How much time dilation is there as time passes?

[Snickers]

I'm not a Physicist, I just have this question that has bothered me for a long time. This came up again recently when I was listening to some people talking about evolution.

We know the universe is expanding, and we know that time is stretched with it. So if I'm looking back a million years, it's not that long a million years ago. As a thought experiment I make a time machine, and set it to travel back in time a million years. When I want to travel forward again, how much time do I set the machine to travel forward back to my own time?

 

[jbriggs444]

I'm not a Physicist, I just have this question that has bothered me for a long time. This came up again recently when I was listening to some people talking about evolution.

We know the universe is expanding, and we know that time is stretched with it. So if I'm looking back a million years, it's not that long a million years ago. As a thought experiment I make a time machine, and set it to travel back in time a million years. When I want to travel forward again, how much time do I set the machine to travel forward back to my own time?

One million years exactly. The length of the [timelike] interval between here/now and there/then is what it is.

The [co-moving] distance between distant galactic clusters may have increased in the interim, but that does not affect the invariant length of a timelike path. That length is whatever a clock traversing the path would have measured.

 

[russ_watters]

Welcome to PF!

We know the universe is expanding, and we know that time is stretched with it. So if I'm looking back a million years, it's not that long a million years ago. As a thought experiment I make a time machine, and set it to travel back in time a million years. When I want to travel forward again, how much time do I set the machine to travel forward back to my own time?

I'm having trouble parsing this...

a) Time is what a clock reads. Period. If you go back in time a million years, to get back to where you started, you go forward a million years.

b) Time is not expanding due to the Big Bang, just space. Perhaps there is an issue here of reconciling time (age) and distance between moving objects. E.G., the light from a distant object may have traveled for a billion years to get to us, but since the object moved in the meantime it is "now" a billion and a half light years away (fake numbers).

Does that help, or could you clarify?

 

[Ibix]

I think you misunderstand what relativity sayys about time.

If you ask me how fast I'm travelling, I might answer

• I'm sitting down - so zero.
• I'm sitting in a train - so about 60mph.
• I'm sitting on a rotating planet - so about 1000mph
• I'm sitting on an orbiting planet - so about 20kps

All of these answers are right. They just have different underlying assumptions about what "not moving" means.

Relativity just takes this kind of thing and amps it up to include the measurement of time. So the answer to your question is, how far back or forward in time you are depends on who how you ask the question. Whose notion of "time" are you using? Time machines are probably impossible to build, so the question as asked isn't really answerable. However, assuming your calendar always uses the same methodology to associate a "now" with a particular date, then it's true by definition that "now" and "a million years ago" are always a million years apart.

 

[Snickers]

Welcome to PF!

I'm having trouble parsing this...

a) Time is what a clock reads. Period. If you go back in time a million years, to get back to where you started, you go forward a million years.

b) Time is not expanding due to the Big Bang, just space. Perhaps there is an issue here of reconciling time (age) and distance between moving objects. E.G., the light from a distant object may have traveled for a billion years to get to us, but since the object moved in the meantime it is "now" a billion and a half light years away (fake numbers).

Does that help, or could you clarify?

Well I have to disagree that time is not expanding. It was my understanding that the space time fabric is described that way for that very reason. MIT did a experiment where they synchronized two identical atomic clocks, placed one at sea level and one on a mountain. After a month the clocks were no longer synchronized. They explained this saying that gravity stretches the space time fabric so time passes differently in the two locations. So my reasoning is that if the universe is really expanding then so is the fabric of space time. As time passes, relative time changes. Does that help?

 

[Snickers]

I think you misunderstand what relativity sayys about time.

If you ask me how fast I'm travelling, I might answer

• I'm sitting down - so zero.
• I'm sitting in a train - so about 60mph.
• I'm sitting on a rotating planet - so about 1000mph
• I'm sitting on an orbiting planet - so about 20kps

All of these answers are right. They just have different underlying assumptions about what "not moving" means.

Relativity just takes this kind of thing and amps it up to include the measurement of time. So the answer to your question is, how far back or forward in time you are depends on who how you ask the question. Whose notion of "time" are you using? Time machines are probably impossible to build, so the question as asked isn't really answerable. However, assuming your calendar always uses the same methodology to associate a "now" with a particular date, then it's true by definition that "now" and "a million years ago" are always a million years apart.

