# B How old is light?

1. Sep 8, 2016

### Francis Ward

If, as Einstein tells us, time dilates as we travel, and that dilation increases massively at speeds approaching the speed of light, then light, which travels at c would, to the static observer, never age. To us, light would take infinitely long to reach us.

2. Sep 8, 2016

### Staff: Mentor

There is no reference frame where light is at rest. "Time that passes for light" is not a meaningful concept.
No. The time dilation you discussed is for the traveling object, not for us. Send a spacecraft at 99.99% the speed of light to Alpha Centauri (4 light years away), and it will be there in about 4 years, while the clocks on board show a much smaller time difference.

3. Sep 8, 2016

### Simon Bridge

Welcome to PF,
You are mixing up reference frames. Try again, but this time be more careful.
Note: light cannot be an observer: it is what we use to observe things. See mfb: it is not meaningful to talk about time that passes for light.

4. Sep 9, 2016

### Francis Ward

Thanks for the replies folks. As you see, I am new to this relativity thing!
According to the 'Thought experiment' which Einstein used to demonstrate the dilation of time, the time dilation is relative to the static observer. The 'person' travelling on the train ages slower when compared to the static observer. However to the person on the train, the 'light clock' ticks at exactly the same rate. Note also the muon decay experiment which 'proves' the dilation of time. As far as the muon is concerned, it lived it's prescribed length of time. As far as we, the static observer is concerned it lived 29 times longer because it was travelling at close to the speed of light.
I am curious about the 'it is not meaningful to talk about time that passes for light' - why not? We talk about light years, and that it takes 4 years for light to travel from here to Alpha Centuri. If light is not subject to time, then it is meaningless to talk about how fast it can travel, or how long it takes for it to get somewhere.
As a final (for now) and related question, in the Thought Experiment, where on the mirror on the opposite side of the travelling light clock would we expect the light to impinge? Does it impinge at the point directly opposite the point it left the other mirror (in other words travel both across the train and with the train) or does it impinge marginally back from that point (ie not affected at all by the movement of the train)? This is relative to the person on the train.

5. Sep 9, 2016

### Simon Bridge

The 4 some years for a light signal to get from here to alpha centauri is time experienced by us, not the light.
The time dilation effect is a comparison between two clocks. The muon experiment is a case in point, the experimenters are comparing the muon clock (its decay time) with a clock on the ground. What is the clock for light?

Also note: all observers are stationary in their own reference frames. All observers measure the same speed for light, but not for muons.
There is a precise language that goes with relativity that is designed to avoid the kind of confusions you have.

6. Sep 9, 2016

### Francis Ward

Again, thank you.
Thoughts on the train question?

7. Sep 9, 2016

### PeroK

In the train's reference frame, it's not moving. The light is bouncing back and forwards in a straight line from the middle of one mirror to the middle of the other.

On your other point. Relativty explicitly precludes light (EM radiation) having a reference frame and having measurements of space and time. And, if anything else could travel at the speed of light, it would also have no concept of space and time. Within relativity, the equations that govern an observer's measurements of space and time are mathematically undefined for an observer moving at the speed of light, $c$, with respect to another observer.

The key thing is the "gamma factor": $\gamma = \frac{1}{\sqrt{1- v^2/c^2}}$.

If you try to put $v=c$ in that equation, corresponding to the reference frame of something travelling at $c$, you get $\gamma = \frac{1}{0}$, which is mathematically undefined.

8. Sep 9, 2016

### Francis Ward

The clock for light? Maybe it is just the same as the clock for any stable particle. To my mind muon is not a clock for muon, it is a measure of the passing of time under a fixed set of conditions based on the life of muon particles. Change those conditions and you change the physical way the muon particles react. As such it would be considered an unreliable clock.

9. Sep 9, 2016

### Francis Ward

10. Sep 9, 2016

### PeroK

That's from your perpective outside the train. From the perspective of those on the train, it's you that is moving sideways.

