# Gravitational Time Dilation and Twin Paradox ?'s

Disclaimer: Im relatively illiterate on the subject so please try to keep explanations generally understandable. Aerospace undergrad programs get zero exposure to relativity I suppose.

[Gravitational Time Dilation]

First I want to know if im correct in saying that the gravitational time dilation is basically: gravity affects time? The closer to a gravitational source the slower the time?

Now I know there is experimental confirmation from atomic clocks but the meaning of the results is what I wish to interpret. I understand that this often get's philosophical but I do not believe that it has to and any suppositions I make are only in effort to understand what is actually happening.

I do not understand how having time implicit in the explanation of the result makes sense (not that anything in the universe has to I just believe that it can). Let me explain: If the interpretation was that radioactivity (or whatever mechanism the clock is using... it does not really matter) is affected by gravity and therefore the atomic clock will work relatively mechanistically....then that would seem reasonable. Analogous to temperature affecting the speed of molecules. Temperature is the conception; molecule speed is the physical nature -> time is the conception; radioactivity rate is the physical nature.
I suppose bunching the underlying explanation of everything into time is reasonable if that was the underlying explanation but I do not think that's what gravitational time dilation is saying.

First what I know and is it right: velocity affects time -> twin traveling near light speed returns younger.

Again I seek what is really happening in our universe not how we come to make predictions of it.

Now I know the paradox is a thought experiment and not likely to actually occur but lets look at the interpretation of what this actually means.

Trying to rationalize: If you are going to say Time is slower for the twin traveling near lightspeed you must be bunching everything that constitutes change in the universe into "time". If you are really going to say the twin comes back younger then you are really saying that every particle that makes up every atom is slowing accordingly and moreover, all different particles are slowing at the same rate (as a result of everything being bunched under the definition of "time"). That seems unlikely but is the only logical explanation I see in changing "time" from a conception to a physical aspect of the universe.

Any thoughts on the subject or explanations on where I may be misinterpreting something are appreciated.
Cheers

[Gravitational Time Dilation]

First I want to know if im correct in saying that the gravitational time dilation is basically: gravity affects time? The closer to a gravitational source the slower the time?

You are correct in saying that.

I do not quite understand what you want to say with the rest of your post, but in a travelling twin kind of scenario of course you also have to take the gravitational potentials at the locations the twins are positioned at into account. If the travelling twin escapes the earths gravitational field his clock will speed up, if he travels away from the sun his clock will speed up, if he gets closer to Jupiter or some star his clock will speed down.

Bill_K
Apophenia, You seem to have the right ideas, just trouble accepting them!
If the interpretation was that radioactivity ... is affected by gravity and therefore the atomic clock will work relatively mechanistically....then that would seem reasonable.
If you are really going to say the twin comes back younger then you are really saying that every particle that makes up every atom is slowing accordingly and moreover, all different particles are slowing at the same rate
The idea that time dilation and Lorentz contraction are real physical effects acting on material objects was historically the first. Einstein's explanation that they were acting on space and time themselves rather than every atom equally was a great simplification.

Dale
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2020 Award
I do not understand how having time implicit in the explanation of the result makes sense (not that anything in the universe has to I just believe that it can). Let me explain: If the interpretation was that radioactivity (or whatever mechanism the clock is using... it does not really matter) is affected by gravity and therefore the atomic clock will work relatively mechanistically....then that would seem reasonable. Analogous to temperature affecting the speed of molecules. Temperature is the conception; molecule speed is the physical nature -> time is the conception; radioactivity rate is the physical nature.
Yes, clocks based on radioactivity slow down, but so do all other clocks. It doesn't matter if your clock uses the strong, weak, or EM forces, all of them slow down and they all slow down by the same amount.

If clocks based on radioactivity slowed down but clocks based on EM did not, then we would probably say that gravity affects radioactivity. But that isn't what is observed. What is observed is that ALL clocks of whatever mechanism slow down. So I will turn the question around to you:

What is the EXPERIMENTAL difference between time slowing down and all clocks of any mechanism slowing down?

