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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79 Responses to Q: Does light experience time?

  1. The Physicist The Physicist says:

    @ Bill S.
    Unfortunately there aren’t enough hours in the day to provide a running dialog for each posted comment. Were you wondering about something in particular?

  2. Bill S. says:

    “Unfortunately there aren’t enough hours in the day to provide a running dialog for each posted comment. ”

    I appreciate that, and am grateful that there are experts who are willing to give their time for the benefit of “hitch-hikers” like me.

    “Were you wondering about something in particular?”

    I wonder about almost everything. :)

    Seriously though, I felt I had raised questions about your original answer, and wondered if I my comments were valid in terms of current scientific thought.

    Also, I had attempted to respond to issues that others had raised, and didn’t like the idea that I might mislead them.

  3. The Physicist The Physicist says:

    Looking back I don’t see anything particularly egregious that jumps out.

  4. Bill S. says:

    Thanks; I take consolation from that. It just seems that two comments such as:

    “When something travels at the speed of light it really doesn’t experience any time.”


    “It seems that the best we can say is that we have no way of knowing if photons experience time, or not.”

    appear to be mutually exclusive

  5. David says:

    What might be some broader implications of this? Entanglement for instance – while the photon appears to us in different places – resulting in ‘spooky action at a distance’, to the photon it’s in the exact same place at the same time, so no surprise a change is affected instantaneously. Might entanglement be really an outcome of how our view of time and distance is different from a particle’s? Do you think a photon wonders the reverse about us?

  6. Bill S. says:

    Hi David,

    I’m not even going to try to comment on entanglement. The chances are you know more about that than I do.

    One thing I think we have to keep in mind is that we observe entanglement. In an observer’s frame of reference a photon cannot be in two places at once. It would seem to follow from that that entanglement should be observable only in the F of R of the photon, and as we have already discussed, we cannot establish a F of R for a photon.

    I think you raise some interesting points, and I hope others will join in so we can have a good discussion.

  7. The Physicist The Physicist says:

    Entanglement is bizarre, but not terribly mysterious. We have found that (despite what popular media overwhelmingly says to the contrary) there is no way for entanglement to be used to send signals faster than light.

  8. Bill S. says:

    Thanks for those links, Physicist, all I need now is time to read them. :)

  9. will says:

    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.

  10. Bill S. says:

    Will, I think your reasoning is somewhere between pre & post Einstein physics.

    In your first example, if you are talking about motion on the Earth, you can say “this person is moving at 100 kph relative to the surface”, thus establishing that it is that person, not you, who is moving relative to the Earth.

    If, on the other hand, you are talking about travelling in empty space, you can reach no such conclusion. All you can say is: “one of us is moving relative to the other”.

    Similarly, in your second statement you mention “the person who passed by”. On Earth, that person would admit to moving at 100 kph; but would be able to give your response in space.

    I think, what you were saying was that perceived motion depends on the individual’s frame of reference, which is, of course, good sound relativistic thinking.

  11. Carmela Chen says:

    If time doesn’t pass for light, how is it that light is red-shifted? Since the distance traveled by a photon that gets red-shifted is essentially 0 — even if the space over that distance is expanding from our frame of reference — wouldn’t this mean that the energy at emission and absorption should be the same? How, then, can light be emitted at one energy level, and then absorbed at a different energy level? Is the energy difference observed for a photon traversing expanding (or shrinking, for that matter) space really just an artifact of spacial expansion: something that shouldn’t really be attributed to the photon to begin with?

  12. Bill S. says:

    In view of the categorical “Nope” of our physicist’s initial reply, hopefully, we can expect him/her to respond to this!

    My own (very amateur) feeling is that if light does not “experience” time, any changes we measure must be in our reference frame only. You have probably gathered from my earlier posts that I am not convinced, either way, on this question – still looking for learning opportunities.

