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

  1. Pingback: Q. If you are a Photon, What time is it? | Musea Zine

  2. Russell says:

    If light has traveled across the universe to get to us but experiences no time, then how can we say that any time has elapsed at all? Is it that light has not experienced time and the rest of the universe has?

  3. Orien Rigney says:

    When in doubt about your theory or analysis, use the following.
    http://www.space.com/52-the-expanding-universe-from-the-big-bang-to-today.html

  4. yoron says:

    “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).”

    Quite so, brave, very few mathematicians that really dare to have a view of it. Then to get to my view of it you just have to exchange a ‘propagation’ for a ‘timing’, locally defined. then we don’t need time dilations for them, we just need local principles and rules. To it you can add scales, creating a structured local time, and a very weird principle presenting us a ‘common universe’ represented by ‘c’.

  5. Parrish says:

    If time dilation is true and photons have mass then… Every photon is a new moment in time. When a photon is created time stops for the photon until it reacts with something. It is then moved to that point in time. Since movement in time is always observed with photons then this governs our perception of time moving forward.
    What if light isn’t the thing moving? The reason you approach infinite energy as you approach the speed of light is because you are really slowing in moving space.

  6. Tom Hendricks says:

    If I am a photon, what time is it?

  7. Orien Rigney says:

    Your question Tom: If I am a photon, what time is it?

    Let me add just a bit more fuel to that fire Tom.~~~~~~~~~~~~~~~~~~~~~~~~
    Can a photon ever be at rest? If so, with respect (relatively) to what? Again, if so, what must a photon weigh at rest, with respect to what? If everything actually increases in mass (weight) with an increase in speed, what will a photon weigh at C?

  8. Rich B says:

    As time is simply a concept we have constructed with our own brains then it’s true to say that a photon experiences no time. I this is the case then it might help us to understand one of the confusing aspects of quantum physics, whereby atoms (and probably photons) can be everywhere at once.

  9. Orien Rigney says:

    We do take a lot for granted when dealing with Quantum Physics or Mechanics. But time itself is not simply a manifestation of our brain to work out unmanageable or complicated problems, time is real! Yet, even though the “Half Life” of elements have been worked out to a certain degree, it still isn’t an absolute science time wise; but darned close.
    http://simple.wikipedia.org/wiki/Half-life_(element)
    An atom or photon etc. seemingly to be in two places at the same time is probably one of natures tricks much like the double slit particle or wave phenomenon that continues to be questioned.

  10. Peter reid says:

    As I see it, a photon is created as a particle but becomes a wave when it travels at light speed. As time stands virtually still for the photon, it would see itself as a particle even though it’s travelling as a wave at LS. Therefore is it both a wave and a particle simultaneously.????
    Is it possible that all objects travelling at LS will then travel as a wave or does the mass of an object change that?
    Any answers please send to my email, I’m actually interested in finding out!!!!
    Peter
    peter999@ live.com

  11. Orien Rigney says:

    Some of the sharpest minds of today are just now digging deeply enough into quantum physics and mechanics to begin getting answers allowing them to dig even deeper. Only through due diligence are these people even allowed to work with the equipment needed to do this job. Meanwhile, most of us have to wait, read and listen to what work is in progress. Here is a good take off on it.
    https://www.boundless.com/physics/textbooks/boundless-physics-textbook/introduction-to-quantum-physics-28/history-and-quantum-mechanical-quantities-182/energy-mass-and-momentum-of-photon-668-6239/

  12. Grahame Thorne says:

    The answer given is clear, but I read the question as asking about the photon’s ability to travel in the absence of time, not the observer’s perception of it, so it is largely an answer to a different question!

    Einstein’s Special Relativity tells us that we see the clocks of things moving at speeds (relative to our own frames of reference) running slower, and lengths shorten; both of these phenomena regardless of whether speeds are faster or slower. Therefore, it isn’t too difficult to conceive that a photon travelling at the speed of light appears infinitely contracted in length, more like a particle than a wave, and, as a clock, appears to have frozen time. Conversely, a photon will see the universe through which it passes as infinitely shortened (but only in its direction of travel, excellently illustrated in the diagram above), and time frozen.

    However, in all frames of reference (at all velocities) all clocks within the same reference are observed to run at their proper speed. Presumably this is the same for the photon at the speed of light: a photon will observe its own clock running at its proper speed. If I’ve understood Einstein’s Special Relativity correctly, then it is incorrect to say ‘at the speed of light no time passes at all’, only that ‘(to all observers at any velocity less than the speed of light) time appears to be stationary at the speed of light’.

    At the speed of light, the photon should see the universe as incredibly short (and be intrigued as to how so much can exist such a small, apparently two-dimensional space), it will perceive it’s own speed as very slow (presumably stationary), not because of aberrations in its time, but because its velocity (v=d/t) will tend towards zero as its measure of distance shortens. It also seems likely it would see its own full length extending from its origin through its own ‘present’ in time, more like a wave than a particle.

    I’m not a physicist or a mathematician, so factual corrections or omissions would be welcome.

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