Say Im on earth with a clock and a photon - with a clock - is emitted from the sun towards me.
Is the following correct: when the photon reach me, my clock ticked for say 1 hour and I see photon's clock ticked for say 1 min. But the photon sees its clock at 1 hour and mine at 1min?
Basically, light doesn't experience time as such, so your question kinda doesn't make sense as written.
I'm glossing over details here, but if I'm wrong I would love to learn why from somebody with more knowledge!
"The photon has a watch, and I have a watch. Both read 12:00.00 when the photon is emitted from the sun. What do the watches read when the photon reaches me?"
Photons take ~8 min to travel from the sun to the earth. Therefore MY watch reads 12:08.00.
The photon's frame of reference is at the speed of light though. So because of time dilation, the photon's watch reads 12:00.00.
The photon traveled all that distance without experiencing any time at all. In fact, if you, a human being, could travel at the speed of light (spoiler: you can't) without running into anything, you would live forever in reference to the universe.
Thats the part I dont understand. From earth point of view the photon (or the spaceship) goes at speed of light (or close to it). But from the photon point of view, the earth is going at speed of light. If thats not the case, then speed isnt relative its absolute: photon goes at speed of light, earth does not.
Second, a photon doesn't have an inertial frame of reference. There is no "photon point of view". The equations that relate the velocities of the same object as seen from different reference frames break down when you plug in the speed of light as relative velocity between the two frames. Put differently, all frames of references have relative velocities that are lower than the speed of light.
Hope this answers your question!
Not quite tbh. What if we are taking a fast spaceship instead of a photon. Spaceship starts from sun toward me. When it reaches me, I see my watch has ticked for an hour and I see the spaceship watch has ticked for 15 min. But from the spaceship point of view, earth was going toward it. From spaceship point of view, its watch must have ticked for an hour and it sees mine has ticked for 15min.
If things are relative why isnt it the case?
The question this paradox comes down to is, when are you comparing your watches? The two observers (the spaceship and you) won't be able to agree on what it means for two events to occur at the same time. Once you take this into account you will realize there is no contradiction and the POVs of the two observers are indeed symmetric as expected from the word "relativity".
For more on this check out e.g. Bernhard Schutz's "A First Course in General Relativity". IMO it's a rather terrible book as far as General Relativity goes but the first chapter, which is about Special Relativity (~25 pages), is fantastic and is was what helped me finally understand SR and, in particular, the paradoxes.
So, the thing the observers disagree in their different reference frames is what “now” means.
Damn its frustrating. Month I ask questions about that. Every time I get: velocity is absolute. Maybe it is, maybe thats the answer to my question, velocity is absolute, meaning the ship is going toward earth in "absolute". But then one could be at rest in the universe and then there is an absolute frame in the universe thats motionless. Therefore we could elect a center.
Let’s turn the problem round. The ship is leaving Earth at some high velocity. In Earth’s frame after 60 minutes have passed the ship’s clock reads as 15 minutes having passed. In the ship’s frame of reference when 60 minutes have passed the clock on Earth will have recorded 15 minutes.
Now this sounds impossible because you think we agree on when events are simultaneous, “A’s clock is at X so B’s must be at Y,” but this is only true when A and B are in the same place, if they aren’t in the same place then you can only say, “A’s clock is at X so in A’s reference frame B’s must be at Y.”
Essential you can divide space time into three regions for a point A. The future is the cone of space time that could be reached from A without exceeding the speed of light. The past is the cone of space time from which an object could reach A without exceeding the speed of light. The third region isn’t the future or the past, and for any point B in this region we can find an inertial reference frame where A and B are simultaneous, or where A happens before B, or where B happens before A.
We can only ever globally agree that points in A’s future cone are after A, and points in its past cobe are before A, and these remain true in every inertial reference frame.
For a 60 min travel between earth and ship, there is a point where ship reaches earth reading ship:60 and earth:15. And there is a point where earth sees the ship land and read earth:60 and ship:15 resp. Those are not the same points.
So when you travel at high speed in a frame A, you're kind of gonna "experience a different story" than objects traveling in low speed in frame A relative to each other.
Edit: do we know an object with a mass thats traveling at high speed in our frame?
Edit2: Oh but the universe is expanding so we do actually.
Mind boggling really.
From earth point of view, when it comes back the passenger has outlived us all whereas from the passenger point of view, when he comes back, he only left for an hour.
Also, if it is the case that from the POV time is not passing at all, all time passes in no time, which would mean perceiving the entirety of time in one static juxtaposition,
but... can information reach a thing traveling at light speed? Does it matter the angle of incidence? (Nothing can catch up, but information-carrying particles like other photons could intersect or approach head on... but with their own relative velocity capped...?)
Time to fire up the Wikipedia...
Really though the physics says nothing can _attain_ the speed of light, so light (or some other photon) could still intercept something with mass traveling at 99.9999999% the speed of light. And that thing with mass will never reach 100% the speed of light.
If you want to look deeper I suggest checking out the accelerating expansion rate of the universe, and what that means for information traveling throughout the universe over the next billions of years. Humanity is actually extremely lucky to have been born at a time when the universe was young enough to still be mostly visible. Civilizations that come about in the next 10s of billions of years might actually assume their galactic clusters are the only ones in existence (much like we thought initially until Hubble found another galaxy).
...if the photon comes to rest on earth, and we compare clocks...
IIRC, the deceleration of the photon relative to me (as it comes to rest on earth as would a returning space traveler) would "reverse" the time dilation relative to me, so both our clocks would read 12:08.
Please, correct me if I'm wrong. I read this many years ago but never dove all the way into the math.
Which is why if you could accelerate yourself to say 90% the speed of light, then decelerate and return to earth, you would have essentially traveled into the future and outlived everyone you ever knew.
It is like the waterplanet in the movie Interstellar (I think it was that planet), where they spent a short while on the surface and the person in space aged decades.
It is called time dilation [0] and there was a rather interesting experiment done by Hafel Keating [1].
* Interesting to know: time slows down to a halt at the speed of light, and the one second time lapse of the photon goes to zero no matter how long you wait on earth.
[0] https://en.m.wikipedia.org/wiki/Time_dilation
[1] https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experim...