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

  1. sumoworm says:

    C does not equal infinite speed, it is a finite number. Therefore isn’t it accurate to say state that C’s time = 0 corresponds to its antithesis that space contraction at C = the plank length? Neither are infinite, both are the maximum/minimims in our universe.

  2. dylan says:

    light from the most distant galaxies has been traveling at 186,000 miles a second for 13.5 billion years, it would take thousands of trillions of years to reach us because it has been traveling at the speed of light, reason being when traveling at speed of light for 30 years 1000 years pass here on earth so the first photon waves to leave those galaxies should not have reached us yet. is this correct, if not, why. two people, one standing on the equator, one traveling around equator at 99….. % speed light, both have flash lights pointing at north star. they switch them on at same moment whose beam will reach the north star first

  3. jiohdi says:

    a photon traveling 13.5 billion years at light speed is just that… it does not take more than 13.5 billion years to arrive at earth from the point of view of a man on earth detecting its arrival… if there were another man traveling on that photon, it would seem like no time had passed at all for him, because he is essentially not moving relatively speaking… but if he were a real man with real mass, he would have become a black hole singularity and would never ever again notice any time at all… hyper-dimensionally speaking he would be precisely where he started in the 4th dimension, even though he would have moved in the other three dimensions… as did the rest of the universe around him, so he is basically down a very very deep gravity well that is now 13.5 billion light years deep.

  4. dylan says:

    very interesting

  5. FD says:

    I still don’t understand.. Let’s focus on length contraction. I am observing a light ray coming in from a distant star. Length contraction tells me that the distance between every pair of photons in the ray should appear to be zero to me, but in fact it doesn’t: I can put a synchronized clock on a spaceship stationary with respect to me, located on the straight line between myself and the star, and I can measure the distance between a pair of photons this way, and it won’t be zero. Where’s the catch?

  6. jiohdi says:

    I am observing a light ray coming in from a distant star. Length contraction tells me that the distance between every pair of photons in the ray should appear to be zero to me…

    1. Length contraction only exists for massive objects that warp space-time.
    2. the reason that length contraction happens at all is to ensure that no object can be observed by anyone as greater than light speed. Photons are traveling at light speed and will be measured as such.

  7. Tracy says:

    Just some thoughts…

    So, from the frame of reference of a photon, the photon as we experience it does not exist. There is only a probablity that energy is transferred from one mass (an atom) to another. The type of energy transferred is characterized by the normal electromagnetic parameters we assign to photons. (Simplified, yes, for the moment let’s not discuss electrons, orbitals, directions, etc.)

    In example, a photon travelling from an atom in a star in galaxy we describe as 13.5 light years away hits a detector on the Hubble Space telescope. The probability of those two masses transferring energy is very low but it happens anyway, and its effect is to lower the energy transferred exponentially (inverse square).

    In the ‘mass world’ the Hubble detects a photon (or a bunch from a very localized area in the blackness of space) with very low energy and we back calculate a distance of 13.5 l/y. Anyway, we assume that si where this really big galaxy is, because that answer is consistent with other measures we have (in other words, it’s not a microscopic low energy mass only 3 miles away, because we could tell the differnce using parallax, etc.)

    Subsequently, if in the photon frame of reference it does not exist, there is no such thing as a free photon just shooting off somewhere. It only exists as a transfer of energy between mass objects.

  8. joey says:

    – The photon does not experience elapsed time and can cover any distance in zero time.
    In effect, a photon can be everywhere at once. This implies that there is no need for more than ONE photon in the universe.

  9. Tracy says:

    Interesting, but pondering what a photon is ‘experiencing’ can only be done when considering the relative frame of reference. In the zero-mass frame of reference: there is no time, there is no distance, energy is transferred between mass objects instantaneously (and all the mass objects are in one place that has no dimension to it), there is not the need for any photons, because they do not exist in that frame of reference.

    However, from the mass world’s frame of reference (where you and I do experience things) photon’s appear to be flinging about everywhere. We perceive the transfer of electromagnetic energy between masses much differently, imparting both dimensions of time and distance between the masses and placing the now existent photon (our definition of the phenomenon) between them.

  10. Ric Adams says:

    Time is a measure of changing relationships. Time tending to zero for massive objects means that the mass tends to infinity and there is no energy or force capable of moving them from that final state. A photon has no mass and so there is no need to say its time is zero. From any perspective you can measure photons do change relationships with the rest of the universe and can always be measured to do so at the speed of light.

  11. Xerenarcy says:

    to help clarify this… if you were to draw the line of simultaneity for a photon on a minkowski diagram (SR only for simplicity), it would be parallel to the photon’s path through spacetime. that easily confirms that the photon experiences a single instant only, what is more difficult to prove is its ‘motion’ whether apparent or physically real.

    well… at rest our notion of ‘simultaneous’ is anything happening parallel along the time axis. since a photon’s time component is zero, its motion (if any) must exist entirely on it’s axis of simultaneity. because photons can travel an arbitrary ‘distance’ and have an observable ‘motion’ (position dependent on or can be a function of time), the path it takes does have to have a ‘length’ or ‘magnitude’ of some sort in all reference frames if you avoid using reference frames that yield singularities (such as zero-distance analogies).

    the motion of a photon, if not space-like, must then be time-like to the point of view of a photon. (to summarize) this is because all apparent motion must have parallels in all reference frames; singularities arise only because of poor choice of coordinate systems, and a zero-distance interpretation implies a singularity has arisen.

    so then to restate the question unrelated to photons specifically – can time-like motion experience time?

  12. Rpahut says:

    It is the perverted corpuscular idea of a photon we get from our daily lives, I have learned, what leads to all the questions about its motion and speed. The answers are not to say simple, but rather questions themselves should have no place if you understand what the photon really is (or what it isn’t).

