# What is it about speed that affects time?

1. Apr 29, 2007

### Poita

We laymen know that high speed travel has an effect on the local time of the traveller. What exactly is it about the speed that does it though? I heard that it is not the actual high speed but the acceleration and deceleration that has the effect. That would mean that it is the change in speed rather than the speed its self. Is this true.

On a related note. I have a question about 'movement' its self.

Is there such a thing as movement? How can there be when the position of every thing is relative? All objects in space are on the move and there is no absolute still thing. Even the calculated center of the universe might be on the move. We don't know if the universe its self is still or moving (or even if that means anything)j.
For example. If i fly counter rotation around the earth at the speed of the earths rotation it could be said that I am not moving. then if i flew around the sun like wise i would be still from the point of view of the solar system. Then if I flew counter around the galaxy, the same. Then i'd have to fly counter around the cluster of galaxys then also inward towards the center of the universe at the exact speed the universe is expanding.
I know all this is actually impossible but what i'm trying to get at is, if i did all those perfectly could it be said that i'm perfectly still? But is there such a think as absolute movment or stillness seeing as everything is on the move?
If there is no such absolute then how can anything be said to be moving and are we not only changing position relative to another object. And if this is so then how can there be an absolute effect of time on us at certain speeds. For example. If there were triplets and two of them flew away from the earth at half the speed of light for ten years then turned around and came back but one flew in the outward direction the universe is expanding and the other flew inward then when they arrived back on earth (i know it would have moved its self by then) then would the time difference between the two travellers and the brother they left on earth be identicle or different?

2. Apr 29, 2007

### HallsofIvy

Staff Emeritus
No, it is not change in speed- it just speed itself. Although it would be more correct to say that the speed of one person relative to that of another affects how they perceive each others "time flow".

Yes position is relative and therefore movement is relative. There is no "absolute still thing" so we can only speak motion relative to some other thing. But that is motion in any realistic sense so, yes, there is motion!

It took me a while to understand what you are saying. I was thinking you really meant moving WITH the rotation of the earth so that you stayed in the same spot relative to the earth's surface, like geo-synchronous satellite. But you are doing the opposite- imagine in an railroad train moving east at the same rate the earth is rotating west. Then, in a sense, you could say you were not moving, but it wouldn't be a very important sense! All of the people around you on the earth would certainly say you were moving! Oh, and of course, the earth is moving around the sun so you would still be moving relative to the sun.

Again, I think you are using the phrase 'from the point of view' incorrectly. If you moved along with everything in the galaxy, then you would be stationary "from the point of view of the galaxy"- i.e. in its frame of reference- relative to the galaxy. If you are part of the galaxy but moving in such a way as to negate its "natural motion", you would be stationary with respect to some completely fictitious point. And what that point was would depend on what YOU thought the "natural motion" of the universe was!

What do you think "motion" means? It is precisely changing position relative to another object!

There can't be an "absolute" effect of time. All motion is relative. Now I see where you got the "acceleration" part from in your first paragraph.

If one of two twins moves away from earth at a constant speed, close to the speed of light and the two were able to observe one another, each of them would observe the other's motion as slowed- time would have slowed down. Of course, then the question "when they get back together, who is older?" doesn't arise because they can never get back together. I am informed that if the traveling twin then decellerates so he can come back, one can show that it will be the twin who has undergone the acceleration and decellertation that will be younger but I don't pretend to know how to calculate that.

Last edited by a moderator: Apr 29, 2007
3. Apr 29, 2007

### Staff: Mentor

Uniform motion (constant speed in a straight line) is relative, but changes in motion (speed or direction) are not. If one observer speeds up, slows down or changes direction, he feels the forces that cause that change; other observers do not.

4. Apr 29, 2007

### Santural

This answers quite a few of my questions.
This is what I infer:
So time is completely relative to another "point." There is no "grand cosmic time" and time can be measured only relative to another place.

5. Apr 29, 2007

### Poita

This is what i'm unsure about.
I already guessed that there is no such thing as absolute stillness and therefore movement its self is one big relative. So in a sense you never move you just alter your position in relation to the position of other things.
If all that is relative then how come the way time affects you is mesurable? I mean, to mesure something we need a standard. One year on Earth takes one year to pass. One year on earth takes one month to pass for a guy in a space ship that is travelling at a certain speed away from then back to Earth. the Brother left on earth aged one year and his sibling who travelled for two weeks away and two weeks return on his ship aged a month but there is not an eleven month time and age difference between them when they next meet. Surely this means that time is absolute acording to the laws of motion. Time is relative to your speed but always absolute to you yourself wherever you are.
Time is not the same for everyone but if you add space and motion into the calculation then it is isn't it? Kind of: one year of time plus no motion equals one year. One month of time plus extreme speed equals one year. Seen from that equation can't you say that time is indeed absolute.
You can not calculate and predict things that are not constant and although time is not constant in a relative way it is constant in it's none relavance. So as long as you factor in space and motion then time always behaves as the laws of physics dictate (at least on the none quantum level).
If this is correct then i was just wondering how this can be so when stillness and motion don't really exist in an absolute sense. If you can never really say anything is absolutly still or anything is independantly in motion and that they are all just 'considered' to be still or in motion depending on their relationship to other things then how is it that motion affects time in an absolute and predictable way?

