# Time Dilation caused by light alone?

1. Oct 16, 2015

### stringpoet

Let me first preface this with a couple facts about me. I am not a physicist, nor have I ever taken any classes in my life to warrant any extensive knowledge on the subject. I'm a software engineer by trade but have a great interest in physics (or at the very least, spacetime and how it all "works"), so I apologize if my question is elementary.

For the past few years, I've been mesmerized by time dilation. I've read a lot about it and I understand what it is, but the "why" it is doesn't seem to add up for me. When I read about time dilation being a result of gravity and speed, I don't understand why those two factors have any influence on the object's time, whether it is in motion or standing still in space.

To me, it seems the only factor here is the speed of light, and without an observer, it's all irrelevant. Let's say I'm moving at 0.7c toward a planet (let's call it B) in another system, and my starting point is Earth (A). Now, I don't know the math here, but I do know that my actual passage of time would not be slower. My perceived time of B would be much faster because I'm receiving the light from it at a much faster pace than the inhabitants are on that world, and if they saw me coming, it would appear I am moving faster than my actual speed. Of course, the reverse of that would be the effects on A, which would appear to be moving much slower, since I'm receiving its light information at a slower speed. Conversely, if I made it to B, only to make a return trip to A, shouldn't my total "trip" time passed equal that of the time passed on A?

If the answer to this question is "yes", which I'm hoping it won't be (and can be explained for my further understanding of this), then why are speed and gravity considered in time dilation at all?

2. Oct 16, 2015

### ZapperZ

Staff Emeritus
The problem here is that you are starting from the consequences of SR/GR, instead of trying to understand the starting point!

You need to start by looking at the postulate of SR first. Try and understand the concept of the speed of light being a constant in all inertial reference frame, and the concept that it is also isotropic. It is only AFTER that will you be able to see that the idea of "time dilation", "length contraction", etc... all came out of logical consequences of those postulates. This is the "why" those effects came about.

Zz.

3. Oct 16, 2015

### stringpoet

I feel like my understanding the concept of the speed of light being an isotropic constant is the basis for my entire argument, although I may have not made it very clear. My concern is with SR/GR being defined as what it is, rather than what seems to me is a postulate for time dilation concerning the effect of light on an observer.

4. Oct 16, 2015

### SlowThinker

After all such effects are accounted for, there still remains the time dilation, length contraction, and most important, simultaneity shift.
If someone was watching your trip from the side, very very far away, they would see your clock tick slowly, as you fly towards B (if they were at rest with respect to the planets A and B). Obviously they would see your image long after you reach B.

5. Oct 16, 2015

### ZapperZ

Staff Emeritus
I have no idea what you just said here.

You need to understand the entire process. You propose a postulate, and then you check the consequences of those postulates via logical derivation, i.e. mathematics. Then you check against observations.

You need to understand that Einstein was trying to deal with a very serious problem in classical electrodynamics back then. There was a problem with the classical Maxwell equations describing classical electrodynamics, and the problem that they are not covariant under the Galilean transformation, something that didn't make any sense since Newton's laws were seen to obey this covariant form. It is ONLY after one apply such postulates and reformulated Maxwell equations can one realize that more generalized version.

THAT is what made it valid and why we accept those postulates.

Now, you may want to ask why those postulates are valid. We don't know, the same way we don't know why electron has an intrinsic spin of 1/2, why there is a magnetic and electric field, why there is charge... etc... etc.

Zz.

6. Oct 16, 2015

### stringpoet

OK. I've decided the main problem here is my lack of knowledge in the mathematics, as well as a lot of the things you're referring to in your first reply. I guess what I was looking for was an explanation in layman's terms as to why time dilation isn't simply explained as a perception alone based on light received by an observer. Either this assumption is completely incorrect, or I'm misunderstanding what you're trying to tell me, or both.

7. Oct 16, 2015

### ZapperZ

Staff Emeritus
Would you say that a muon, traveling through the atmosphere, would also have an issue with "perception" the way we do? Yet, we observe a muon life time so much longer than what it should be when it is traveling very close to c.

It is more fundamental than just "perception" when elementary particles also undergo the same effect.

Zz.

8. Oct 16, 2015

### harrylin

The short answer is NO: what you describe is (mostly) the classical Doppler effect.

In classical physics, using your words, your perceived time of B would be much faster because you're receiving the light from it at a much faster pace than the inhabitants are on that world. However, if they saw you coming,they would measure the same relative speed as you would measure - that is so in classical physics and also in relativistic mechanics. Of course, the reverse of that would be the effects on A.
Conversely, if you made it to B, only to make a return trip to A, your total "trip" time passed according to your wrist watch is according to classical physics equal that of the time passed on A.