 

[Snickers]

We know that time passes differently for different relative positions. NASA took twin brothers, and were able to determine their age by some kind of DNA analysis. They sent one to the international space station for a year and when he returned, they examined the brothers again and they were no longer the same age. If you travel to another solar system light years away, when you return your twin brother will be long gone and you will have hardly aged at all because of your acceleration stretching the space time fabric.

So are you suggesting that gravity (acceleration) stretches space time but the expansion of the universe does not?

 

[jbriggs444]

We know that time passes differently for different relative positions. NASA took twin brothers, and were able to determine their age by some kind of DNA analysis.

Reference, please.

 

[russ_watters]

Well I have to disagree that time is not expanding. It was my understanding that the space time fabric is described that way for that very reason. MIT did a experiment where they synchronized two identical atomic clocks, placed one at sea level and one on a mountain. After a month the clocks were no longer synchronized. They explained this saying that gravity stretches the space time fabric so time passes differently in the two locations. So my reasoning is that if the universe is really expanding then so is the fabric of space time. As time passes, relative time changes. Does that help?

Time dilation is what is measured between two clocks at different locations in a static gravitational field. There's no stretching of either time or space involved. It's a very different thing from a single clock moving in time or the expansion of space.

 

[Orodruin]

Well I have to disagree that time is not expanding.

It is not an issue for opinion. You are simply wrong.

 

[Ibix]

Well I have to disagree that time is not expanding.

Space is expanding (or that's a reasonable-ish natural language description). Time is not. It's not clear to me what such a thing could mean.

MIT did a experiment where they synchronized two identical atomic clocks, placed one at sea level and one on a mountain. After a month the clocks were no longer synchronized.

True. These days, clocks are good enough that you can spot the difference between clocks on a table and on the floor.

They explained this saying that gravity stretches the space time fabric so time passes differently in the two locations.

Again, this is a passable description of what's going on. A better one would be that the curvature of spacetime is such that the elapsed time between "now" and (wait a second) "now" depends on height.

So my reasoning is that if the universe is really expanding then so is the fabric of space time. As time passes, relative time changes. Does that help?

That makes no sense, I'm afraid. As noted above, I'm not sure what "time expanding" could really mean.

We know that time passes differently for different relative positions. NASA took twin brothers, and were able to determine their age by some kind of DNA analysis.

Um... no. Two physicists, Hafele and Keating, flew an atomic clock around the world (first class, under the name Mr A Clock - I kid you not). There were a few hundred nanoseconds difference in the elapsed time on this clock and one left at home. In principle this also works for people, but no one can personally count time accurately enough to detect less than a microsecond difference.

 

[phinds]

Well I have to disagree that time is not expanding.

Interesting point of view. You are not a physicist so you, quite reasonably, come to a forum full of people who understand physics, and then you decide that you know better than us after all.

Reread post #10

Forget about insisting that you are right. Try to understand why you are wrong, but don't lean on pop-science to get your answers.

 

[Snickers]

Space is expanding (or that's a reasonable-ish natural language description). Time is not. It's not clear to me what such a thing could mean.
True. These days, clocks are good enough that you can spot the difference between clocks on a table and on the floor.
Again, this is a passable description of what's going on. A better one would be that the curvature of spacetime is such that the elapsed time between "now" and (wait a second) "now" depends on height.
That makes no sense, I'm afraid. As noted above, I'm not sure what "time expanding" could really mean.
Um... no. Two physicists, Hafele and Keating, flew an atomic clock around the world (first class, under the name Mr A Clock - I kid you not). There were a few hundred nanoseconds difference in the elapsed time on this clock and one left at home. In principle this also works for people, but no one can personally count time accurately enough to detect less than a microsecond difference.

I saw a new report on tv that was probably incorrect - now I can't find anything about how they determined the ages of the twins. I'm still reading but I don't have much hope finding anything about that. Thanks for your help.

https://www.nasa.gov/feature/nasa-twins-study-confirms-preliminary-findings

 

[Snickers]

Interesting point of view. You are not a physicist so you, quite reasonably, come to a forum full of people who understand physics, and then you decide that you know better than us after all.

Reread post #10

Forget about insisting that you are right. Try to understand why you are wrong, but don't lean on pop-science to get your answers.

