Q: Does light experience time?

The original question was: Given that light is moving at light speed, and time slows down as a massive object approaches the speed of light, does light travel through time?  Does the whole time slowing down thing just not apply to massless particles, and if not why not?  If light doesn’t travel through time, how does anything make sense, since clearly light moves but movement is dependent on time?

Physicist: Nope!

There are some things that behave differently when investigated from an “approaching light speed” way of thinking and the “being at light speed” way of thinking.  In this case there’s no difference.  When something travels at the speed of light it really doesn’t experience any time.

On the flip side of that coin, it also doesn’t experience any distance.  The time and location of its emission and the time and location of its absorption are the same from a photon’s perspective.

This may not make sense, and it’s a little mind bending, but consider this:

Movement isn’t dependent on the time experienced by the moving thing, it’s dependent on your time.  If you see someone pass by, you can say (for example) “that person is moving at 100 kph, because during one of my hours they’ve traveled 100 of my km”.  That may seem a little over-exact, but the time and distance between things changes for observers that are moving differently, so you have to be especially careful.

If, however, you were to ask the person who passed by “how fast are you moving?” they’d say that they’re not moving at all.  They’d say that during one of their hours they traveled zero of their km.  These different measurement systems / perspectives are called “reference frames”.

Here on Earth we feel like there’s such a thing as “non-relative movement”, since we all agree (very naturally) on the same reference frame: the (local) surface of the Earth.  That is, you probably frequently refer to yourself as moving, while you rarely think of the Earth as moving.  You’d have to be pretty full of yourself to drive down the street and claim that you’re stationary and that the rest of the world is moving past you.  But at the same time: you’d be right.

Smug drivers: technically correct.

The point is: everything always thinks of itself as stationary (you don’t move with respect to yourself), and movement is a property assigned to other things based on each observer’s reference frame.  So light may not experience either time or distance itself, but to move, all it needs to do is get from one point in your spacetime to another point in your spacetime.

Answer Gravy: As a needless side-note: when physicists talk about the path of an object through spacetime they usually “parametrize” it using that object’s on-board (or “proper”) time.  That is, you give them a time on the on-board clock, and they’ll tell you where the object is at that time.

Using on-board time is convenient for a number of subtle reasons.  It even makes one of the derivations of E=MC2 run a lot smoother!

But a photon can’t have an on-board-clock, so physicists instead use an “affine parameter”, which is fancy-speak for “screw it, we’ll just use my clock”.

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116 Responses to Q: Does light experience time?

  1. Bill S says:

    Marcos, it’s never too late to join a thread, especially when you have a positive contribution to make.

    “Photon may or may not experiment time. We can´t know but we can assume one of these two possibilities.”

    No argument from me, there. There are plenty of scientists who make an assumption on either side of this question. The important thing seems to be to keep in mind that it is an assumption; not a proven fact (if there is such a thing in science).

    “…….it seems to me that a photon MUST be everywhere at the same time at it´s reference time frame.”

    Intuitively, that would seem to be so, but can you say why?
    How do you define a reference frame for a photon.

  2. Marcos says:

    Bill, firstly thanks for your answer and good words.

    I am assuming things and I know that it can´t be assumed. But sometimes this is good to know if a certain way of think is better then the other.

    Your first question is very hard : “Intuitively, that would seem to be so, but can you say why?”
    The problem is the word “why”….

    I don´t have so much to add to argumentation.

    My point of view is that IF we assume the photon don´t experience time at it´s own frame then it is useless our time frame.Because our time frme can´t influence the time frame of the photon. My time frame can only influence how I see the events, but not the events by themselves.

    In a given trip of the photon from point A to B, if the photon doesn´t experience time brings me two possibilities (and they are independent from an observer time frame):
    1. In fact there´s no point A or B. (weird…)
    2. The light is along all the way path from point A to B forever. Which drives me crazy…

    I am sorry about my english.
    I don´t know if my words can misunderstood as a kind of arrogancy or being rude. It´s not my intention. I don´t know how to write a more polite and better english.

  3. Bill S says:

    Marcos, don’t apologise for your English; one thing of which I can assure you is that your command of English is better than my command of your language, whatever that language might be.

    There are one or two points I would like to come back to, but am a bit short of time at present.

  4. Marcos says:

    Bill, relax. I understand that is a lot of things in our day life that happens at the light speed, to them there´s no time, but to us they happen very fast. 🙂
    I´m pleased for your answer until now because I´m trying to understand a little bit better things that are unclear in my head and you helped me in this way.

