# If I were light

1. Apr 26, 2010

### xcrunner2414

I think I already know the answer, but a friend of mine disagreed with me on this point:

If I were to become light, i.e. my consciousness was transferred to a photon moving with speed c, would it then seem to me as if I existed outside of time? I think it would because of the fact that moving clocks seem to travel slower. If you do a time dilation calculation you get the time interval in the moving system to be infinite compared to the proper time interval. It seems to me as if you would paradoxically experience two different things:

1. The universe would stand still (i.e. time would stop) for the rest of eternity.

2. All events in the future would happen instantly.

2. Apr 27, 2010

### Nabeshin

Re: If I was light...

You're not going to like this answer but it's the only there is:
You cannot become light. More specifically, photons have no reference frame so to ask what you would experience "riding on" a photon is a meaningless question. Essentially, it amounts to asking "what would relativity predict about this if relativity were wrong?"

3. Apr 27, 2010

### xcrunner2414

Re: If I was light...

- I know. It was a hypothetical question.

- I don't understand. If they have a specific velocity, how can they not have a reference frame?

- I never said anything about riding on a photon, I asked what it would be like if I was a photon. That's definitely a valid question.

4. Apr 27, 2010

### Dmitry67

Re: If I was light...

1 Even hypotetical question must be valid
2 Because you get /0 when you try to build such frame
3 It is even worse, and the question is not valid. Photons are structureless, so they dont have memories, they cant observe or think.

5. Apr 27, 2010

### JesseM

Re: If I was light...

1. You get division by zero if you try to plug v=c into the Lorentz transformation
2. If a photon had an inertial rest frame, it would violate the first postulate of relativity which says the laws of physics must work the same way in every inertial frame, since in every sublight frame all photons must move at c
3. There is a specific physical definition of the space and time coordinates of an inertial reference frame in terms of a network of rulers and clocks at rest in that frame, but it wouldn't make sense if you tried to imagine the rulers and clocks moving at c, since rulers approach zero length as they approach c, and clocks approach being stopped
Not if by "what it would be like" you mean what would be true in a photon's inertial rest frame. If you mean something else, you need to specify.

6. Apr 27, 2010

### bapowell

Re: If I was light...

Relativity does not apply to photons, since it breaks down for speeds v = c. This is why Nabeshin says 'it amounts to asking "what would relativity predict about this if relativity were wrong?"' We don't have a theory with which to answer your questions.

7. Apr 27, 2010

### JesseM

Re: If I was light...

It's not that relativity doesn't apply, just that they don't have their own inertial rest frame. Relativity still applies in the sense that the laws governing photons have the same form in all inertial frames.

8. Apr 27, 2010

### bapowell

Re: If I was light...

OK, yes. I agree with this and wasn't careful with my language. What I should have said is that the Lorentz transformations are singular at v = c.

9. Apr 27, 2010

### bcrowell

Staff Emeritus
Re: If I was light...

FAQ: What does the world look like in a frame of reference moving at the speed of light?

This question has a long and honorable history. As a young student, Einstein tried to imagine what an electromagnetic wave would look like from the point of view of a motorcyclist riding alongside it. But we now know, thanks to Einstein himself, that it really doesn't make sense to talk about such observers.

The most straightforward argument is based on the positivist idea that concepts only mean something if you can define how to measure them operationally. If we accept this philosophical stance (which is by no means compatible with every concept we ever discuss in physics), then we need to be able to physically realize this frame in terms of an observer and measuring devices. But we can't. It would take an infinite amount of energy to accelerate Einstein and his motorcycle to the speed of light.

