Q: If a photon doesn’t experience time, then how can it travel?

Physicist: It’s a little surprising this hasn’t been a post yet.

In order to move from one place to another always takes a little time, no matter how fast you’re traveling.  But “time slows down close to the speed of light”, and indeed at the speed of light no time passes at all.  So how can light get from one place to another?  The short, unenlightening, somewhat irked answer is: look who’s asking.

Time genuinely doesn’t pass from the “perspective” of a photon but, like everything in relativity, the situation isn’t as simple as photons “being in stasis” until they get where they’re going.  Whenever there’s a “time effect” there’s a “distance effect” as well, and in this case we find that infinite time dilation (no time for photons) goes hand in hand with infinite length contraction (there’s no distance to the destination).

At the speed of light there's no time to cover any distance, but there's also no distance to cover.

At the speed of light there’s no time to cover any distance, but there’s also no distance to cover.  Left: regular, sub-light-speed movement.  Right: “movement” at light speed.

The name “relativity” (as in “theory of…”) comes from the central tenet of relativity, that time, distance, velocity, even the order of events (sometimes) are relative.  This takes a few moments of consideration; but when you say that something’s moving, what you really mean is that it’s moving with respect to you.

Everything has its own “coordinate frame”.  Your coordinate frame is how you define where things are.  If you’re on a train, plane, rickshaw, or whatever, and you have something on the seat next to you, you’d say that (in your coordinate frame) that object is stationary.  In your own coordinate frame you’re never moving at all.

How zen is that?

Everything is stationary from its own perspective.  Only other things move.

Everything is stationary from its own perspective.  Movement is something other things do.  When you describe the movement of those other things it’s always in terms of your notion of space and time coordinates.

The last coordinate to consider is time, which is just whatever your clock reads.  One of the very big things that came out of Einstein’s original paper on special relativity is that not only will different perspectives disagree on where things are, and how fast they’re moving, different perspectives will also disagree on what time things happen and even how fast time is passing (following some very fixed rules).

When an object moves past you, you define its velocity by looking at how much of your distance it covers, according to your clock, and this (finally) is the answer to the question.  The movement of a photon (or anything else) is defined entirely from the point of view of anything other than the photon.

One of the terribly clever things about relativity is that we can not only talk about how fast other things are moving through our notion of space, but also “how fast” they’re moving through our notion of time (how fast is their clock ticking compared to mine).

 

The meditating monk picture is from here.

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223 Responses to Q: If a photon doesn’t experience time, then how can it travel?

  1. Pingback: What Does Science Explain? Part 3 – The Mythos of Objectivity | Mythos/Logos

  2. craig says:

    Sorry, but for me the original question was not answered – either by the Physicist or the subsequent replies . The Physicist says that at the speed of light “there is no time to cover any distance and no distance to cover”. So enough already with the dilations and relative observers – if a photon is travelling at the speed of light – why does it not arrive on the other side of the universe at the same time it left. Does it move or doesn’t it !

  3. The Physicist The Physicist says:

    @craig
    From the perspective of anything else in the universe it clearly travels a distance and it clearly takes time to do so. But from the “perspective” of the light, it travels zero distance in zero time.
    Frustratingly, no one is “correct” and surprisingly this doesn’t cause a problem.

  4. David says:

    The problem arises when calculations explode to values of infinity near the speed of light and become infinite at the speed of light. The equations do not hold at that speed.

  5. Sairam Subramaniam says:

    I am a novice, but I am curious. I had the same question in mind and your post clarifies it nicely for me. Thank you. However, another question that arises in my mind is – Does that mean for the photon, there is no space / distance at all and its actually not even moving (from photon’s point of view). Its simply present encompassed within a point?! (because all distance and space has contracted for the photon)

  6. James Becker says:

    I think this is a good description.

  7. craig says:

    Re David. If we are just discussing light travelling zero distance in zero time, I’m not sure what equations and infinities you mean, The equations I’ve seen don’t allow me to travel at the speed of light (they end up with a zero denominator).

  8. Pingback: A corolally to The Theory of General Relativity | Thoughts as they happen

  9. Orien Rigney says:

    Other than reading or listening to what scholars speculate, I can only assume to have an argument in trying to understand light speed or the ramifications involved in its known processes. But, from what I’ve read, light moves at different speeds depending upon the medium it which it moves. In a vacuum, air, the void between objects in our universe, water, a prism, glass, etc. Might it be assume that our universe is constantly traveling at light speed even as we speak, while the movement happening about us is matter raised to different levels above the rest speed of light?

  10. Orien Rigney says:

    You’ll have to excuse me David, but my intellect only allows me to ask many more questions than I can give answers due to my lack of mathematical knowledge. But, if you wish to chase this brain twister farther, I’ll give it a shot..

  11. Orien Rigney says:

    A Center/less Universe????
    Many, if not most Scientists involved in cosmological physics, believe or say that the universe has no center and expansion began as a singularity less the size of an atom. Somehow saying, “the universe has no center” doesn’t make it so or mean it is without one. The mistake is in using a loaf of raisin bread to demonstrate the point. Yes, the loaf will expand uniformly in all directions while baking. Thus the raisins will move away from each other at comparable rates of speed and distance while this process is going on.
    To make the picture even more vivid, let’s say that, “hypothetically”, the raisin bread is perfectly spherical (globular) and at its very center is a single raisin. In which direction will that raisin be moving as the bread begins to expand? It won’t and can’t, and why? Because, being at the very center of the bread, any movement is neutralized in any direction by the laws of physics. Looking from that central position other raisins will be moving away as we see galaxies moving away from us. But, if you ere situated on a galaxy other than the central stationary galaxy, it will appear that all other galaxies are moving away at the same rates of speed. The same would appear to an observer on any other galaxy even though your central galaxy isn’t moving at all. Think about it?

