To an extent, from a layman’s perspective, I htink I have done ok at understanding and accepting some of the counterintuitive bits that Relativity and Quantum Mechanics throws at us. By no means do I claim a deep understanding of it but I think I can at least cope.
One thing that puzzles me though is considering what happens from the point of view of a photon (or any “object” that moves at light speed).
Assume I am a conscious photon, able to observe what is happening to me. As I undestand it if I moved at the speed of light the universe would shrink to zero distance in the direction of travel. So, from here to the edge of the universe is infinistesimally close. Also, time ceases to pass for me. So travel from “here” to “there” (wherever “there” is) takes no time for me.
So how is it for the rest of you that you can observe my travel through the universe? It takes minutes from your perspective for me to leave the sun and arrive to you. More to the point, what happens when you capture me (the photon me)? If I can travel from anywhere to anywhere in zero time what happens when I get stopped somewhere along my travels? I had no time to be stopped, there is no “in between” time for someone to capture me.
Sorry if this is not clear but I am curious just the same.
I think this is key to your misunderstanding. There’s no one universal notion of time. To you (the photon) there is zero elapsed “time” between the events of emission and absorption. To me (not a photon) there is some elapsed time. We’re moving at different rates so we make different measurements.
Ok…but say I am the photon. The entire universe passes me by in no time. I travel to the end of the universe (or wrap around it or whatever) in no time. Yet you stop me with your photon collector. To me you are somehow in-between a place that, to me, has no in-between. Is the assumption then that the Universe to me exists only between me and your photon collector? It makes no difference that to me your photon collector is as close (or as far away) as the edge of the universe? That you are somehow smushed in the “middle” of a place that has no middle?
(No need to discuss the Universe having no edge…figure of speech.)
I admit it’s a bit of a mind-bender, but what you have to do is think of points in spacetime as being only labelled by “what happens there” and the elapsed time on a path between events having no meaning beyond the particle travelling that path.
Try this: A photon is emitted from a source at event A, reflected at a mirror at event B back towards the first device, and absorbed by a target on the first device at event C. No coordinates, no labels on anything but A, B, and C. A is “this point in spacetime”, B is “that point in spacetime”, and C is “another point in spacetime”. Don’t think of any outside structure you don’t absolutely have to. In particular, C is not “event A a bit later”, since there’s no way to say events are “in the same place at different times” because that assumes a canonical separation of space and time.
Now, we know that particles of light travel along “lightlike” paths, meaning that if we measure the spacetime length of the photon’s path from A to C the answer is zero. You, the experimenter, standing at rest relative to the device containing the source and target also follow a path through spacetime to get from A to C, though you see yourself as standing still. If we measure the spacetime length of that path, it’s not zero. Two different paths between the same points have different lengths. That’s all that’s going on.
Even beside Mathochist’s correct statement that you are in different reference frames, your language betrays why your explanation doesn’t work.
You cannot talk of “passes” or “in-between” or “middle” to something that has no time and no internal existence. That’s why quantum theory describes photons as neither a wave nor a particle, merely something whose behavior we can interpret as being like one or the other after different effects.
A photon blinks on and blinks off from an outsider’s reference frame. Internally, it has no existence as we massive bodies normally define the term. No time, no distance, no mass, no endurance, no between. No point of view. A photon is a quantum thing unto itself without a good analogy in classical terms. Yes, that’s very weird, but that’s why people always describe the quantum world that way. You can’t think yourself into a proton’s reference frame. You can only study it from the outside.
Oh, very much so. I’m wondering, though, whether the problem is specific to “photons”, which allow this “quantum is weird” back door. I’m trying to approach this as pure GR, and these together lead to an interesting question: Would you assert that the only things that travel along lightlike trajectories are quantum systems (ignoring for the moment that everything is a quantum system)? In other words, could there be some non-quantum thing we can put in place of “photon” that still leaves the GR problem to be answered?
Not an answer to your question but if you’re looking for something cool to think about, I was on the subway the other day and I realized that if Planck’s constant had a different value we could diffract around corners and through doorways. Now wouldn’t that make for a neat universe?
How does the OP’s example play with the double-slit experiment?
Does that mean that the photon is actually everywhere at once, but collapses on the photon collector based on some probability (or even completely randomly)?
Yeah, pretty much. I think that much of this sort of philosophical question gets a lot easier when you stop thinking quantum mechanics and start thinking quantum field theory. There’s only one photon field in spacetime, and it has various probabilities of affecting other fields (say, those describing photoreceptors) at various points in spacetime. The field as a whole and its interactions with other fields are described by a horrendously complicated system of differential equations, but we can in principle say that given certain input information (photon fired at this point in spacetime with this momentum) we can predict probabilities for various output states (photon observed at that point in spacetime hitting the film behind the slits).
Well, if you want to be purely GR about it, you could use a gravitational wave, which, if it can be quantized, we don’t know how yet. But if you want to talk about a “thing” (depending on your definition of “thing”), then you’re back to gravitons, which are quantum thingies whether we want to admit it or not.
But I would agree that the problem lay right at the beginning, in trying to consider things “from the photon’s point of view”. 'Tain’t none such. I don’t think the problem is inherently quantum-mechanical, though, just relativistic.
Congratulations! From what I have read, this is the question that Einstein asked himself as a young boy. From this question a lot of good science has come.
Perhaps someone else can ask this question and come up with an even better answer than before.
Actually, as Chronos said, a photon can’t have a frame of reference. Light must travel at c with respect to all frames of reference and light can’t travel at c wrt light.
However, this is also kind of a nitpick, because these kind of gedanken experiments can lead to some pretty neat insights such as Einstein’s Riding on a light beam. He realized that if he could travel at the same speed as light he would see a non time varying spatial waveform. Since light consists of time varying electric and magnetic fields it would cease to exist.