    Note that the photon is from the bosons family. Photon carries light just as gluon “carries” the strong force, or higgs boson “creates” the mass. They are used in physical models and proved themselves useful as proxies in interactions (for the lack of better alternatives), but outside of the equations they make very little sense.

    No one have ever observed a boson for example. The only way to know there is a photon is to get it absorbed by an electron – but then there is no photon any more, and it is still a question if there was one to begin with. All we know is that one electron somewhere emitted a portion of energy and then another somehow received it. There is definitely no ball of light traveling through the space between them, as was demonstrated by the double-slit experiment all the way back in 1800′s, so the question “how can photon travel” actually became invalid long before it could have come to any of our heads.

  13. Bryan says:

    Photon’s don’t travel. Photons are everywhere at the same time as they are infinitely big.

    If you want to see Einstein’s thoughts on this then google “spooky reaction at a distance”. The misnomer in that name is “distance”, not “spooky”, because there is no distance. As long as you keep that in mind you’ll be able to a) understand “perfectly”, no PhD required, and b) appreciate the genius Einstein really was (well beyond e-mc2).

  14. Orien Rigney says:

    If a photon experiences no time in its travel between objects and is every where at once, doesn’t that make using “red shift” as a means of measuringe distance between stars and galaxies a waste of time? Now I am just a bit confused as to why it isn’t light all of the time? Perhaps my ignorance is causing me to miss something here???

  15. Xerenarcy says:

    redshift is used in conjunction with brightness to determine distance. at first we only had brightness to go on, but after noticing that things appear to be receding from us the further you go (correlation with redshift) we realized we could use this fact backwards – that the redshift indicated the recession velocity and due to the correlation, distance as well.

    after thinking on this for a while i’ve come to believe that ‘photon’ is a label in the same way that ‘wave’ describes a propagating distortion. perhaps a simple way to explain this distinction is conways game of life. specifically gliders (http://en.wikipedia.org/wiki/File:Gospers_glider_gun.gif)

    in the same way a photon is, in one sense, an self-sustaining oscillation of the electric field which due to the nature of the oscillation itself, ends up moving through the field (if it were stationary or could slow down, the oscillation would likely disappear / disperse). what this would imply is the field itself must experience time, since without time-evolution occurring the photon could not propagate to begin with.

    in that respect it makes no sense to talk about ‘the experience of a photon’ for the same reason it makes no sense to talk about the motion of a dot of light from a laser pointer – regular matter cannot hold itself together from exchanges via ripples in a field if it begins moving at the same speed as these ripples. in fact it is academic to show how this slows down time for matter as it speeds up relative to something, without having to touch the rate of time-evolution of the matter or the relative observer.

    basically the rate of time evolution looks to be independent from time dilation of electromagnetic interactions, commonly attributed to actual time passing. due to this it is not impossible for a photon to experience time, however this notion of time is quite abstract – it is not the same notion of time you would ascribe to anything of human experience; if there is a locally absolute time in some form, this would be it.

  16. Orien Rigney says:

    Thanks Xerenarcy! I do get the gist of your statements: But, it seems the deeper I dig, the wetter I get. To me, light is light no matter at what part of the spectrum(freq,) we are looking. Then I see the superluminal properties Einstein speaks of, and knowing nothing of calculus or physics, I’m stuck. Perhaps you might run this by me so I may get a better understanding. Thanks http://www.nature.com/nature/journal/v406/n6793/abs/406277a0.html.

  17. The Physicist The Physicist says:

    @Orien Rigney
    There’s a post here that talks about some of those superluminal experiments. You’ll be happy/disappointed to know that no physical thing actually goes faster than light, even in these bizarre experiments.

  18. Orien Rigney says:

    Thanks again Xerenarcy!
    Am I thrilled that I’m ignorant, of course not! But it does open an even larger can of worms for me to contemplate. In a sense then, we’re earth bound other than visiting our relatively small solar system? Trying to stay within the subject matter, knowing that C is max., how can we even think of traveling to another star system. While I’ve heard of a generational transport ship of some sort getting us possibly to Alpha Centauri B, time wise it reminds me of an old TV series starring Lorne Greene. In just so many words, wouldn’t it be far more conducive for everyone involved to use their knowledge making our planet a more habitable world?

  19. Xerenarcy says:

    yup. so far there has not been a single physical thing conclusively observed to move faster than the speed of light. but in that experiment and in the linked post explaining it, we are not dealing with a physically real thing, so conventional notions of ‘speed’ and ‘position’ (amongst other things) would not always or necessarily apply.

    and for the record our planet is the most habitable we’ve found, after all, we’re on it. we don’t really know how to improve it, and when we think we do, we either screw it up royally or can’t agree on what to do. (im a social cynic, don’t listen to me)

    just thought this section on good old wiki would help explain my argument a bit better – http://en.wikipedia.org/wiki/Maxwell%E2%80%99s_equations#Vacuum_equations.2C_electromagnetic_waves_and_speed_of_light

    the very short version is: if photons couldn’t experience time in some way, they couldn’t have a phase.

    which leads to an interesting perspective – when two things interact, if one experiences time so must the other.

  20. Orien Rigney says:

    It doesn’t take long to see that I’m only a thinker not a scientist, does it? So, let me hit you with what I think is my very best shot ever. Your statement gave me the reason to even consider it. Quote: the very short version is: if photons couldn’t experience time in some way, they couldn’t have a phase. Unquote What is the possibility of a wave or system of waves that link all atoms in the universe? And are systemic, intrinsic, self sustaining and endowed to each sub-particle at inception. Personally I see this as the Holy Grail of nature. If science can even remotely consider such a phenomona, I think they will have begun to understand creation itself.

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