Do different frames of reference experience time dilation at the same rate?

[kweagle]

..in your reference frame the impact and the sound generation are simultaneous. In other words, you must take the delay in the respective observations of light and sound into account when deciding when something happened.

Exactly. I understand that perfectly, but it seems to contradict some of the notions about relativity and time dilation. Perhaps it is because I do not understand it fully.

If two observers are at rest with respect to each other and a light year apart, and they exchange signals, then these signals take a year before they are observed, but neither observer is "in the past" with respect to the other.

So what you are saying is that a moment in time is the same for everyone no matter where you are, correct? Then how is it that time can pass slower for someone traveling compared to someone stationery? Would that not mean they would be in different moments in time? Again, I understand they will perceive things differently relative to each other due to the speed of light, but they must be in the same moment in the universe.

In this case, if he travels to a point 1 light year from Earth ( as measured by the Earth), he will see the Earth age 3.156e-8 sec during the 1.411 sec he measures for the trip. On the return leg he will see the Earth age 63103756.7084 sec while he ages another 1.411 sec. Total amount he ages 2.822 sec, total time he sees the Earth age, just a tad over 2 years.

So even though it takes ~1 light year to reach the destination, he is unable to observe anything during that time? Why is that?

 

[PeroK]

Exactly. I understand that perfectly, but it seems to contradict some of the notions about relativity and time dilation. Perhaps it is because I do not understand it fully.

It contradicts nothing about relativity, as the finiteness of the speed of light delaying observations is not a factor in relativity. And, yes, that's because you don't yet understand it fully.

So what you are saying is that a moment in time is the same for everyone no matter where you are, correct? Then how is it that time can pass slower for someone traveling compared to someone stationery? Would that not mean they would be in different moments in time? Again, I understand they will perceive things differently relative to each other due to the speed of light, but they must be in the same moment in the universe.

Not at all. The critical difference between relativity and classical physics is when you have two reference frames moving with respect to each other. If you have two observers at rest with respect to each other, then they will agree on measurements of length and elapsed time.

But, if the two observers are moving with respect to each other, then they will not agree on lengths and elapsed times.

Notice that I say "elapsed time", as the concept of an absolute "moment" in time is essentially meaningless. Elapsed time as measured between two events is the vital concept.

 

[Ibix]

With all due respect, how is "instantaneously" not clear?

It means "you depart and arrive simultaneously". But "simultaneously" is only an unambiguous concept if there is an absolute universal definition of time - which is not the case in relativity.

I know it is impossible, but we are talking hypothetically to make it easier to explain what I am asking. I could rephrase the same question to ask what does an observer 1 light year away from the Earth see? They see what Earth looked like 1 year ago, right?

Typically, we would say yes, what you are seeing now is what happened one year ago. But in doing so we would have implicitly chosen to use Einstein's definition of simultaneity. It's the most natural one in this circumstance and perfectly reasonable, but it's just a choice. That's why I was criticising your teleportation - because "simultaneous" (even using Einstein's convention) is different for observers in relative motion. Some people would say your "instantaneous" transit took time to complete; some would say it took negative time to complete.

Relating that to you sitting a light year from Earth looking at it through a telescope, someone traveling at 0.866c would say that the Earth is only half a light year from you, so you are observing it six months ago. Such an observer would have a coherent explanation in terms of your clocks being set differently from the ones on Earth (because his definition of "simultaneous" is different from yours) for why you are seeing the calendars showing a year ago, even if it's only six light months to Earth in his frame (6 months' travel time plus six months offset).

Again, I know traveling at the speed of light is impossible. If it makes the question any more acceptable, let's say you are a photon.

How is the photon going to measure time or distance? It can't build rulers or clocks.

I'm not just being glib here - the operational definition of time is "whatever it is clocks are measuring". Since you can't have a clock moving at light speed, you can't define time at the speed of light. That's a kind of mirror version of my earlier argument. I like the first one better because it's more or less a direct statement of the relevant truth: it makes no sense to talk about the perspective of anything traveling at the speed of light. You will only get nonsense if you try.

