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.

This entry was posted in -- By the Physicist, Relativity. Bookmark the permalink.

250 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:

    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 ?? 😉


  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/

    Photo Electric Effect

  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?

  24. Tom Hendricks says:

    The Heisenberg Uncertainty principle says we can’t know both the position and the momentum of a quantum particle.
    My question is this, if you know that a photon is going the speed of light, the momentum, then does that mean you can never know the position of the photon?

  25. Mike Melgaard says:

    I have to interject there, David. Einstein’s Relativity holds steady and very strong. Your comparison to the speed of sound is not a valid comparison. We have observed physical phenomenon for hundreds of years which show an object traveling faster than the speed of sound. Take for instance the crack of a whip; that crack at the end is essentially a minature sonic boom.

    The problem with breaking the light speed barrier (beyond what the obvious mathematics show) is that no object containing mass seems to travel beyond the speed of light. I’m not saying you’re absolutely wrong (because who knows, maybe there is something we’re all missing here) but the evidence thus far supports relativity to an absolute certainty if the theory truthfully describes our reality. And trust me, there have been a plethora of methods which have been dreamt up to test the theory and its validity.

    As for your other ideas, as the once great Carl Sagan put it: Extraordinary claims require extraordinary evidence.

    The evidence for your ideas is lacking, and strongly favors the modern theories which currently are used to explain what we observe.

    But don’t stop thinking and being creative with your ideas. That’s half the fun of all of this if you ask me; and new ideas are what propel science forward.

  26. Sam Claret says:

    There are many ways of looking at a photon. If you regard the photon as an exchange particle when the emiter drops in energy and the absorber increases in energy. In the frame of reference of the photon (by virtue of its velocity) then there is no separation in time or space (in the frame of reference of the photon) between the emiter and the absorber. That is simple maths. But you say if you take a photon that was emitted 17 odd billion years ago and we capture it as back ground radiation we know it has travelled a long way for a long time – in our frame of reference. So in common parlance does it travel a distance or not – the answer is it does and it doesn’t…..it depends who is looking and where they are sitting relative to the photon.

    @Tom – the Heisenberg Uncertainty Principle states that you can only know the position / momentum to within a certain precision. Ie the more precisely you know the momentum of something the less precisely you know the position of it. Now you may know fairly precisely the momentum and also the position between where it was emitted and absorbed – however that is only on a straight line there are 2 other spacial co-ordinates perpendicular to the line of travel where the particle can be probably located. Note the probably bit thats the important bit when dealing with quantum particles.

  27. Spoutinwzye says:

    if matter moving above the speed of sound creates a sonic boom, then why does normal light photons not make noise? They are infinite mass, traveling faster than the speed of sound
    Also if the sound of a whip cracking is the tail (or now thought to be the arch of the loop) traveling faster than speed of sound, then why is the end of the tail get frayed after use of cracking? Simply being in motion shouldn’t cause damage.
    Is the crack perhaps the end touching itself at the speed?


  28. vivetha kanagaraj says:

    You said that light travels zero distance in zero time. If that is the case, how we are saying that sunlight reaches earth in 8 min 16 sec? Since light travels through vacuum to reach earth’s atmosphere, how can we determine this time calculation for sunlight?

  29. The time calculation is for the non-photon observer. For light itself no time has elapsed from its origin in the nuclear fusion until reaching our retina. In fact it is an instant connection between fusion and retina.

  30. craig says:

    So to address the original question – “how can it travel”?

  31. For the slow us it travels at the speed of light, and from one place to another.

    For the foton itself it is mere interaction between fusion atom and retina molecule, without distance and speed.

    A stick is long thing looked at it from the side, and a narrow dot when looked at in the axial direction.

    Well this is my penny thought on this…..

  32. James Becker says:

    Craig: it does not travel. Photons do not “travel”, they are instead everywhere at once. It is only our perception of photons that creates the concept of travel. As a previous commenter mentioned, when you look at the sun, any photon linking your retina and the sun is simultaneously present on your retina and on the sun, and in fact there is no distance at all between your retina and the sun, to the photon. It is a difficult concept to understand, but there is just no simpler way to explain it.

  33. craig says:

    Sorry James – I can’t buy it. If photons do not “travel”and are everywhere at once – then every photon has instantaneously “seen” the entire history of the universe from beginning to end.

  34. But James Becker, when a photon enters a black hole, what time is it? Or is there time? Does that mark a finite time?

  35. Craig, That’s what I had said. Go further, the photons from the Big Bang have traveled everywhere in the Universe endlessly over and over.

  36. James Becker says:

    Craig: Spot on. You’ve figured it out (and summed it up quite nicely, I might add). Yes, the universe is weird.

    Tom Hendricks: it depends what time zone you’re in. EST? Central time? Jakarta? Or BHST (Black Hole Standard Time)? Keep in mind that BHST does not incorporate daylight savings time.

    There is no such thing as “time” to a photon.

  37. James Becker. When a photon enters a black hole it stops being on this side and starts being on the other. That suggests a moment in time, and a before and an after doesn’t it?

  38. Or invert this: photon entanglement enables us to look into the black hole…..

  39. Wow! Hubert Pellikaan, that is a very exciting thought!

  40. craig says:

    In response to James (Sept 27) – if I am right, and every photon has instantaneously “seen” the entire history of the universe from beginning to end – then wouldn’t this explain some of the wierd aspects of tunnelling, or how biological stuff like leaves seem to create themselves. Maybe the probabilty waves collapse and turn themselves into particles at the right moment cos they’ve already bin there and done it.

