5 ms·

What I think you're asking: "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 w

What I think you're asking: "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.

quadcore 4y ago
The photon's frame of reference is at the speed of ligh 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.
- aaroninsf 4y ago
  Thank you for this framing. I haven't thought about this particular corner before! 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...
  - Ancalagon 4y ago
    Nothing can travel faster than light, so no, nothing can catch up to something traveling at the speed of light. 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).
- cdelsolar 4y ago
  From the photon's perspective, the entire universe is compressed to a line of infinitesimal length, from the place it was emitted to the place it is ultimately absorbed, if any. In its perspective, it takes 0 time to travel through that line (really more of a dot).
- codethief 4y ago
  The velocity of objects moving through spacetime is relative but there are certain quantities that aren't. The fact whether or not an object is traveling at the speed of light is one such quantity (called the interior product of the 4-velocity vector). That is, it is independent of the observer. So in all frames of reference Earth is traveling at sub-lightspeed velocities and the photon is moving at, well, the speed of light. 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!
  - quadcore 4y ago
    Nice. Thanks. 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?
    - aardvark179 4y ago
      Right. So you see the ship taking an hour to travel from the Sun to the Earth, and the ship’s clock only counts fifteen minutes. But you turned this round and ask how much time the ship would see our clock tick during its journey? Now if everything is symmetrical then it would see our clock tick 3.75 minutes. So, the thing the observers disagree in their different reference frames is what “now” means.
      - quadcore 4y ago
        I thought A goes toward B at C is undistinguishible from B going toward A at C. So if A sees A:60min and B:15min, therefore B should see B:60min and A:15min. Anything other than that means something was absolute. 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.
jjslocum3 4y ago
Also what I think is being asked: ...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.
- Ancalagon 4y ago
  No. This difference in the clocks will remain even if you could somehow slow down the photon without it being absorbed. This is why if you sync two atomic clocks, bring one atomic clock up to orbiting speed (such as on a gps satellite), then decelerate it and return it to earth the clocks will still read different numbers. 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.
karmakaze 4y ago
Do photons have to slow down to change? Change requires time to pass doesn't it? So how does a photon change polarization (without slowing down)?

quadcore 4y ago

The photon's frame of reference is at the speed of ligh 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.

aaroninsf 4y ago
Thank you for this framing. I haven't thought about this particular corner before! 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...
- Ancalagon 4y ago
  Nothing can travel faster than light, so no, nothing can catch up to something traveling at the speed of light. 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).
cdelsolar 4y ago
From the photon's perspective, the entire universe is compressed to a line of infinitesimal length, from the place it was emitted to the place it is ultimately absorbed, if any. In its perspective, it takes 0 time to travel through that line (really more of a dot).
codethief 4y ago
The velocity of objects moving through spacetime is relative but there are certain quantities that aren't. The fact whether or not an object is traveling at the speed of light is one such quantity (called the interior product of the 4-velocity vector). That is, it is independent of the observer. So in all frames of reference Earth is traveling at sub-lightspeed velocities and the photon is moving at, well, the speed of light. 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!
- quadcore 4y ago
  Nice. Thanks. 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?
  - aardvark179 4y ago
    Right. So you see the ship taking an hour to travel from the Sun to the Earth, and the ship’s clock only counts fifteen minutes. But you turned this round and ask how much time the ship would see our clock tick during its journey? Now if everything is symmetrical then it would see our clock tick 3.75 minutes. So, the thing the observers disagree in their different reference frames is what “now” means.
    - quadcore 4y ago
      I thought A goes toward B at C is undistinguishible from B going toward A at C. So if A sees A:60min and B:15min, therefore B should see B:60min and A:15min. Anything other than that means something was absolute. 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.

jjslocum3 4y ago

Also what I think is being asked: ...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.

Ancalagon 4y ago
No. This difference in the clocks will remain even if you could somehow slow down the photon without it being absorbed. This is why if you sync two atomic clocks, bring one atomic clock up to orbiting speed (such as on a gps satellite), then decelerate it and return it to earth the clocks will still read different numbers. 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.

karmakaze 4y ago

Do photons have to slow down to change? Change requires time to pass doesn't it? So how does a photon change polarization (without slowing down)?