  12. Pedro says:

    Photons don’t need to travel … they are everywhere at once.

    If you’re looking for a singularity with infinite mass where time stands still…. you should ride a photon.

    A “singularity with infinite mass where time stands still” …. now where have we seen one of THOSE before ?? 😉

    https://en.wikipedia.org/wiki/Big_Bang

  13. craig says:

    That answer just returns us to the original question. If photons don’t travel and are everywhere at once, because time for photons stands still – then how did they get to that “everywhere” in the first place.

  14. Photon is outside of time. But if it enters a black hole, does it do so at a specific time?

  15. YES!

    That means that time and space are not as local as we tend to think. That is what Einstein (and Podolski and Rosen) were struggling with!

    Stricktly speaking this whole discussion is not new, but the consequences are in the order of “resistance is futile”

  16. Orien Rigney says:

    Somehow the photon seems to add much more mystery as each day passes. Yes, over the past 13.5 billion years it’s possible to believe that a photon is limitless in its life span and action, capable of leaping tall buildings with a single bound and correcting all types of wrongs in this wilderness universe. But as is so in any other wave action nothing seems extra special about a photon. A radio wave generated by Marconi a hundred years ago is still zooming through space at the speed of light and will probably do so until the end of time. I can’t possibly, with my lack of math give you a better explanation of the critter, but these two short articles can.

    How are Photons created and destroyed/
    http://curious.astro.cornell.edu/about-us/137-physics/general-physics/particles-and-quantum-physics/805-how-are-photons-created-and-destroyed-advanced

    Photo Electric Effect
    https://www.asu.edu/courses/phs208/patternsbb/PiN/rdg/photoelectric/photoelectric.shtml

  17. Chad Maddox says:

    Concerning the big bang, is it possible that before matter was created from the singularity that there did exist energy on a massive quantum scale? Energy does seem to exist everywhere and in everything even as gravity. And since energy travels at the speed of light and is therefore timeless it could have and still does exist before time was created as experienced by matter. Therefore, as Einstein proved correctly, you can turn matter into energy, is it also true that energy can be turned into matter? Remember that only mass less particles can travel at light speed. So if these timeless particles of pure energy are somehow slowed to less than light speed did they erupt into the big bang of matter we call our universe ?

  18. Pedro says:

    The way that I imagine this is that there was no “Before” because time stands still as one inertial frame is viewed from another.

    The whole universe looks like a “projection” to me .

    And I strongly doubt that the “Big Bang” theory is true. Or at least it didn’t look much like a “Big Bang” if you were there :-)

  19. Orien Rigney says:

    Because I believe the universe is an eternally cyclical event, to me the Big Bang is totally passé. A singularity on the other hand, while consisting of a single source of material may not have been the size of an atom or less, but perhaps the mass of a billion galaxies? Possible? Why not. Let’s say a cycle of this universe covers a hundred billion years, or so? Next, perhaps it, (the universe) slowly dies through the decay of energy/matter due to attrition. Then, this defunct matter/energy somehow returns to the core of creation on the shuttle of dark matter and dark energy where it began to be transformed into thea precursor of materials necessary to begin all over again? Believe me, I’ve read and seen enough scientific jargon to think this is possible.

  20. Photon Alpha, is outside of time, we agree. But it enters a black hole.
    From the point of the photon, was there a specific time when it entered the black hole?
    From the point of the photon, is it still outside of time when it passes across the event horizon of the black hole?

  21. David Frank says:

    light is sensual to us; we may not be able to observe an object going faster than light, but that dose not mean that their is not an object, or that it cannot go faster than light. Many assumed we could not go faster than the speed of sound before the sound barrier was broken, but latter we did. Einstein’s theory must be wrong. Matter cannot be contingent upon the speed at which it moves, nor can it disappear into another time zone because of the speed at which it is moving. My theory: The fabric of space is both space and matter at the common distances of the galaxies that we observe. The universe follows the study state theory rather than the big bang model. The universe is infinite in both time and space. There is no end and was no beginning. This fits the grand propose. Otherwise, logic dictates “where did it all come from” if there was ever a time and place of creation such as the big bang theory.

  22. Torsh Johansen says:

    I go by David’s POV, and this hasn’t been answered. My dad has his PhD in Theoretical Physics and can’t answer it (although he doesn’t do research over the years; just teaches).

    It’s not that a photon merely “experiences” no time — Literally No Time has elapsed. Time may elapse only when it changes state (interacts with something to change its state). Which means, when you split a photon and they go in opposite directions and one hits something to change it’s polarity and the other’s (gasp) changes — “faster than light” — instant, that should be no surprise.

    The photon was at the end of it’s road in the same moment it split with the other photon. It doesn’t matter whether it’s 20, 2000, or 2 million miles later. It’s in both places at the same time, but Not relative to us. That’s what makes it “spooky”. It’s polarity changed the moment it split. RELATIVE to US, we have to wait until it travels that distance and hits the polarity changer to see the results on the other photon.

  23. Orien Rigney says:

    Having read probably hundreds of questions, answers and statements with many of them presented as fact, I’ve yet to read one that makes much sense as written. As a layman I can’t disparage anyone’s intelligence, but it seems the more we dig, the more puzzling the photon becomes. Saying that, if a photon or (photons) were isolated in the vacuum of a spherical reflective spherical, would they bounce around in it forever?

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