4 ms·

This was perhaps my favorite part of Physics 390 ("modern physics") which was about quantum dynamics and relativity. The speed of light is defined in terms of a

by ChuckMcM 11mo ago

This was perhaps my favorite part of Physics 390 ("modern physics") which was about quantum dynamics and relativity. The speed of light is defined in terms of a velocity (~300,000,000 m/s) but if you were traveling at the speed of light time stops (which keeps the rule that its constant in all frames of reference). That and time passes more quickly at higher altitudes and these days we can actually measure that. Wild stuff.

tacker2000 11mo ago
Also see the twin paradox, which is related to this and equally fascinating. If one twin stays on earth and the other makes an intergalactic trip (with the speed of light), upon return, the one on earth will have aged much more than the one on the trip.
- Tor3 11mo ago
  The paradox isn't actually when the twin is returning, the paradox is seen during the trip away from earth. Both are observing each other through telescopes, and both can see the clock on the wall in each other's home (a house, or for the twin, his space ship. Which has got a big window). And both can observe that the other twin's clock is moving at a slower pace than their own clock. And that's the paradox. (p.s. the spacefaring twin doesn't have to move at the speed of light, and indeed cannot, it's enough to move at a "relativistic" speed, i.e. fast enough that this is actually measurable. With today's clock that doesn't have to be very fast actually)
MaxikCZ 11mo ago
The fact that at speed of light time stops is just so bonkers to me. A photon is, from its point of reference, at the point of creation and at the point of destination at the same "time". Its literally seeing both parts of the universe at the same time, and since its traveled some distance over that time it cannot perceive, its essentially connecting 2 points in spacetime. If I understand it correctly, every photon exists, from its point of view, for only infinitely small amount of time (similar to how virtual particles do exist from our point of reference), but for us its so easy to "play" with the photon along its path, giving us plenty of time to even decide what we want to do with it after it has already been created. Its just so bonkers that time can be perceived such differently depending on frame of reference.
- gyomu 11mo ago
  > A photon is, from its point of reference, at the point of creation and at the point of destination at the same "time" > every photon exists, from its point of view, for only infinitely small amount of time Why is that amount “infinitely small” and not 0 since photons travel exactly at the speed of light?
  - RobotToaster 11mo ago
    Existing for zero time would imply it never existed.
    - skissane 11mo ago
      > Existing for zero time would imply it never existed. In the mathematics of infinity, something can have exactly zero probability yet still happen, and exactly one probability yet nonetheless fail to happen (hence the standard term “almost surely”). If something can have literally zero probability of existing yet still exist, why can’t it exist for literally zero time yet still exist?
- nrhrjrjrjtntbt 11mo ago
  It sort of makes sense to me. It means effectively there is no speed limit. At least from the reference point of who is moving.
- kburman 11mo ago
  Have a question related to this, if a photon has zero proper time between emission and absorption, how should I think about the influence of later-created photons or fields on it? In our frame, we can interact with a photon long after it's emitted send it through a filter, bounce it off a mirror, measure it, etc. But from the photon's own “no proper time” perspective, does it make sense to ask how something created after its emission could affect its path?
  - hhjinks 11mo ago
    >does it make sense to ask how something created after its emission could affect its path The problem here is likely the concept of "after". It's relativity; what's "after" in our frame of reference isn't after in all frames of reference.
gilbetron 11mo ago
My favorite explanation: https://www.youtube.com/watch?v=Vitf8YaVXhc https://www.youtube.com/watch?v=Vitf8YaVXhc
euroderf 11mo ago
> but if you were traveling at the speed of light, time stops Are we saying then that it is theoretically possible that (a) photons have a zero or near-zero lifetime, but that also (in our frame of reference) (b) they exist for a finite duration of time and are visible during that time simply because time stops for them ? Or did someone manage to slow down light to some fraction of light speed ?

tacker2000 11mo ago

Also see the twin paradox, which is related to this and equally fascinating. If one twin stays on earth and the other makes an intergalactic trip (with the speed of light), upon return, the one on earth will have aged much more than the one on the trip.

Tor3 11mo ago
The paradox isn't actually when the twin is returning, the paradox is seen during the trip away from earth. Both are observing each other through telescopes, and both can see the clock on the wall in each other's home (a house, or for the twin, his space ship. Which has got a big window). And both can observe that the other twin's clock is moving at a slower pace than their own clock. And that's the paradox. (p.s. the spacefaring twin doesn't have to move at the speed of light, and indeed cannot, it's enough to move at a "relativistic" speed, i.e. fast enough that this is actually measurable. With today's clock that doesn't have to be very fast actually)

MaxikCZ 11mo ago

The fact that at speed of light time stops is just so bonkers to me. A photon is, from its point of reference, at the point of creation and at the point of destination at the same "time". Its literally seeing both parts of the universe at the same time, and since its traveled some distance over that time it cannot perceive, its essentially connecting 2 points in spacetime. If I understand it correctly, every photon exists, from its point of view, for only infinitely small amount of time (similar to how virtual particles do exist from our point of reference), but for us its so easy to "play" with the photon along its path, giving us plenty of time to even decide what we want to do with it after it has already been created. Its just so bonkers that time can be perceived such differently depending on frame of reference.

gyomu 11mo ago
> A photon is, from its point of reference, at the point of creation and at the point of destination at the same "time" > every photon exists, from its point of view, for only infinitely small amount of time Why is that amount “infinitely small” and not 0 since photons travel exactly at the speed of light?
- RobotToaster 11mo ago
  Existing for zero time would imply it never existed.
  - skissane 11mo ago
    > Existing for zero time would imply it never existed. In the mathematics of infinity, something can have exactly zero probability yet still happen, and exactly one probability yet nonetheless fail to happen (hence the standard term “almost surely”). If something can have literally zero probability of existing yet still exist, why can’t it exist for literally zero time yet still exist?
nrhrjrjrjtntbt 11mo ago
It sort of makes sense to me. It means effectively there is no speed limit. At least from the reference point of who is moving.
kburman 11mo ago
Have a question related to this, if a photon has zero proper time between emission and absorption, how should I think about the influence of later-created photons or fields on it? In our frame, we can interact with a photon long after it's emitted send it through a filter, bounce it off a mirror, measure it, etc. But from the photon's own “no proper time” perspective, does it make sense to ask how something created after its emission could affect its path?
- hhjinks 11mo ago
  >does it make sense to ask how something created after its emission could affect its path The problem here is likely the concept of "after". It's relativity; what's "after" in our frame of reference isn't after in all frames of reference.

gilbetron 11mo ago

My favorite explanation: https://www.youtube.com/watch?v=Vitf8YaVXhc https://www.youtube.com/watch?v=Vitf8YaVXhc

euroderf 11mo ago

> but if you were traveling at the speed of light, time stops Are we saying then that it is theoretically possible that (a) photons have a zero or near-zero lifetime, but that also (in our frame of reference) (b) they exist for a finite duration of time and are visible during that time simply because time stops for them ? Or did someone manage to slow down light to some fraction of light speed ?