I Does time dilation cause different aging rates for people?

[richengle]

TL;DR Summary

if you could leave the planet, and wait a year in space... would you age much much faster than a person on earth because the earth was moving relative to you and you are out of earths gravity field [like a higher up clock]?

if you could leave the planet, and wait a year in space... would you age much much faster than a person on Earth because the Earth was moving relative to you and you are out of Earth's gravity field [like a higher up clock]?

 

[DaveC426913]

You're sort of mashing up special relativity (extreme velocities) and general relativity (extreme masses) but OK.

Yes. Astronauts age more slowly in the ISS than on Earth - by a few nanoseconds per year.

 

[PeroK]

Summary:: if you could leave the planet, and wait a year in space... would you age much much faster than a person on Earth because the Earth was moving relative to you and you are out of Earth's gravity field [like a higher up clock]?

if you could leave the planet, and wait a year in space... would you age much much faster than a person on Earth because the Earth was moving relative to you and you are out of Earth's gravity field [like a higher up clock]?

The effects of the Earth's gravity are very small in this respect, so that we are very close to moving inertially. Technically, someone could age more by getting further away from the Earth, but any difference in age would be small.

On the other hand, there is no limit to how much less you could age by taking a near-light-speed journey away from the Earth and back again.

 

[Nugatory]

if you could leave the planet, and wait a year in space... would you age much much faster than a person on Earth because the Earth was moving relative to you and you are out of Earth's gravity field [like a higher up clock]?

Gravitational time dilation would cause you to age slightly (not "much much") faster. Although very small, this effect is routinely measured with clocks on satellites - for example the GPS system wouldn't work if it didn't correct for gravitational time dilation.

The effect of relative velocity is a bit trickier. You are suggesting that space guy would age faster than Earth guy because Earth guy is at rest relative to him, but we could just as easily say that Earth guy is the one who is at rest while space guy is moving so Earth guy should be aging faster. This is the apparent paradox of velocity-based time dilation: A says that B's clock is running slow, B says that A's clock is running slow, and they're both right. The explanation is relativity of simultaneity - google for "Einstein train simultaneity" if you are not familiar with that concept, and then look for threads here about how relativity of simultaneity ties in with time dilation.

Also, do not confuse your thought experiment with the classic Twin Paradox in which the two observers separate and reunite so that all comparisons of age and time passed are done while they are at the same place at the same time. We have even more threads on that, but it's important to understand how your setup is different from the Twin Paradox.

 

[richengle]

The effects of the Earth's gravity are very small in this respect, so that we are very close to moving inertially. Technically, someone could age more by getting further away from the Earth, but any difference in age would be small.

On the other hand, there is no limit to how much less you could age by taking a near-light-speed journey away from the Earth and back again.

thanks, but moving away from earth, the clock should appear slower, and moving towards us, the clock should appear faster? In the end, it should cancel out? isn't dilation based on appearance? Like, there should be no preferred reference frame. the sun is moving thru the universe, that doesn't slow our clocks, right?

 

[Ibix]

isnt dilation based on appearance?

No. That's the Doppler effect. Time dilation is what's left over once you correct for the changing distance.

 

[Ibix]

the sun is moving thru the universe, that doesn't slow our clocks, right?

Slow them relative to what? Any clocks in inertial motion with respect to the Sun will see our clocks slowed and vice versa.

 

[PeroK]

isn't dilation based on appearance?

No, it isn't. It has nothing to do with the delay in light signals from an event reaching a specific observer.

 

[richengle]

Slow them relative to what? Any clocks in inertial motion with respect to the Sun will see our clocks slowed and vice versa.

exactly, so how can they say that if you go on a ship and move fast, your clocks actually slow... but like u said, each frame would see the other ones slower, right?

 

[Ibix]

exactly, so how can they say that if you go on a ship and move fast, your clocks actually slow... but like u said, each frame would see the other ones slower, right?

Who is "they"?