First I want to say that if this gets out of the discussion I welcome any moderators to point it to an appropriate forum.
Apophenia, You seem to have the right ideas, just trouble accepting them!

The idea that time dilation and Lorentz contraction are real physical effects acting on material objects was historically the first. Einstein's explanation that they were acting on space and time themselves rather than every atom equally was a great simplification.
It seems impossible to test but are there any experimental confirmations of Lorentz contraction?

Yes, clocks based on radioactivity slow down, but so do all other clocks. It doesn't matter if your clock uses the strong, weak, or EM forces, all of them slow down and they all slow down by the same amount.

If clocks based on radioactivity slowed down but clocks based on EM did not, then we would probably say that gravity affects radioactivity. But that isn't what is observed. What is observed is that ALL clocks of whatever mechanism slow down. So I will turn the question around to you:

What is the EXPERIMENTAL difference between time slowing down and all clocks of any mechanism slowing down?

Ok, very good point. I dont wish to muddy physics with philosophy but it may get that way so I apologize beforehand.

Whenever I hear someone say "time changes" I think "what the hell are you even talking about". Does anyone really know what they are saying? (not being condescending to anyone; I will be the first to admit I do not). I think it was Feynman who said something to the effect of "if a physicist tells you he understands quantum physics, he is lying".

Im sure they have, but are there gravitational time dilation experiments using clocks other than atomic clocks? Also, is the experimental difference between different clock mechanisms undoubtedly attributed to experimental error? Again, Im sure people testing these are cognizant of this.

Now, let me respond to your question:
If the mechanism behind the clocks all slow accordingly, then there is NO experimental difference between saying time slows or saying the mechanism slows as you would suggest. Then what is all the fuss about? you ask.

The fuss is in the interpretation of reality. Saying time (as if it were a thing) changes seems like a huge misconception. There is a difference is saying time slows and the mechanism slows. If a clock measurement depends on temperature and two clocks in two different temperatures are clearly running at different rates saying that time is slower for one over the other when we know the mechanism is dependent on temperature does not seem correct. However, I suppose that if gravity does affect all matter saying time slows may be logical if it is all encompassing; i.e., all mechanisms of matter slow.... even specific to neurological processes (which is what Im doing to you all with this philosophical BS...haha).

This could also have actual implications such as: if all the mechanisms of the universe were not slowed accordingly (only the mechanisms we measure or can currently measure are) then is it really correct to say time is what's variable? No, it is not. Specific example: if the cell processes in your body do not conform with slowing of mechanisms of clocks used to experimentally verify GTD then saying you would return younger is not correct. To say time is slowing it also MUST be the case that all mechanisms must slow at an according rate so that there is no experimental difference between saying time slows and the mechanism slows (like you were specifically referring to in your response).

I may be off on this but was it not that Einstein did not like the idea of a pulling force at a distance? So, relativity get's rid of a pulling gravitational force and instead says: space-time pushes you or you just move through variable space time. I would argue that you can make variable time and space in the event of any pulling force? I would suppose a way to do this specifically with magnetic forces although there would have to be some ridiculous caveat to take into account that the magnetism is specific to electron/protons whereas gravity to all matter. It seems "magical" force at a distance is a fundamental aspect of the universe. Magic is only magic because there's something we are not seeing.

Im like that evangelical astrophysicist who writes physics papers accepting theories that the universe is 13.5 billion years old but then privately believes it is 6000. LoL.....
except that Im the ultimate skeptic.

Thanks for the replies guys; I appreciate it.

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Dale
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It seems impossible to test but are there any experimental confirmations of Lorentz contraction?
The Michelson Morely experiment.

Im sure they have, but are there gravitational time dilation experiments using clocks other than atomic clocks? Also, is the experimental difference between different clock mechanisms undoubtedly attributed to experimental error?
Time dilation has been measured using gravity, the strong, weak, and electromagnetic forces. There are currently no other known forces to account for.