Since arguments from positivism can often kill off perfectly interesting and reasonable concepts, we might ask whether there are other reasons not to allow such frames. There are. One of the most basic geometrical ideas is intersection. In relativity, we expect that even if different observers disagree about many things, they agree about intersections of world-lines. Either the particles collided or they didn't. The arrow either hit the bull's-eye or it didn't. So although general relativity is far more permissive than Newtonian mechanics about changes of coordinates, there is a restriction that they should be smooth, one-to-one functions. If there was something like a Lorentz transformation for v=c, it wouldn't be one-to-one, so it wouldn't be mathematically compatible with the structure of relativity. (An easy way to see that it can't be one-to-one is that the length contraction would reduce a finite distance to a point.)

What if a system of interacting, massless particles was conscious, and could make observations? The argument given in the preceding paragraph proves that this isn't possible, but let's be more explicit. There are two possibilities. The velocity V of the system's center of mass either moves at c, or it doesn't. If V=c, then all the particles are moving along parallel lines, and therefore they aren't interacting, can't perform computations, and can't be conscious. (This is also consistent with the fact that the proper time s of a particle moving at c is constant, ds=0.) If V is less than c, then the observer's frame of reference isn't moving at c. Either way, we don't get an observer moving at c.

10. Apr 27, 2010

### xcrunner2414

Re: If I was light...

1. This is not a physics question, it is a question about perception of time.
2. I know, I'm a university physics major.
3. No sh!t. Tables can't observe or think either, but I can still imagine a question like "according to relativity, how would a stationary table perceive time if it was a personal, conscious being?"

1. I know
2. I guess my question is, "how would a conscious being perceive such a state?"
3. I understand what happens when you approach c. My question is, "what do you think that would be like?" or put another way, "how would you describe such a state of existence?"

My question is philosophical, not physical.

11. Apr 27, 2010

### xcrunner2414

Re: If I was light...

Let me rephrase my question:

Suppose photons are conscious, personal beings, just like us, i.e. they have thoughts and are aware of themselves and their surroundings (I know, it's completely ludicrous and impossible). Just PRETEND. Use your IMAGINATION, not your physics degree. How would you describe their perception of reality, specifically their perception of time?

12. Apr 27, 2010

### HallsofIvy

Re: If I was light...

Well, I don't have a physics degree so I don't have to worry about that. Using my imagination, I imagine that if I were a photon, I would see pink dragons and golden unicorns playing leap frog (which, given the unicorns' horns, and the dragons' tendency to blow flame when annoyed, is a dangerous game!).

Not what you wanted? Then what do you want when you ask a physics question and say "use your imagination, not your physics degree"? This is after all, a physics, specifically relativity, forum. Why would you come here and say "ignore physics"?

Once again, you are essentially asking "imagine that relativity is wrong, so we can have this situation- what would relativity say about this situation." And that is a question that simply makes no sense.

13. Apr 27, 2010

### bapowell

Re: If I was light...

So you're hoping to win an argument by saying, "some guy used his imagination on physicsforums and it agreed with my imagination"? What you should be gathering from everyone's helpful responses (especially bcrowell's) is that your question is poorly posed from a physical standpoint. If you want a philosophical answer, why not take it to the philosophy forums?

14. Apr 27, 2010

### xcrunner2414

Re: If I was light...

- How exactly am I asking you to pretend that relativity is wrong? Correct me if I'm wrong, but light travels at the speed of light, c. All I'm asking you to do is pretend you are light, just like in a physics lecture a teacher might say, "pretend that you are in a space ship moving at half the speed of light. The rest of the world would appear...".

So, pretend you are light. The rest of the world would appear (or wouldn't appear at all)...?

Last edited: Apr 27, 2010
15. Apr 27, 2010

### xcrunner2414

Re: If I was light...

- No, I'm not interesting in winning an argument, I simply want to read other people's thoughts.

- Because I'm willing to bet that a large percentage of philosophers can't do Lorentz transformations.

16. Apr 27, 2010

### bapowell

Re: If I was light...

You are lacking an important understanding of relativity. What would life be like....relative to what? Relative to people at rest? In order to relate life as a photon to life as a person at rest, you must carry out what's called a Lorentz Transformation. These transformations are undefined for reference frames that are moving with relative velocities = c. The question about the spaceship is perfectly consistent and one can answer it using the Lorentz transformations. Not so for light. This is what people have been trying to tell you for several posts now.

