[pdonis]

> If they are moving with the speed of light no time passes for photons

No, the concept of "time passing" for an object moving at the speed of light has no meaning. That's why "lifetime" has no meaning for a photon if it has zero mass.

[omarchowdhury]

Wouldn't c have to be infinite for "no time to pass", since light is still traversing space at a finite speed?

[ben_w]

No, because the rules of Lorenz invariant field theories mean that the infinity you’re (presumably) expecting is mapped, via the joy of hyperbolic geometry, onto the propagation speed of that field.

Or, to put it another way, the speed of light is where time dilation and length contraction run into each other and it all goes zero-divided-by-zero.

[pdonis]

> Wouldn't c have to be infinite for "no time to pass"

You're using a different notion of "time passing". Yes, we, observing photons, certainly observe them to take a finite time to cover a finite distance. But that is not the same as the concept of "time passing" for the photons themselves, for example according to a clock that the photons carried along with them; that is the concept that has no meaning.

[omarchowdhury]

If the photons were a conscious subject, would that mean, from their point of view, they are everywhere (everywhere being defined as the entire path the photon will take from its genesis to its final destruction or absorption), all at once? Because if no time is passing for them, doesn't that mean no space is being traversed either?

[etaoins]

The flip side of time dilation is Lorentz contraction. As you approach the speed of light objects in your direction of motion will become shortened from your frame of reference.

For example, there are particles from cosmic rays that should not be able to make it to the surface without decaying. However, they're detected all the time. Two valid ways to think of this are:

1. From the Earth's frame of reference time moves more slowly for the particle. This slows down the process of decaying.

2. From the particle's frame of reference the Earth's atmosphere is considerably shorter so it doesn't need to travel as far.

Things get a bit hairy to talk about once you actually reach the speed of light. One way to think of it might be from the photon's frame of reference its entire path has become infinitely short so it had no distance to travel at all.

[omarchowdhury]

> One way to think of it might be from the photon's frame of reference its entire path has become infinitely short so it had no distance to travel at all.

From the photons frame of reference, then, they do not move at all?

And the environment that photon "experiences", being the path in the universe that it traverses from our point of view; is the past, present, and future (from our point of view) all in instant simultaneity for the photon?

[pdkl95]

Unfortunately when talking about physics, sloppy human languages and our tendency to anthropomorphize when describing very non-human-like things cause a lot of communication/learning problems. Concepts like "experiencing an environment" and "time" don't make sense for the photon, which is sort of equivalent to "moving at c" because experiencing something like "time" requires interactions (events) at different places in spacetime.

Saying "neutrino has a very small mass" is roughly equivalent to saying "neutrinos very rarely experience an oscillation event (changing into a different flavor)". The distance between the rare events is the "time" it experiences. These are so far apart in spacetime for the neutrino it's experience of time (the way it evolves over spacetime) is extremely slow. More massive particles are "more massive" because they frequently interact with the Higgs field. More interaction events means their experience of time happens faster.

The photon (and anything else with 0 mass) only experiences two events: it's creation and destruction. It moves at c because it's never being slowed down by experiencing interactions.

For a very good explanation of this (with helpful animations) this[1] short playlist (6 ep) of PBS Spacetime episodes.

[1] https://www.youtube.com/playlist?list=PLsPUh22kYmNCLrXgf8e6n...

edit: TL;DR - When speed-of-light particles pause to interact with things (thus moving < c)slowing it down), we say that particle "has mass". Mass is a measure of how frequently those interactions occur (aka how much "time" it experiences).

[pdonis]

> From the photons frame of reference, then, they do not move at all?

There is no such thing as "the photons frame of reference". It is not even a well-defined concept.

> And the environment that photon "experiences", being the path in the universe that it traverses from our point of view; is the past, present, and future (from our point of view) all in instant simultaneity for the photon?

No, none of this is correct. The reason I keep insisting that all these concepts are not well-defined for a photon is to make it clear why you cannot draw all these inferences that you are trying to draw--they are all wrong. The only way to stop drawing them is to recognize what "not well-defined" means. It means the questions you are trying to ask about photons are meaningless; they are like asking how long the color red is or how much time passes for it. Photons are simply not in the category of things for which those questions make sense.

[pdonis]

> One way to think of it might be from the photon's frame of reference its entire path has become infinitely short so it had no distance to travel at all.

No, that is not a correct way to think of it. See my other responses to the poster you responded to.

[pdonis]

> If the photons were a conscious subject

They can't be. You can't make a conscious subject out of something that has zero mass and moves at the speed of light, and therefore does not have a well-defined concept of "time passing".

> if no time is passing for them, doesn't that mean no space is being traversed either?

It is not the case that no time is passing for a photon. What is the case is that the concept of "time passing for a photon" is not well-defined. That concept not being well-defined means that "space being traversed for a photon" is not well-defined either. That does not mean "no space is being traversed". It means the concept of "space being traversed for a photon" is not well-defined.

[Yajirobe]

No, outside observer will measure clocks of photon run slower by a factor of γ (Lorentz factor), which (in scalar form) is equal to 1/sqrt(1 - v^2 / c^2 ). It diverges at v = c.

[pdonis]

> outside observer will measure clocks of photon run slower by a factor of γ

This is not correct since "clocks of photon" is a meaningless concept (at least if photons have zero mass, which they do according to our best current models).

[Yajirobe]

Obviously, I did not mean to imply that you can attach a clock on a photon. I thought this goes without saying.

[pdonis]

> I did not mean to imply that you can attach a clock on a photon.

The issue isn't that you can't attach a clock to a photon in a practical sense. The issue is that even in principle, the concept of "the clock of a photon" is not well-defined. For it to be well-defined, there would need to be an inertial frame in which a photon was at rest. But that is impossible.

[Florin_Andrei]

Speed of light is for-all-practical-purposes infinite in the reference frame of the moving object (i.e. photon), thanks to special relativity.

[pdonis]

> Speed of light is for-all-practical-purposes infinite in the reference frame of the moving object (i.e. photon), thanks to special relativity.

This is not correct, because there is no "reference frame" for a photon moving at the speed of light. The concept of "reference frame" is not well-defined for such objects.