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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118 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.

  6. Orien says:

    Quite an interesring way of putting it. But! Whether ruminating or discussing time, time travel, space, space travel, matter, energy, light, light speed and/or any combination of them, it’s like shoveling smoke against the wind. Without the super glue of imagination none of them hang together. Since no human, not even an Einstein or a Tesla could answer those questions, give them your brest shot. Until someone comes along with a tangible and concrete answer, the human brain while composed of multiple billions of neurons and non-neuronal cells can as yet do little more than guess or speculate.

  7. The Physicist The Physicist says:

    And then verify or refute those guesses and speculations experimentally.

  8. Tero says:

    Yes there are small parts still to be discovered. Could also say mass requires time travel of particles or small pieces. Let’s say without a mass, all would move at speed of light to every direction (chaos). But when we take mass, tiny particles start to time travel near mass. Delay the time. So particles would return time and time again and create same time again and again making mass possible to exist.

    If thinking like this, in emptynes there would be chaos and time would be superfast. So there would be only randomness. Then at random particles would create mass and start to slow down time by doing the time travel so basically repeat movement in speed that would create cycle of time. All the way to the black hole which would be another point where is almost none random movement and particle are cycling almost infinite times at same time. Though also if thinking further could mass be born by particles moving through space time so that same particle would multiply by traveling back in time again and again multiplying and starting to create a mass.

  9. Enn Norak says:

    Chaos is a description of real tangible stuff located in space that is behaving in an unpredictable chaotic way due to helter-skelter forces acting on it. Pure empty space by itself is not a real tangible thing — it’s just a concept of empty volume where real chaotic stuff can be located.

    If you take all energy, matter, forces, fields and stuff yet to be discovered out of space you are left with complete homogenous nothingness which cannot possibly be described as chaotic.

  10. Orien Rigney says:

    In answer to the Physicist: What is found to be without question scientifically concerning all of the above is still only a search mission for answers. While we know different speeds of light under controlled conditions, none fit a category demanding they remain unquestionable. Time is still an unknown factor other than as we perceive it to be. What I mean is, how do we compare the life cycle of an ameba to that of a giant sequoia, a human or turtle? How and why the differences, There are no concrete answers. What is pliable living flesh and bone when compared to a stone or fossil that has been in the earth millions or perhaps even billions of years? No! Without bringing something into the mix far greater than all of its parts, we best let the guessing games go on.

  11. Tero says:

    Sure all is seeking. And quessing like a big bang, in area of physic where something can be calculated and be seen as fact. But example empty space and chaos and again what can come from chaos. If we say that things born randomly. More we have empty space, more likely something like chaos will exist there. We can say it is totally random, but also there might be some forces effecting the change when do things start to happen. It sounds quite clear that deeper the empty space is, more likely there can be activity. But also would sound clear that if materia conditions in black hole we accept as idea, why the opposite, emptyness, we dont see as opposite compared to black hole.

    Now we say materia to build universe came from big bang. I can say materia comes from chaos happening in empty space. Gas, stars and there forward to galaxies. Again if materia starts to form in emptyness, materia would need to form back to small particles we dont yet know well. And that would be in black hole. Though it would sound too simple. From emptyness as we see empty space, materia would start to build to hevier forms. And at the end collapse to small particles in black hole again. And random effect of the smallest particles would create the space as we see it.

    Again back to time and life cycle. What would be the time. It would be a life cycle of materia created with great random effect. But materia itself (like rocks) would have less random factor. As those would be built in time. Meaning steps galaxies are born. Starting from random gas cloud. So time would be built with series of things needs to happen.

    If we think again big bang. Example space would start to collapse. What would that mean in practise. Mass would start to increase. And in any current knowledge that is not possible. Basically space can’t expande or collapse. It can do that in area we observe it. I just can’t get it how big band is accepted as best theory to explain space. It is just the same as say earth was created in seven days. Anyway current knowledge show there are small parrticles those will behave in random way. And in the way we don’t understand. And there are materia we quite much understand. Time is nothing, basically a concept that tells us how materia does stay versus chaos as example. And heavier materia is, more stable it is. And somehow that is effecting also all materia near heavy materia.

    Interesting though as it is quite difficult to prove example particle movement in empty space where no gravity exist. As from gravity how do we know how it is effecting to particles in size we don’t know much but think exists. Though best way to prove example that galaxies are born from empty space would be to locate a calaxy or punch of stars from empty space. Where we could say gas has not blown there from blown supersun as example. Or appeared there from big bang where infinite amount of gas was exploded in infinite small amount of time to infinite space.

  12. Found a reference to quantun teleportation as depicted below.
    I see this as evidence that light is simultaneously at the beginning and the end of its “travel” so there is no time (and place?) difference between both places where the foton is. That is for the foton itself. For the observers c would apply making the speed a number and non infinite. For the massless amongst us there is a kind of infinite speed. And no, the ones that carry mass can not attain c or infinite speed. That makes sense in a way. In E=1/2mv2 there will be needed a lot of energy to get your rocket producing c at the speed dial. To get instantanity you will need infinite energy.

