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Comment by chrisweekly | Hacker News Reader

> "A photon instantly travels to the end of time"

Please explain this. TIA

First set gamma as being 1/sqrt(1-v^2/c^2), with "c" being the speed of light. The factor for time dilation and distance contraction in special relativity is gamma and 1/gamma respectively.

That means that when you get to speeds equal to c, your time runs infinitely slower and the distances are infinitely shorter. So if your clock is infinitely slower, so every travel at "c" speeds means that no time passes for you. And if your distances are infinitely shorter, all travels at "c" speeds cover any distance as immediate. So you could reach every point of the universe as if it was immediately closer and in no time at all.

So in the frame of reference of the photon, the moment it is created it has already reached its destination, be it wherever it is on the universe.

Of course we can never reach "c" as beings with mass, but we can get closer to that. So for example if you get to 99.99999999999999% of the speed of light, you could travel a distance of 54,794,520 ly and only one year would pass to you, while 54,794,520 years would pass on earth.

thanks, great explanation

Follow up question from someone who's mostly forgotten his university physics.

Do photons actually exist, in the traditional sense of physical matter.

Or are they just a convenient short hand to describe the transfer of energy via waves in the fabric or space time, if they dont experience the universe when passing through it but only when interacting with matter and matters "dents" in space-time.

As a non-physicist, my understanding is that they actually exist, but can't be thought of as flying around like ping pong balls. I think it's one of those things that comes down to interpretation though, where the math is very clear but how you think of what it "means" lies beyond science.

Went on a slight rabbit hole reading after posting this and seems like we're all just excitations in a stack of quantum fields :-D

A very layman’s explanation I read a long time ago. Imagine you are in a car driving in a big open desert. You can drive in any direction you want. You can drive East or South or West or North, or some degree between them. But the more you travel East, the less you can travel North or South. The more you travel North, the less you can travel East or West. If you drive completely North, you aren’t traveling at all in the East/West direction.

Spacetime is like that, except North/South is space, and East/West is time. The more you travel through the spatial dimension (the faster you go) the less you travel through the time dimension (the less time passes for you). Photons are traveling completely through the spatial dimension (North/South) and so aren’t moving through the time (East/West) dimension at all, so from a photon’s perspective (if they had perception) no time ever passes so they would zip around “instantly” and never “experience” anything.

It is also why the speed of light is the fastest you can go. Once you are going 100% North, you cannot be going any more North.

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