  13. Stuart says:

    I would like to know why light from, say a star a light year away, would take a year to reach us, if it is not subject to a travel time. In other words, why, if the photons are not experiencing time should they not reach us instantaneously?

    As an observer, I can see why it might look like it took a year to us, but are we actually seeing an event a year old or one which is taking place now?

    I also realise that concepts like ‘now’ aren’t necessarily valid but we are often told that light from stars takes a certain time to reach us, so it seems a reasonable question.

  14. Bill S. says:

    You are right, Stuart, it is a reasonable question, in the reference frame of us hitch-hikers on the journey of scientific discovery.

    To some extent you have answered your own question – always a sign of a thinker.

    You acknowledge that time and simultaneity may not appear the same in different reference frames, but seem not to trust your own line of reasoning. Time and simultaneity don’t just seem different, they are different. When astronomers look at distant stars they are seeing them as (but not where) they were when the light left them. If (and it is a very big IF) the light doesn’t “experience” time, then the photons would not experience that time, in their F of R, and both would be right.

    If we were talking about some futuristic space craft travelling at an appreciable fraction of “c”, it would be possible to calculate the difference in time in each F of R, but once the speed hits “c”, those calculations are no longer valid.

    The only answer to your question is that, in our present state of knowledge, light takes one year to travel one light year in our F of R, and that is our reality. If light experiences anything, we have no way of knowing what might be.

    Obviously, our Physicist will disagree with that, so I continue to live in hope of a counter argument. Having our mistakes explained is a good, if painful, way of learning. :)

  15. Stuart says:

    #Bill S. Thanks for your answer. Watching the recent news, I guess the fact that data is taking a certain time to reach us from, say, the vicinity of Pluto’s orbit is a good demonstration of the answer to my question.

  16. Bill S. says:

    It answers half your question, Stuart, in that it demonstrates that EM radiation needs time, in our reference frame, to make the journey, and that when information arrives it is information that was current at the time of the signal’s departure. It still tells us nothing about what you would experience if, as in Einstein’s thought experiment, you were keeping pace with a photon. Some scientists will point out that this is an invalid thought experiment because nothing massive could keep pace with light. True as this is; what’s good enough for Einstein……… :)

  17. George says:

    I say SR does not apply to light itself because from the frame of reference of light, we are traveling at the speed of light too. We all know that leads all kinds of absurdities. What is so puzzling is that light is so special and it is right in front of my eyes, just like a simple life form like a seed or our brains full of miracles all the time. When will we figure these out?

  18. Bill S. says:

    Hi George.

    If we could assign a reference frame to light, your first sentence would be correct, but your statement that “SR does not apply to light” does indicate that we cannot do that.

    I would be fascinated to know what the absurdities are that you refer to.

  19. George says:

    Hi Bill,

    I do not have much to say about it. As you know, by SR, in the eye of light, our time is not ticking. Our space is not there. Our mass would be infinite. It almost sounds like we are in a black hole. But I am not convinced that black hole can exist. Does this sound weird enough?

  20. Bill S. says:

    Hi George,

    If you extrapolate the equations of SR to the point where v = c, then you would be right, there would be no time or space in that reference frame. There are experts who maintain that you can do that, and other experts who claim you can’t, because at v = c the equations of SR break down and there is no valid reference frame.

    My own feeling is that each of us should look as carefully as we can at the evidence on both sides, then say an honest “don’t know”.

    I would be interested to know the line of reasoning that leads you to say: “Our mass would be infinite.” I have a reputation as an “infinity crackpot”, so I’m always looking for people’s thoughts on that subject.

    I’d also love to know why you think black holes can’t exist.

  21. George says:

    Hi Bill,

    I see that stationary mass in our space is definitely not zero. So at the speed of light, the mass must be infinite in the eye of light, right?

