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Comment by Filligree | Hacker News Reader

It’s intuitively a reach, because photons don’t move.

From a four dimensional perspective—which is the correct one, let’s not forget—the distance between a photon’s emission and absorption event is always zero.

“Tired light” would require the photon to spontaneously lose energy based on the Manhattan distance of the path it takes. That distance can be arbitrarily large, sure—a photon can cover the entire universe before absorption—but it isn’t the path the photon actually takes, so it’s difficult to imagine why it would matter.

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> From a four dimensional perspective—which is the correct one, let’s not forget—the distance between a photon’s emission and absorption event is always zero.

That would require the emission and absorption events location in the space to be the same would it not? Or do you mean that a photon does not experience time, so relative to the photon it simply travel instantaneously from emitting coordinate to absorption coordinate?

The distance metric for flat spacetime is given by:

   ds^2 = dx^2 + dy^s + dz^2 - cdt^2

For a particle traveling at the speed of light, this gives ds=0.

> That would require the emission and absorption events location in the space to be the same would it not?

I think they're referring to the distance in Minkowski space,[0] where the time component and space component cancel out for light.

[0]: https://en.m.wikipedia.org/wiki/Spacetime#Spacetime_interval

Where does your claim that photons don't move come from? Very simply, the Sun is 8 minutes from the Earth. The photon is most definitely moving from the Sun to the Earth.

They are talking about "intervals" in 3+1 dimensional space time. To explain it rigorously, one needs to define a few terms like "metric" or "light cone". One defining property of the photon is that its space-time interval between any two events in its history is zero. You can think of it as the dual of the fact that a clock moving at the speed of light would be completely stopped.

Intervals are the square of the distance, so not quite, but… yes.

There’s a tendency to treat spacetime intervals as something abstract and possibly metaphysical, because we can’t see or touch them, and they work counterintuitively.

They’re actually the opposite. Our normal experience of 3D space is what is unphysical; the (square root of the) spacetime interval is simply distance, measured in meters. Or seconds, of course.

The literal, physical distance between an absorption and emission event is always zero. Reality is non-Euclidean. The Manhattan distance is larger than the actual distance, which is usually true anyway.

Well, I suspect I just spent a lot of words telling you something you already knew, so I’ll leave off with a book recommendation. Dichronauts, by Greg Egan.

Between that and the Clockwork Rocket trilogy, I don’t know a better way to inculcate an intuition for special relativity.

One is set in a 2+2 spacetime; the other in 4+0. The differences are instructive.

This made me laugh given that, actually, Greg Egan is by far my favorite author. But I also think you are going way too far by saying that our normal 3D space is what is unphysical. The fact that our everyday reality is the limit of a more interesting reality when some parameter becomes small, does not make it any less real, for any reasonable definition of "real".

Well, it's certainly consistent. And for the most part you're right; it's just light that gets hard to understand from this perspective.

Unfortunately photon behaviour is fairly important...

Briefly, from special relativity. As something approaches the speed of light, time slows down for it. At the speed of light (through vacuum) time stops, and in that reference frame there is no such thing as distance.

Thoughts on gravitational lensing point above? From the photon's point of view, it is experiencing changes through time and space.

That point misses (or rather ignores) the larger point. Ignoring special relativity because you think Einstein was "lost in the notation" is an extraordinary claim for which no extraordinary proof exists.

Never said that. I'm solid on speed of light as a max, gravitational lensing, gravitational redshift, and all the real world measurements that are in total sync with relativity.

The point is that I think claims like "distance doesn't exist" is the kind of thinking that comes from people lost in notation.

It's 8 minutes for an "external" observer. Physicists say the photon itself does not experience time or space.

The photon does experience change over time and space. Take gravitational lensing by the Sun. The photons that we see from behind the Sun, are definitely affected by the Sun. From the photon's point of view, it is experiencing changes through time and space. Thoughts?

Not from the photon's point of view. It is emitted and absorbed immediately. The distance traveled (in 4d spacetime) of a photon is always 0, no matter (pun intended) what's in between or what you observe.

I don't think it is true to say that "it is emitted and absorbed immediately", because there is a very real difference from a photon going from the Sun to the Earth, to a photon going from the Sun to Jupiter.

I know the theory the view you're saying comes from, and the weight it has in academia, but it always struck me as useless. And that whoever came up with it was lost in notation.

>because there is a very real difference from a photon going from the Sun to the Earth, to a photon going from the Sun to Jupiter.

Not from the photon's point of view. This is very crucial and it is not because of notation or maths. The real difference is from your point of view. I know it's counter-intuitive but that's different from being useless.

Sun->Earth or Sun->Jupiter is a small beer. It is the same for photons from the CMB being recorded. For them, they have been emitted and absorbed at the same instant and experienced no passage of time or distance traveled

"Sun->Earth or Sun->Jupiter is a small beer"? How do you mean? This is a real physical example with real physical consequences for example for how much dimmer the light is per area (by 1/r^2) and by how much redder it is (by 1/r).

What are the real physical consequences of thinking that the photon is "emitted and absorbed at the same instant"?

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