My apologies - I'll try to omit my opinions in future posts

 

[Ibix]

I saw a new report on tv that was probably incorrect - now I can't find anything about how they determined the ages of the twins. I'm still reading but I don't have much hope finding anything about that. Thanks for your help.

https://www.nasa.gov/feature/nasa-twins-study-confirms-preliminary-findings

This has nothing to do with time dilation and everything to do with low gravity having odd effects on humans. See here: https://www.nasa.gov/feature/nasa-twins-study-investigators-to-release-integrated-paper-in-2019

Reasons for the lengthening of Scott’s telomeres in space are still under investigation, but may be from his rigorous exercise regime and restricted caloric intake while on the space station.

You can estimate the effects of time dilation over a year in space. It's less than a second.

 

[jbriggs444]

I saw a new report on tv that was probably incorrect - now I can't find anything about how they determined the ages of the twins.

https://www.nasa.gov/feature/nasa-twins-study-confirms-preliminary-findings

That study was aimed at examining the consequences of the more mundane aspects of space travel. Effects of weightlessness, radiation, etc. A few microseconds of gravitational and/or kinematic time dilation would have been lost in the noise. Far too small to measure by their biological effects.

 

[Snickers]

Reference, please.

Something I saw on a report on tv - Maybe I didn't see what I think I saw. This is what I found online:
https://www.nasa.gov/feature/nasa-twins-study-confirms-preliminary-findings

 

[phinds]

Something I saw on a report on tv - Maybe I didn't see what I think I saw. This is what I found online:
https://www.nasa.gov/feature/nasa-twins-study-confirms-preliminary-findings

I don't see anything in that link but I'm quite confident that you did in fact read somewhere (and it is legitimate) that NASA made a statement that the space twin had aged differently than the Earth twin and they even stated a number. I don't recall what it was but as jbriggs said, it was quite small.

BUT ... that is differential aging, not a difference in the RATE of passage of time for the twins. For each of the twins, their clocks ticked at exactly one second per second. It's just that they took different paths through spacetime so they ticked a different number of times, not at a different rate. Your confusion about this is quite common.

It's exactly analogous to saying that people traveling at 60mph going from Washington to Boston via different routes tick off a different number of miles even though they are both traveling at 60 mph

 

[Snickers]

Space is expanding (or that's a reasonable-ish natural language description). Time is not. It's not clear to me what such a thing could mean.

I appreciate your patience with me. I'm still tumbling this around - but here's where I'm going ... space expanding: Meaning that the bodies as observed by Hubble are moving apart and the things that are further away are moving away faster then the things that are closer by. Time by me (Mr Clock A) will occur/pass differently than in the past (Mr Clock B). Since I can't talk to Mr Clock B, Can I make an assumption based on Mr Clock A and the amount the space time fabric has expanded for the amount of time passed? So you're saying that it's incorrect to say that time "expanded" from Mr Clock B, right? In other words time passed more quickly for Mr Clock B because Mr Clock A was stretched out more.

 

[Snickers]

I don't see anything in that link but I'm quite confident that you did in fact read somewhere (and it is legitimate) that NASA made a statement that the space twin had aged differently than the Earth twin and they even stated a number. I don't recall what it was but as jbriggs said, it was quite small.

BUT ... that is differential aging, not a difference in the RATE of passage of time for the twins. For each of the twins, their clocks ticked at exactly one second per second. It's just that they took different paths through spacetime so they ticked a different number of times, not at a different rate. Your confusion about this is quite common.

It's exactly analogous to saying that people traveling at 60mph going from Washington to Boston via different routes tick off a different number of miles even though they are both traveling at 60 mph

I understand that both clocks tick once second each second. But when they came back together their "clocks" no longer matched. The explanation was that gravity and acceleration are the same and they bend the space time fabric. since the guy in orbit was more "bent" than his twin on the ground his "clock" ticked more slowly. Am I getting anywhere?

 

[phinds]

Space is expanding (or that's a reasonable-ish natural language description). Time is not. It's not clear to me what such a thing could mean.

I appreciate your patience with me. I'm still tumbling this around - but here's where I'm going ... space expanding: Meaning that the bodies as observed by Hubble are moving apart and the things that are further away are moving away faster then the things that are closer by. Time by me (Mr Clock A) will occur/pass differently than in the past (Mr Clock B). Since I can't talk to Mr Clock B, Can I make an assumption based on Mr Clock A and the amount the space time fabric has expanded for the amount of time passed? So you're saying that it's incorrect to say that time "expanded" from Mr Clock B, right? In other words time passed more quickly for Mr Clock B because Mr Clock A was stretched out more.