This is a critical issue regarding the notion of relative vs absolute motion. Many people think this relates to Einstein's relativity, but in fact it was Galileo who first realised all motion is relative.

Let's suppose that the light on the "moving" train doesn't behave in the same way as the light on a "stationary" train. Well, the "stationary" train is orbiting with the Earth. So, we ought to see all sorts of strange effects here on Earth, with light and other objects trying to move "properly" relative to a stationary observer at rest with respect to the sun, say. You might point a torch in one direction, but the light would go off in another because you are moving with the Earth.

But, the sun itself is moving round the galaxy and the galaxy is moving towards Andromeda.

So, who's actually at rest here and who is actually moving with what absolute speed in which absolute direction?

11. Sep 9, 2016

### phinds

You misunderstand what a clock is. This is a VERY common misconception. A clock, first and foremost, is something made of matter. Matter cannot travel at c, so there is no such thing as a clock traveling with light, thus light has no clock.

12. Sep 9, 2016

### jbriggs444

Suppose that you are in a jet airplane flying west from New York to Los Angeles at noon.

You are flying first class and pick an olive from your complimentary beverage, tilt your head back and toss it in the air, catching it in your mouth. Do you expect to be able to do so?

Is the olive travelling vertically upward from your hand and vertically downward to your mouth?
Or is it travelling on a diagonal path at ~400 mph westward relative to the rotating earth?
Or is it travelling on a diagonal path at ~300 mph eastward relative to the earth's center?
Or is it travelling on a diagonal path at some thousands of miles per hour westward relative to the sun?

Yes, it is doing all of these.

13. Sep 9, 2016

### PeroK

I'll try that next time I'm flying first class!

14. Sep 9, 2016

### Francis Ward

I do that every time I fly first class.
But we cannot attribute the actions of a material item, such as an olive, to those of light. Light, we are told, cannot be influenced by the speed of it's source (we cannot add to the speed of light by putting the torch on the front of the train). Hence it should not be influenced either by the sideways motion of the train. As a person sitting on a train I would expect, within my frame of reference, for a ball I throw against the opposite wall to hit where I aim and bounce back to me. That is because it has applied to it the two motion components of the train and my arm. Light should not, we are told, behave like that. So why does it (according to the thought experiment) do so? Could it simply be that in fact it does not, and that in fact we simply do not have the apparatus to be able to detect what light is in reality doing? Could it be that the theoretic universal speed limit, c, is only there because all of our observational capabilities are constrained by light? Who really knows what the movement of the earth, galaxy etc is doing to the light that is all around us? It is all a matter of perception, and probably very little to do with reality.

15. Sep 9, 2016

### Staff: Mentor

No. It doesn't matter how you actually accomplish the measurement of the time passed for an object or the measurement of its velocity. You will still have time dilation and there will still be a speed limit of c.

If what we regularly observe doesn't coincide very closely with reality, then reality is very strange indeed. That would mean that the laws of physics that we have now and that we've tested to a very high precision are grossly incorrect despite working extremely well for us here on Earth. Our observations of distant places in the universe would somehow make sense according to these incorrect laws, even though they would be very wrong. That's a serious contradiction.

16. Sep 9, 2016

### Francis Ward

Why then does it seem that the thought experiment contradicts what we are taught about the movement of light? It seems that the light IS being influenced by the velocity of the train, because otherwise it would not, to the observer on the train, impinge at the centre of the opposite mirror. It would impinge a distance (albeit very small) behind the centre of the mirror.

17. Sep 9, 2016

### jbriggs444

True enough, its speed is not influenced by the speed of its source. But its direction can be influenced by its source. If you have ever used a flashlight and shone its beam first left then right, you may be aware of this.

It may be worth paying close attention to the design of the aiming device that you use to emit the pulse of light on the train. How exactly is your hypothetical emitter designed? [I would suggest a flashlight, a fast shutter and a blackened tube through which the pulse travels -- that should be simple to analyze].

