This question is kind of a mash between physics and philosophy. If I should delete this post and put it on the physics stackexchange, let me know.

If light travels at 299,792,458 m/s, then does it exist?

I will elaborate.

Time passes slower as you get closer to the speed of light. If you somehow have all of your atoms and particles accelerate to the speed of light instantly, then time moves so slowly for you that if you stay at that speed for any amount of time, you would come out exactly the same as before.

Since no time passed for you, even though it passed outside of you, that time that you were at the speed of light did not exist for you.

Now imagine that light particle. No time exists for it, even though time exists for us, and it exists for us. So, if time does not exist for it (it never gets slower than c), then to what degree does it exist?

So, assuming we know that things exist and this is not a simulation/brain in a vat/hallucinated/dreamed-up universe, could we prove that light exists/does not exist?

  • Light exists in a very different sense in which a living things exist: we "live in time" because we live. Maybe a particle of light does not "live". See Heidegger Dec 30 '20 at 16:20
  • A ray of light does exert pressure, so I do not really see the point you are getting at. Do you suggest that the theory that a photon would, as far as we know, not experience time if it could experience anything, does have any bearing on the existence of photons in our own frame of experience?
    – Philip Klöcking
    Dec 30 '20 at 16:31
  • 1
    Does this answer your question? Does the electromagnetic field physically exist?
    – Dave
    Dec 30 '20 at 20:30
  • What is the "degree" in "to what degree does it exist"? Time is relative and frame dependent in relativity. Objects "experience" different local times depending on how they are moving, photons do not "experience" it at all. If you look at a stick from different angles you will see it have different lengths, and if you look vertically down it will have none. That's how time works in relativity. What does this have to do with existence? Photon could equally "ask": if time does not exist for me but exists for you then to what degree do you exist?
    – Conifold
    Dec 31 '20 at 0:00

You are correct that a photon exists only instantaneously within its own Relativistic reference frame. This was a key insight of Einstein's. For a photon emitted from a star, propagating across intergalactic space to a camera at an observatory on Earth, within its own reference frame no time elapses and no distance is traversed.

It certainly has a physical existence. Four-dimensional spacetime is sometimes understood as a "block Universe", with the whole of its past, present and future forming a single continiuum. However, in the Relativistic model, the exact direction and rate of passage of time are not absolute, they depend on the particular observer who is measuring it. Any observer must follow their own timeline too, and can see only what is within their "light cone", the region of spacetime which is accessible below lightspeed. The rest of the block universe is inferred only because the laws of physics assume a symmetry which requires its existence. A photon follows a straight line or "geodesic" across the block landscape. So if the block universe exists and can support structure, then the photon also exists within it.

To the photon, neither space nor time exist. Yet it must undergo an absolutely fixed and finite number of oscillations (i.e. the same for all observers) during its journey, so it has some "knowledge" of its adventure and is more than just a structureless point.

So yes, on that basis the photon exists all right, but the nature of its existence is not easy to unravel.

For example we may want to cut an instantaneous spatial slice across block spacetime, call it "now", and declare that only "now" truly exists. But where the slice falls is relative to the observer. If I cut the slice relative to myself, the photon is a point in an identifiable location (or maybe a probability distribution across a certain patch of space). But relative to the photon itself, block spacetime has no extent, the slice falls... well, let's think about that.

Perhaps a clue is to be found in twistor space, a transform of block spacetime formulated by Roger Penrose, and which he has described loosely as "light-ray space". In this model, the photon is a point in twistor space and any given point in block spacetime, through which it has passed, is smeared out across twistor space, among all the other photons which passed that way. Any slice of "now" must be smeared out in similar fashion, as all the photons it crossed paths with on its journey. Twistor space is of major importance in high-energy particle physics, where it greatly simplifies the calculation of particle-particle interactions. That is to say, particles which met at a common here-and-now point in the block Universe.

All this is a "classical" or large-scale model of existence. On the quantum scale things behave very differently and the nature of existence has its own problems. According to the mainstream "Copenhagen" model, nothing exists except measurements and their probabilities. But that is really another question.

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