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

  1. ScienceInterested says:

    From the perspective of a photon, there is no time duration and no distances. This means the photon and the entire Universe are in one and the same place. Therefore the photon does not need to travel anywhere, because it is already there. From the photons perspective, to travel from that ‘distant’ star to your retina, it travels no distance in no time because the ‘distant’ star, the earth and you and the rest of the Universe are all together with the photon in one place -because there are no distances at light speed.

  2. David says:

    We know the rules for matter, for instance with time dilation. They’ve been confirmed by experiment very accurately. Attempts to apply the same rules to light are not necessarily justified, and there’s a lot of reason to think that light goes by different rules from matter. In fact, we know it does. For one example, light has energy without having mass, and that breaks the rules for matter.

    But the present view of SR, which is wrong, suggests that the rules of SR apply to everything, not just matter. They look like universal rules in that view, so you get a lot of misguided people trying to apply them to light, and getting nonsense back out. But because the nonsense always has a faint, obscure self-consistency, as in the infinite shrinking of distance and the infinite shrinking of time mentioned in this article, it’s comparatively easy for people to believe, or to convince themselves, that this makes sense. But it’s conjecture, it hasn’t been tested, and many good physicists think it’s false.

  3. Pedro says:

    Photons exhibit “relativistic mass” and momentum.

    https://physics.stackexchange.com/questions/2229/if-photons-have-no-mass-how-can-they-have-momentum

    They do not break any known rules, I promise 🙂

  4. Peter Grimshaw says:

    Science Interested yes this is what I am holding in my mind.
    In SR – which could be, and probably is wrong …. – the photon is everywhere in the whole universe at the same time. Yep, SR says ‘The photon and the entire universe are in the same place’
    All the time we experience in our co-ordinate frame, happens in the null-time of one photon.
    What light does that shed on Quantum Entanglement ?
    If the photon is everywhere and throughout all time, viewed from our co-ordinate frame, it’s properties are weird.
    We, in our co-ordinate frame, experience it either as a ‘wave’ or ‘particle’.
    Does this say more about our perception, than the essential quality of the photon.
    ‘Perception’ itself starts to become a metric of some sort.
    Worth a ponder.

  5. My problem is this. Photon is outside of time. When it enters the event horizon of a black hole, does it enter time? When it goes into a black hole and leaves the universe, has it entered at a specific moment in time?
    Does the photon that entered a blackhole, have a before it entered the black hole – time, and an after it enters the black hole – time?

  6. Pedro says:

    I have “pondered” this for several years.

    What I came up with is that from the photon’s perspective there is a singularity where all the mass of the universe is concentrated in one place.

    Sound familiar ?? 🙂

    I further conclude that the universe is a “projection”.

  7. Peter Grimshaw says:

    Pedro
    Ok so Photon up close = Black Hole viewed from a distance

    Ps, I can’t do SR math yet, but getting there slowly.

    I’m wondering if EM ratiation (ie ‘light’ and EM waves) is actually somehow ‘Ether’.
    From an EM perspective, EM is immanent.
    Ie, it is in all places at all times.

    Isn’t it a perennial question – if light is in ‘waves’, what is being ‘waved’?
    Maybe the question is – is the perception of waves caused by the moving position of the observer through time rather than the motion of light?

    Wonderful science

  8. Hakon says:

    I was thinking about this too, and I was philosophisingg along the same lines as the original poster. From the previous responses, there are two ways to look at this: 1) a photon has mass, or 2) a photon does not have mass. I don’t know how to do the math, but – and correct me if I am wrong – you’d need infinite energy to propel anything with mass up to light speed, so anything travelling at light speed either has no mass or infinite energy. But what is mass? If you break particles of mass down into their most basic components, do they not start assimilating massless waves moving at light speed? Is not then all mass just the inertia of confined elemental particles moving at light speed?

  9. Tom Hendricks says:

    Let’s say photon A – a photon is outside of time because it is going the speed of light – crosses the event horizon of a black hole and enters it, then my question is
    Was there a time before entering the black hole, and a time after entering the black hole, or is the photon still outside of time.

  10. The Physicist The Physicist says:

    @Tom Hendricks
    It’s not quite fair to say that a photon is “outside of time”. Different observers disagree about how much time passes between pairs of events. A photon (or anything else traveling at the speed of light) is basically just the most extreme case of that. While you might say that the time between the emission and absorption of a photon was years, the photon will always say that the amount of time was zero (black hole or not).