If the mechanism behind the clocks all slow accordingly, then there is NO experimental difference between saying time slows or saying the mechanism slows as you would suggest.
Exactly. There is no difference. You are free to adopt either of the following two propositions:

1) Time slows down as predicted by relativity.
2) Some mysterious unknown mechanism which cannot otherwise be measured or detected (e.g. the luminiferous aether) causes EM processes to slow down and coincidentally it just happens to slow down by the exact same amount predicted by relativity, AND a second mysterious unknown mechanism which cannot otherwise be measured or detected causes strong-force processes to slow down and doubly-coincidentally it just happens to slow down by the exact same amount as the EM processes and relativity, AND a third mysterious unknown mechanism which cannot otherwise be measured or detected causes weak-force processes to slow down and triply-coincidentally it just happens to slow down by the exact same amount as the previous two processes and relativity, AND a fourth mysterious unknown mechanism which cannot otherwise be measured or detected causes gravitational processes to slow down and quadruply-coincidentally it just happens to slow down by the exact same amount as the previous three processes and relativity.

You are free to believe proposition 2, no evidence will contradict you, but that is too many mysterious mechanisms and lucky coincidences for most scientists, myself included.

Im the ultimate skeptic.
Actually, if you believe 2 then you are quite far from being a skeptic. You are, in fact, a firm believer, willing to accept a belief in the constancy of time despite the lack of evidence supporting it, the abundance of evidence against it, and the many other fantastic things that you must believe in (without evidence) in order to maintain the constant-time belief in the face of the existing evidence.

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The Michelson Morely experiment.

Time dilation has been measured using gravity, the strong, weak, and electromagnetic forces. There are currently no other known forces to account for.

Exactly. There is no difference. You are free to adopt either of the following two propositions:

1) Time slows down as predicted by relativity.
2) Some mysterious unknown mechanism which cannot otherwise be measured or detected (e.g. the luminiferous aether) causes EM processes to slow down and coincidentally it just happens to slow down by the exact same amount predicted by relativity, AND a second mysterious unknown mechanism which cannot otherwise be measured or detected causes strong-force processes to slow down and doubly-coincidentally it just happens to slow down by the exact same amount as the EM processes and relativity, AND a third mysterious unknown mechanism which cannot otherwise be measured or detected causes weak-force processes to slow down and triply-coincidentally it just happens to slow down by the exact same amount as the previous two processes and relativity, AND a fourth mysterious unknown mechanism which cannot otherwise be measured or detected causes gravitational processes to slow down and quadruply-coincidentally it just happens to slow down by the exact same amount as the previous three processes and relativity.

You are free to believe proposition 2, no evidence will contradict you, but that is too many mysterious mechanisms and lucky coincidences for most scientists, myself included.

Actually, if you believe 2 then you are quite far from being a skeptic. You are, in fact, a firm believer, willing to accept a belief in the constancy of time despite the lack of evidence supporting it, the abundance of evidence against it, and the many other fantastic things that you must believe in (without evidence) in order to maintain the constant-time belief in the face of the existing evidence.

The Michelson-Morley supported Lorentz contraction as an explanation to a null result? Correct. Im not really familiar but there is no direct way to measure Lorentz contraction currently?

Questioning makes me a skeptic. Postulating how this problem could be physical makes just makes me a thinker; I did not say I believe 2) as there is no evidence to support it. I accept 1) but think how it can translate to physical nature. Everything we know has a physical nature does it not? Massless particles...anything.

However, if anyone is content with saying Time and not thinking what that would/should physically imply then that's fine; I understand perfectly the theory works and has progressed science. In this economy, I have time on my hands (unfortunately and fortunately)....but Time, it would seem, must physically imply something if you are going to say it is a variable aspect of the universe.

Another way is to say: OK, space is variable via Lorentz contraction. Material shortens with velocity. So rather than saying or even acknowledging that it must have an effect on the physical nature of the material (atoms and any matter composing it) the explanation is bunched into saying space. That's fine and extremely elegant if so, but who would venture to say that this would have no effect of the material properties of the contracted object? Id suppose the explanation is space itself is something and it's contraction does not have any affect upon the material; a material shortens without changing its physical composition ! That is pretty mind blowing if true and understandably hard to rationalize. The universe does not have to be rational; yes I know, but I think most anything observable can be reduced logically. And logic makes sense. If space and time are somehow physical culprits in this problem I would hope that someday they are reduced logically. If they are physical, they should be reducible in the context that anything else in the universe is.

