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

  1. ScienceInterested says:

    From the perspective of a photon, there is no time duration and no distances. This means the photon and the entire Universe are in one and the same place. Therefore the photon does not need to travel anywhere, because it is already there. From the photons perspective, to travel from that ‘distant’ star to your retina, it travels no distance in no time because the ‘distant’ star, the earth and you and the rest of the Universe are all together with the photon in one place -because there are no distances at light speed.

  2. David says:

    We know the rules for matter, for instance with time dilation. They’ve been confirmed by experiment very accurately. Attempts to apply the same rules to light are not necessarily justified, and there’s a lot of reason to think that light goes by different rules from matter. In fact, we know it does. For one example, light has energy without having mass, and that breaks the rules for matter.

    But the present view of SR, which is wrong, suggests that the rules of SR apply to everything, not just matter. They look like universal rules in that view, so you get a lot of misguided people trying to apply them to light, and getting nonsense back out. But because the nonsense always has a faint, obscure self-consistency, as in the infinite shrinking of distance and the infinite shrinking of time mentioned in this article, it’s comparatively easy for people to believe, or to convince themselves, that this makes sense. But it’s conjecture, it hasn’t been tested, and many good physicists think it’s false.

  3. Pedro says:

    Photons exhibit “relativistic mass” and momentum.

    https://physics.stackexchange.com/questions/2229/if-photons-have-no-mass-how-can-they-have-momentum

    They do not break any known rules, I promise 🙂

  4. Peter Grimshaw says:

    Science Interested yes this is what I am holding in my mind.
    In SR – which could be, and probably is wrong …. – the photon is everywhere in the whole universe at the same time. Yep, SR says ‘The photon and the entire universe are in the same place’
    All the time we experience in our co-ordinate frame, happens in the null-time of one photon.
    What light does that shed on Quantum Entanglement ?
    If the photon is everywhere and throughout all time, viewed from our co-ordinate frame, it’s properties are weird.
    We, in our co-ordinate frame, experience it either as a ‘wave’ or ‘particle’.
    Does this say more about our perception, than the essential quality of the photon.
    ‘Perception’ itself starts to become a metric of some sort.
    Worth a ponder.

  5. My problem is this. Photon is outside of time. When it enters the event horizon of a black hole, does it enter time? When it goes into a black hole and leaves the universe, has it entered at a specific moment in time?
    Does the photon that entered a blackhole, have a before it entered the black hole – time, and an after it enters the black hole – time?

  6. Pedro says:

    I have “pondered” this for several years.

    What I came up with is that from the photon’s perspective there is a singularity where all the mass of the universe is concentrated in one place.

    Sound familiar ?? 🙂

    I further conclude that the universe is a “projection”.

  7. Peter Grimshaw says:

    Pedro
    Ok so Photon up close = Black Hole viewed from a distance

    Ps, I can’t do SR math yet, but getting there slowly.

    I’m wondering if EM ratiation (ie ‘light’ and EM waves) is actually somehow ‘Ether’.
    From an EM perspective, EM is immanent.
    Ie, it is in all places at all times.

    Isn’t it a perennial question – if light is in ‘waves’, what is being ‘waved’?
    Maybe the question is – is the perception of waves caused by the moving position of the observer through time rather than the motion of light?

    Wonderful science

  8. Hakon says:

    I was thinking about this too, and I was philosophisingg along the same lines as the original poster. From the previous responses, there are two ways to look at this: 1) a photon has mass, or 2) a photon does not have mass. I don’t know how to do the math, but – and correct me if I am wrong – you’d need infinite energy to propel anything with mass up to light speed, so anything travelling at light speed either has no mass or infinite energy. But what is mass? If you break particles of mass down into their most basic components, do they not start assimilating massless waves moving at light speed? Is not then all mass just the inertia of confined elemental particles moving at light speed?

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