[henry2023]

From the perspective of a photon, there is no such thing as time. It's emitted, and might exist for hundreds of trillions of years, but for the photon, there's zero time elapsed between when it's emitted and when it's absorbed again. It doesn't experience distance either.

[dataflow]

Something about this doesn't make sense. This means that, from its own perspective, every photon is emitted at time T at location x, then absorbed at time T at location y, meaning there is no time "in between" those... yet somehow, at a distinct point in between those events, an object can get in the way and block the photon from reaching point y. Meaning you have two points in time that have the same value but aren't identical? How does that work, mathematically? Do you need nonstandard calculus or something to make sense of it?

[fnordpiglet]

This is because a photon has no mass and always travels at the speed of light in all reference frames. A photon never observes time because the effects of time dilation at the speed of light is infinite and due to relativity they don’t experience the events between their emission and absorption as they don’t decelerate when they’re absorbed. In a very real sense from the photons perspective they never exist and don’t experience a temporal dimension. However from a frame of reference less than the speed of light - which all objects of mass experience - we can observe their creation, propagation, and absorption.

Reality is stranger than fiction!!

[favflam]

Think of time as defined by how many times a photon bounces between two mirrors. If the two mirrors move at the speed of light with the photon, the photon will never reach the mirror to bounce. Therefore, the photon experiences no time.

There is no universal time T in space time. Time is relative to the observer.

I suggest reading https://en.wikipedia.org/wiki/Minkowski_space for the mathematics.

[cmsj]

I am not a Physicist, so please don't rely on this, but my understanding is that this comes down to the frames of reference in special relativity.

If you're in a car on the highway, an observer standing still outside the car sees you moving at 50mph, but an observer inside the car with you, sees you not moving at all, but the world rushing past at 50mph. These are two different reference frames, and the second one is called a rest frame - any object can be considered to have a rest frame of reference where it isn't moving at all. Any object that is, which has mass.

Massless objects are always observed to be moving at the speed of light by any observer in any frame of reference. This is one of the most mind-bending parts of relativity, but it ultimately connects to the part of special relativity where speed affects time - the faster you go, the slower your clock runs compared to things moving slower than you, and the closer you get to the speed of light, the closer your clock gets to zero. Massless particles only travel at the speed of light, and thus have no clock, therefore they experience no time.

I hope I got that right, but even if I did, I still don't feel like I actually understand it at all!

[Leftium]

If something blocks the photon, that was "agreed" upon before the photon was created.

A photon travels between an electron that emits it and an electron that absorbs it. These two electrons must agree on the photon transfer beforehand. This has some strange implications:

- When you see a star at night, it's because photons from the distant star hit your eyes.

- These photons started traveling billions of years ago, when neither you nor your eyes existed yet.

- Yet the electrons in your eyes and the electrons in the star (which may no longer even exist) must have agreed to transfer these photons.

- From the photon's perspective, it traveled instantaneously from the star to your eye; it traveled through billions of years of time!

Source: https://www.youtube.com/watch?v=bAedYtUredI&t=2630s

[fnordpiglet]

While I think you can conceive of it this way I don’t think it’s precisely right. Photons don’t experience time at all and never exist from their own perspective as they have no mass and always travel at the speed of light in all reference frames. However causality at speeds less than the speed of light is a real thing and due to special relativity we can interact with photons with a different simultaneity. This is partially because they can’t experience the events of the universe going less than the speed of light due to the fact they absorb rather than decelerate.

This doesn’t mean there’s some cross time agreement between particles mediated by the photon because the reference frame of those particles are different than the photons and are necessarily slower than the speed of light.

[akomtu]

If time doesn't exist objectively, then there's no puzzle. What if photons are thin threads connecting distant things. The entire web of these light threads remains still, like an art sculpture. We may explore thin slices of this sculpture, one after another, and this slideshow will create an illusion that a few points, where the light threads cross the slices, move. We may try to find such a slice of this sculpture that the light threads would be seen in their entirety. Such a slice exists, but the light thread on it appears so densely packed that it fills the entire slice. It's truly everywhere on the slice. It's quite possible that in this sculpture all the light threads are really one thread.