# B Time Dilation and velocity

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1. Oct 22, 2016

### Faiq

In explaining time dilation we usually say, if velocity of A is greater than velocity of B, then time is slower for A as compared to B. However, if vA> vB, using the equation for time dilation, tA>tB. So if 60 seconds passed for B, 100 seconds passed for A. How does that imply that time slowed down for A? Time, in above example, increased for A. So doesn't it means that time becomes faster for A?

2. Oct 22, 2016

### Staff: Mentor

No. We say more like: If B is moving with respect to A, then A will 'measure' B's clocks to run slow (compared to A's). And vice versa, of course.

3. Oct 22, 2016

### Ibix

Also, I think you might have your equation back to front. If you, A and B agree at some point that t=0 then if (according to you) $v_A>v_B$ then (again according to you) A's clock will thereafter always have a lower reading than B's - A's clock ticks slower.

4. Oct 22, 2016

### Andrew Mason

You must use the Lorentz transformations and keep the concepts of proper time and time dilation straight. You have to be clear on what times you are referring to when you speak of tA and tB.

Proper time is the time measured by an observer on a clock that is at rest in the observer's frame of reference. This is necessarily a time between two events that are separated by time only, ie. not spatially separated, in the observer's frame.

The time intervals that observers in other inertial reference frames measure between two events are determined by the Lorentz transformations. The variable t represents time of an event in the "stationary" frame and t' represents time co-ordinate of an event measured in the frame moving at velocity v relative to the first.

Consider an observer in inertial reference frame A and an observer in inertial frame B whose origin is moving at velocity v in the direction of the x axis relative to the origin of A. The proper time interval that observer in A measures on its own clock between event E1(0,0,0,t1)and event E2(0,0,0,t2) is t2-t1. An observer in B observes the same two events at E1(γ(x-vt1),y, z, γ(t1-vx/c2) and E2(γ(x-vt2),y, z, γ(t2-vx/c2) where x=y=z=0. So the time interval that B measures on its own clock between these events is $\gamma (t2-t1)$ which is greater than the time measured on A's clock. The result is that from B's perspective the time between events measured at the same location in A's frame of reference (A's clock at A's origin), which is moving at velocity -v relative to B, appear to be moving more slowly than B's.

If two events were to occur at the same location in B, A would observe the events to occur at different locations in A and at a different time interval as measured by A on A's clocks. So it is perfectly symmetrical between the two inertial frames. It is just that what one observer sees as a pure interval of time the other sees as an interval of distance and a different interval of time.

Time dilation may be best demonstrated by the muon decay phenomenon. The proper lifetime of the muon is measured in the muon frame of reference. This lifetime can be determined in the laboratory from observing slow moving muons. Muons created in the upper atmosphere by cosmic ray bombardment create muons moving at relativistic speeds relative to the earth. We observe them reach the earth surface. In order to do that they must survive for a time as measured on earth that is significantly longer than their proper lifetimes. Based on our time measurements in the earth frame of reference we say that the muon's clock appears to run slow. That is to say that the time between two events (muon creation to muon decay) is measured to be longer on earth than it is in the frame of reference of the moving body. So, it appears to us that the fast moving muon's clock is running "slow". That is time dilation.

AM

Last edited: Oct 23, 2016
5. Oct 22, 2016

### Faiq

But then why do we say time slows down at higher speeds if you yourself said "the time interval measured by B is shorter than the time interval measured by A. " when A is moving at a greater speed than B ?

6. Oct 22, 2016

### Ibix

According to each observer, they themselves are stationary and everybody else is moving. So whose clock is ticking slowest depends on who is doing the measuring. "Time slows down at higher speeds" is something of an over-simplification, I'm afraid.

7. Oct 22, 2016

### pervect

Staff Emeritus
Saying that the velocity of A is greater than the velocity of B is a frame dependent statement, not an absolute statement. When you say the velocity of A is greater than the velocity of B, you are implicitly creating a frame "C" , in which this statement is true. This is important because time is a frame dependent quantity, and so is time dilation.

The fundamental issue is that saying "time slows down" isn't quite right, it usually indicates a bad model of time, one based on what is commonly called "absolute time". The problem that I have responding to such statements is that frequently people who use absolute time often don't know what I mean when I say the words "aboslute time". The meaning of the phrase doesn't get through :(. Unfortunately, using a lot of words doesn't seem to work well either - the key points seem difficult to convey. Perhaps they seem irrelevant, or perhaps it's a case of tl;dr. One approach I take is to refer readers to Scherr's paper, "The Challenge of Overcoming deeply-held Student Beliefs about the Relativity of SImultaneity" in the hopes that they'll actually read it. I don't get a lot of feedback from people saying that the paper actually helped them, or people who give feedback saying that they read the paper and had questions. So I suspect it often doesn't get read often. Still, it's worth a mention - and perhaps I'm being too pessimistic, I can hope that mentioning the issue could plant a seed that will, at some later date, flower.

8. Oct 22, 2016

### Faiq

Okay then consider my problem this way. Using the equation of time dilation, the answer is always greater than t0 value ( if v>0). What does this increase in time indicate?

9. Oct 22, 2016

### robphy

10. Oct 22, 2016

### Faiq

11. Oct 22, 2016

### Ibix

It isn't.

12. Oct 22, 2016

### phinds

We DON'T say it. Pop-science treatments say it and it is not correct. Time dilation is an observational phenomenon, as has already been pointed out in this thread, not something that actually happens to anyone. Doc Al explained it perfectly well in post #2.