 

[Stephanus]

What do you mean by 'time changes with the speed of light' ? Whose time ? It is important to realize that every clock measures its own passage through spacetime.

I think what kweagle meant is this. If you don't have a measuring time device and you only have sand clock. How can we determine how much grain of sands we need to measure 1 second. So all you have to do is put a mirror at 150000 km away and send a light and also put some sands in the sand clock. And when the light comes back it is how 1 second is. Or you want to calibrate your other time measure device, is through light and distance.

 

[Stephanus]

It is not possible to travel at the speed of light. Rephrase the question in terms of sub-light speed travel and the answer is 'the person whose worldline had the shorter proper time will age less.'.

The longer worldline you mean? Come on, it's you who taught me space time diagram.

 

[Stephanus]

It is not possible to travel at the speed of light. Rephrase the question in terms of sub-light speed travel and the answer is 'the person whose worldline had the shorter proper time will age less.'.

Sorry, you're right. The shorter proper time = the longer worldline.

 

[Stephanus]

Ok... Let me rephrase some of that.

Lets say someone is able to teleport instantaneously from Earth to 1 light year from earth. If they look at earth, they will only see the light from the Earth from one year ago, would you consider that person to have traveled a year into the past?/QUOTE]
If I may rephrase your question.
Not teleported. But two person. A on earth, B 1 light year away from earth. At the same time B will see the clock on Earth is 1 year late. On the other hand, A will see B clock is 1 year late also.
Let's say if there's an astronout on the moon. The astronout would see the Earth as it was 1.3 seconds ago. But we see the moon as it was 1.3 seconds ago, too. It doesn't mean that from the moon we are 1.3 sec in the past and vice versa.

 

[Stephanus]

Lets say someone leaves Earth traveling at the speed of light for one year. While they are moving, it will appear time has stopped on earth. If they return to Earth at the speed of light, will time on Earth appear to move twice as fast? Two years will have passed on earth, but no time would have passed for the person who left? Even though they are able to see time passing on earth?

You can work it out fro any speed up to, but not including c, Say for instance 0.999999999999999 c.

In this case, if he travels to a point 1 light year from Earth ( as measured by the Earth), he will see the Earth age 3.156e-8 sec during the 1.411 sec he measures for the trip. On the return leg he will see the Earth age 63103756.7084 sec while he ages another 1.411 sec. Total amount he ages 2.822 sec, total time he sees the Earth age, just a tad over 2 years.

I think the answer of your question is this...
$63103756.7084 / 1.411 = 44,722,719$ Not twice as fast, but 44 millions faster. It's just a simple math

 

[Stephanus]

So what you are saying is that a moment in time is the same for everyone no matter where you are, correct? Then how is it that time can pass slower for someone traveling compared to someone stationery? Would that not mean they would be in different moments in time? Again, I understand they will perceive things differently relative to each other due to the speed of light, but they must be in the same moment in the universe.

Perhaps this can answer your question...

Consider Einstein's Train example...

https://www.physicsforums.com/threads/length-contraction.817911/page-2#post-5135255

 

[Stephanus]

So even though it takes ~1 light year to reach the destination, he is unable to observe anything during that time? Why is that?

Oh, he is ABLE observers anything, all right. During his return trips, he is ABLE to see everything on Earth moves 44 millions times faster! But on away trip, he is ABLE to see everything on Earth much slower. How much slower? I think it is $\frac{1}{1.411}$ slower. And when he stops, he'll see everything on Earth moves normally. During the return trip -> 44 millions faster.

 

[PeterDonis]

The shorter proper time = the longer worldline.

No, the shorter proper time = the shorter worldline, hence less aging. The amount of proper time, the amount of aging, and the length of the (timelike) worldline are all just different ways of describing the same thing.

 

[PeterDonis]

Gravity affects physical process, that is intuitively correct I think. I read somewhere that giving birth in space is impossible if not difficult. The baby can't orient his/her head to birth canal. Is just one example.