  11. Peter Grimshaw says:

    As I get it, all our time, from the big bang, to the big crunch, fit inside the null-point of the photon. And, actually, kinda, we are the photon because of space contraction at the speed of light. A bit like a hologram. The photon is simultaneously at all points in our universe at the same moment, if it is a wave.
    Of course, if you were ‘in’ the photon (as a particle), time would pass, but an infite number of lives, an infinite numbe of big-bangs would be lived ‘outside’ your frame of motion.
    At least that is what the maths points to, and what ‘relative’ time means. Time passes at different rates in different time-frames.
    I haven’t done the math because I have drunk too much whisky and spoilt my brain-cells but at the level of Einsteins thought-experiments this is what is suggested?
    Of course, there could be some new mathematical paradigm needed here and I’m sure folk are working on it ..

  12. The Physicist The Physicist says:

    @Peter Grimshaw
    Relativity is a bit more straightforward than that. Time dilation is basically “moving clocks tick slower than yours”.

  13. Tom Hendricks says:

    Thanks for the comments all. Yet The Physicist, I think you have dodged the question a bit. Saying the time was zero, does that mean the photon is both outside the black hole, and across the event horizon and inside the black hole, and is always and has always been in both places?
    That suggests that for the photon it is both of this universe and the singularity of the black hole – and that at the big bang it was in the black hole too.

  14. The Physicist The Physicist says:

    @Tom Hendricks
    The photon’s “clock doesn’t tick”, but that doesn’t mean our clocks don’t. According to any given observer, a photon is at a given place at a given time and a little later it’s somewhere else.

  15. Peter Grimshaw says:

    I think we are all right – and it all agrees with the Physicist’s original article.

    There is no single ‘truth’.
    There are multiple ‘truths’ all happening at the same time.

    It all depends on your ‘frame of reference’.
    I don’t think there is anything simple about this !
    (but as usual, could have got it wrong ..)

    For a photon at the ‘speed of light’,
    (the photonic frame of reference) :-
    Time slows to super-slow.
    Dimensions expand to include the whole universe, including the black-hole and the big bang.
    (Like – wow.) (This is conceptual – I haven’t done the math)

    From outside the photon, the perception is that a ‘photon’ is a discreet event.
    Space and time are effectively witnessed by NOT travelling at the speed of light.
    (So this is from a different ‘frame of reference’ as Einstein would have put it.)
    So the photon is seen as a discreet object, and is seen to be ‘moving’ through our time.
    This is the effect of the geometric position of the observer.
    Spacetime is a deterministic geometry.
    There are multiple viewpoints, and multiple time-frames.
    One viewpoint is not ‘better’ or more valid than another.

    Is the photon everywhere, or is it discreet ?
    Both ?

  16. The photon’s clock doesn’t tick … OK, but is it ever in a black hole? If it is ever in a black hole, when did it enter?

  17. Jeremiah says:

    David is absolutely right. In fact, we already have tested the fact that photons undergo time evolution and we use this in digital processing. The wave of a photon has the same time element as an electron at rest. The only difference with electron is it needs a lorentz factor to account for special relativity. Things without invariant mass are outside of the scope of special relativity.

  18. Tero says:

    When photons don’t have time element, those have always existed and always will. And only mass can create distances and time. Still seems odd. There are two ends. No mass, no time. Endless mass (black hole), time stops. And between we think it is quite constant relative to earth (basic understanding from time without relative point of view, max speed and max mass we could think).

  19. Perhaps I am not asking the question right, but I don’t think it has been answered.
    Let me try again. Let’s say there is a photon called Alpha. Alpha is outside of time. Alpha also crossed over the event horizon of a black hole and entered into the black hole.
    Did Alpha have a time before the black hole, and a time after the black hole?

  20. The Physicist The Physicist says:

    @Tom Hendricks
    From the photon’s “perspective”, no.
    From the perspective of anything that experiences time, yes.

  21. Peter Grimshaw says:

    But Tom it is weirder than that isn’t it?
    From within the photon’s time-frame time goes on as normal.
    The photon does not know it is travelling at the speed of light, does it?