13. Oct 22, 2016

### Faiq

So time doesn't slows down at higher speeds?

14. Oct 22, 2016

### Ibix

Are you reading the replies you are getting? #3, #6 and #7 all address this. You have the maths wrong and your description is over-simplified.

15. Oct 22, 2016

### Staff: Mentor

Listen carefully to what @pervect is saying, that "time slows down" is a bad way of thinking about it; almost certainly you're thinking that there is a correct, not-slowed-down, "absolute" or "real" time that we're comparing with.

Forget the time dilation formula for a moment, as it is tempting you to think about "time slowing down". Instead, work on understanding the relativity of simultaneity (Google for "simultaneity Einstein train" - no math required), as both time dilation and length contraction are consequences of relativity of simultaneity. Don't move on until you clearly understand that the statement "X and Y happened at the same time" is frame dependent; if two things happened at the same time according to Alice, and Bob is moving relative to Alice, then the two things did not happen at the same time according to Bob; and the things that did happen at the same time according to Bob did not happen at the same time according to Alice.

Once you understand that, you are in a position to understand what's really going on with time dilation, and it's not "time slows down". Say Alice and Bob are moving relative to one another, but at some point they meet - either Alice passes Bob or Bob passes Alice, but either way they're at the same place vat the same time at the moment of passing. As they do, they both set their clocks to 1200 noon.

Later Alice looks at her clock and sees that it reads 1300. At the same time that her clock reads 1300, Bob's clock reads 1230 (How does she know this? Well, suppose their relative speed is .5c, so Bob is now .5 light-hours distant. If she's watching Bob's clock through her telescope, what she sees at 1330 is the image of Bob's clock at the same time her clock read 1300 - it took the light 30 minutes to get to her telescope), so she concludes that Bob's clock is slower than hers.

However, relativity of simultaneity means that although "Alice's clock reads 1300" and "Bob's clock reads 1230" happened at the same time according to Alice, these two events did not happen at the same time according to Bob. According to Bob, who is also watching Alice's clock through his telescope, the event "Bob's clock reads 1230" happened at the same time as "Alice's clock reads 1215", and it is Alice's clock that is running slow.

If you are really serious about understanding this.... The best thing you can do is to try to forget everything you think you know about relativity and start over with Taylor and Wheeler's book "Spacetime Physics" or equivalent.

16. Oct 22, 2016

### Staff: Mentor

It does not, at least not the way you're understanding it.

17. Oct 22, 2016

### Faiq

Wait a minute so both times 1230 and 1330 are correct? If no then, what is the absolute time at the moment Alice looks sees 1300 at her clock?

18. Oct 22, 2016

### Ibix

As pervect said in post #7, there is no such thing as absolute time. So your question is meaningless.

19. Oct 22, 2016

### Faiq

This is some little vague thinking I made after analyzing above posts. Please tell me if it's close to correct

Let's for some simplicity consider time "a line" which is 1 cm long. So 1 cm will be 1 second and so on. This line is different for stationary objects and objects moving.
So let's say va >vb. According to the time dilation equation ta>tb but the conventional books/youtube videos tell us that time slows down for moving objects. So here's my idea behind it
The time that books are mentioning is in my opinion the "line" idea provided above. If objects are moving, the length of their "time line" will decrease so in their case 1 sec will be like 0.9 cm, 0.8 cm.
The time that equation refers to is the number of "time lines" one experiences in a time interval.

Now I will refer to an example to be a little clear.
In a 10 second interval measured in a inertial frame
Suppose B is at rest and experiences 10 second.
We can say his length of time-line is 1cm. So he experiences 10 time-lines.

A is moving and by using the time dilation equation we see that he experiences 20 seconds.
We know that the "correct" (I know this is the wrong word here but just for sake of simplicity) time is 10 seconds.
We can arrive at this conclusion if we consider his time-line to be 0.5 seconds. In this way he will experience two times more time-lines in 10 seconds and may measure his time to be 20 seconds.

I know this is too much over-simplified but contradiction between the posts here and the SAT books are driving me crazy.

20. Oct 22, 2016

### Staff: Mentor

Will you please stop saying this without some context? As has been pointed out several times, such a statement is frame dependent.

21. Oct 22, 2016

### Ibix

For the third time, no! If, in some frame, $v_A>v_B$ then the time elapsed on A's clock over some time interval (measured in that frame) will be less than that on B's.

22. Oct 22, 2016

### Faiq

A spaceship of alien sports enthusiasts passes by the Earth at a speed of 0.8c, watching thefinal minute of a basketball game as they zoom by. Though the clock on Earth measures aminute left of play, how long do the aliens think the game lasts?

This is a question taken from a SAT book. The answer is 100 seconds. Can you tell me what does 100 seconds and 60 seconds mean if they are referring to the same event? And how does that relates to the slowing down of time which is again written in the start of the same book?

23. Oct 22, 2016

### Ibix

According to the alien the stadium clock ticks every 1.667s because, to it, the Earth is moving at 0.8c. So it is the Earth's clocks that tick slowly from the alien's perspective.

Edit: ...as I said in #6.

24. Oct 22, 2016

### phinds

AND THEY ARE WRONG, as has been explained several times already. FORGET what they say.

25. Oct 22, 2016

### Faiq

Okay, thanks for spending time on me. It was just those books which got me all messed up.