We say light is something ; light is reducible -> made of photons or whatever we postulate, though it may be inconceivable in relation to what we commonly think or pure theory because it is currently unobservable.

It's clear that this discussion in a general framework does not really lead anywhere but I do see the questions brought up in the discussion as 1) not philosophical and 2) essential to progress science.

Thanks for the input and patience DaleSpam.

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First what I know and is it right: velocity affects time -> twin traveling near light speed returns younger.

This is not quite correct. Velocity is relative, and when two observers have a relative velocity approaching c, each sees the other as time dilated. So the travelling twin sees the stay-at-home twin's clock running slow as well.

It is the whole combination of relative velocities and accelerations that result in the travelling twin having aged less when he returns. His deceleration/acceleration as he turns around plays a crucial part in this.

Mike

Would someone like to define exactly what "time" is and tell us what device you use to measure it? Scientifically speaking. Because I don't know the answer to either and I'd
like to comment on the thread once I do.

Nugatory
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Would someone like to define exactly what "time" is and tell us what device you use to measure it? Scientifically speaking.

"Time is what a clock measures".
Now all we need to do is to decide what a clock is.

A pretty decent ideal clock would be two fixed mirrors facing each other and at rest relative to each other, with a flash of light bouncing back and forth between the mirrors, and with the physicist who is using the clock to measure time located at one of the mirrors (and hence also at rest relative to both of them). Now one tick of the clock is counted each time the light flash hits the physicist's mirror.

We can argue about whether what this device is measuring is "really" time, but so far no one has found a better definition that is also physically measurable without sneaking in some hidden assumptions.

You may also object that this device assumes that the speed of light is constant. It does, and that's OK because this assumption is one of the two explicit postulates of relativity. (The great success of Maxwell's equations and the negative results of Michelson-Morley experiments are what makes this postulate attractive).

Nugatory
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The Michelson-Morley supported Lorentz contraction as an explanation to a null result? Correct. Im not really familiar but there is no direct way to measure Lorentz contraction currently?

Length contraction provides one of the simplest and most elegant explanations of how moving electric charges produce magnetic effects; look in the FAQ at the top of this forum for a pointer to Purcell's work here.

Of course, that's still an indirect effect, whereas to be really convincing, we'd have to take a meter stick, examine it and arrange a second stick of equal length, and then accelerate the first in such a way that it flies past the second at relativistic speed... And then live to tell about the observation. That's not a practical experiment with anything macroscopic, so we may have to settle for indirect results.

mfb
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The Michelson-Morley supported Lorentz contraction as an explanation to a null result? Correct. Im not really familiar but there is no direct way to measure Lorentz contraction currently?
Time dilation (due to relative motion) and Lorentz contraction are basically the same thing, just in different reference frames. So every measurement of time dilation (and that is done on a daily basis) in some frame is a measurement of length contraction in another frame.

Length contraction in the lab frame has to be taken into account in the interpretation of heavy ion collisions.

Dale
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2020 Award
The Michelson-Morley supported Lorentz contraction as an explanation to a null result? Correct. Im not really familiar but there is no direct way to measure Lorentz contraction currently?
What is indirect about measuring length using an interferometer? An interferometer is the most precise method of measuring length that we have.

I accept 1) but think how it can translate to physical nature. Everything we know has a physical nature does it not?
I have no idea what the term "physical nature" means. Please define it scientifically. I.e. what experiment can be performed to determine if something has a "physical nature" or not.

The universe does not have to be rational; yes I know, but I think most anything observable can be reduced logically. And logic makes sense. If space and time are somehow physical culprits in this problem I would hope that someday they are reduced logically.
The test for whether or not something is logical is whether or not it can be expressed in terms of a self-consistent set of mathematical statements. Math is the language of logic. "Makes sense" is not the correct criterion. Relativity is a prime example of how some features of the universe are emminently logical, but so far removed from ordinary human experience that it may not "make sense".