17. Apr 27, 2010

### Dmitry67

Re: If I was light...

Appear = to be able to observe something. Observe = to change the internal state. Objects flying at c can't change their internal state. They cant observe.

18. Apr 27, 2010

### xcrunner2414

Re: If I was light...

- Do you really think I don't know what a Lorentz transformation is?

t' = t0 / sqrt( 1 - v2/c2 )

let v = c

t' = t0 / 0

I know! It's division by zero! I'm asking you to use your abstract thinking skills to make a conjecture about what that would be like.

19. Apr 27, 2010

### bapowell

Re: If I was light...

Who cares? It's not division by zero, because the transform itself isn't defined there!!

20. Apr 27, 2010

### Dmitry67

Re: If I was light...

abstract thinking skills you say?
Abstract logic tells us that from FALSE you can derive anything, so there is absolutely no value in trying to make such self-contradictory 'conjectures'.
Are you working for the Hollywood? Do you need to show what astranauts would see when they flight faster than light? :) Use your imagination, show some color splashes you know, but this is physics forum...

21. Apr 27, 2010

### JesseM

Re: If I was light...

But what does "perceive" mean? Do you understand that when we talk about what is observed in ordinary slower-than-light frames, this is not the same thing as what slower-than-light observers at rest in these frames actually see with their eyes? For example, although moving clocks run slow in the coordinate system of a slower-than-light observer, if the clock is moving towards the observer they'll see it running faster than their own clock due to the Doppler effect.

If by "perceive" you're talking about what's true in a coordinate system where the photon is at rest, then the problem is that there are an infinite number of ways to construct such a coordinate system, and there's no particular physical reason to prefer any of them (unlike with inertial observers where inertial frames are preferred because the laws of physics are symmetric under the Lorentz transformation). If you're talking about what's seen visually, I suppose we can figure out how light rays from other objects intersect the worldline of a photon, but there'd be no way to decide how fast the photon is seeing successive intersection-events since a photon doesn't have any proper time (no internal processes that could function as a clock that could be compared with slower-than-light clocks so we can decide how fast one second 'for a photon' is)

The closest you can come to answering this question in the context of relativity is thinking about what things look like in the limit as you approach the speed of light (relative to external landmarks like the galaxy), though not all quantities are well-defined in this limit. In such a limit, the traveler will see all the clocks at rest relative to the galaxy (or close to it) as approaching zero rate of ticking, and also sees the length of the galaxy in the direction of motion as squashed down to near zero. Then there is also the question of simultaneity, I wrote about this a while ago on this thread:
And another post on how the limit isn't well defined because you can't decide how fast other photons should be traveling in the photon's own limit-frame:

22. Apr 27, 2010

### xcrunner2414

Re: If I was light...

- Right... because it's division by zero.

- Who said anything about false?

Perhaps what I'm asking, quite simply, is how light and time are related.

23. Apr 27, 2010

### xcrunner2414

Re: If I was light...

"So in the limit, perhaps you could say that the photon's entire history is traversed instantly, since it's going zero distance and all the different clock-readings it passes are squashed together on this zero-length path."

- That's the kind of stuff I was looking for. Thank you JesseM. Perhaps I should have been discussing the limit as v approaches c.

24. Apr 27, 2010

### bapowell

Re: If I was light...

Nice. Way to be snippy with people when it was you who posed a poorly thought out question

25. Apr 27, 2010

### Fredrik

Staff Emeritus
Re: If I was light...

Yes, that would have made a lot more sense. But note that it's not at all obvious that it makes sense to define that limit to be "the photon's point of view". I prefer to just leave it undefined. (When we're talking about massive objects there's a specific method we can use to associate an inertial frame with the object's motion and orientation in space, but that method doesn't work for massless particles).