All that is based on the classical assumption that time is NOT a function of speed; in this account there is NO time dilation; the time of signal transfer has nothing to do with time dilation. Also when you hear the thunder a few seconds after a lightning strike, that is not time dilation but simply delay time.

In special relativity the same effects are accounted for, but on top of that, there is a difference in measured times. And if you arrive back at A you will find that LESS time has passed on your clock than on the clocks that remained at A (with some common assumptions such as that you went very fast).

9. Oct 16, 2015

### stringpoet

Thank you for your post. This was by far the easiest response to read!

However, don't these two statements contradict each other? Perhaps I need to do some more reading, but my understanding is that no matter how fast I'm going, if I go somewhere and come back, the time passed on my wrist watch and at the time at the original (and destination) location would be the same on arrival. Does this change if, say, I'm moving at 0.95c? If so, then I've got this thing all wrong! Also, why?

10. Oct 16, 2015

### phinds

What you are not understanding is the whole concept of world-lines. Yes, your clock would tick at 1 second per second exactly was would that held by someone at A. What you are missing is that when you return, you have followed a different world-line and your clock has ticked fewer times. You really DO, as Zapper keeps telling you, need to study the fundamental postulates and the math. It's all very simple, just REALLY counter-intuitive.

EDIT: Oh, and one thing that is key. For you to have gone and come back you have had to accellerate, which means you and A were not in the same inertial frame all of the time. Study the Twin Paradox.

11. Oct 16, 2015

### Janus

Staff Emeritus
As already mentioned, what you are describing is the classical Doppler effect. An example of this is when you here the sound of an approaching object at a higher pitch than when it is receding. With this Doppler effect, the speed of the sound is measured with respect to the medium (air) and the frequency received depends on both the receiver's and sender's motion with respect to the medium.

In this case, the change in frequency is by the factor of
$$\frac{c+v_r}{c+v_s}$$

Where c is the speed of sound with respect to the medium, vr is the velocity of the receiver with respect to the medium, and vs is the speed of the sender with respect to the medium.

However, with light in a vacuum, things are different, there is no medium, and everyone measures light as moving with respect to themselves. This changes the factor by for the change in frequency to

$$\sqrt{ \frac{1+\frac{v}{c}}{1-\frac{v}{c}}}$$

Where c is the speed of light in a vacuum. and v is the relative speed between sender and receiver (positive is they are approaching and negative if they are moving apart)

So let's take that 0.7c trip from A to B and back to A and determine what you will see. Moving away from A, you will see its clock run at 0.42 the rate of your clock. If the trip to B takes 1 yr by your watch, then you will see Earth age 0.42 yrs.
On the return trip, you will see the Earth age at a rate of 2.38 times your own, thus in the year it takes by your watch to make the return trip, you will see the Earth age 2.38 yrs. So you during the two years by your watch will see the Earth age a total of 2.8 years.

As far as what the Earth sees:
The Earth sees your clock run at a rate of .42 while receding up until the point you turn around when your watch reads 1 yr. Thus is takes 2.8 yrs own the Earth clock before they see you turn around. They then see your watch run at a rate of 2.38, until it ticks of another year, which takes 0.42 yrs by the Earth clock. So the Earth sees your clock tick off 2 yrs, while its own clock ticks off 2.8 yrs.

You both end up agreeing that you aged 2 yrs while the Earth aged 2.8 years.

12. Oct 16, 2015

### stringpoet

Janus, how cool! Thank you for that. Where can I find out more about the equation you showed me, and these postulates? I'm incredibly intrigued by the numbers you gave me, not being anything close to what I expected.

13. Oct 18, 2015

### harrylin

Yes, special relativity disagrees with classical physics on that point.
Classical physics is usually approximately correct at low velocities; as a result it is still used a lot. It does appear that you had that main point wrong.
As a matter of fact, the first explicit time dilation prediction was for such a case, by Einstein in 1905: he predicted that if a clock is rapidly moved around (and not even needed to go extremely fast), if we can neglect the effect of acceleration forces on the clock, then that clock will delay on a clocks that remain steady at their place.
- §4 of http://www.fourmilab.ch/etexts/einstein/specrel/www/

14. Oct 18, 2015

### Staff: Mentor

That's not right, and you're describing the classic Twin Paradox (which is not really a paradox at all, but a problem that is easy to analyze incorrectly to produce apparently paradoxical results). Start here: http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_paradox.html and pay particular attention to the section on the Doppler explanation.