To be pedantic, photons never accelerate. They only ever travel at one speed in one direction
And as they’re massless, photons do not experience time. Regardless of how far a photon travels, from its perspective, the journey takes no time.
It also does not experience space, as the entire universe has been length contracted in its direction of motion into a 2d plane. It is simultaneously occupying every point along its path. So it doesn’t need to experience time.
It also does not experience, as it lacks consciousness
Well now we’re getting into philosophy
I’ve been trying to wrap my head around this part and what it means for cause and effect for a while. I think Feynman said something like a photon is only ever emitted when the source and destination agree to exchange one. Which makes sense if the exchange is instantaneous to the photon. But how can billions of years pass for us in the mean time?
Because there isn’t perspective of a photon. It doesn’t experience because it doesn’t change like mass does.
I’m not sure Feynman was right. Most photons are emitted and never absorbed by anything.
Photons exist, so there is the perspective of a photon. Most may not be absorbed but that’s irrelevant because some are. And when they are, their perspective - like them - ends. Like yours does when you die.
The photon does not experience time, but we do, so from our perspective they can be emitted and absorbed even though from their perspective they are timeless. Again, like us. Before you were born, you didn’t experience being not alive. From your own perspective, you’ve always existed, even though from the perspective of someone older than you, there was a time when you didn’t.
I was using the wrong term. Photons don’t have a frame of reference.
But even from the colloquial definition, photons don’t have perspective. They don’t live and die because they never experience time. If you had their point of view, your beginning and end would happen simultaneously, meaning you wouldn’t experience anything. They are immutable particles whose only interactions are emission and absorption.
I just want to say how much I appreciate those discussion. They remind me of how little I know even though I’m considered an “expert” in my field of work.
Open loop dispersion vs closed loop absorption, in either case they are a distinction of low energy observer bias. They are functionally equal because the waveform is a projection through a open feature of a manifold bound by a topological inversion that intersects it.
So the photon never really goes anywhere, we just see its shadow cast across a screen that moves from our perspective.
Most photons are emitted and never absorbed by anything…
Yet
Eventually all photons will hit something. Even if it’s a trillion trillion trillion years in the future when nearly everything in the universe has decayed into irony.
Someone correct me if I’m wrong, but eventually the universe is going to be expanding faster than the speed of light. At that point all interaction ceases, and any photons that didn’t get absorbed by something yet never would.
Sort of. The expansion of space causes (and is measured by) redshift. The photon that doesn’t get absorbed “exists” until its wavelength is not measurable (as its wavelength approaches infinity).
The cool thing about this is that it is identical to what happens in a black hole. Spaghettification. This also has the fun consequence of us possibly existing inside of a black hole, and black holes themselves are entire universes. Because of the breakdown of physics beyond the event horizon its not exactly easy to confirm or deny this either.
Edit: redshift not redshirt. Startfleet personal aren’t dying here, it’s photons
Genuine question, how do we know that photons are being emitted that never get absorbed if observing them requires absorbing them? Is it an energy loss type of thing with the emmiter where we have to assume x many photons had to have been emitted to explain the loss?
Because we know how different things emit photons. We know a light bulb emits photons in all directions because we can move around and measure it. And we can see the photons being emitted from objects receiving the initial light bulb’s light as well so we know it’s emitting light in that direction as well.
The idea that photons are only emitted if they hit something also doesn’t make sense because of power usage and how we know particle physics work.
Obviously it doesn’t experience space if it doesn’t experience time. It’s emitted and absorbed simultaneously in its frame.
Relativistic effects are cool
A photon would somehow experience the big bang, the heath death of the universe and everything in-between all at the same time.
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They don’t accelerate, but can travel at different velocities in different mediums.
For example light travels faster in air than in water and fastest in a perfect vacuum.
In aggregate, yes, but any individual wave of light is still traveling at c. You get the appearance of a slower wave because secondary waves are generated that cancel the original one in such a way that it makes a combined wave that appears to be slower.
Not quite.c is the speed of light in a vacuum. It’s more accurate to say c is the speed of causality.
Velocity/speed isn’t very useful with photons either - its a wave-particle.
Light in changing mediums is a separate but related phenomenon. The photon essentially doesn’t continue on its same path, it gets absorbed by the particles in the medium. This leads to changing states (of usually an electron in an atom) which may emit another photon, remain stable but increase the atom’s kinetic energy (I can’t remember how likely that is, if at all), or it may eject the electron, ionizing the atom. In any case, the state changes, because the whole system (the atom, electron, and photon) can’t have net energy gain or loss.
I believe they still travel at the speed of light, but are regularly absorbed and re-emitted in a way that makes the effective speed less than c.