First, there is no "fabric" of spacetime and talking about it is not helpful. It is a pop-science misrepresentation of what's happening. Space-time is geometry. Things are moving apart. Nothing is "stretching". Google "metric expansion". I also recommend the link in my signature.

You have it right that things farther away from each other are receding from each other faster than things closer together. You will notice that I said they are RECEDING from each other, NOT that they are "moving" apart. There's a difference, but I don't want to go into that whole discussion right now.

Again, time does NOT pass differently for different objects or for different position. Time is what a clock measures (locally). You, right now as you read this, are traveling at 99.9999% of the speed of light according to a particle in the CERN accelerator, and according to that particle, you are massively time dilated. Do you feel slowed down at all? Does your clock tick differently? It would be very weird if it did since you are ALSO traveling at about 200mph relative to a bullet train in Japan and according to that train you are time dilated by a really trivial amount. You are also not moving at all relative to your chair and according to it you are not time dilated at all. For all of these, your clock just keeps ticking at one second per second, BUT ... they see it differently than you see it

 

[phinds]

I see our posts crossed paths.

No, the clocks did not tick more quickly or more slowly. They both ticked at exactly one second per second. Reread post #17

Again, you are confusing differential aging (more/fewer ticks) with the local passage of time (one second per second)

 

[Snickers]

I see our posts crossed paths.

No, the clocks did not tick more quickly or more slowly. They both ticked at exactly one second per second. Reread post #17

Again, you are confusing differential aging (more/fewer ticks) with the local passage of time (one second per second)

I meant relative to each other. If it were possible I mean since we can only compare the two "clocks" after the fact(I'm kind of taking a little liberty with the clock idea). Like in the MIT experiment didn't match in the end. If one were to observe the clock of the other he might say the clock was ticking different relative to his position.

 

[russ_watters]

I meant relative to each other. If it were possible I mean since we can only compare the two "clocks" after the fact(I'm kind of taking a little liberty with the clock idea). Like in the MIT experiment didn't match in the end. If one were to observe the clock of the other he might say the clock was ticking different relative to his position.

Yes, so...I suppose you could take your OP and combine it with what @phinds said about driving from Washington to Boston and say that on your return trip you can take any path you want and make it take any length of time as long as it's more than a million years.

 

[Snickers]

Time dilation is what is measured between two clocks at different locations in a static gravitational field. There's no stretching of either time or space involved. It's a very different thing from a single clock moving in time or the expansion of space.

At first I was going to reply then started to think my thinking of space time was way off. So what I was trying to convey is that if I were able to observe some time measurement somewhere else from where I am it would appear different. Know what I mean? IF time were stretched and I were able to able to observe that other clock in that other place it would appear to measure time differently than my clock. But if time isn't ever stretched then I've misunderstood the relationship between time and space. Let me know if I'm making any sense.. thanks

 

[gmax137]

I too have recently become interested in understanding time dilation, and at the recommendation of @Dale, something called "radar coordinates" or "Bondi K-calculus." Though I am still working my way through it, this is where I started:

https://en.wikipedia.org/wiki/Bondi_k-calculus

I hope you find it as interesting as I do.

 

[Snickers]

I too have recently become interested in understanding time dilation, and at the recommendation of @Dale, something called "radar coordinates" or "Bondi K-calculus." Though I am still working my way through it, this is where I started:

https://en.wikipedia.org/wiki/Bondi_k-calculus

I hope you find it as interesting as I do.

That looks helpful - thanks very much

 

[gmax137]

I think it appeals to me because I'm a visual learner. Things don't gel for me until I can draw myself a diagram. And that is just what Bondi's approach does.

This is a good one, too, by @robphy

https://www.physicsforums.com/insights/relativity-using-bondi-k-calculus/

 

[PeroK]

That looks helpful - thanks very much

You probably need to decide whether first to learn a bit about Special Relativity, time dilation and differential ageing (and not worry too much about the expanding universe); or, first to learn about the expanding universe (and not worry too much about astronauts on the ISS).

There is a significant difference between the popular science sources (even if they are produced by professional physicists or NASA) and these subjects as taught and studied at university. This forum is more focused on the "real" subject that is taught and studied than the "popular" interpretation.

One good example is that spacetime "fabric" being "stretched" is pure popular science terminology. No textbook on Relativity would talk about "fabric". There is no fabric. Instead there are the delights of differential geometry!

 

[Nugatory]

I understand that both clocks tick once second each second. But when they came back together their "clocks" no longer matched. The explanation was that gravity and acceleration are the same and they bend the space time fabric.