18. Sep 9, 2016

### Francis Ward

Yes, the light on the wall will move as the flashlight is moved. But the light that is emitted at the instant the flashlight is at 90 degrees to the wall, should not deviate from that path, regardless of how fast I move the flashlight. Einsteins thought experiment implies (to my feeble mind) that it does.

If the train is going 300 km/h and the distance across to the opposite mirror is 10m (very big train) then the light will take 33.356 nano seconds to travel across. In that time the train will have progressed a massive 2.78 micro-meters relative to the starting position of the light. So, to the person on the train, should the light not impinge 2.78 um behind the position it would if the train was stationary?

19. Sep 9, 2016

### jbriggs444

You must analyze the scenario carefully. The above is not careful. How does the aiming on your hypothetical flashlight work, exactly?

20. Sep 9, 2016

### PeroK

You're simply misinterpreting the statement 1) "the velocity of light does not depend on the velocity of its source". It's interesting, because a while ago someone else had exactly the same thought as you.

With you're interpretation, I guess, all light beams must move in a single direction. Let's assume you set up a target and shine a light beam towards it. That establishes the velocity (speed and direction of light). Then, all other beams of light must go in that direction. Even if you turn round and shine your torch in the opposite direction, the light ought to come out of the back of your torch and towards your target.

Or, if a friend stood 1m to your left and shone their torch at the target, you would have violated the above statement, as you now, patently, have two different velocities for light. The same speed perhaps, but definitely differenet directions, hence different velocities. So, light does not always have the same velocity, only the same speed.

Clearly, however, that is not what is meant by statement 1). A beam of light from a torch can be sent out in any direction you like. Now, imagine the beam is shone along inside a tube (let's say, by an observer standing next to the tube). He will see the light move along inside the tube without touching the sides. But, so will an observer moving with respect to the tube and the light source.

Now, you could argue that the direction is "the same" in each case (it's physically in the direction of the tube). But, clearly, light cannot be moving "straight in front of" both observers at the same time. That is, of course, impossible.

21. Sep 9, 2016

### Francis Ward

Not sure quite what you mean by careful analysis :)
You are correct, velocity is vector, speed scalar. However, that aside, I am not talking about flashlights, which emit a continuous beam of light, I talking about a small pulse of light emitted from a source across a 10 m train, which is travelling at 300 km/h. If that light hits the opposite wall precisley opposite the source, then it has travelled not only 10m across the train but 2.98um from the point at which it left the source. That, as I understand it, is the premise of the thought experiment. Is that correct?

22. Sep 9, 2016

### jbriggs444

A starting point would be to answer the question I've asked three times now: What is the design of your aiming device?

Edit: That's a dead serious question. I am prepared to discuss at least three possible designs: A blackened tube, a parabolic reflector or the flat end of a ruby laser. Without having accepted the theory of special relativity to start with, the blackened tube model is the best choice since its operation is simple and uncontroversial.

23. Sep 9, 2016

### PeroK

It makes no difference whether it's a single pulse or a continuous beam. No, it hasn't moved $2.98 \mu m$ in the reference frame of the train, it has moved precisely 10m. In the reference frame in which the train is moving, it has moved slightly further. So, if the speed is the same in both frames, then less time must have elapsed in the train's frame when measured from the platform frame. And there you have time dilation.

24. Sep 9, 2016

### Francis Ward

But I didn't say it had moved 2.98 um in the reference frame of the train, I said from the point at which it left the source. I know that this is in fact the stand-point of the observer outside of the train, and so to them it is perceived to have travelled further. But I am trying to understand the real situation, not the one of perception. Perception is simple to grasp but has little bearing on reality.

25. Sep 9, 2016

### PeroK

The reality is both. Distance travelled is relative to reference frame. Nothing (whether it's light, cars, penguins or whatever) can move the same distance in all reference frames. That would be impossible. Just imagine yourself sitting still. You have not moved at all, as far as you're concerned. But, to someone on a train you have move at 200km/h perhaps. And, to take the example from earlier posts, you are spinning through space.

The reality is that distance traveled is relative to reference frame and that has been known since Galileo.