# Photon's Point Sized Universe or plane sized?

1. Aug 5, 2013

### aayushgsa

Hello,
I recently read about a photon's point of view according to relativity(which is a fascinating thing for a high school boy), It said to photon's perspective the universe is point sized. But recently I learnt that lorentz contraction occurs in the dimension in which the object is travelling. If this is so, Imagine a photon travelling in the z direction according to lorentz transformation formulas, Contraction would only be in the Z direction and the photon will see the universe as an Infinite plane with an infinitesimal thickness. isn't it?. But the thing that photon sees universe as point sized seems contrary. Where I am Wrong? Please use some maths(but simple because i am still a high schooler).
Thank-You

2. Aug 5, 2013

### ZapperZ

Staff Emeritus
This is making the assumption that you CAN transform to the rest frame of the photon and then see what's going on. This contradicts our understand of physics.

Zz.

3. Aug 5, 2013

### aayushgsa

So we cannot apply lorentz transformation to photons? Then how we say that it's perception of universe is of a point sized universe?

4. Aug 5, 2013

### Staff: Mentor

It's either just plain wrong (there are, sadly, many bad and misleading popular explanations of relativity out there) or you're misunderstanding it - and I'd bet on the first.

But do take a look at the FAQ at https://www.physicsforums.com/showthread.php?t=511170

5. Aug 5, 2013

### ghwellsjr

Think about this: whenever you turn on your flashlight, untold billions of photons travel at exactly the same speed, no photon overtakes another photon. Photons are what we use to see things. If no photon from you or your flashlight can overtake or even catch up to any of the other photons, by what mechanism do they "see" what's behind them?

6. Aug 7, 2013

### aayushgsa

I can't get it I am not talking what is behind them rather I am talking about the sideways. Photon's perspective of the universe must be planar so why it is point sized?

7. Aug 7, 2013

### ZapperZ

Staff Emeritus
I'm not sure why you still don't get it. The question has no answer because you are asking for something that doesn't occur. Our physics does not have a transformation to the reference frame of the photon. So asking what is the view from the photon's perspective is meaningless!

That's like me asking you "why did the unicorn lay a blue egg?"

Zz.

8. Aug 7, 2013

### ghwellsjr

First off, I never heard of a book that claims that the universe is point sized to a photon. I have only heard of claims that the universe is planar to a photon. So what book did you read that made this claim? Can you quote the passage?

Secondly, what we can say is that when a fast moving observer analyzes stationary objects according to Special Relativity, the observer determines that they are Length Contracted only along his direction of motion, as you pointed out. But we are not talking about how stationary objects look or appear to the observer, we are talking about a determination that the observer makes based on timing measurements using his clock followed by calculations. These are what we call radar measurements requiring the round trip passage of light (or radar) signals from the observer to the objects and reflected back to the observer.

Now if an observer could travel at the speed of light, he could not conduct any radar measurements, could he? His radar signals could not travel away from him, could they?

You should also be aware that no matter how fast an observer is traveling, spherical objects will still appear to him as spherical, not flattened. This is because the light from the various points of a spherical object have different distances to travel to get to his eyes. So if an observer flies past our sun at a very high speed, he will always see it as a sphere.

So now do you see why your book has misrepresented both the facts of what a high speed observer can see and the calculation of what a high speed observer can determine based on his radar measurements? And do you see that the author has jumped to the conclusion that what an observer can do as he approaches the speed of light is what a photon can do at the speed of light?

Last edited: Aug 7, 2013
9. Aug 7, 2013

### BruceW

As others have said, asking 'what is the perspective of a photon' is a fairly meaningless statement. I am trying to think what this article you read could possibly mean. Maybe they mean the photon travels on a path whose proper time is zero. For massive objects, the proper time is the time which passes according to the object. So in a way, you could say that time does not pass for a photon. So in a certain sense, maybe they mean the universe is point sized according to a photon because for the photon, zero time passes while it is travelling around in the universe.

10. Aug 7, 2013

### ghwellsjr

Time is also meaningless for a photon. You need to carry a clock to measure Proper Time and no clock can travel with a photon and you can't make a clock out of just photons. So that can't be the "meaning" that the author was intending.

This, by the way, is another reason why a photon cannot determine that length contraction is going on, it has no clock with which to measure the time interval between sending out a radar signal and receiving its echo (ignoring the fact that photons cannot engage in radar activity).

11. Aug 7, 2013

### BruceW

Is there really any difference between saying "time does not pass for a photon" and "time is meaningless for a photon"? I would have thought they are the same statement. In both cases, what we really mean is that a beam of light travels along a null geodesic. Maybe you are thinking that "time does not pass for a photon" is a bad way to word it (maybe the statement seems to imply more than it actually means?). Anyway, I definitely agree that the perspective of the photon is pretty meaningless. But I think the author was trying to attach this meaning of "no time passes". I have seen many meaningless statements in magazines that are explaining science in a poetic, or non-rigorous way.