Note that there's a distinction between time dilation, which is a symmetrical effect, and diffetential aging, which is not. The latter is relevant for round trips - look up the twin paradox. In the case of a round trip, clocks will in general show different elapsed times.

It isn't entirely clear to me whether you are talking about gravitational time dilation, kinematic time dilation (as seen in inertial travel in flat spacetime), or differential aging (as seen in round trips). The circumstances are different.

 

[richengle]

Who is "they"?

Note that there's a distinction between time dilation, which is a symmetrical effect, and diffetential aging, which is not. The latter is relevant for round trips - look up the twin paradox. In the case of a round trip, clocks will in general show different elapsed times.

It isn't entirely clear to me whether you are talking about gravitational time dilation, kinematic time dilation (as seen in inertial travel in flat spacetime), or differential aging (as seen in round trips). The circumstances are different.

Thanks for the TWIN PARADOX ref. The history section explains it well. I wish i had the experiments that einstein mentions. but if there is no preferred ref frame, they should age the same. after all, light is the same for each frame, so they would age the same?

 

[PeroK]

exactly, so how can they say that if you go on a ship and move fast, your clocks actually slow... but like u said, each frame would see the other ones slower, right?

There are a number of sources that give that impression: that the one who accelerates away from Earth is "really" time dilated. But, that's wrong.

The key difference is the turnaround. In order for the traveller to return to Earth, they must turn round (or move in a circle). In either case, they are no longer moving inertially throughout the experiment. When they get back to Earth, their path through spacetime has been shorter than the path taken by those who stayed on Earth.

The Twin Paradox is really about differential ageing (the different lengths of spacetime paths) - and not, strictly speaking, about time dilation.

 

[Ibix]

I wish i had the experiments that einstein mentions.

I don't know what experiments you mean.

but if there is no preferred ref frame, they should age the same.

No. Time dilation is a coordinate effect and must be symmetric between inertial frames, yes. But the twin paradox is not about time dilation. It's about differential aging, which is not a coordinate effect. It is a single measurementn a direct comparison of co-located clocks. There is no room for ambiguity and there must be a single unambiguous answer which, in the case of the standard twin paradox setup, is that the traveller is younger.

In fact, it turns out that your clock measures the interval (the spacetime equivalent of distance) along your path. That they can end up with different ages if they follow different paths is then no more mysterious than the observation that two routes from A to B can have different lengths.

 

[richengle]

I don't know what experiments you mean.

No. Time dilation is a coordinate effect and must be symmetric between inertial frames, yes. But the twin paradox is not about time dilation. It's about differential aging, which is not a coordinate effect. It is a single measurementn a direct comparison of co-located clocks. There is no room for ambiguity and there must be a single unambiguous answer which, in the case of the standard twin paradox setup, is that the traveller is younger.

In fact, it turns out that your clock measures the interval (the spacetime equivalent of distance) along your path. That they can end up with different ages if they follow different paths is then no more mysterious than the observation that two routes from A to B can have different lengths.

like, people quote and use time dilation as if it really changes the second[ship] observers time... but isn't it just apparent ...

like i was reading Twin paradox - Wikipedia ... and it said that all aging difference happens on the way back, as listed in the section callled -A Non space time approach
=============
If, instead of incorporating Einstein's clock synchronization (lattice of clocks), the astronaut (outgoing and incoming) and the Earth-based party regularly update each other on the status of their clocks by way of sending radio signals (which travel at light speed), then all parties will note an incremental buildup of asymmetry in time-keeping, beginning at the "turn around" point. Prior to the "turn around", each party regards the other party's clock to be recording time differently from his own, but the noted difference is symmetrical between the two parties. After the "turn around", the noted differences are not symmetrical, and the asymmetry grows incrementally until the two parties are reunited. Upon finally reuniting, this asymmetry can be seen in the actual difference showing on the two reunited clocks
============
This seems to be like saying only on the way back would they truly age differently, right?

