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From my understanding in space-time everything moves at the speed of light $c$ in some direction of this four dimensional space. Light itself moves only through space, so it doesn't move at all in time. So if a photon is 'created' in the sun (I have no idea how that works and if it even makes sense for a photon to be 'created'), how can we see/perceive this photon? Because the photon must first travel the 8 minutes from the sun to the earth it must have been 'created' in the future, since at the instant we see it, it already traveled for 8 minutes, but also it didn't move through time. Is this correct? I find it difficult to understand how things that move through time interact with things that do not.

Qmechanic
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timtam
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  • I’m not moving at $c$ in any frame; in fact I’m sitting at my desk right now so not moving at all. So: can you clarify your “everything moves at the speed of light $c$”. Light moves at $c$ in any frame.. – ZeroTheHero Jul 22 '21 at 12:54
  • @ZeroTheHero I am not a physics expert and I might be wrong about this, but I thought that like I described above everything moves at the speed of light in spacetime. So since you are sitting at your desk you are moving only slowly through three dimensional space but very fast through time. Light on contrary doesn't move at all through time since it moves through space at $c$. – timtam Jul 22 '21 at 12:58
  • not sure what you mean by moving “very fast through time”. Can you provide an example of something moving slowly (but not zero velocity) through time? – ZeroTheHero Jul 22 '21 at 13:00
  • To get this straight, I have never studied physics and I am an absolute layman. But I think "moving fast through time" is what we normally do, since we move very slowly through space. If you were to accelerate an object close to the speed of light in space only, then this object would move very slowly through time. This video explains it very well, although it is not very scientific: https://www.youtube.com/watch?v=au0QJYISe4c. – timtam Jul 22 '21 at 13:06
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    Moving in time. What do these three words mean? In my opinion, nothing, but maybe you could explain. – GiorgioP-DoomsdayClockIsAt-90 Jul 22 '21 at 17:12
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    @GiorgioP - It means progressing from earlier time to later time. Similar to what moving in space means - progressing from position 0 to position 1. Though time is different from what classical physics would lead you to think. This may be of interest - What is time, does it flow, and if so what defines its direction? – mmesser314 Jul 22 '21 at 18:47
  • @mmesser314 the real problem when discussing time is a dangerous tendency to use words in an uncontrolled way with a high risk of circularity. Most of the answers in the link have this defect. Also progressing in time as an explanation of moving in time does not escape the same problem of circularity. – GiorgioP-DoomsdayClockIsAt-90 Jul 22 '21 at 21:32

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Light itself moves only through space, so it doesn't move at all in time.

I think this is where your misunderstanding lies. In relativity, elapsed time depends on the reference frame in which it is measured. It is true that the proper time along a path followed by a light ray in vacuum is constant. So, relative to its own reference frame no time elapses for a photon, no matter how far it travels in space. But relative to any other reference frame the photon (in vacuum) travels a distance $d$ in time $\frac d c$ where $c$ is the speed of light. So the time taken relative to the Earth's reference frame for a photon to travel the eight light minutes from the Sun to the Earth is eight minutes.

gandalf61
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  • So the photon does not come from the future, but from eight minutes ago? :( – timtam Jul 22 '21 at 13:00
  • also, do you have an article/book/video recommendation to further understand reference frames? – timtam Jul 22 '21 at 13:07
  • @timtam it's a question of reference frame. In your frame on Earth, the photon does come from 8 minutes ago, but in the frame of the photon no time elapses. I highly recommend : A Traveler's guide to SpaceTime by Thomas A. Moore, https://www.amazon.com/Travelers-Guide-Spacetime-Thomas-Moore/dp/0070430276 – ZeroTheHero Jul 22 '21 at 13:53
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    There is nothing like the photon reference frame. That is the key misunderstanding. – GiorgioP-DoomsdayClockIsAt-90 Jul 22 '21 at 17:08