If a photon does not experience time and exists in its full trajectory at a given instant, how does it “perceive” a body moving to cut its path?



OK maybe ELI12 would do.

In: Physics

It doesn’t perceive it. It just interacts with a body somewhere on its path. From the POV of the photon, no time has passed or will ever pass. But that doesn’t mean that every photon everywhere exists forever; from the POV of the body in its path; there was no photon at one moment, then there was a photon hitting it and being absorbed. The photon experienced no time, but the electron cloud of the body that absorbed it did.

In this way of envisioning the universe from the point of view of the photon, “movement” does not exist because time doesn’t really have meaning. The object that intersects its path can be visualized as a 3D object smeared through the 4th dimension, and that smear at some point intersects the ray of the photon in the 4th dimension.

To make an analogy, imagine you are looking at a road that crosses a railroad line. How does the road “see the rail line move and interact with it”? It doesn’t, they exist simultaneously with a particular point of intersection.

It can’t interact with anything, so it doesn’t experience anything, it just goes from point in space-time to point in space-time instantly, there is nothing in between.

If you think of time as a series of 3d snapshots, then for photons it breaks down as from their perspective, they never exist, they just are created at point A and instantly get absorbed at point B.

Taking a step back, nothing can cut a photon’s path, its path ends at its final destination, the destination was determined the instant the photon was released.

The instant the photon was released was the same instant that something blocked its path millions of light-years away, the two timelines are just offset by millions of years.

Don’t take the analogy and GR thought experiment of the photon not “experiencing time” too far. Photons don’t experience _anything_, they don’t. An object moving at the speed of light can still run into something, quite obviously. It still moves from point A to point B, and that takes a very finite amount of time.

IF you were a photon moving at the speed of light, you’d perceive the rest of the universe passing by in an instant because of the exponential nature of the Lotentz curve which dictates the effect of time dilation on a moving object. However, you’re not, and you don’t, and you can’t, and massless photons aren’t conscious.

For you to perceive, information must reach you, but no information can reach you because it can’t go faster than you.

For a particle that moves at light speed, there is no time. It simply exists everywhere on its path instantly and simultaneously. Nothing ever changes.

If we are talking about something moving at “the speed of light” (or the local invariant speed), then either the thing hits the body or doesn’t. If it hits the body, that is because it was already there. If it doesn’t, the body was never there, but always to the side.

Or to put it another way, the object either is instantaneously absorbed by the body if it hits it, or it doesn’t need to “perceive” it because the body is out of its path.

Disclaimer: the rules for SR don’t apply to things travelling at the invariant speed. Also photons don’t always travel at this speed, for given definitions of “photon”, “travel” and “this speed.”

A photon perceives nothing. It’s a simple particle. There isn’t much value in imagining the perspective of a photon because it has no perspective, and it isn’t possible for something that does have a perspective to move like a photon to experience it.

The faster you travel, the smaller distances seem. This can be hard to envision, since it takes a pretty good bit of speed to actually see this in action, but imagine a car speeding past you really fast. The faster it goes by, the less time it takes to pass you, right? This is partially a function of speed, but it’s also because the car literally looks smaller. However, this is only from the point of view of someone observing the car. From the guy driving the car, he’s sitting still and not moving. In fact, YOU are the one moving very fast indeed. He’s simply sitting still while the world zooms by. As a matter of fact, you are the one who shrunk a bit while he drove by. This is because while fast things shrink, fast things also view everything else as shrinking instead. This all comes to a point when you reach the speed of light. At the speed of light, everything else has shrunk so much that distance is meaningless. Everything is at the exact same point and can’t shrink anymore. Therefore, movement is meaningless. Movement is just moving some distance over time, and distance no longer exists. So a photon wouldn’t “perceive” a body moving to cut its path. To a photon, it doesn’t have a path because a path implies some distance to be travelled, some point B to get to from point A. Point A and B are the exact same point, as is everything else, because everything is shrunk. Either there is an object for it to collide with, or there is not, and the photon would never know the difference, because it would happen instantly by its personal timescale.

Can you explain this question to me like I’m 5?

What a bunch of bologna in this thread!

* *”A photon experiences no time”*

* *”To a photon, the universe is compressed into a flat 2D space”*

Nonsense! This is not physics!

A photon does not experience anything, and we can’t construct a photon frame of reference. People are taking the “limit” solution of relativistic effects, but you can’t just do that. One of the fundamental postulates of relativity itself is that the speed of light is the same for everyone (= there is no photon reference frame). The rest of relativity follows. You can’t use those *new* rules, based on the assumption that there is no photon frame, and apply them to the photon frame, and expect a meaningful answer. The whole machinery breaks down in the process. As is evident by all the wishwashy explanations in here to the thousands of paradoxes which emerge as soon as you try to talk about photon frames.

It can be fun to *speculate* about photon reference frames, but it has *nothing* to do with relativity. It is the complete *opposite* of relativity. And unless someone has some groundbreaking physics to share, this thread belongs in the philosophy department.