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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66 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.”

    and

    “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……… :)

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