1. Apr 17, 2006

### UglyEd

I know that the faster something moves the slower relatively it experiences time. As in if i had a twin brother and at birth he was put into a spaceship traveling at half the speed of light, and I stayed on Earth moving alot slower. When I reached 30yrs old. My brother would have only aged 15yrs? Right. If so is my brother really 30yrs old?

Last edited: Apr 17, 2006
2. Apr 17, 2006

### -Job-

How do you know which twin is moving at c/2? All we know is that there is a difference between the velocities of the twins relative to each other, so which should get older and which should get younger?
The twin in the spaceship is in an environment moving at c/2 relative to the other twin at home. However, for this twin in the spaceship light still moves past him at c, and his experience inside the spaceship is the same, some objects in the spaceship move slightly slower/faster than him, and his cell metabolism is carried out at the same rate as otherwise so he would age as fast, relative to his surroundings, as the twin on earth. Their perception of time wouldn't be different i believe, and they both would age at the same rate relative to their immediate surroundings. I guess i don't really understand the twin paradox.
What's leading me to this conclusion is the fact that, biologically speaking, it's the interaction between each cell with its environment that produces the aging (if you're frozen then you won't age, no matter how fast you're travelling). From this perspective then, the aging of an organism is dictated by the degree of interaction with itself & the environment. As long as organism 1 is interacting with itself and its environment at the same rate as organism 2 is interacting with itself and its environment, then they both will age at the same rate, even if the environments themselves are moving through space at very different velocitites. So i think the rate of aging is dictated "locally" if that makes any sense.

Last edited: Apr 17, 2006
3. Apr 18, 2006

### pervect

Staff Emeritus
Your twin brother would have to travel at .866 c, not the speed of light, to age at half the normal rate.

Your question as-is is rather philosophical. We can say for certain that any clock your brother carried on his journey would read 15 years, and that the physiological changes in your brothers body would be consistent with a 15 year journey through space.

4. Apr 18, 2006

### -Job-

If two spaceships A & B, each carrying a twin, start at the same point, move away from eachother at .866c, then at some point move towards eachother, again at .866c, then, when they meet again:
1. is A physiologically older than B?
2. is B physiologically older than A?
3. are A and B, physiologically, the same age?
4. we can't predict, need more information (i.e. the speed at which each spaceship is moving)

5. Apr 18, 2006

### yogi

If they both accelerate at the same rate for the same duration, but in opposite directions, and they similarily turn around under the same local deceleration dynamics, things are symmetrical and both will be the same age, notwithstanding the fact that each will measure the clock on board the other space ship to be out of sync during the exercise

6. Apr 18, 2006

### UglyEd

This is totally hypothetical, but what if at the start of the universe there were 2 clocks. One clock was launched at and continued to move close to the speed of light. The other clock remained totally stationary. Then after a long period of time, the time on the 2 clocks were checked. Of course the clocked that had been moving would show less time had passed. Which clock though would be more accurate in showing the true age or amount of time the universe existed?

7. Apr 18, 2006

### Giulio B.

I'm not a physician (not yet) but from the small lines of relativity i know i can tell you that there is not a favourite referring system...at least you can turn the question in: -what is the best way to calculate age of universe relatively to us- , at this question i would suggest -the clock that was moveing at our speed-

8. Apr 18, 2006

### ZapperZ

Staff Emeritus
Er... remain stationary to what? And the one that is moving close to the speed of light, it is moving with respect to what?

Zz.

9. Apr 18, 2006

### JesseM

Presumably you mean that both clocks are checked at the same moment? The problem with this is that different reference frames in relativity define simultaneity differently, meaning that two events that happen at the same time-coordinate in one frame happened at different time coordinates in another. So in one frame the event of clock A ticking 10 o'clock could happen "at the same time" as the event of clock B ticking 11 o'clock, meaning that in this frame clock A ticked slower, but in another frame the event of clock A ticking 10 o'clock could happen "at the same time" as the event of clock B ticking 9 o'clock, meaning that in this frame clock B ticked slower.

10. Apr 18, 2006

### Giulio B.

what about if the 2 clocks at the moment of measuring would be stiked together (maybe for a strange nonlinear geometry of the universe...that allowed them to meet after that time and that distance)
however in that case we could measure the 2 events like almost simoultaneous right?

Last edited by a moderator: Apr 18, 2006
11. Apr 18, 2006

### JesseM

Yes, if the clocks are in the same location then all reference frames will agree on which one is behind. But if they were originally flying apart as UglyEd said, then for them to meet again at least one would have to accelerate in order to turn around (which involves changing velocity), so then you have another version of the twin paradox, where it will always be the clock that accelerated that's behind when they meet. You mention the geometry of the universe, and it's true that it is possible for two clocks to fly apart and meet again without either one accelerating if spacetime has an unusual geometry--in this case it'll depend on how their paths relate to the geometry of space, see this thread for a discussion of the issue.

12. Apr 18, 2006

### -Job-

The only thing that troubles me about the twin paradox is the question of why one twin would age faster than the other. I understand that only one twin is accelerated, but i thought acceleration, like velocity, was a relative concept. In this manner it seems that acceleration isn't relative but more like a property of matter, a hidden variable, so you can understand my confusion. Would a relative concept be responsible for a non-relative effect?

Last edited: Apr 18, 2006
13. Apr 18, 2006

### ZapperZ

Staff Emeritus
Acceleration isn't a relative concept. You can perform an experiment to distinguish if you're accelerating at 10 m/s^2, 20 m/s^2, 30 m/s^2, etc. Don't believe me? Try looking at the period of the same simple pendulum in each of these accelerating frame.

Zz.