This is an example of proper acceleration affecting physical processes, not gravity. Giving birth in a rocket accelerating at 1 g would work the same as giving birth in a room at rest on the surface of the Earth. But giving birth in a rocket in free fall in space would have the difficulty you describe. That difference has nothing to do with gravity; it has to do with the presence or absence of proper acceleration.

 

[A.T.]

...impossible if not difficult

I like that.

 

[harrylin]

[..] First let me explain what my personal concept of time has always been. To me, 'time' is a constant. It is linear, and it always passes at the same rate, no matter where you are, what you are doing, or what is around you, time never stops or slows down. What does change is what you are able to observe happening in time as a result of the time it takes for light to travel. Time keeping, however, is something developed by humans as a way to measure the passage of time. It has always been my thought that it was the measuring devices (the physical/quantum world) that is affected by speed/acceleration and gravity. I accept that this view is not correct, which is why I am here to try and understand it.
Relativity does make perfect sense to me, but it is the time dilation that does not. It seems to me that time dilation is simply a result of the time it takes light to travel based on relative speeds of two objects, therefore, I think a more accurate name would be 'light dilation'.
[..]
If the speed of light is considered to be a constant speed, no matter who is observing it, and no matter what velocity they are traveling or what gravitational forces they may be experiencing, would it be accurate to say that time as we know it in physics is derived directly from the speed of light, lightspeed being the base line for how time is measured, and this is why time changes with the speed of light?

Instead, all physical processes are affected the same - the speed of light was merely a known fact of observation that was used as input to the derivation (and perhaps you overlook that the estimated travel time is accounted for in the calculations).

This is just as when you have an equation 2x + c = 5 and you know that the constant c equals 3. With that information you can solve for the correct value for x, but that doesn't mean that x is physically a result of c!

If the speed of light is constant, why do we observe a redshift in stars that are moving away from us? Shouldn't they appear to look the same no matter how they are moving if light is always traveling at a constant speed?

The "constant speed of light" means that the speed of light is a constant, independent of the speed of the source - just like the speed of sound in air at standard conditions. The Doppler effect is well known with sound.

This is often confounded with the invariance of the speed of light: standard inertial reference systems that are in relative motion will each measure the speed of light as the same constant (this follows from the relativity principle). Those two facts of observation seem in contradiction to each other.
This is well presented in the intro here: https://www.fourmilab.ch/etexts/einstein/specrel/www/

If two people left Earth traveling at near the speed of light, one going in a straight line and back, and the other simply orbiting the earth, both returning to the surface at the same moment, would time have passed the same amount for both of them when they got back? How much time would have passed for them and how much time would have passed on earth?

In his 1905 paper to which I gave you the link, Einstein gave a prediction that a clock that is moved in a circle will be slow compared with a clock that stays in place. The answer to your questions are very well explained there (in the second half of §4): it makes no difference at all.

Similarly, Langevin gave in 1911 an example of an astronaut who travels fast to a nearby star and back; upon arrival on Earth he may find that all his friends already died of old age.

 

[Janus]

Thank you everyone for the replies, I really appreciate it. I have been scouring the web trying to learn more about time and what it really is, and I think I am starting to get some of the theories a bit more. I do have some follow-up comments/questions if you don't mind.

First let me explain what my personal concept of time has always been. To me, 'time' is a constant. It is linear, and it always passes at the same rate, no matter where you are, what you are doing, or what is around you, time never stops or slows down. What does change is what you are able to observe happening in time as a result of the time it takes for light to travel. Time keeping, however, is something developed by humans as a way to measure the passage of time. It has always been my thought that it was the measuring devices (the physical/quantum world) that is affected by speed/acceleration and gravity. I accept that this view is not correct, which is why I am here to try and understand it.

Relativity does make perfect sense to me, but it is the time dilation that does not. It seems to me that time dilation is simply a result of the time it takes light to travel based on relative speeds of two objects, therefore, I think a more accurate name would be 'light dilation'.

With that being said, here are my questions...

If the speed of light is considered to be a constant speed, no matter who is observing it, and no matter what velocity they are traveling or what gravitational forces they may be experiencing, would it be accurate to say that time as we know it in physics is derived directly from the speed of light, lightspeed being the base line for how time is measured, and this is why time changes with the speed of light?

