 hours.

Also, what ghwellsjr and other commenters more knowledgeable than I am have said. I don't think I've contradicted any of that. I wanted to respond to your post because I too had doubts about the reality of differential aging. But the preponderance of experimental evidence supporting it is pretty convincing ... strange as it might seem, at first anyway. And differential aging actually becomes sort of inuitive when you consider that what seems like empty space interspersed with ponderable objects is actually a seamless hotbed of all sorts of underlying interactional activity. So the periods of atomic oscillators are physically related to their speed of movement, and changes in their frequencies are due to accelerations involving changes in speed. Still something of a mystery as to exactly how that works, but not really strange or weird.
 
  • #31
ghwellsjr
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By "neutral perspective" I assume you mean some common referent. Like say A is on Earth, so he's stationary wrt the Earth, and B is moving at .9999c wrt A and the Earth. Ok, so when the Earth-Sun system marks 10 years, then A will have aged 10 years and A's clock will have accumulated 10 years, but B will have aged (and B's clock will have accumulated) only about 51 days and 15 hours.
But, assuming that you did the calculation correctly, you could also say that from a reference frame in which B is at rest, A and the Earth-Sun system are moving away at 0.9999c and when B will have aged 10 years, A's clock will have accumulated only about 51 days and 15 hours and the Earth will have only progressed about 1/7 of a revolution around the Sun.

Or you could pick a Frame of Reference midway between A and B such that both of them are traveling at exactly the same speed in opposite directions and in which they both age at the same rate. Why wouldn't this FoR be a better "neutral perspective" than one in which either A or B was at rest?

And there are a gizzilion other Frames of Reference that establish totally different accumulated times on A's and B's clocks, none of which can be called a "neutral perspective". That's just a euphemism for "preferred reference frame" of which there are none.
 
  • #32
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But, assuming that you did the calculation correctly, you could also say that from a reference frame in which B is at rest, A and the Earth-Sun system are moving away at 0.9999c and when B will have aged 10 years, A's clock will have accumulated only about 51 days and 15 hours and the Earth will have only progressed about 1/7 of a revolution around the Sun.
I assume you're talking about what B would see and not what actually is the case, because if B is moving away from the Earth-Sun system and A at .9999c, then during the interval that B's clock goes from 0 to 10 years, then the Earth would have gone around the sun 707 times. But yes I understand that B would see the Earth as only having progressed about 1/7 of a revolution around the Sun when he (B) marks that his clock has accumulated 10 years.

The reason I framed the scenario the way I did was to illustrate the expected reality of a situation where A is stationary on the surface of the Earth and B is moving at .9999c wrt A and the Earth (because akshayxyz seemed concerned with the physical reality of the situation and not just how things might appear to A and B).

akshayxyz specified that A is "stationary". But stationary wrt what, I wondered. Then, reading a bit further, he mentioned "from a neutral perspective", and I supposed that he meant some referent, like say, the Earth, wrt which A is at rest and wrt which B is moving at .9999c. In which case, B would be aging considerably slower than A.

But yes I understand what you're saying about the notion of a "neutral perspective", and I don't really know what that might refer to wrt the discussion in this thread. I agree with your statement that wrt SR the term "neutral perspective" is essentially meaningless, but I don't think that it's necessarily a euphemism for "preferred reference frame". At least I don't think that that's what akshayxyz meant by it. Hence my conjecture, in the absence of input from akshayxyz, regarding what might contribute to furthering his understanding of differential aging, as he seems to be having some difficulty understanding that it's a prediction of SR and a pretty well established experimental fact.

Or you could pick a Frame of Reference midway between A and B such that both of them are traveling at exactly the same speed in opposite directions and in which they both age at the same rate. Why wouldn't this FoR be a better "neutral perspective" than one in which either A or B was at rest?
Because it wouldn't illustrate differential aging, which is what akshayxyz seemed to me to be concerned about, and which I take to be the basic theme of this thread (ie., a discussion about the relationship between speed and timekeeping/aging).

And there are a gazillion other Frames of Reference that establish totally different accumulated times on A's and B's clocks, none of which can be called a "neutral perspective".
Yes, point taken. So I just specified a common referent, the Earth. I thought it would then be clear(er) to akshayxyz that as long as A was at rest wrt the Earth and B was moving at .9999c wrt the Earth, then B would be aging, and his clock accumulating time, slower than A by a factor 70.7

That's just a euphemism for "preferred reference frame" of which there are none.
Maybe that's how some people use the term "neutral perspective" in these sorts of discussions. I don't know. But, as I mentioned above, I don't think that "preferred reference frame" was what akshayxyz was referring to wrt his use of the term "neutral perspective".
 