    I thought in additional thing to try to explain my reasoning :
    – To us, on the earth, things passes at the same speed time (or time frame). I know that time depends of the speed, but it´s an insignificant difference for now.
    I mean, a second is a second, an hour is an hour and so on…

    I tried to imagine a space ship, at almost light speed, crossing the horizon.
    My chronometer shows me that it took 60 minutes to move from the left side untill the right side of the horizon.
    Desconsidering th precion of the calcules, I can say that, maybe, for the voyagers inside that space ship the path took one second.
    Until here is clear to me and I agree with everything.

    More than this I would be capable to understand IF (very, very big IF) the space ship was moving faster than the light speed, probably I would see it crossing from the right to the left side of the horizon. (I know that is impossible to overcome the light speed (as far as we are under it), is just an hypothesys.)

    To me are clear what happens at the two situation above.Please, let me know if my undertanding about these situations are wrong !!

    But, at the exactly light speed my brain crashes.

    Look, if under light speed the object moves from left to right and opposite occurs when something is above the speed light, what can I imagine ?
    In my imagination I would be able to see the space ship along all the way path from left to right, or the opposite (doesn´t matter because the space ship would be at every single point at the same time to me !!!).

    Going far away : space is something that can be compressed or inflated by the speed. In another words, space adjust itself to the speed.

    Let´s do an abstraction :
    Imagine three axis corresponding to height, widht and depth. Height is pointing to up and the other two axis to the right separeted by an angle of 90 degrees.
    Think at the same three axis, but now they are mirrored. Now the height is pointing down an the other two are pointing to the left.
    In both cases there is just ONE POINT joining the three axis.
    (please, don´t take into consideration that in space this referencial is not valid – it´s just an abstract way to try to show what I´m thinking)

    Please, construct this picture in your brain to understand how my conception is.

    If we are under the speed light we experience the first picture of the space axis, so height is pointing up and the other two to the right.
    If we are above the light speed we experience the mirrored axis picture.

    So, in consequence, what do we would experience if we are at the exactly light speed ?
    In my mind we will experience is that space will be just that point that joins all the axis. Or, there´s no space, there is a single point.

    Speed, just an only speed, can make that single point expands to the one or to the other side, creating extension to what we call axis.
    Speed can be felt by our senses (sight or hearing). But the expansion of the space caused by speed, called also TIME, is not easy to feel. From certain way we feel it, but we can´t recognize it like we do with the other senses.

    Due to it we have a sort of misunderstangs in what TIME is.

    Bill, I really don´t know if these kind of reasoning makes some sense to physicist, but is in this way that I am trying to reach the compreension about what is disturbing me : what happens at the light speed ?

  5. Marcos says:

    Bill, probably my summary is a bullshit, but it started to make some sense to me (a layman).

    Einstein said that there is space and time, and he said that both are joined in space-time.
    I agree wit Einstein.

    Oh, my god…who do I think that I am to disagree with him ?!

    My point is : what creates space and time ?
    The answer that is making some sense to me is : speed.

    Speed needs space to exist, and time to be measured.
    So, without speed there is no space neither time.

    Is it a very big bullshit Bill ?

  6. Bill S says:

    Marcos, you said: “I tried to imagine a space ship, at almost light speed”

    That should be reasonably straightforward as long as you use the relativistic velocity addition formula to calculate time and space transformations. Relativity covers this and defines reference frames for travellers and observers. The trouble comes when you try to extrapolate the equations of relativity beyond their legitimate scope. As far as I am aware, there is no RF defined for anything travelling at “c”, so, although anyone can hold an opinion as to what would happen, or what conditions would be like in such a RF – if one could be defined – until someone can come up with the necessary equations/scientific evidence, it’s all speculation.

    Having said that; it does seem very odd that the relativistic velocity addition formula should work up to the “threshold” of “c”, then suddenly not work anymore. I’m not comfortable with this, and, seemingly, you are not, either.

    Ethan Siegal says, at:


    “When you move at the speed of light, this means the following:

    You absolutely cannot have a mass; if you did, you’d carry an infinite amount of energy at the speed of light. You must be massless.

    And you will not experience the passage of time; your entire journey will appear to you to be instantaneous.

    It’s a long time since I read this article, but if I remember correctly, it’s worth looking at.