I'm going to try and paint a picture of what is going on through an analogy.
Imagine two men( M1 and M2) walking side by side on a featureless plane. Without changing his pace, one of the men(M2) changes the direction in which he is walking. Each man judges forward progress as progress in the direction he is facing. So from each Man's perspective, the other man is now making less forward progress (even though by the other man's judgement nothing has changed and he is still walking at the same rate as before.), and falls further and further "behind".
Now M2 changes direction again, turning so that he is walking in the same direction as M1. What does he perceive? As he turns, the M1's apparent position with respect to the direction M2 is facing changes and goes from being "behind" M2 to Being "ahead" of M2, as judged by M2. ( as far as M1 is concerned, M2 remains behind him). Both men are now walking in the same direction, and their relative positions are constant and bot Men agree that M1 is ahead of M2.

If M2 continues his turn until his forward path intersects M1's path, As he turns, M1 will move a bit more ahead of him. After the turn, M1 will be ahead of him, but now not progressing "forward" as quickly. M2 will start to gain on him a bit. However, by the time he intersects M1's path, he will not have caught up to him. If he then turns to match M1's walking direction he will still find himself behind M1.

The above analogy is like the twin paradox, While moving at different speeds (walking in different directions), Time in the other frame runs slow ( the other man makes less forward progress). If a spaceship travels away from the Earth at some speed, turns around and comes back, (M2 changes direction so that he returns to the path of M1), It will find that it has aged less than the Earth has. (M2 finds himself behind M1)

In the analogy I gave, there is no universal fixed direction to which the concept of "forward progress" can be applied. each man has his own measurement of forward progress. In Relativity, there is no universal fixed "direction" to time measurement. Put another way, instead of a universe with 3 spatial dimensions and 1 time dimension which are fixed and separate from each other, we have a universe of 4 dimensional Space-time, where the "direction" for time depends on the relative motion's of the frame measuring it. Two observers with relative motion with respect to each other will judge the position of an event in time and space differently. Observer 1 could say that two events are separated by x meters and y sec, while Observer 2, with a relative motion with respect to Observer 1 would judge the same two events as being separated by w meters and z sec. Much as the way that two people facing in different directions would disagree as to how far to the right or left and how far to the front or back two objects are from each other.

The speed of light comes in in that it determines just how these differences perspective relate to each other. ( or conversely, the relationship between space and time determines what the speed of light is.)

If the speed of light is constant, why do we observe a redshift in stars that are moving away from us? Shouldn't they appear to look the same no matter how they are moving if light is always traveling at a constant speed?

I've heard this question a lot, and I've never quite understood the reasoning behind it. Doppler shift is not a result of how fast the light is traveling with respect to you, but how closely the Light waves are bunched together according to the observer.
Consider the following animation. In the first, the light source is stationary wit respect to the observers. The waves move out as circles, and hit both observers at the same frequency

In the second one, the source is moving towards the blue dot and away from the red.

The waves still move out as circles and at the same speed as they did before. ( the first wave even hits both observers at the same time, just as in the last animation.) However, as each successive wave is emitted, the source is closer to the blue dot than it is the red dot and the waves are more crowded together in this direction and more spread out in the other the other direction than they were in the first animation. The blue dot sees an Increase in frequency, and the red dot a decrease in frequency.

If two people left Earth traveling at near the speed of light, one going in a straight line and back, and the other simply orbiting the earth, both returning to the surface at the same moment, would time have passed the same amount for both of them when they got back? How much time would have passed for them and how much time would have passed on earth?

As long as we ignore gravitational effects and assume that the orbital speed and "straight line" speeds are the same, Then both people will have aged the same and less than someone on the Earth. The main difference is that the straight line traveler does all his acceleration turning the turn around while the "orbiting" traveler is under constant acceleration. (he wouldn't be in a true orbit as he would have to be constantly firing his engines inward towards the Earth in order to maintain a circular motion around it at such a high speed. )