 

[PeroK]

like i was reading Twin paradox - Wikipedia ... and it said that all aging difference happens on the way back, as listed in the section callled -A Non space time approach
=============
If, instead of incorporating Einstein's clock synchronization (lattice of clocks), the astronaut (outgoing and incoming) and the Earth-based party regularly update each other on the status of their clocks by way of sending radio signals (which travel at light speed), then all parties will note an incremental buildup of asymmetry in time-keeping, beginning at the "turn around" point. Prior to the "turn around", each party regards the other party's clock to be recording time differently from his own, but the noted difference is symmetrical between the two parties. After the "turn around", the noted differences are not symmetrical, and the asymmetry grows incrementally until the two parties are reunited. Upon finally reuniting, this asymmetry can be seen in the actual difference showing on the two reunited clocks
============
This seems to be like saying only on the way back would they truly age differently, right?

Not really. The simplest geometric approach has no such asymmetric in the outbound and inbound journeys.

Using light signals is a good way to analyse the problem, but that introduces more concepts and complexities.
The main issue that the concept of what "age" someone is when they are not local to you and especially when they are moving relative to you is not so simple to define. Plus, the turnaround is not as symmetrical as it first appears, if you look at it more closely.

 

[richengle]

Not really. The simplest geometric approach has no such asymmetric in the outbound and inbound journeys.

Using light signals is a good way to analyse the problem, but that introduces more concepts and complexities.
The main issue that the concept of what "age" someone is when they are not local to you and especially when they are moving relative to you is not so simple to define. Plus, the turnaround is not as symmetrical as it first appears, if you look at it more closely.

Thanks pal. the tough part is that when i read the wiki article, there seems to be multiple views on it, interwoven, yet inconsistent. in one part they say its the acceleration that changes age; in another they say it is the velocity. which do you favor?

 

[phinds]

This seems to be like saying only on the way back would they truly age differently, right?

Thanks pal. the tough part is that when i read the wiki article, there seems to be multiple views on it, interwoven, yet inconsistent. in one part they say its the acceleration that changes age; in another they say it is the velocity. which do you favor?

The different ways of looking at the issue seem to me to be irrrelvant (and I also find them confusing). What matters is the space-time diagram and the fact that they take different paths through space-time thus leading to differential aging.

 

[PeroK]

Thanks pal. the tough part is that when i read the wiki article, there seems to be multiple views on it, interwoven, yet inconsistent. in one part they say its the acceleration that changes age; in another they say it is the velocity. which do you favor?

I personally favour the geometric approach, as that is the simplest.

In Euclidean geometry we have the triangle inequality, which means that the sum of two sides of a triangle is always longer than the third side.

In Relativity, spacetime has a hyperbolic geometry, where the sum of two sides of a triangle can be shorter(!) than the third. Study spacetime geometry and the twin paradox is trivial!

Apart from that, I think the important point is that if you do not move inertially (i.e. the traveling twin), then you cannot simply use the rules of SR as though you were traveling inertially. This is the mistake everyone makes. In short: analyse the problem from a suitable inertial frame and you have your answer.

If you are not satisifed with either of those, then you need to analyse things from the traveller's perspective and that's when things get a little complicated. A lot of people are not happy until they have an analysis from the traveller's non-inertial frame. And, the simplest way to do that (IMHO) is the light signal analysis.

 

[Ibix]

Thanks pal. the tough part is that when i read the wiki article, there seems to be multiple views on it, interwoven, yet inconsistent. in one part they say its the acceleration that changes age; in another they say it is the velocity. which do you favor?

There are different ways of explaining it. The fundamental truth is just that the "distance" along different paths is not necessarily the same.

That said, the velocity relative to some inertial frame is all you need to calculate the age difference. If one of the twins accelerated and the other didn't then you will always find that the one who accelerated is younger. That's not because of the acceleration, but because a worldline that includes acceleration is not straight, and in spacetime a straight line is the longest distance between two points.