It's clear that this discussion in a general framework does not really lead anywhere but I do see the questions brought up in the discussion as 1) not philosophical and 2) essential to progress science.
And I am skeptical of both those claims. If you have two positions which cannot be distinguished experimentally then it sure seems philosophical to me, and I don't really see the questions as beneficial for scientific progress, let alone essential. I have my own opinion on what is most essential right now.

Time dilation (due to relative motion) and Lorentz contraction are basically the same thing, just in different reference frames. So every measurement of time dilation (and that is done on a daily basis) in some frame is a measurement of length contraction in another frame.

Length contraction in the lab frame has to be taken into account in the interpretation of heavy ion collisions.

For an atomic clock on or near earth experiment (Hafele-Keating for instance) tells us that the faster a clock moves in relation to the center of the earth the slower the clock will tick.

Also the higher up from the earth the clock is situated the faster it will tick.

Do you really need length contraction to tell how fast the clock will tick?

Nugatory
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Do you really need length contraction to tell how fast the clock will tick?

In the rather weak sense that the formula you would use to calculate the change in tick rate doesn't include a contracted length, no. But you won't be able to (no one has ever succeeded and there are good reasons to think it's not possible) construct an internally consistent theory that includes time dilation but not length contraction - as mfb suggested above, they're two sides of the same coin.

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mfb
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It is a tricky analysis with clocks (as their length is irrelevant), so let's consider another example: Cosmic muons. They have a lifetime of some microseconds. At highly relativistic speeds, but without time dilation, this corresponds to a decay length of about 500m. Many of them can travel through the atmosphere (>>500m) before they decay, however, due to time dilation.
What happens if we analyze the problem in the system of the muons? There is no time dilation, so how can they travel through the atmosphere? Well, the answer is length contraction: For high-energetic muons, the atmosphere has a length of less than a kilometer.

Dale
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2020 Award
Do you really need length contraction to tell how fast the clock will tick?
In the rather weak sense that the formula you would use to calculate the change in tick rate doesn't include a contracted length, no.
It is a tricky analysis with clocks (as their length is irrelevant), so let's consider another example: Cosmic muons. They have a lifetime of some microseconds. At highly relativistic speeds, but without time dilation, this corresponds to a decay length of about 500m. Many of them can travel through the atmosphere (>>500m) before they decay, however, due to time dilation.
What happens if we analyze the problem in the system of the muons? There is no time dilation, so how can they travel through the atmosphere? Well, the answer is length contraction: For high-energetic muons, the atmosphere has a length of less than a kilometer.
Hi mfb, Nugatory, and Agerhell,

mfb is correct, and the analysis he gave for muons is applicable to clocks as well. An atomic clock passes by two landmarks at some speed. Both a ground observer and a clock observer agree on the relative speed. The ground observer says the landmarks are a certain distance apart and the clock observer says it took a certain time. The ground observer distance divided by the clock observer time is not equal to the relative speed. The ground observer attributes that to time dilation and the clock observer attributes that to length contraction.

ghwellsjr
Gold Member
Here is at thread that deals with the tradeoff between Length Contraction and Time Dilation for an incoming muon.

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Nugatory
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Hi mfb, Nugatory, and Agerhell,

mfb is correct...
Of course he is - I don't know about agerhell, but I'm pretty sure that you, mfb, and I are all saying the same thing.

Dale
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Yes, I just wanted to clarify it for Agerhell so that he wouldn't think that mfb was backpeadling or being forced to recant.

I have no idea what the term "physical nature" means. Please define it scientifically. I.e. what experiment can be performed to determine if something has a "physical nature" or not.

Physical -> something that is potentially reducible (made of something) although it need not be necessarily. Is it awkward that someone says "something" exists but this "something" is completely intangible and seemingly made of nothing, imo.

Now when we measure clocks, of course the mechanism has to slow/speed for a different measurement. You pointed out that different mechanisms slow accordingly whether using radioactivity, EM, etc. Also, how it would be illogical to believe that all mechanisms "magically" coincided to fit the reduction in time; but that is precisely what they seem to do which is why the theory is amazingly mathematically elegant. I see the hesitation here both from you and from myself, but if gravity is an effect on all matter is it not a uniform effect on these mechanisms which all share at some basis of similiar composition of matter.
Get rid of sting theory (0 evidence) and unify that. Im kidding. Haha.