  7. Bill S says:

    Marcos, our posts of yesterday seem to have appeared in the wrong order. Mine is certainly not a reply to your last one.

    You ask: “Is it a very big bullshit Bill ?”

    I’m not qualified to make that judgement, and if I were, I would hesitate to label anyone’s ideas as “bullshit” – well, almost :).

    “Speed needs space to exist, and time to be measured.
    So, without speed there is no space neither time.”

    At first glance, that looks like a circular argument, but I’ll give it some thought and come back to it later.

  8. Marcos says:

    Bill, maybe is better to change “speed” for “movement”…
    “Movement needs space to exist, and time to be measured.
    So, without movement there is no space neither time.”

    By the way, I sent you an e-mail regarding your penultimate e-mail (the one that you sent me an article too). I did it because I sent you 3 diagrams to ilustrate my reasoning.

  9. Bill S says:

    Changing speed to movement in the first part of your reasoning was something I thought might help. Paraphrasing your original I came up with:
    “Movement needs space in which to move.”
    “Movement through space needs time in order to me measured as speed.”

    Is I see it, the problem lies in your conclusion: “So, without speed there is no space neither time.”

    This is a bit like saying:
    All quadrupeds have four legs.
    All cows have four legs.
    Therefore, all quadrupeds are cows.

    The mistaken logic, here, is obvious, but yours is a bit more subtle. Possibly a better conclusion would be:
    “Therefore, movement requires space and speed needs time (d/t and all that), so neither movement, nor speed can be precursors of space and/or time.”

    Space and time must come first in order to make motion and speed meaningful.

    BTW, I’ve not received an email from you. I thought email addresses were not “published” on this forum.

  10. Marcos says:

    Bill, I thought for a long time and unfortunely I was not able to find the logic error that you you pointed in my reasoning.
    We agreed to change the word “speed” for “movment”, then the phrase is : “Without movement there is no space neither time.”
    Comparing this sentence with your analogie, my phare would put movement, space and time as the same thing. I can´t realize that I did that.

    I agree that your reformulated conclusion is much better than mine.

    These sentences catched my attention :
    ““Therefore, movement requires space and speed needs time (d/t and all that), so neither movement, nor speed can be precursors of space and/or time.”

    Space and time must come first in order to make motion and speed meaningful. ”

    If big-bang theory is right, then at beggining there was no space nor time. The expansion of the universe created them. The expansion is nothing else than put things in movement.

    But here I can see clearly a logic mistake like the dog that tries to catch his tail.
    The problem with it is : were space and time created since the begining of the expansion?
    The big-bang theory says NO. It took some “instants”.
    So, how could there be expansion without space? This question is not answered yet.

    If someday scientists review the statement above to “Space and time were created since the early beggining of the expansion”, then my reasoning makes sense.

    But, I agree that we can´t work with so many variables and IFs.

    Obs.: my purpose is not to change the subject, but to give subsidies for what I think.

    You are right about the e-mail. No problem because during the weekend I thought about the diagram and I realized that I need to review my argument.

  11. Bill S says:

    Marcos, never mind the logic thing, for the moment. Let’s look at the Big Bang. You said:

    “If big-bang theory is right, then at beggining there was no space nor time. The expansion of the universe created them. The expansion is nothing else than put things in movement”.

    One thing to remember about the BB theory is that however much backward extrapolation we do, we cannot reach the “instant” of the BB. We might be able to approach to an unthinkably small fraction of a second, but, like trying to approach zero K, we can never actually get there.

    Consequently, we have no real evidence to support or refute the idea that time and space were created at the BB.

    A few decades ago, most cosmologists would have said that it made no sense to ask what came before the BB, because the answer would have to be “nothing”. I think there is a tendency, now, for science to acknowledge that the BB might have been only the start of our observably Universe, and that there might have been something before it.

    To me, this makes very good sense, because if there had been nothing before the BB, there would have had to be a change from “nothing” to “something”. Change cannot happen without time, so time must have existed before the BB.

    If there had ever been nothing, there would still be nothing now.

  12. Marcos says:

    Bill, I am not up to date with the BB theory, but what you wrote about what cosmologists are thinking now is just a circular reasoning. Which means that BB is just an explanation that, in fact, doesn´t explain anything.
    Im not disagreeing with you, I´m disapointted with your update about the subject.

    I´m not a scientist nor work with physiscs, cosmologie, and so on…Then I used and argumentation that I was thinking is valid nowadays.