    Tele happens, chats happen, babies happen.
    Within the photonic time-frame, time passes as normal.
    Within any relatavistic time-frame, time passes as normal.
    Ok, babies might not happen, because there is no mass (?) within the photonic time-frame (?)
    From within the photonic time-frame the exterior universe, us guys, outside the photonic time-frame do not exist because there is an identity in space and time. Light is at all places at the same point in time, simply because it travels at the speed of light.

    But outside the photonic time-frame, it appears the photon exists as, perhaps a particle, perhaps a wave, anyway a ‘thing’ that passes through the ‘time’ of the exterior universe.

    (? !)

  22. Peter Grimshaw, and The Physicist, so our photon, Alpha, is both in the universe and in the black hole at the same time, even before the black hole is created? Further with our photon being “at all places at the same point in time”, it is in all black holes too.

  23. What if I take this a step further. Before the big bang there was no photon. So from the point of view of the photon, Alpha, was there a time before existence (pre Big Bang) and a time after existence (post Big Bang)?

  24. The Physicist The Physicist says:

    @Tom Hendricks
    Where is all of this extra stuff coming from?
    If you shine a light at a wall, the photons don’t experience any passage of time as they cross the room. That doesn’t involve existence before and after the universe or being in every black hole. They never leave the room, and don’t exist other than the brief (by your clock) time between the flashlight and the wall. Nothing fancy.

  25. Tariq khan says:

    Let me add my two cents about the photon being “out of time”. Sorry if someone else said the same thing as I did not go through all the comments.

    Let’s say that I am watching a beam of light traveling from my left to my right and cross two imaginary points A and B in space which are 186,282 miles apart. From my perspective, every individual photon will take one full second to reach point B from point A.

    Now let’s assume that you are riding on one of those photons and you. Let’s also imagine that distance between point A and B is actually a rod which is 186,682 miles long. So from your perspective, that rod zoomed passed you with the speed of light. If you try to calculate the length of that rod, what length would you see according to the special theory of relativity? The answer is that you will see a rod of zero length. Which means from your POV, point A and B will exist at the same location, or you can say that you are at point A and B at the same time.

    You can put point A and B on two extreme ends of the universe, and the answer will not change. That is what means by “a photon is out of time”.

  26. Peter Grimshaw says:

    It’s the two perspectives isn’t it Tariq?
    I don’t have the maths yet, but may try to.
    But this is exactly what ‘relativity’ means in terms of time?
    For some observers, time goes quickly, for others slowly.
    It is really mind-bending.

  27. Peter Grimshaw says:

    Tom it looks to me like the Photon creates it’s own dimension.
    From the photon’s perspective, at the speed of light, it does not realize it is travelling at the speed of light. In fact inside it’s ‘relativity box’ or ‘frame of reference’, the photon is stationary.
    Whatever a ‘photon’ is. Maybe a portal to a new dimension, for us.

    The two ‘realities’ exist side-by-side.
    A bit like a multiverse.

    By making “c” a constant, and a speed limit, from our perspective we perceive photons as objects travelling through our space.
    “c” is a barrier into another world.
    On the other hand, from the photon’s perspective it does not see us at all.
    It inhabits all our time and space due to time dilation and space shrinkage.
    We are shrunk to nothingness, no space, infinite time (from our perspective).
    We will never be able to see inside the ‘box’ or ‘reference frame’ of the photon because we can reach the speed of light. We have mass, we can never travel fast enough to enter the reality of the photon.

    I guess most of us are not good enough at maths yet to get to the bottom of this but it is fun to play with ideas. I can’t do Riemann maths yet, but I think I get Einsteins thought-experiment in it’s simplest form.
    What Einstein had was a simple idea he then extrapolated the Math from.

    Peterg

  28. Peter Grimshaw says:

    Yep.
    That is what my take is.
    So that is how we describe it from our perspective.
    How does the photon describe it?
    Is this modernism?

  29. Peter Grimshaw says:

    Yes.
    I think you got it.
    Can one ride a photon?
    Is that a useful question?
    What would it be like?

  30. Jay VanSant says:

    In Quantum Wave Theory, a photon travelling at the speed of light is not a particle or an energy wave, it is a probability wave that represents the likelihood of a “particle event” happening at a point in spacetime (for example, the absorption event changing an electron shell level of an atom). I liked an earlier post that referred to the speed of light as “the speed of causality”.

