How the world changes from the point of view of a photon [duplicate]

Question

Imagine a universe without time, or more specifically without the Flow of Time. Everything will be a 2D projection and nothing more. No movement, no interaction, and in other words no Change.

But our universe is not like this. We have movement, we have interaction, and our world changes in many ways. But there is a catch. We have some 0 mass particles in our universe, which move at the speed of light e.g. photons and, according to the theory of relativity, these particles are not affected by time. So the universe is static from the point of view of a photon.

But it can't be like this. Our world is dynamic. And this kinda raises a contradiction. On the one hand, the universe is static and in another the universe is dynamic. Can anyone explain this?

Answer 1

It's simple: there are only two elementary actions that can happen to a photon - it is generated at one point and then it is consumed at another. Its whole life consists of just these two events, two "instants" if you will - the "travel" is really just the delay between those two occurrences with regard to everyone else. Two instants, each of zero duration, comprise zero time: $0 \cdot 2 = 0$.

Because of this, it doesn't have the capability to define an "experience" that is anything more than those two events - and thus you cannot ascribe to it a "view" of the Universe. This is why, in a sense, you cannot define a reference frame for a photon that "makes sense", as you're running into. The "world view" is "I get emitted, I get absorbed". Boom. That's it.

Another thing that is often missed is that "reference frames" do not exist as an entity in their own apart from spacetime and its contents. What they are, instead, are just systems of coordinates, that is, ways of labelling events i.e. points in spacetime, and in that case are wholly arbitrary. The Lorentz frames in special relativity are arbitrary - what is not is the Lorentz symmetries that relate them, which are self-maps of the points on spacetime as points regardless of labels, and the fact that the dynamics, the physics, on that spacetime, respect those symmetries, meaning that if we want to describe a moving observer's view of the world, we can do so without altering the form of the equations we were already using - which is how you can talk hypothetically of a Lorentz symmetry violation without it tripping over itself in a contradiction. (Note that you can always create a "moving reference frame" in any sort of universe that has a notion of space and time, but it may be that you need to use different "physics" to talk about this: toy example is Conway's Game of Life. Its rules are not motionally symmetric in any way.)

And to link that back to the idea of "experience", any "way of experiencing" spacetime can be related to some coordinate system. In fact, the Lorentz frame is not any human's experience of spacetime: instead, what you want is a "light cone frame" where the "present" is taken as one's past light cone. The Lorentz symmetry still relates it to the moving case, but the coordinates are all different and the symmetry takes a different form. (Whose experience is it? Well, because its "present" involves knowledge of spacelike-separated events, a patch of it can only be seen by an observer well into the future of the happenings in question, that has reconstructed the view from suitable observations.)

Hence, the failure of the Lorentz transformation to meaningfully define a frame means that the situation in the case of a photon is fundamentally different from the situation in case of everyone else, and its experience is likewise different.

Answer 2

There are various ways to answer this. See Would time freeze if you could travel at the speed of light?

I agree with David Z's answer, but I would put it differently. In short, there is no view of the world as seen by a photon.

There is an argument that as the speed of an object increases with respect to me, its clocks slow down with respect to mine. A faster object is closer to catching up to a photon. A photon must be like the limit of such objects. Its clock must be stopped.

This argument is misleading. Taking the limit does not lead to anything physical. There is no photon's view of the world. That is, there is no inertial frame of reference at which a photon is at rest. Saying that time does not pass in that non-existent frame is wrong.

You can get a hint of this by looking at photons in our frame. In the classical view, light has a wavelength and frequency. Light travels, and the phase changes as it does so. This is not consistent with photons seeing our world as compressed to a plane and having no time.

The quantum view is a little different, but the same argument can be made to apply.

---

This is not to say that the question deals with a completely unphysical situation. Consider an atom falling into a black hole. As it crosses the event horizon, it emits a photon straight up.

You can consider the world from the point of view of that atom. But the atom is not moving together with the photon. The atom is being sucked into the black hole. Naively, the photon hovers forever from the point of view of a distant observer. What that really means it the photon gets to the observer only after an infinite time.

The observer can assign coordinates to the event horizon. It stays at a constant location in his frame. But he cannot transform to find a momentarily comoving inertial frame of reference at the horizon. He gets a coordinate singularity if he tries.

A minor point is that a photon does not just exist in a plane. It is described by a wave function that would be partly inside the event horizon, and partly outside. Thus it would have a probability of being sucked in, and a probability of escaping. Taking a long time to escape corresponds to being in a narrow region just above the horizon. Taking an infinitely long time means being in an infinitely narrow region. The probability goes to $0$ as time to escape goes up.

---

There is a view of the world as a Block Universe. Time does not flow in this view. This is consistent with physics. See What is time, does it flow, and if so what defines its direction? But so is a view in which time does flow.