  41. Supercharlie says:

    Light only travels at the speed of causality (“speed of light”) in a vacuum. It slows down when passing through matter as photons are absorbed and re-emitted. Light could experience time during these slowdowns. It can transform at these times too, such as changing polarity, intensity, frequency/color, or direction.

  42. Student says:

    I found this web page after my Chem teacher went to explain the potential energy of electrons in their various shells. I suggested that the inner electrons should have a larger kinetic energy based on Kepler’s Laws and our known laws of physics (that tighter orbits=a higher speed and thus more energy) . I know that it was wrong, but it started my investigation into the topic of this web site. I am now thinking that the Human race need to create a new set of Laws for light (the Physics of light speed) see as how our current law of physics seem to break down at the electron scale. I am filled with more and more questions I would love to solve, such as why can’t electrons escape their orbit with the energy that they already posses? If electrons can ‘quantum leap’ why don’t / can’t they escape the small magnetic field produced by protons? What stops them from jumping across the universe? I can guess that these questions have already been answered in a similar way to the first post or that they are incredibly simple questions, but any response would be much appreciated.

  43. Danny Byrge says:

    There was a post above referring to photons slowing down in different mediums, opposed to a vacuum. I don’t know if someone else answered the question or not, but photons actually travel at the speed of light, no matter what. What he observe as a photon slowing down, is in fact, it being absorbed and emitted back out by an electron. Let’s say we have two photons running a race starting at the same starting line. If they have a distance of 3 gigameters to cover, it will take 10 seconds. If photon A. travels in a pure vacuum, he arrives 10 seconds later at the finish line. If photon B. encounters one atom on his journey, and is absorbed then emitted 1 second later, he will arrive at the finish in 11 seconds. If we divide the distance by time of each photon, photon A. will travelled at the speed of light. Photon B. will have seemed to travelled slower than the speed of light, but since it was absorbed and basically did not exist for one second, except as energy inside an electron, it never stopped travelling at the speed of light. So the 10 seconds out of 11 seconds ot existed as a photon, it travelled at light speed. They only way to change it speed was to change it to energy, in which case it didn’t exist as a photon anymore. So it covered the same distance in the same amount of time why it was a photon. I hope this helps.

  44. Maksym Edel says:

    The so called photons cannot travel because they have zero mass, nothing can affect nothing thus nothing can make photon move. So this is like you imagine a friend and try to push it – this will happen only in your imagination, but not in real, because your friend = nothing, but your imagination.

    So literally – photons are nothing = zero. If you add something – this is no more nothing, this will the the something you have added and it’s no more a photon. So easy like 1 + 0 = the same 1, but not 10 or 0.

    The light is a wave which only live forms with eyes (or other visual sensing detectors) can see or detect, in terms of any non live (not being able to process EM wave information) light does not exists – all darkness = nothing. Like some crabs on the bottom of the ocean – they have no eyes thus they don’t even have the term “Light”.

    The eyes (or other sensing detectors) can see the rest of absorbed so called light (because it’s EM) waves and transform the signals into a picture in our brain. No brain – no picture – no light.

    The same happens with time – there is no time, time is what we, humans, use to measure the duration of processes (life, years, days, hours, rotation of planets etc = actions). Because all of the processes start and in most cases have tangible related to us, living forms, end – this period we call time.

    If simple and covering a bit more.

  45. Sirius45 says:

    Following up on the thread, can I view a photon as an eternal particle, or something with a lifetime that can be created/destroyed. Does it make sense to say there are and will be a finite number of photons in the known universe?

  46. Brad says:

    Isn’t relativity symmetrical? From the light’s frame it is everything else in the universe moving while the light is stationary. In the light’s frame everything within its frame would be occurring normally. I know light can’t observe, but if one could hitch a ride on the light and had a clock, one wouldn’t notice anything different and time would pass subjectively normally. These assertions are just statements of how I’m understanding relativity. Please comment if they are wrong. I love “breaking” my mind with this stuff. Science can be so bizarre.

  47. A new phenomenon is theorized on the hidden power of entanglement. Read

    Very briefly Erik Verlinde states :
    The observed phenomena that are currently attributed to dark matter are the consequence of the emergent nature of gravity and are caused by an elastic response due to the volume law contribution to the entanglement entropy in our universe.

    Or even shorter (and maybe wrongly since by me;-))

    Entropy in quantum entanglement causes gravity

  48. wesley smith says:

    Would photons experience time relative to a new particle discovered traveling faster than the speed of light? Or is the speed of a light the point where distance and time mathematically become zero?

  49. Coulomb Jean - Paul says:

    ” If a photon does not experience time then how can it travel ?
    Interesting question, …. it is may be because photons are ”located” outside our
    4-dimensional spacetime. They are in a 5 dimension. Equivalent to an energy
    dimension …. Take a look to the research of P. S. Wesson : ” Modern five
    dimensional Gravity ” ( P.S. Wesson – Wikipedia ).
    Regards from Marseille – France.

    Jean – Paul

  50. Hakim says:

    Well technically nothing travels anywhere from the perspective of the thing itself. You are always the still frame of reference relative to you and everything else is moving past you. So you don’t actually travel anywhere..things come and go from you…physics supports your narcissism.

Comments are closed.