 

[jbriggs444]

If you are not satisifed with either of those, then you need to analyse things from the traveller's perspective and that's when things get a little complicated. A lot of people are not happy until they have an analysis from the traveller's non-inertial frame. And, the simplest way to do that (IMHO) is the light signal analysis.

Count me as one of the people who was not comfortable with the Twin Paradox until I had an accounting for it from the non-inertial twin's perspective. The idea of a "hyper-plane of simultaneity sweeping forward along the inertial twin's world-line" resonated well.

With that understanding in hand, I could look at the geometric interpretation with fresh and accepting eyes and acknowledge that it is simpler.

 

[robphy]

Indeed, the geometrical approach has the result, clearly and unambiguously...in accordance with relativity.

From there,
one can find different ways of analyzing and interpreting the same diagram.

 

[Ibix]

With that understanding in hand, I could look at the geometric interpretation with fresh and accepting eyes and acknowledge that it is simpler.

It was the other way around for me. A former poster, @ghwellsjr, used to post spacetime diagrams in every twin paradox thread. Once I grokked them, all the rest fell into place.

There're loads of ways to explain it, and once you "get it" you can understand and explain it by any of them. But which explanation first clicks for you appears to be fairly personal. Reminds me of the Monty Hall problem in that respect.

 

[Nugatory]

Thanks pal. the tough part is that when i read the wiki article, there seems to be multiple views on it, interwoven, yet inconsistent. in one part they say its the acceleration that changes age; in another they say it is the velocity. which do you favor?

There’s an easy cure for that - try learning from a real textbook instead of Wikipedia,

 

[richengle]

The different ways of looking at the issue seem to me to be irrrelvant (and I also find them confusing). What matters is the space-time diagram and the fact that they take different paths through space-time thus leading to differential aging.

an interesting way of putting things. so one can only change so much stuff with respect to time. the stuff is xyz and time? but the relative Velocity shouldn't change anything, no... so is it you can't acellerate things, else you retard time?

 

[phinds]

an interesting way of putting things. so one can only change so much stuff with respect to time. the stuff is xyz and time? but the relative Velocity shouldn't change anything, no... so is it you can't acellerate things, else you retard time?

Well, "retard time" is certainly not the way I would put it since that makes it sound as though time is progressing at a different rate, which it is not. What I would say is "change the spacetime path" or something like that.

 

[Nugatory]

an interesting way of putting things. so one can only change so much stuff with respect to time. the stuff is xyz and time? but the relative Velocity shouldn't change anything, no... so is it you can't acellerate things, else you retard time?

No, you are never changing time. Both the stay-at-home twin and the traveller’s clocks tick once every second, as surely as the odometer of a car ticks off one kilometer for ever kilometer of road that passes under the wheels.

What’s happening in the twin paradox is that the two twins are following different paths through spacetime, and there are more seconds on one path than the other. It’s like two cars driving different routes to the same destination: if the different routes have different lengths their odometers will count off different distances having passed under their wheels, except that here we’re counting seconds on different paths through spacetime instead of kilometers on different routes through space.

The acceleration only comes in because we can’t send them on different paths without accelerating at least one of them - so you’ll always find some acceleration somewhere in any twin paradox scenario (there’s another way of doing this in GR, but let’s get the easier SR case down first).

 

[jartsa]

if you could leave the planet, and wait a year in space... would you age much much faster than a person on Earth because the Earth was moving relative to you and you are out of Earth's gravity field [like a higher up clock]?

So you and your clock stop traveling around inside the solar system. (Note the word 'year')

Well, that would allow your blood cells to travel around your body some more. And that would allow your clock's clock hand to travel around the clock face some more.

Not being in the Earth's gravity well ... Well I'm not saying whether that allows the clock hand to travel more times around the clock face or not. Because I always get this patrticular thing wrong.

 

[Herman Trivilino]

thanks, but moving away from earth, the clock should appear slower, and moving towards us, the clock should appear faster? In the end, it should cancel out?

Go to YouTube and search for Hewitt Twin Trip. The two effects do not cancel each other out because the speed of light is invariant.