I sort of get that light doesn’t age - after all, a clock attached to a photon would never tick. If I think through the whole clock moving relative to an observer, I can see the conclusion it leads to.
I’ve heard it said that therefore all light is the same age as the big bang. I don’t know if that’s glib and oversimplifying the truth, or if it is meaningful to say the photon is as old as the big bang.
From the photon’s point of view, what does it mean to not age? A photon from our point of view is created, say, by an electron in a hydrogen atom in the sun, and 8 minutes later, it hits some pavement and gets absorbed and transforms into heat. It existed for 8 minutes. It did not exist yesterday, and it does not exist now.
Does the photon experience that? Did it exist for the entire 93 million mile path all at the same time? Did it experience existence as a 93 million long line, rather than as a quasi-particle moving from the sun to the earth? Does it just sort of ‘exist’ in that slice of the spacetime loaf, as something that neither moves forward nor backward through time? Is there any intuitive way of understanding a photon’s life experience?
You are misunderstanding the notion of relativistic time dilation. Differences in the ticking of a clock moving at relativistic speeds are noticed only by observers moving relative to each other. The idea that “light doesn’t age”–i.e. that a clock attached to a photon moving from, say, the sun to the earth does not tick–is from the perspective of the earthbound observer only. An observer riding along with the photon (if such a thing were possible) would see his/her clock tick as expected for the entire 8-minute journey.
The OP is right in that a clock somehow riding along with a photon would not experience ANY time. The emission of the photon and it’s absorption would be a single point in space time for it.
Time dilation on a ship traveling near the speed of light from the sun to the earth would make the trip seem incredibly short, while observers on Earth would see the ship travel for about 8 minutes before passing by.
It’s important to note that not only does no time pass for the photon between its creation and destruction, but to it, both events happen at the same location, too, since the entire universe is contracted to nothing in its direction of propagation. It’s not like the photon is moving and experiencing zero time while it’s doing something – from its point of view it’s experiencing no change in time or space.
Yup, or as I like to say, “Every photon that has ever existed has believed that it is the entire universe, that the universe started when it was born, and ended when it was gone, and that this all happened instantaneously.”
See Flatland, Edwin A. Abbott, 1884, who understood these things way back then. From a chapter near the end, “How the Sphere encouraged me in a Vision”:
Maybe it’s a philosophical rather than physics question (not sure) : since the photon appears and disappears at the same instant, doesn’t it mean that it doesn’t exist at all from its point of view? And if so, how comes that something that doesn’t exists from its point of view can exist for an outside observer?
Or aybe to say it another way : isn’t a photon nothing? And if so, how nothing can have properties?
You’re confusing “point of view” for a photon with your experiences as a massed object. Just because certain physical events are interpreted by you in specific ways doesn’t make them apply to photons. Virtually everything in relativity and quantum mechanics that people find interesting, baffling, or odd resides in how our everyday experiences don’t apply to much of the universe.
Sorry if this is a hijack - but what of those photons that travel very very slowly? I believe a scientist a few years back slowed light to “walking pace”. If no longer traveling at 186,000 miles per second would not relativistic effects be absent?
So for that instant, from the photon’s POV, the sun and the earth are in the same point? Or does the photon experience one dimension - its length - as existing in that same instant for 93 million miles?
By the way, thank you for your response. What you posted made some new synapse patterns emerge - I could totally feel it.
I guess I’m seeing the photon as a particle there. If it’s a wave, does it sort of exist with some probability very close to zero everywhere in the universe, and very close to 1 along the line between the sun and earth?
I’m trying to tie this in with my “understanding” of quantum physics, too, where in some sense photons are aware of each other, even if they don’t exist at the same moment; e.g. you get the ‘wave’ pattern in the two-slit experiment even firing the photons in minute amounts, separated by time. I put understanding in quotes because I accept the results of the experiment but have no sense of what the hell is going on.
Someone correct em if I’m wrong, but I believe this isn’t a case of photons traveling slowly, in fact I believe photons MUST travel at C and cannot travel any slower.
When light enters a medium one of the possible interactions is transmission. Light is absorbed and re-emitted by atoms that make up the substance it’s traveling through. This causes light to propagate slower, but the actual photons traveling from atom to atom, are still traveling at C.
Is this why the fact that neutrinos can change “flavor” proves that they must have mass. For if not, they travel at the speed of light and cannot experience time and therefore could not change. Is this correct?
I think the discussion is a bit clouded by the discussion of a photon, which never really “experiences” anything, and is the most extreme case of the effect.
Consider instead, the classic “person in a spacecraft moving at 0.9999999c relative to earth towards some star 1000 light years away”. Someone on earth observes the guy on the spaceship and think that the trip still takes just over 1000 years to make, but the guy on the spaceship nevertheless makes it in his lifetime because time has slowed down so much for him.
But the guy on the spaceship doesn’t perceive his own time as going any slower (after all, he is at rest relative to himself), and he perceives the distant star as coming at him at 0.9999999c, not any faster, so how does he reconcile the fact that he’s going to meet up with the star in his lifetime?
The answer is that for him space is contracted in the direction of his motion, so for him the star is not 1000 light years away, but a lot closer. In fact, everything in the universe in that direction is a lot closer. What is “spaceship guy is going to make it because of time dilation” for the earthbound observer, becomes “spaceship guy is going to make it because of space contraction” for spaceship guy.
The photon behaviour is just the limit of that. We observe the photon as having time stopped. The photon would, if it could observe anything at all, observe the universe as being compressed into a two-dimensional blob in its direction of motion.
Kinthalis had it right: The bulk motion of the light can be slowed down, but any individual photon, in the interval between its emission and absorbtion, is traveling at c.
That’s part of it. Massless neutrinos could not oscillate in vacuum, for the reason you state. But you could still have oscillation between a massive neutrino and a massless one, if you’re traveling through some medium. The other piece of it is that the frequency of the oscillations depends in part on the difference between the squared masses of the two neutrino states (i.e., m2^2 - m1^2), so even traveling through a medium, you’d have to have at most one of the neutrino states be massless.