12. Aug 7, 2013

### Naty1

hey aayushgsa............

Correct.
The Lorentz transforms only apply to objects with mass and they can never reach speed 'c'.

While it is true that as you [an observer with mass] goes faster and faster, length exterior to your own frame of reference does become contracted....forshortened....more and more, it can never approach a 'point' because you can never reach speed 'c.'.

So the explanation is an exaggeration, but your interpretation of such an exaggeration, as a 'plane', is not so bad given an incorrect, unrealistic input.

It takes a while to get used to such news way of thinking....so give yourself a bit of time to develop new perspectives..maybe come back and read these posts a few more times.....

13. Aug 7, 2013

### pervect

Staff Emeritus
You can apply the Lorentz transform to the worldline of a photon. But you can't make it travel at any speed other than "c" by applying the Lorentz transform.

The idea of the "point of view" of a photon presupposes a frame where the photon is at rest. This idea is of course not compatible with relativity. People seem to get fixated on the idea of the "frame of a photon" any way and won't listen when you tell them there is no such thing. I'm not quite sure why.

14. Aug 8, 2013

### ghwellsjr

Yes there is a difference. Saying "time does not pass for a photon" can mean that time for the photon is zero, like we could say "zero time passes" between AM and PM. It would be better to say "time does not apply for a photon".

You should not think of a null geodesic as being the same as a timelike geodesic but with the magnitude equal to zero any more than you should think of it being the same as a spacelike geodesic with a magnitude of zero. They are three different kinds of geodesics.

You also suggested that "proper time is zero" applies to a photon. But this is also wrong. Proper Time does not apply to a photon.

He may have been trying to say "no time passes" which is also something we could say about the interval between AM and PM. You seem to be reluctant to disassociate time from a photon.

15. Aug 8, 2013

### BruceW

well, there is zero change in proper time along the path of the photon. A photon could be emitted by a far-away star. Then the change in proper time along its path from that star to us is zero. But I think I see what you mean. It is not possible to choose a rest frame for the photon (as pervect has just said). So it is meaningless to think of things in the perspective of the photon. And time which the photon experiences would be something 'in the perspective of the photon', so it is not a meaningful thing at all to talk about.

16. Aug 8, 2013

### ghwellsjr

If you have become convinced that time for a photon is not meaningful to talk about, then you should not start your post with the statement that "there is zero change in proper time along the path of the photon". That also is a meaningless statement. Proper Time is what a clock measures as it travels between two events. No clock can travel between the event of the emission of a photon by a far-away star and the event of us detecting that photon.

Maybe another way to convince you is that if you pick any two events as defined by the coordinates of an Inertial Reference Frame, you can calculate the Spacetime Interval between them and it will fall into one of three categories: time-like, light-like or space-like. Spacetime Intervals that are time-like are called that because they can be measured by a clock. Spacetime Intervals that are space-like are called that because they can be measured with a ruler.

If you start with two events that are space-like and you move the event by changing one of the coordinate values so that the Spacetime Interval gets smaller until it passes through zero and then starts becoming larger again as a time-like Spacetime Interval, you can think of the measuring device starting out as a ruler and ending up as a clock. At the point where the Spacetime Interval is zero, would you say that the device was a ruler or a clock? It is neither, because the interval cannot be measured by either. In fact, it cannot be measured by any means whatsoever. That is why a zero Spacetime Interval is also called a null interval. It's neither a zero space-like interval nor a zero time-like interval. It is a different category.

17. Aug 8, 2013

### Naty1

That's a slightly different perspective ... and one I have wondered about....

In these fanciful descriptions of 'moving at the speed of light'....a 'photon's perspective.....why things instead wouldn't instead be described as going black for a significant portion of such an observer's view...since photons from behind can't catch her to provide any 'vision' [detection]....more broadly speaking, seems a much reduced information would be received....

also, if photons get infinitely redshifted out of observation from behind an such a fictional observer, why not hypothesize incoming photons would be infinitely blue shifted and 'fry' an observer....it's gamma radiation or more.....Why decide the 'point' view is the only fiction involved.

18. Aug 8, 2013

### ghwellsjr

.
I didn't make this comment to offer another perspective for a photon--I was hoping to dissuade the OP from thinking in terms of any perspective for a photon.

There can be no such observer traveling with a photon or at the speed of a photon.

The 'point' view is not the only fiction involved--and no view for a photon should even be classified as fiction, just meaningless and such discussions should all be dissuaded.

19. Aug 8, 2013

### Naty1

ghwells....
good luck with that effort.

I think it wiser to explain why such views fall outside factual science....which you did.

20. Aug 8, 2013

### BruceW

When I say proper time, I just mean the spacetime interval. We are using different definitions for the term 'Proper Time'.