14. Apr 18, 2006

### -Job-

I don't have a pendulum else i definitely would, in the meantime i'll take your word for it. I have a question though. I'm assuming that time slows down for an object only while the object is accelerating and that whenever the object stops accelerating and reaches a constant velocity, time stays "slow", otherwise if it didn't stay "slow" and i were to survive an acceleration to c taking place in less than a minute, then no matter how long i might travel at c, i would age at the same rate as a stationary individual because time would go from "slow" to "normal" since my acceleration would now be 0.
But this suggests i'm able to measure the acceleration that got me to my current velocity by examining the swing of my pendulum and determine the velocity that i am traveling at, and velocity is a relative concept so i don't believe it is determinable without a reference object, or no?

15. Apr 19, 2006

### JesseM

Just think of the fact that you experience G-forces when you accelerate, like how you're thrown forward when a car brakes, or pushed backwards into your seat when it speeds up (both are kinds of acceleration, since in physics 'acceleration' just means your velocity is changing, whether increasing or decreasing or just changing direction at constant speed). If you're in a capsule in space moving at constant velocity you'll be weightless, but if it accelerates, you'll feel these G-forces, you could measure them just by standing on a scale.
In any given inertial reference frame (inertial = not accelerating), the rate a clock is ticking depends only on the clock's speed at that moment in that frame. But since different frames see the same object having different frames, they also disagree about the rate a given clock is ticking. If you have two clocks A and B moving relative to one another, you can find one frame where A is moving faster and therefore ticking slower than B, and another frame where B is moving faster and ticking slower than A. Both frames are equally valid, there's no single truth about these things according to relativity.
You can certainly use your acceleration to figure out how much your velocity has changed in the frame you started out at rest in--for example, if your ship starts out docked to a space station and then accelerates away, you can figure out your speed relative to the space station by measuring your acceleration. But there's nothing that says you have to analyze the situation from the space station's frame, you could also analyze it from the point of view of a frame where the station and the docked ship were originally travelling at 0.8c, then the ship's acceleration caused it to come to rest while the station continued to move away at a constant velocity of 0.8c. In this frame, the ship's clock sped up during the acceleration, since its speed was decreasing, while the station's clock ticked at the same constant slowed-down rate.

16. Apr 19, 2006

### yogi

Acceleration per se is not what causes the time differences in the twin thing - or any other experiment - acceleration is a necessary circumstance that is involved in changing the velocity (speed) of one clock relative to another - when one clock is accelerated for a short time, and then then left to cruise for many years at a uniform velocty, the amount of time difference between the two clocks is determined by the time spent during the cruising period where the clocks traveled at a relative uniform velocity - the acceleration is required to develop a relative velocity - and it is the relative velocity between the two clocks that is dealt with in Special Relativity. Having said that, there are some who prefer to treat the age difference between the two twins as a problem in General Relativity - in which case extra pseudo gravitational fields are introduced to provide an accelrating turnaround frame

Last edited: Apr 19, 2006
17. Apr 19, 2006

### ZapperZ

Staff Emeritus
I think, before you move on to Special Relativity, you need to understand newtonian mechanics first. If not, all of us here have to double-back and keep explaining elementary stuff.

In kinematics, remember that you cannot perform ANY experiment to detect inertial motion, i.e. moving with constant velocity. If I put you in a box, and tell you to detect that you are moving with constant velocity, you can't. However, if I put you in the same box, and ask you to detect that you are moving with an acceleration, you can! And in fact, you CAN even detect if you are moving with one value of acceleration versus another. Forget about the simple pendulum, just hang a mass on a spring! The change in the equlibrium position of the mass on the spring from one acceleration value versus another is sufficient to tell you that.

So you cannot equate constant velocity and acceleration.

Zz.

18. Apr 19, 2006

### UglyEd

I mean as in. When ever the universe started. Say with th BB. One clock never moved. The other was launched out at near C. Like I said this is hypothetical.

Also is it true or do I have a misunderstanding? That if something moves at C time stops for it?

19. Apr 19, 2006

### ZapperZ

Staff Emeritus
I still don't understand. The BB isn't just an "explosion" into an already existing spacetime. It CREATES spacetime!

You cannot say something is stationary without any reference point. And once you think you can do that to an "absolute" reference point, you will have proposed a new hypothesis which isn't validated by physics.

Zz.

20. Apr 19, 2006

### whizmd

Most explanations of twin paradox invoke acceleration to explain time dilation. Although it is clear that accleration is not the cause of time dilation and the real cause is the velocity. Time dilation is calculated using velocity and not acceleration. The original time dialtion equation is totally independent of accleration and gives the precise amount of time dilation.

Another way to eliminate the role of acceleration is to have the twins both start the journey together. Both twin A and B can accelerate to near speed of light say in one week. After one week of accleration twin B immediately decelerates at the same rate and acclerates back to return home and then decelerates to stop. Twin A continues for a journey of 10 years and then does the deceleration accleration deceleration routine to return home after another 10 years. In this way the accleration is taking only 4 weeks total for each twin (acclerate decelerate accelerate decclerate one week each = 4 weeks) However it is exactly symetrical for both the twins. Only difference is that Twin A differes from Twin B in that he travels for 20 years at uniform velocity. Now you expect Twin A to be younger due to effect of time dilation, don't you? We have succesfully removed the effect of accleration (it is same for both the twins). So what causes time dilation then? Motion through space? I have the answer to that But I would like to see yours? I do really believe in time dialtion. This explanation also tells us that accelration in gravity is not the cause of time dilation in gravity as also populary believed.

It is well known to particle physicists that the accleration in cyclotrons do not explain the time dilation of particles but it is the velocity. So trying to explain twin paradox on accleration does not work. And trying to confuse the issue by using dropler effect is also wrong.
Good luck

Last edited: Apr 19, 2006