If any of you guys would comment on this thread for Lorentz Contraction I would appreciate it. It does not add so much of the speculation but is more for learning. There is one question on implication however:

Thanks for the input; Im clearly out of bounds on this stuff !

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Dale
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Physical -> something that is potentially reducible (made of something) although it need not be necessarily.
That is a pretty poor definition. Not only is it non-scientific (no experiment), it essentially negates itself with the "although..." phrase.

However, if that is your definition of "physical nature" then the answer to your previous question seems to be "no, not everything we know has a physical nature". Mass, charge, energy, momentum, and many other quantities are not made of something.

That is a pretty poor definition. Not only is it non-scientific (no experiment), it essentially negates itself with the "although..." phrase.

However, if that is your definition of "physical nature" then the answer to your previous question seems to be "no, not everything we know has a physical nature". Mass, charge, energy, momentum, and many other quantities are not made of something.

No but they are all the result of particles (which are "physical" and potentially reducible). Time is mentioned in the likes of mass, energy but supposedly is not the result of physically reducible stuff (you dont say matter has time or matter is made of time. matter does have mass, energy, momentum,...etc.) You would not put distance into the category of mass, momentum. Distance is not inherent result of matter but a measurement of it; likewise time merely a measurement of the change in particles which accounts for all processes.

*Is it not interesting that different mechanisms for clocks slow in gravity all by the same time? The more I think of that, the more it seems pretty amazing actually. The result of these physical processes (changes in particles or matter) are what define time...likewise, the amount of matter and empty space of an object define distance.

As for the although in my statement: Atoms are divisible into sub-atomic particles and sub-atomic particles into quarks or whatever it may be. The although is referring one of two possibilities of reducibility:

1) atoms are infinitely divisible, i.e. made of something which is made of something..,,,and so on.....or
2) atoms are made up something which is indivisible. something made of nothing.

Both 1 and 2 are outcomes that are completely out of comprehension to human brains (nothing and infinity are mysterious because we never encounter them) but one of which must be the result of reducing a particle. So although you can reduce it, it does not necessarily need to be comprehensible or made of something as in 2). Although the constituents of atoms are physical, they need not be necessarily reducible by 2). I guess you can say reducibility potentially is not a test of physicality if 2) is true) but reducibility seems to be a test of physicality is a more precise way of saying what I was attempting to say.

And if it is not directly physical then it must be an inherent property of matter (which is physical) and it should fit in the statement: matter has ____. Not a good statement but similiar to what I was referring in the first paragraph of my response. Mass has a physical basis because it is the result of matter. Energy does because it is the result of the movement of matter (or whatever mechanism)
I suppose you can turn it around and say: distance does because it is the measurement of matter. But if you apply that to time then you are acknowledging time is the result of the measurement of particle change and should have interest in * (about clock mechanisms not about time).

Im not that good at trying to convey my meaning but I think it is more evident in the examples I give; they are an effort to work around definitions and such.

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"Time is what a clock measures".

No sorry. I can't accept that. Nor the fudged suggestion.
Clocks don't measure anything.
I'm aware of 2 uses for clocks : time of day and interval.

If there was just one clock in the entire world you would never know
if it was telling you the right "time" or not because it doesn't "measure"
anything in that sense.
All a clock "displays" is a number related to all the other clocks each
of which is just set to indicate an arbitrary relationship between
the earths orbit around the sun.

Making them "tick" faster or in sync does not change that whether
they are clockwork or atomic ticks so an interval timer is not related
to any external thing either - no measurement there that I can see.
Just sync.

To be able to say that time is "dilated" surely there must be a
scientific explanation of what exactly is being dilated?

mfb
Mentor
To be able to say that time is "dilated" surely there must be a
scientific explanation of what exactly is being dilated?
The clock, relative to other clocks.
You do not need regular clocks for that, anything with a time-evolution will work: Humans, radioactive atoms, the solar system, ...