    The main reason for me to say that black hole cannot exist is that it is too strange and it could exist in theory but not in reality. First, even if our current knowledge about basic particles could not provide enough resistance to forming black hole, deeper knowledge may help. If all that fails, I still say that SR will come into play. As we know at the core of the sun, the density is about 150 times of water. That is not a lot. Based on this, I say other stars are similar. Now when it starts to collapse into a little dot first, it will need a lot of matter, at least a size of earth and more. As the density builds, the gravity becomes large such that the falling speed will approach the speed of light in a few thousand kilometers journey. By SR, that will never happen. In the process, matter and energy will build up and we will have a big explosion and no black hole. What do you say?

  22. George says:

    If the process that I outlined above is correct, then there are several more things we can say about supernovas. First it will not happen for small stars. Second no matter how big a star we start with, the core ending process is the same, the same explosion process will blow the whole star into bits. Here, of course, due to the property of gravity, a star cannot be infinitely big as outside will drift away, condense and form their own objects. So I do not understand why some people say that if you start with a bigger star, you will get a black hole and if you start with a small one you will get a neutron star in the end.

    Also, it is clear that nuclear reaction cannot convert matter into energy completely. In the end, it seems, it may be the converting gravity into energy powers the explosion. I saw some talking about antimatter process in stars. If that is true, we should not have collapsing step. But then I am not clear about antimatter stuff…

    Comments, anyone?

  23. Bill S. says:

    “I see that stationary mass in our space is definitely not zero. So at the speed of light, the mass must be infinite in the eye of light, right?”

    I don’t follow that, George.

    I think we may have drifted off topic somewhat by wandering into black holes, but as we are there it is probably worth mentioning that the physical evidence in favour of the existence of black holes is considerable. There’s a lot we don’t know about them beyond mass, charge and spin, but there are things out there that would need a lot of explaining if they are not BHs.

    I’m sure there must be threads in which this discussion would be more appropriate, but let me leave you with one thought about gravity.

    If gravity creates more gravity, what stops runaway gravity from turning the whole Universe into a giant black hole?

    Just a thought.

  24. George says:

    If I am photon, I consider myself to be stationary and the rest to be moving past me at the speed of light. So I see you as a two dimensional object. From my point of view, your clock is not ticking and your mass is infinite by relativity. How does this sound?

  25. Bill S. says:

    It sounds like a logical extrapolation of relativity. I think there are plenty of scientists who would agree, at least to some extent. However, as we are using relativity, there are a couple of things to consider.

    1. If you are a photon, relativity does not say anything about a frame of reference for you.

    2. If you had a F of R you would consider yourself as stationary, but how would you see any other photon that was in your F of R? Think about that; there could be a problem.

  26. George says:

    I agree with you that it is difficult to establish frame of reference for photon. Now by relativity, to define time and space we have to select a FR first because time and space are all relative to a FR. Since we cannot define a valid FR for light, by relativity, we cannot define meaningful time and space for light itself using relativity. Therefore I say time and space for light are out of the realm of relativity, right?

    By the same reasoning, all the massless light traveling particles share this same property. What do all these imply?

  27. Bill S. says:

    “Therefore I say time and space for light are out of the realm of relativity, right?”

    That seems to be the case.

    “By the same reasoning, all the massless light traveling particles share this same property. What do all these imply?”

    You really need a particle physicist to answer that. Bumbling amateurs like me can get into a mess. :)

    When I joined this thread I expected a lot of counter arguments to my post; they have not materialised, so I started trying to think of my own objections.

    One thing that occurred was that photons do not decay. This might be used as an argument in support of their not “experiencing” time. However, you mention other massless particles. Presumably gluons (for example) travel at c, but I believe they decay. I know a straightforward comparison oversimplifies the situation, but it’s food for thought.

  28. George says:

    Are you misinformed? The current thinking is that for any particle, if it decays, then it must have mass and as a result, it cannot travel at C. Which particle are you saying that it decays and travels at C? This would be a very interesting case.

  29. Bill S. says:

    The usual example is the gluon.

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