  • #33
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This whole discussion about "preferred reference frames" not existing seems a moot point after the Hafele-Keating Experiment. Please correct me if I'm wrong but according to http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/airtim.html" [Broken], a clock moving easterly "gained" time while the clock going westerly "lost" time. IF there was no "preferred" reference frame, then we could look at both clocks and see relatively the same time. Moreover, both clocks' reference frame could be chosen as "correct" and all the clocks timing would line up, eventually. However, we see that since one clock gained time and the other lost time, there must be some absolute to which everything is measured to. I'm NOT comparing them to the Earth fixed reference frame, but rather the net change. Again, please point out the flaw in my logic if it is there and show how it is wrong.
 
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  • #34
phinds
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This whole discussion about "preferred reference frames" not existing seems a moot point after the Hafele-Keating Experiment. Please correct me if I'm wrong but according to http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/airtim.html" [Broken], a clock moving easterly "gained" time while the clock going westerly "lost" time. IF there was no "preferred" reference frame, then we could look at both clocks and see relatively the same time. Moreover, both clocks' reference frame could be chosen as "correct" and all the clocks timing would line up, eventually. However, we see that since one clock gained time and the other lost time, there must be some absolute to which everything is measured to. I'm NOT comparing them to the Earth fixed reference frame, but rather the net change. Again, please point out the flaw in my logic if it is there and show how it is wrong.
I may be misreading you but it seems you believe the experiment DOES show that there is a preferred frame of reference when it fact it shows the opposite.

You seem to be leaving out the middle clock. There were three clocks and their motion was relative to each other and the results were compared relative to each other and were consistent with relativity. There was no measurement against anything "absolute". Each of the 3 would see the other two exactly as predicted by relativity (within the margin of error of the measurement).
 
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  • #35
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I may be misreading you but it seems you believe the experiment DOES show that there is a preferred frame of reference when it fact it shows the opposite.

You seem to be leaving out the middle clock. There were three clocks and their motion was relative to each other and the results were compared relative to each other and were consistent with relativity. There was no measurement against anything "absolute". Each of the 3 would see the other two exactly as predicted by relativity (within the margin of error of the measurement).
I'm not sure that it does. Remember, you can choose any of the clocks to be your reference frame. If that is the case, I can take the westerly plane as Reference (A), the earth as (B) and the other plane as (C). If we can take any of the three of them to be the reference frame, then let us choose C as our frame. If we assume that C is at rest, then the other frames are moving "faster". This means that they should experience less time passing. Now if we assume reference (A) as the stationary frame, the same comment holds. This is, unquestionably, a contradiction. Since there is a direction for time elapsed, A>B>C, there must be an absolute.
 
  • #36
ghwellsjr
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But, assuming that you did the calculation correctly, you could also say that from a reference frame in which B is at rest, A and the Earth-Sun system are moving away at 0.9999c and when B will have aged 10 years, A's clock will have accumulated only about 51 days and 15 hours and the Earth will have only progressed about 1/7 of a revolution around the Sun.
I assume you're talking about what B would see and not what actually is the case, because if B is moving away from the Earth-Sun system and A at .9999c, then during the interval that B's clock goes from 0 to 10 years, then the Earth would have gone around the sun 707 times. But yes I understand that B would see the Earth as only having progressed about 1/7 of a revolution around the Sun when he (B) marks that his clock has accumulated 10 years.
No, I was not talking about what B would see and there is no "what actually is the case", unless you consider what each person sees as "what actually is the case". I discussed this back in post #21 where I said:
"...each observer will read the other one's clock as ticking slower by exactly the same amount but here we are not talking about time dilation, we are talking about Relativistic Doppler which is what each observer actually sees of the other one's clock. The amount of slowdown that they see is not the amount of time dilation."​
The formula for the Relativistic Doppler Factor for observers moving away from each other is:

√[(1-ß)/(1+ß)]

Since ß = 0.9999, we can calculate the factor as:

√[(1-0.9999)/(1+0.9999)]
√[(0.0001)/(1.9999)]
√[(0.0001)/(1.9999)]
√[(0.00005)]
0.00707

You said back in post #30:
Like say A is on Earth, so he's stationary wrt the Earth, and B is moving at .9999c wrt A and the Earth. Ok, so when the Earth-Sun system marks 10 years, then A will have aged 10 years and A's clock will have accumulated 10 years, but B will have aged (and B's clock will have accumulated) only about 51 days and 15 hours.​
Here you were talking about time dilation based on an Earth-Sun reference frame. But this is not what A will see of B's clock after 10 years. Instead A will see B's clock to have advanced only 0.00707 times 10 years or .0707 years or 25.8 days, roughly half of the time dilation.