    In my exposition (if it was still true) I said that was a problem in it because nobody knows what happened at that firsts “moments” (whatever “moments” could mean at that condition).

    I totally agree wit you that we will not be capacle to reach a conclusion of what happen at “instant” zero which caused the expansion. There will be never a way to test if a thery is true or not. But, BUT, sometimes is necessary to extrapolate our commom sense an if someone could develop a theory with mathematical proofs will be better than nothing.

    Back again to the subject :
    So, for the cosmologists, time is something that has always existed?
    It rememebers me Lavosier.

    Well, in the near future we will reach the conclusion that : “Nothing is lost, nothing is created, everything is transformed.” And God will be laughing of Stephen Hawking…

  13. Bill S says:

    Marcus, I’m not a scientist, either. Any knowledge I have comes from Pop Sci books, on line discussion and that sort of thing. I could be wrong about current beliefs among cosmologists.

    We have drifted some way from the original question, but that question involved time, and time touches on practically everything, so let’s justify it that way. 🙂

    We have already considered that the Big Bang may the best theory we have for the origin of the Universe. It is widely believed that it needs inflation in order to iron out some of its problems. Eternal inflation (Alan Guth) seems to be popular at present. It involves universes being created eternally into the past and future. (In order to look seriously at this, I have, at least temporarily, to suspend my reservations about infinite successions and the like).

    To have a situation in which this eternal inflation can operate, we must have an infinite vacuum, with a non-zero vacuum energy.

    Quantum mechanics provides us with this, because it forbids us from having the classical vacuum, which can be identified as absolutely nothing. Absolute nothing provides absolute information about its state, and the uncertainty principle does not give us that luxury. There has to be the possibility that the vacuum contains something, and therefore, is something. In fact, to the best of our knowledge, the vacuum is, on the scale of the Planck’s length, a very active and energetic place.

    An additional requirement, one, that for reasons which I find inexplicable, some experts hesitate to agree with, is that that this vacuum must be eternal.

    On the principle of taking one step at a time, let’s break here to get your thoughts this bit.

  14. Ron Williams says:

    I am not Bill or Marcos. Time does not exist as an entity. It is a construct of man for convenience. There is only the present and and a record of the past. There is no future. We exist in the present but cannot exist in the past. We cannot know the future as it is hidden from us. Time cannot be defined except by what we experience in the now. We cannot reprise the past. Time is measured by artificial means but cannot predict the future nor allow the past to be recaptured. Time is an artificial means of measurement developed for the convenience of man. Time does not actually exist in nature or in science except for an artificial measurement procedure developed to record events. Otherwise there is no evidence that Time exists.

  15. Ritin says:

    Speed of light is constant for all observers. At the speed of light time stops and distances are shrunk to zero. How can then light maintain its speed in its own frame of time? Because time does n’t move and no distance to cover to maintain its constant speed .

  16. Bill S says:

    Ron, you say: “I am not Bill or Marcos. Time does not exist as an entity.” From this, I assume, you have me tagged as one who considers time to be an “entity”; defined as “a thing with distinct and independent existence”. Is that the same as saying it is something “real”? Obviously, one would have to be clear about the definition of “reality” to answer that “safely”.

    I try to keep an open mind regarding the nature of time, and am inclined to argue from different perspectives to further my own understanding.

    My current position is something like this:

    1. There can never have been “nothing” or there would still be nothing now.
    2. Something must always have existed.
    3. Discounting the supernatural; I consider the best contended for that to be the cosmos (sensu John Gribbin).
    4. Given that the cosmos is eternal, it must also be infinite. (I’ll justify that if necessary).
    5. If the cosmos is infinite, it can have no change, passage of, or through, time; nor can it have any divisions. (I’ll justify that if necessary).
    6. Our Universe changes, as far as we are aware, so we need time in order to make sense of the perceived changes.
    7. It would follow from point 5 that our Universe, must be a feature of the cosmos, and must, therefore, be infinite, changeless and timeless.
    8. At about the time at which I reached this point in my thoughts, I met David Bohm’s concept of the implicate and explicate orders. The implicate order would be roughly equivalent to the eternal cosmos. The explicate order would be that which we experience as our changing Universe.
    9. This leads to the idea of a timeless cosmos, of which the Universe is a 3+1 dimensional “shadow”.

    That lot should set the cat among the pigeons, – I hope. 🙂

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