You've been reading some very misleading explanations.

Gravity and acceleration are not the same thing at all, and only gravity has anything to do with spacetime curvature (and even then, gravity doesn't curve spacetime, instead gravitational effects appear when spacetime is curved in certain ways) . The famous thought experiment in which we compare the experience of someone in a windowless compartment at rest on the surface of the Earth with their experience inside the same windowless compartment being accelerated through empty space at 1G does not show that acceleration is gravity, but rather that gravitational forces, like Coriolis and centrifugal forces, can be made to appear or disappear depending on what you choose to consider to be at rest.

since the guy in orbit was more "bent" than his twin on the ground his "clock" ticked more slowly.

Actually the person on the ground is moving through a more strongly curved spacetime, and their path is less straight. The orbiting clock ticks off fewer seconds for the same reason that my car's odometer ticks off fewer kilometers when I drive directly from point A to point B than when I take a longer detour through point C on the way - the orbiting clock has traveled along a shorter path through spacetime, the length of a path through spacetime is measured in seconds the same way that the length of a path through space is measured in kilometers, so there are fewer seconds along the shorter path.

At this point I suggest that you not take on gravitational time dilation until you have a solid grasp of how the Twin Paradox of special relativity is resolved, and are comfortable drawing Minkowski diagrams in flat (no curvature, no gravity) spacetime. A very good starting point is Taylor and Wheeler's "Spacetime Physics".

 

[Nugatory]

o what I was trying to convey is that if I were able to observe some time measurement somewhere else from where I am it would appear different. Know what I mean? IF time were stretched and I were able to able to observe that other clock in that other place it would appear to measure time differently than my clock.

Think about exactly what you're asking for when you speak of observing the other clock: You are making an observation that permits you to say something along the lines of "At the same time that my wristwatch reads X the other clock reads Y". For example, if the remote clock is ten light-minutes away and at 3:10 PM you observe (really powerful telescope?) that it reads 2:30, you will conclude that the remote clock read 2:30 PM at the same time that your clock read 3:00 - the remote clock was 30 minutes behind yours and then it took ten minutes for the light to get to you.

The key phrase here is "at the same time". Observers who are in motion relative to one another will not agree about which spatially separated events happen at the same time; this is the relativity of simultaneity (google for "Einstein train simultaneity"), absolutely essential for understanding relativity.

We can compare how much time passed for two clocks that start together, separate and follow different paths through spacetime, then come back together because they're colocated when we compare them.

But anything you say about whether one clock is running slower than anpother when they're not colocated is going to depend on an "at the same time" assertion. Your clock reads 3:00 at the same time that the remote clock reads 3:00; later, at the same time that your clock reads 3:40, the remote clock reads 3:20; you will be tempted to say that the remote clock is running slow and measuring time differently from yours. But that's not what's going on. Instead, you're using a definition of at the same time that makes it come out that way. In fact, if the two clocks are moving relative to one another, someone at rest relative to the remote clock will find that at the same time the remote clock read 3:20 your clock read 3:10 so yours is the slow one. They're both right.

 

[Snickers]

This seems to be contrary to what some of the other posters in this discussion have been saying. I would like some others responding to help clarify what you suggest. Can you expand on your thoughts with an example? Thanks.

 

[jbriggs444]

This seems to be contrary to what some of the other posters in this discussion have been saying. I would like some others responding to help clarify what you suggest. Can you expand on your thoughts with an example? Thanks.

There is no conflict.

@sweet springs is suggesting using different coordinates so that instead of thinking in terms of an "expanding universe" we can think about a "slowing universe" where cosmological distances are constant but where time is slowing down.

But it is just a game played with coordinates. It changes nothing physical. Every possible experiment gives the same physical answers no matter what coordinate labels you choose to stick on events. The elapsed time measured by an atomic clock going from a place like "the birth of the sun" to "the first man on the moon" is going to be a fixed value, no matter what coordinate values you choose to place on those two events.

 

[phyzguy]

What NASA saw for the two twins was a difference in biological age. caused by the different environments the two twins experienced. It has nothing to do with time dilation, which, as has been pointed out, was less than 1 second for the time spent in orbit

 

[nitsuj]

... started to think my thinking of space time was way off.

For Einstein too! The "visual" I came across is envisioning a long (one light second maybe) "perfectly rigid" pole/bar next to Earth (inertial/freefall). the curvature of the pole/bar compared to the path of a photon closely beside it are different.