6. Apr 29, 2007

### Santural

Before anything else, let me say I'm not a physicist by profession, and if I sound ignorant, correct me rather than ignore. I really might be wrong.
All I can say, however, is that you really can't say that the brother in the space ship traveled one earth year in one month. It would be one earth year in one earth month. However, we must measure that time relative to the speed the spaceship he is traveling in, and that would be one earth year, due to the speed.
So I personally think that time can't be measured in earth units when not on earth.

Sorry if I'm off, and do correct me if I am.

Regards,
Santural.

7. Apr 29, 2007

### Poita

Well, when they meet again and compare atomic clocks or whatever then as Mother earth was the starting point they will use Earth time as the base and compare everything to that. so the earth clock will say one year has passed adn the brother in the space ship's clock will show one month to have passed. Looking at them side by side that's what they will show.
You can describe it any way you want of course. You can say that time on earth slowed down as time on the ship was nomal or time on the ship speeded up as time on earth was normal but when you just compare the difference you will seet the more time passed on the earth bound clock than passed on the space ship clock.

8. Apr 29, 2007

### Santural

Ok, what you're doing here is using earth time as time for the space ship (base time), which you simply can't, because time on earth does not apply for every journey. The two clocks will read different, because they don't account for the geometry of the journey, or the speed. Let me quote a reply I got on a similar question:
Once again, I suggest you check this out with prevect or someone who knows more than me, I'm just going as far as I know, forgive me if I'm wrong.
Hope that helps,
Regards,
Santural.

9. Apr 29, 2007

### JesseM

You can measure in units of the Earth's frame when not on Earth. In relativity an inertial frame is basically just a way of assigning coordinates to different events using an imaginary (although it could be constructed in principle) grid of rulers and clocks which fill space. So imagine a giant grid of rulers which are at rest with respect to the Earth, and at regular intervals along the rulers are placed clocks which are also at rest with respect to the Earth, and which have all been "synchronized" in the Earth's frame using something called the "Einstein synchronization convention" (basically the idea is that if you set off a flash at the midpoint of two clocks, if both clocks read the same time at the moment light hits them, they're defined as synchronized in their rest frame...however, if frames in motion relative to one another each use this convention to synchronize their own clocks, they will measure the clocks of the other frame to be out-of-sync.) Then for any event which occurs far from the Earth, you assign it position and time coordinates locally, by noting the position markings on the ruler right next to the event, and the time reading on a clock that was also right next to it as it happened.

For example, if you want to judge how fast a clock moving relative to the Earth is ticking in the Earth's rest frame, then you might notice the moving clock read T=1 second as it was passing right next to a clock which read t=1 sedond in the Earth's ruler/clock grid, and then later you might see the moving clock reading T=2 seconds as it was passing next to a different clock in the Earth's ruler/clock system which read t=3 seconds at that moment, in which case you'd conclude that the moving clock was slowed down by a factor of 2 in the Earth's frame, since it only advanced by 1 second between events which were 2 seconds apart according the the Earth's rulers and clocks.

10. Apr 29, 2007

### Poita

Once all the clocks that have left earth return to earth then they can compare themselfs with the clock on earth. It doesnt actually matter if the earth clock is or isn't a universal standard as the purpose of the comparison of the two clocls that left earth and returned is to see how they differ in the way in which they compare to the earth clock.

My question still stands. Is there such a thing as movement or stillness in an absolute sense? If there is not then how is it that time can be affected by movement? If you can not be said to be moving or still in reality and just changing position relative to other objects then how is it that what you do can affect time?
If i fly towars the center of the expanding galaxy at the exact speed it is expanding then i am basically not moving at all. Then why is it that time will be speeded up with me relative to soemone on Earth. It's the guy on Earth who is actually moving with the rotating galaxy that is expanding outward with the universe and I who am still. Yet the illusion is that i am flying throgh space and the Earth is still.
If all movement is just relative and an illusion then why does it have an absolute and mesureable effect on time?