And, by the same token, during the time that B's clock has advanced 10 years, B will see the Earth as having gone only 1/14 of its way around the sun, the same 25.8 days that A sees of B (except, of course, there's no solar system at B, but he does see his clock and keep track of the days).

Remember two important things about what each observer sees of the other one's clock: It has nothing to do with any specific theory or any particular frame of reference, and, secondly, it's a combination of time dilation and the propagation of light. That's why it's different than just time dilation.
The reason I framed the scenario the way I did was to illustrate the expected reality of a situation where A is stationary on the surface of the Earth and B is moving at .9999c wrt A and the Earth (because akshayxyz seemed concerned with the physical reality of the situation and not just how things might appear to A and B).
What each person sees is physical reality.
akshayxyz specified that A is "stationary". But stationary wrt what, I wondered. Then, reading a bit further, he mentioned "from a neutral perspective", and I supposed that he meant some referent, like say, the Earth, wrt which A is at rest and wrt which B is moving at .9999c. In which case, B would be aging considerably slower than A.
It's perfectly OK to say that A is "stationary" and that B is moving with respect to A. You don't need to have a solar system nearby to validate A's situation.
But yes I understand what you're saying about the notion of a "neutral perspective", and I don't really know what that might refer to wrt the discussion in this thread. I agree with your statement that wrt SR the term "neutral perspective" is essentially meaningless, but I don't think that it's necessarily a euphemism for "preferred reference frame". At least I don't think that that's what akshayxyz meant by it. Hence my conjecture, in the absence of input from akshayxyz, regarding what might contribute to furthering his understanding of differential aging, as he seems to be having some difficulty understanding that it's a prediction of SR and a pretty well established experimental fact.
It's not just in the context of SR that a "neutral perspective" is meaningless, that also is a well established experimental fact.
Or you could pick a Frame of Reference midway between A and B such that both of them are traveling at exactly the same speed in opposite directions and in which they both age at the same rate. Why wouldn't this FoR be a better "neutral perspective" than one in which either A or B was at rest?
Because it wouldn't illustrate differential aging, which is what akshayxyz seemed to me to be concerned about, and which I take to be the basic theme of this thread (ie., a discussion about the relationship between speed and timekeeping/aging).
Differential aging is what happens after the two observers with their clocks are reunited. Until that happens, you either talk about what each observer sees of the other one's clock (Relativistic Doppler which is symmetrical while they continue to move apart), or time dilation based on an arbitrary Frame of Reference and it doesn't matter which Frame of Reference you use, but most will have a non-symmetrical time dilation.

So let me repeat so you don't miss this very important point: any Frame of Reference will illustrate the exact same differential aging as any other Frame of Reference.

And there are a gizzilion other Frames of Reference that establish totally different accumulated times on A's and B's clocks, none of which can be called a "neutral perspective".
Yes, point taken. So I just specified a common referent, the Earth. I thought it would then be clear(er) to akshayxyz that as long as A was at rest wrt the Earth and B was moving at .9999c wrt the Earth, then B would be aging, and his clock accumulating time, slower than A by a factor 70.7
That's true but it's only a half-truth as I hope you are well aware of by now because you can equally establish B as the "neutral perspective", if by that you merely mean a reference frame, but you seem to think the Earth makes one reference frame more significant than some other reference frame.
That's just a euphemism for "preferred reference frame" of which there are none.
Maybe that's how some people use the term "neutral perspective" in these sorts of discussions. I don't know. But, as I mentioned above, I don't think that "preferred reference frame" was what akshayxyz was referring to wrt his use of the term "neutral perspective".
And as I stated in post #31, you could use an imaginary point midway between A and B as the basis for your reference frame and it would give both of them the same time dilation. Why wouldn't this one be the neutral perspective?
 
  • #37
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ghwellsjr said:
...each observer will read the other one's clock as ticking slower by exactly the same amount but here we are not talking about time dilation, we are talking about Relativistic Doppler which is what each observer actually sees of the other one's clock. The amount of slowdown that they see is not the amount of time dilation.
Ok, thanks ghwellsjr. This corrects part of my thinking on this. I'll get up to speed on it, eventually.

There are some other things you mentioned that I might want to nitpick from your post #36. But I'll have to do it this evening or later.
 

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