jb1991 4 days ago

You are saying, from the perspective of light, whether it travels 1 mile or a trillion miles, that journey takes the same amount of time?

▲

sh-run 4 days ago | parent | next [-]

I'm not a physicist, but I believe that's the exact insight that led to special relativity. It goes something like: If your moving at 1,000kmh next to a jet moving at 1,100kmh then the jet is moving at 100kmh relative to you. Eventually people realized those wasn't the case with light. No matter how fast the observer is, light still moves at 299,792,458m/s. Einstein figured out that if the speed of light is fixed despite relative motion, then time must slow down as you move faster. So from the perspective of a photon no time has passed since its departure.

▲

cryptonector 4 days ago | parent | prev | next [-]

Time does not pass for photons. So yes, that is exactly what GP is saying.

There's also a different thing that GP might be hinting at, which is that by convention we assume that the speed of light is the same in all directions, but there are other conventions we can use as long as the round-trip speed of light agrees with that which we've measured (and yes, we can only ever measure the round-trip speed of light, FYI). Another convention is that all the light we see takes zero time to get to us but the light we emit goes out at half the speed one would expect with the standard convention (known as the Einstein synchronization convention). So instead of "light we see from Alpha Centauri is 4 years old" or "we see Alpha Centauri as it was 4 years ago" we can say that we see it as it is right now, but this is not a very commonly used convention.

▲

jb1991 3 days ago | parent [-]

interesting, why would this be?

> we can only ever measure the round-trip speed of light, FYI

	▲	cryptonector 3 days ago \| parent [-]
		Because say you fire a photon from an emitter to some target and you want to know how much time elapsed for that flight, but how would you find out? The target will have to communicate to the observer (you in this case) when it received the photon, but that communication will require.. more photons, and a trip back to you. If you're colocated with the emitter then it's a round-trip, else you need a photon from the emitter and the target, as well as the one from the emitter to the target, and this amounts to a round-trip anyways. Therefore you can't measure the speed of light in any one direction. You can only measure the round-trip time of flight (e.g., if you have the detector at the emitter and use a mirror).

▲

seabass-labrax 4 days ago | parent | prev | next [-]

I'd really highly recommend the Uncle Albert series of novels by Russell Stannard:

https://booksforkeeps.co.uk/article/visiting-uncle-albert/

The intuition you can develop about special and general relativity from these books is pretty amazing!

▲

oneshtein 4 days ago | parent | prev [-]

Yep, this is what he saying, but this is not what photon does. Photon must perform different amount of wave cycles to reach 1 meter or 1 trillion metters. These cycles can be counted.

▲

cryptonector 4 days ago | parent [-]

> These cycles can be counted.

In a lab setting, yes, but across such distances, no. Photons don't have a cycle counter on them, so they don't keep a cycle count and can't reveal that cycle count. All we can do is measure frequency/wavelength (spectrum, really, since we're going to see lots of photons, not really onesie/twosies) and intensity, and we can use the astrophysical distance ladder to figure out roughly where the emitter must have been.

	▲	oneshtein 3 days ago \| parent [-]
		Red shift allows us to roughly calculate distance and time, so we can multiple time by frequency of light to calculate number of oscillations or cycles and then calculate loss of energy per oscillation at average.