    The thing that most astounds me about this conversation is that it doesn’t seem to be represented in any physics text. There are numerous books that point out that Einstein began thinking about relativity as a child wondering what it would be like to ride the leading edge of a light wave from a flashlight. However, it looks like neither Einstein nor any of the physicisists interpreting his theory actually went back to the original inspiration and looked at what it is like for a photon on the leading edge of a flashlight beam. Of course, I haven’t read everything so maybe there is some article or text book discussing this that I have not seen.

  31. Peter Grimshaw says:

    Thanks Jay VanSant
    What a lucid description of Quantum Wave Theory as a wave of heightened probability of particles popping out of the Quantum Foam where the wave is, if that is right !

    We are still talking about the probability waves being viewed from the perspective of someone outside the photon, aren’t we?
    Maybe there is a whole new set of probablity waves viewed from within the frame of reference of the photon though.
    As I see it the photon still thinks it is stationary within it’s own frame of reference – the frame that outsiders think is travelling at the speed of light.

  32. craig says:

    I’m worried that if there’s ever gonna be a big crunch after the big bang – do the photons instantly fly to the outer limits of the universe and then instantly go back.
    What kind of life history is that for a photon? As the man said in Princess Bride – “Inconceivable!”

  33. From the point of view of Photon Alpha, there is no time between when it is created and when it ends. Thus from Alpha’s point of view the beginning moment is also the end moment.
    That suggests that if Photon Alpha is one of those created near the Big Bang – those that make up the radiation that fills the Universe – and it continues to exist until the universe ends, that from the photon’s point of view the entire history of the Universe from beginning to end happens at the same moment.

  34. Photon Alpha looses energy, the wave length increases, and becomes weaker as the universe expands. But that suggest that there was a change in the energy level of the photon. But change requires time. How can Photon Alpha both be outside of time, and change?

  35. Aavi says:

    Does the universe exist in frames from moment to moment and one cannot go faster than the frame , are we in frames per second , even if one is sitting still he is in the frames per second

  36. James Becker says:

    @ Aavi

    There are no such things as “frames”—the idea of “frames” is from the motion picture industry, and has nothing to do with physics. Neither does the universe consist of “moments”. “Moment” is a purely human notion of dividing time. But time is indivisible, and consists of eternity in one piece—not separate moments.

    JB

  37. Denes VanaLava says:

    My only question is, if the observer can enter beyond time?
    What we really want to do with math?
    Time necessary to create separation. First we need space to not be one and time to travel on speed what is limited. Just to keep it in one piece. Now its look like real.
    So we believe this separation and We believe we are something different.
    Im fascinatied by math and art and all this creative variation of the same one.
    But to understand we have to be that one. Behind time. This beauty is just proves how beautiful and infinite dimension was created by that one movement. To be One and the exactly same One what created all variation infinity..

  38. KT says:

    There must be a 4th (or higher) dimension that governs photons. A dimension that we cannot perceive with any instrument available; nor do we contain the language necessary to even begin conceptualizing such a phenomena. In this higher dimension photons are the ones that bend to gravity and experience their own movement as result of being trapped in a greater gravitational field then that of the the higher dimension. Or maybe it’s late, and I’m just tired ..

  39. Kristopher says:

    If time doesn’t exist independently of space, why do we keep talking about time as if it is it’s own “thing”? Isn’t the present essentially infinity (no time) and our current location in spacetime a fixed absolute relative to everything else? A photon of light not experiencing time is the same exact thing as us not experiencing time, as to the photon time=0 and for us at any given moment of our conscious being, time also equals zero. If a photon of light has a “birth”, “events”, and a “death” (absorption) from our perspective, but the photon is not aware of the separation of any of that and experiences only one single present moment, isn’t that pretty much the experience that we have as well? After all, from the photon’s perspective our entire “time” is an instant flash as well; from the photon’s perspective, “our time” certainly equals zero. It seems to me that although light is separated from us by dimension, relativity means that the separation is only relative to each other, and both light and man experiences the present moment and the present moment only. The fact that a photon of light trades “time for space” when there is really no difference between time and space indicates to me that it doesn’t trade anything at all. My line of thought is that the present moment is exploding instantly and is the “end all be all”, as no person can ever directly experience the past or future, and a photon cannot directly experience the past or future either.

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