# Time Dilation Arguement

1. Jan 20, 2012

### apinkpwny

Me and a few friend are currently having a large argument whether time is actually changing when a shuttle is moving near the speed of light.

so the scenario is a shuttle moves near the speed of light for 100 years. with a clock set to 0:00:00 and a person on earth also has a clock at 0:00:00.

after 100years the shuttle stops at the stationary person, would their clocks read the same time or will the person on the shuttle be younger and have a clock with a lesser time recorded.

2. Jan 20, 2012

### ghwellsjr

You didn't explain how they started out but if you meant that the shuttle left earth when both clocks read zero, then a hundred years later when the shuttle stops at the stationary person, their clocks will not read the same time, the person on the shuttle will be younger and have a clock with a lesser time recorded.

3. Jan 20, 2012

### apinkpwny

this is exactly what i meant thank you!one of us feels that as soon as the person traveling at a high speed comes to rest the clock will snap back to the stationary person's time.

if you could please explain in a simple way why one of us is wrong i would be much appreciated.

4. Jan 20, 2012

### Jobey

But time itself would not actually change? correct? It would just appear that it has changed.

5. Jan 20, 2012

### Symbiosis

Time relative to the person on the shuttle appears normal, just as time standing still on Earth appears normal. My question would be: is the OBSERVED time less and not the ACTUAL time?

How does the actual time change when it's relative to the observer (guy on the ground or shuttle, not third party)?

Basically, what I'm asking is doesn't time only appear to go slower from guy's perspective from Earth, but once the shuttle guy stops, it should catch up to him (Earth guy), because the information is no longer traveling at near the speed of light (a.k.a. delayed).

Last edited: Jan 20, 2012
6. Jan 20, 2012

### Staff: Mentor

The relativistic effects of time dilation, length contraction, and relativity of simultaneity are not visual effects or optical illusions. They are what remain even after correctly accounting for the appearances.

7. Jan 20, 2012

### Jobey

But can the measurement of say one second change if you are moving faster or slower? 1 second is always 1 second no matter what... yes?

8. Jan 20, 2012

### Ryan_m_b

Staff Emeritus
If we both count 1 Mississippi, 2 Mississippi...and then you jetted off far away at near light speed and came back some time later I would have counted far more Mississippis than you even though we have both been counting at the same rate. If Whilst I was counting I used a telescope to observe you I would see you counting far slower even though for you nothing has changed.

9. Jan 20, 2012

### Jobey

Yes, but one second for me is still one second and one second for you is still one second. I'm not trying to relate the two different scenarios.

10. Jan 20, 2012

### ghwellsjr

Wow, I feel like I'm being ganged up on. Do you guys all know each other?

OK, apinkpwny also didn't explain how the shuttle was going to travel, he just said it traveled near the speed of light. So I cannot tell you about the OBSERVED time because that depends on whether the shuttle traveled 50 years away from the Earth and then turned around and traveled another 50 years back, or maybe the shuttle took a big circular path so that it was always "turning around" the whole trip, or maybe it was in orbit around the Earth for the whole time.

As far as the ACTUAL time goes, in order to address that issue, we have to settle on a theory that includes the definition of time. Einstein's Special Relativity is one such theory that does this and the one that this forum is dedicated to providing an environment for learning. So using SR, you need to settle on a Frame of Reference in which time has a defined meaning. You can pick any FoR but it always makes sense to pick one in which the computations will be easier and that would be one in which the Earth remains at rest. (We always ignore the rotation of the Earth and its orbit around the Sun when we do these kinds of thought experiments.)

So according to how SR defines time, it progresses for each observer at a rate determined by the speed of that observer in the FoR. The faster the observer is traveling, the slower his clock ticks. If an observer remains stationary, his clock ticks normally. So the Earth observer is stationary, time progresses normally for him, and the shuttle is traveling at a high speed so time progresses at a slower rate for him. It's just as simple as that. If you analyze the situation from any other FoR, you will get the same answer, as far as the final result, but the computations will be much more difficult, in fact, so difficult that you can't do it in your head, you will have to get out your calculator and follow a bunch of equations, etc, etc, etc.

Last edited: Jan 20, 2012
11. Jan 20, 2012

### Staff: Mentor

Suppose you have a sheet of paper and you measure its size along one edge and get 11", then suppose your friend measures it along the diagonal and gets 14". You can describe this one of two ways, you can say that tilting his ruler along the diagonal made his ruler shorter in the vertical direction, or you can say that his ruler is the same as it always was, but just that by tilting his ruler he was measuring something different.

A clock is a ruler that measures time. The kind of time that is measured by a clock is called "proper time", this is like the distance measured by the ruler, a tilted ruler being like a clock which is moving. When you take a system of clocks and synchronize them that is like the lines on the piece of paper, it is called "coordinate time". The time dilation formula is about the relationship between coordinate time and proper time.

12. Jan 20, 2012

Staff Emeritus
What is the difference? More specifically, what experiment could you make that would tell the difference.

13. Jan 20, 2012

### ghwellsjr

All the clocks moving together will tick at the same rate and all the time related processes will progress at the same rate so you will have no way of knowing that time is doing anything differently for you. But when you have things moving with respect to each other at different speeds, you need to establish a definition of time in order to make sense out of what is really happening.

And we have just one scenario, not two. That scenario can include any number of observers, all traveling in different directions and different speeds.

14. Jan 20, 2012

### apinkpwny

thank you all for posting i really appreciated all the help you provided me and my friends, i think this is a very hard topic to grasp and even though we know it happens ( in theory) it is still something some people just have to dispute to maintain a sound sense of the universe. thanks again!

15. Jan 20, 2012

### Jobey

Yes we are all friends :) Just with different opinions. But back to what I was talking about. SO what you are saying is proper time cannot change, this is the time we use on earth its a measurement of an event and it always remains the same? This can never change?

16. Jan 20, 2012

### Staff: Mentor

17. Jan 20, 2012

### ghwellsjr

The theory is there to make sense of what is really happening. A shuttle that takes a high speed trip away from the Earth for a hundred years will really return with its crew (and clocks) having aged less than the people that remained on Earth.

18. Jan 20, 2012

### Staff: Mentor

Proper time is the time measured by a clock. Any clock anywhere. It has nothing to do with whether or not the clock is on Earth or not.

As far as the rest of your question, it is too vague for me to answer. What do you mean by "remains the same" and "never change"? I described how you could look at a ruler and say that it had become shorter or how you could look at the same ruler and say that it had stayed the same. You need to be specific about what you are asking about when you are asking about changes.

19. Jan 20, 2012

### Jobey

See this is where I start to get confused on how this can happen, because the same amount of time has passed (from a 3rd party standpoint) on earth, and in the shuttle.

20. Jan 20, 2012

### Jobey

Proper time (the time measured by a clock) cannot change is what im asking.