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

  1. The Physicist The Physicist says:

    @Hubert pellikaan
    Quantum teleportation is remarkable, but is definitely not an instantaneous “action over distance”. There’s an old post that talks about it here. Thankfully, teleportation (as a technique) is simple enough that it’s more or less understandable.

  2. Enn Norak says:

    Everything experiences time but everything does not have a clock that can accurately measure the passage of time. For the vast majority of observers, time dilation is nothing but an illusion relative to their own clock based on mathematical conclusions from observational illusions about motion and distance.

    We really have to investigate the possibility that photons themselves experience a very slow loss of energy due to a tiny amount of hysteresis between the magnetic and electrical components of electromagnetic radiation such as light. We might be surprised to learn that cosmic redshift due to that loss may be large enough to overwhelm a blueshift due to distant galaxies actually moving closer to us rather than further apart.

    The conclusion then would be that, notwithstanding an observed net redshift, we are already in the process of a big crunch instead of the widely believed accelerating cosmic inflation.

    The concept of space is not a tangible real thing and cannot have any properties that allow it to expand , twist, contract etc. Space is simply a concept that allows us to visualize the relative positions of matter, energy, forces, fields, quantum particles and stuff yet to be discovered in an otherwise infinitely large volume of emptiness..

  3. Tero says:

    I just can’t think universe as net that is collapsing or expandimg. I see that what we are able to observe seems like there is a starting point which is now thought as a big bang. Just like seems photon does not experience time. I would say if we see universe as net with beginning and the end, it would have limits. What we now say it is a time how far we see. So most distant calaxies seems to be certain time at past that we say is somewhere after big bang. But let’s say if we are in the center area, it probably has something to do how we are able to observe space. Not actual facts of space (collapsing or expanding). I don’t know what did happen to general space time theory. But maybe it has more to do with the fact in where do we observe a space. And only place we can observe is near earth. If we look at some calaxy, as far as I know, we have only information is calaxy moving further away or towards us. Though I don’t know actual details is there also measurements galaxy moving left, right, up and down direction in space. Or is it just further and towars information estimated from light etc. And if it is from light shifts, do we really know is direction of movement only thing effecting the light shift.

    As example used to explain this is a ambulance coming toward us and going away from us. We can hear difference in sound frequence. But if we take time to that as well. So play sound in different speed (ambulance coming toward or going away). It is not clear anymore from sound to hear is ambulance coming or going or staying still.

  4. Enn Norak says:

    Space is a concept of infinite empty volume with no identifiable center and no borders. Time also had no beginning and will never end. Everything that exists in space has always existed in one form or another. A period of time such as a decade, or century, or an epoch can be said to have a beginning and an end. When one defined period ends, another period of time begins. When we speak about our universe beginning at the moment of the big bang, we are really referring to the beginning of a period during which our universe began to expand from a very tiny “singularity”. The period immediately prior to the big bang ended with a big crunch.

    We may be in an endless cycle of big crunches and big bangs or there may even be many universes (multiverse) with some big bangs occurring when two or more universes collide.

  5. Barry Foster says:

    I think that when a photon “exits” matter what really happens is that nothing really exits but a “vibration” occurs in spacetime. This vibration expands in a spherical way until the surface meets matter and immediately the vibration collapses into the matter giving the sensation of a quantum of light. Thus, if single “photon” was emitted, and observer B was just behind observer A so that the spherical front met A first, observer B would not “see” the photon.

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