11. Apr 29, 2007

### Hurkyl

Staff Emeritus
The relationship between speed, coordinate duration, and proper duration is the same as the relationship between direction, how far North you went, and distance travelled.

Last edited: Apr 29, 2007
12. Apr 29, 2007

### JesseM

There is no such thing as absolute velocity, so if two people are moving apart at constant speed in a straight line there's no truth about who's really still and who's really moving, but acceleration (change of velocity) is absolute in SR.
As long as two clocks are moving apart at constant velocity, there is no absolute truth about which one is really ticking slower, either. In each clock's own rest frame, the other clock is the one that's ticking slower. But if one clock turns around to reunite with the other, then that clock accelerated, which is absolute; and when they meet, the clock that accelerated will have elapsed less time, an absolute truth they both agree on. For more on this, you could read up on the twin paradox.

13. Apr 29, 2007

### masudr

No. Changes in time are measured relative to paths in spacetime.

14. Apr 30, 2007

### rbj

it has to do with how we observe the other's time. this is because each of us observe the speed of light as the same, even if we are moving relative to each other and we're looking at the very same beam of light.

i am observer A and i am watching some observer B whizzing past me with his "light clock" at some very fast speed. this light clock is a simple pair of mirrors spaced some distance (let's say 1 meter) apart with the light beam bouncing up and down between them. observer B sees the round trip distance the light travels as 2 meters, and if the mirrors are oriented so that the line connecting them is perpendicular to the direction that B is whizzing past me, there is no reason why i would see the spacing between the mirrors differently. but i see the distance that light has to travel to bounce between the two mirrors as longer. now if that beam of light got an extra boost in speed (from my perspective), because this B guy with the mirrors was flying by me, then maybe our clocks would tick at the same rate. but since we both see light moving at the same speed, and i see it moving a larger distance than observer B does, then i must think that Observer B's light clock is ticking more slowly than observer B thinks.

15. Apr 30, 2007

### Ookke

Observing/calculating the other's time in somewhat "academic" and not very important actually, I think.

Two persons in rockets could both observe the other's clock to be slow, but this has not much importance if they never meet again. They can really compare their clocks only if they meet again.

16. Apr 30, 2007

### Hurkyl

Staff Emeritus
A lot of people get it wrong; that's reason enough to make a big deal about it.

In coordinate Euclidean geometry, suppose we drew two nonparallel lines. Then, we
(a) Set up a coordinate system with the first line as the y-axis, and with the origin where the lines intersect. Measure the y-coordinate to distance from origin ratio for points on the second line.
(b) Set up a coordinate system with the second line as the y-axis, and with the origin where the lines intersect. Measure the y-coordinate to distance from origin ratio for points on the first line.

Do you think it important that a student of coordinate geometry should have absolutely no trouble with the fact that both of these ratios are less than 1?

If so, then why do you think it unimportant in Minkowski space?

Last edited: Apr 30, 2007
17. May 1, 2007

### Poita

you guys keep missing my point.
I'm not asking how each views each others time and speed while they are on the journey. If each fly in opposite directins away from each other then turn around and come back then they have both travelled the exact same distance and speed etc. I don't want to know how they see each other while they are travelling. I want to know about when they meet again at the original departure point.
They guy flying in the direction of the center of the universe at the same speed that the universe is expanding could be said to be effectivly standing still and not moving where the guy flying away fromt he earth at the same speed is really shifting.
Then when they turn back and meet in space where the earth used to be time should have effected both of them in identical ways because both ahve traveled the same and at the same speed. The third brother they left behind has aged a year for example but both of the travellers have aged just one month due to the effects of speed on time.
My questin is. If there is no such absolute as movement then why does one affect time and not the other. Seen from one point of view, the guy flying towards the center of the univers is not moving at all and has just effected a stationary position. Seen from another point of view he is traveling. Why does he age at the same rate as the out ward bound traveller when they meet up again and why do they and the earth bound guy age differently when niether standing still or traveling is not actually moving because there is no such thing as movement, only changing position relative to other objects?

18. May 1, 2007

### JesseM

The one that moved moved non-inertially (that accelerated) in order to turn around and approach the other after they had been moving apart will have aged less.
The universe doesn't have a center in modern cosmology, space is expanding so the distance between all the galaxies is increasing, but this looks the same no matter where you are. In any case, expanding space puts us in the realm of the curved spacetime of general relativity, while the twin paradox can be understood in the flat spacetime of special relativity.
If they both turn around and accelerate symmetrically towards each other, then yes, they will have aged the same amount. But in the twin paradox, it's assumed that one twin moves inertially--constant direction and speed--while the other accelerates to catch up with the first after they'd been flying apart for a while.
Again, there's no center of the universe, and if one twin accelerates, there is no valid "point of view" where he didn't accelerate, acceleration is absolute. He either feels G-forces, or he doesn't!
If one twin accelerates, there is no inertial frame where he was standing still the whole time. Although different inertial frames disagree about which twin was moving during different phases of the journey, they all agree about whether a given twin's velocity was constant, or if it changed sometime between the moment the twins departed and the moment they reunited.

Draw two points on a piece of paper, and then draw two paths between them, one that goes straight from one point to the other, and the other that has a bend in it. Now, if you draw a coordinate system with an x axis and a y axis, you can define the slope of each line at different points, and you have a choice to orient your axes at different angles, with the slopes being different in different coordinate systems. But all coordinate systems will agree that the straight-line path between the two points had a constant slope, and that the bent path had a changing slope. And if you do an integral to calculate the length of the paths in different coordinate systems, you'll always get the same answer for the total length, finding that the straight-line path was shorter than the bent path. It's basically the same idea for different paths through spacetime in SR, with constant velocity being analogous to constant slope, and the "proper time" of a path through spacetime (the time measured by a clock that follows that path) being analogous to the length of a path drawn on a piece of paper. Just as the geometry of the 2D paper ensures that the straight line path always has a shorter distance than any non-straight path between the points, so the geometry of spacetime ensures that a straight path through spacetime always has a greater proper time than any non-straight path in SR.

19. May 2, 2007

### Poita

So my earlier question about whether it was speed or acceleration that affects time is answered? So acceleration affects time and not speed? does decelleration affect time too and in the same way or conversely?

It's interesting that there is no absolute definition of movement but there is an absolute definition of changing location.
Also, if there is no absolute definition of speed or movement then how can a change in something (ie acceleration and deceleration) that has no absolute definition be messured?

The faster you move the more mass you aquire. Does this mean that it is simply mass that affects time and that speed or acceleration facilitate this cause rather than are the cause in themselfs?. Time passes slower the greater the mass. In a particle accelerator is the speed of the electrons that orbit the nublia of the accelerated particles slower than their stationry counter parts due to the accelrated particles speed and acceleration or is it unchanged?

20. May 2, 2007

### JesseM

It is the shape of the entire path through spacetime that determines how much time elapses on a clock that follows that path. But as I said, the geometry of spacetime is such that a path with one or more bends (accelerations) in it will always have less proper time than a straight (constant velocity) path between the same two points in spacetime.

In ordinary 2D geometry, it is clear that a straight line between two points will always have a shorter length than a bent path between the same to points. And yet it wouldn't be accurate to say that all the extra length accrues in the bend itself.
See above, that's too simplistic. In fact, if you have a ship following non-straight path through spacetime that's made up of a bunch of straight line (constant velocity) segments joined by instantaneous accelerations, you can figure out the total time elapsed on the path by picking a particular frame, finding the velocity v for each segment and the time t between the beginning and end of that segment in your frame, and adding $$t*\sqrt{1 - v^2/c^2}$$ for each segment to find the total time elapsed on the ship's clock (this will work out the same even if you pick different reference frames where the v and t for each segment are different). The acceleration is only important insofar as it allows the ship to shift velocities as it travels along, but you can see that the actual time elapsed is just a function of the velocities of each segment and the time of each segment.
Since there is no absolute velocity in relativity, there's no absolute notion of whether an accelerating object's speed is increasing or decreasing, and physicists use the term "acceleration" to mean any change in velocity.
An absolute definition of changing velocity, not location (for any period of acceleration, you can find a frame where the location at the beginning of the acceleration is the same as the location at the end, although it will have changed in between).
By measuring the G-forces, like if you're in an accelerating car and you feel pushed back into your seat.
Only relative to a particular frame, there is no absolute definition of your "relativistic mass"...in your own rest frame your "rest mass" is constant.
In general relativity there is gravitational time dilation, but the difference between the rate of ticking of a clock with a 2-pound rest mass and a clock with a 1-pound rest mass would be miniscule, whereas a clock with a 1-pound rest mass would have to travel at 0.866c to have a 2-pound relativistic mass, and at this speed it would be ticking twice as slowly. So it doesn't really make any sense to "explain" velocity-based time dilation in terms of changes in mass.
I assume you meant "nucleus"? It's just atoms that have a nucleus with orbiting electrons, and in quantum physics "orbits" don't really involve circling around like a planet, the uncertainty principle means you can't really pinpoint the electron's position and velocity too precisely. But any time-based property of a particle, like how quickly it decays, will be stretched out if you get it moving at relativistic speed in a particle accelerator.