    Here is the thing about teleportation:
    Physicists at the University of Geneva have succeeded in teleporting the quantum state of a photon to a crystal over 25 kilometers of optical fiber.

    The experiment, carried out in the laboratory of Professor Nicolas Gisin, constitutes a first, and simply pulverises the previous record of 6 kilometres achieved ten years ago by the same UNIGE team. Passing from light into matter, using teleportation of a photon to a crystal, shows that, in quantum physics, it is not the composition of a particle which is important, but rather its state, since this can exist and persist outside such extreme differences as those which distinguish light from matter. The results obtained by Félix Bussières and his colleagues are reported in the latest edition of Nature Photonics.

    Quantum physics, and with it the UNIGE, is again being talked about around the world with the Marcel Benoist Prize for 2014 being awarded to Professor Nicolas Gisin, and the publication of experiments in Nature Photonics. The latest experiments have enabled verifying that the quantum state of a photon can be maintained whilst transporting it into a crystal without the two coming directly into contact. One needs to imagine the crystal as a memory bank for storing the photon’s information; the latter is transferred over these distances using the teleportation effect.

    Teleporting Over 25 Kilometres

    The experiment not only represents a significant technological achievement but also a spectacular advance in the continually surprising possibilities afforded by the quantum dimension. By taking the distance to 25 kilometres of optical fibre, the UNIGE physicists have significantly surpassed their own record of 6 kilometres, the distance achieved during the first long-distance teleportation achieved by Professor Gisin and his team in 2003.

    Memory After Triangulation

    So what exactly is this testing of quantum entaglement and its properties? One needs to imagine two entangled photons -in other words two photons inextricably linked at the most infinitesimal level by their joint states. One is propelled along an optical fibre (the 25 kilometres mentioned earlier), but not the other, which is sent to a crystal. It is a bit like a game of billiards, with a third photon hitting the first which obliterates both of them. Scientists measure this collision. But the information contained in the third photon is not destroyed -on the contrary it finds its way to the crystal which also contains the second entangled photon.

    Thus, as Félix Bussières the lead author of this publication explains, one observes “that the quantum state of the two elements of light, these two entangled photons which are like two Siamese twins, is a channel that empowers the teleportation from light into matter”.

    From there, it is a small step to conclude that, in quantum physics, the state takes precedence over the ‘vehicle’ – in other words an item’s quantum properties transcend classical physical properties. A step that maybe now one can take.

    Explore further: Picturing Schrodinger’s cat: Quantum physics enables revolutionary imaging method

    More information: Quantum teleportation from a telecom-wavelength photon to a solid-state quantum memory, Nature Photonics, DOI: 10.1038/nphoton.2014.215

    Journal reference: Nature Photonics search and more info website

  13. Enn Norak says:

    Re photons, there is speed, yes; however there is no way a photon can travel any actual distance instantly. That is one of the problems with the theory of relativity. Instant travel is nothing more than the result of a mathematical calculation that does not correct a flaw in the theory.

  14. Orien Rigney says:

    To Hubert: There is no denying that a lot can be said for teleportation. But I see no way that a physical piece of information such as a photon can be everywhere at once. Our own sun is one such example. The light from it to reach earth takes over eight minutes to make the 93 million mile journey. Pulsars, making trips of billions of light years and rotating at regular speeds in the u/sec range showing their energy (light) as they pulse.

  15. Well its a thought this instanteous thing, derived from that the obesrved speed of light is alway c, whatever the speed of the observer. at 99,9% of c, the observed speed of light is still c.
    On the other hand this entanglement of foton airs facilitates information travel instantaneously, so at infinite speed. When you see infinite as “speed is non applicable since there is no time” then you get a really weird universe where all kinds of fenomena have time and speed, and others dont have these. A large bunch of wires with a lot of knots in it or so….!?

  16. Tero says:

    Could I say this more like cloning. Let’s say I would like to teleport myself to other planet. So I could not send mass as it can’t trval at speed of light. But I would send information at speed of light. And that information would not get older when it travels through space and I would be cloned at same age to other planet. But from where I observe cloning it would not be instant. Again from end point information would be seen first time when information received. From outside there would be delay in cloning and that would be the time information travels from start point to end point. But for the clone itself it would feel cloning was instant. Again to outside observer how exact time of cloning could be defined. As information trvalling to observer would take time as well. And if observer would be behind the clone, could see clone at same time then original.

    For the zero time teleport there would need to be channel passing information faster then light. What is I think what now is thoughted have been done and as general concept of wormhole.

    Again interesting does light and information get older. Would see as point when most distant items of space is studied. If now seems there is a beginning. But can that also be the limit we can observe in space and time.

  17. Emmington says:

    I sitll don’t quite understand;
    Since it moves at the speed of light,it is (from what I’ve understood) not experiencing time,but how then would it be affected by spacial (is that even a word?) changes that take place as it travels,example;
    1.Photon leaves star
    2.Photon is heading towards pohotn detector located at earth
    3.The moment the photon leaves the star,its path to the photon detector is unobstructed
    4.Once the photon has left the star/its source and is heading to the photon detecor located some distance away from it,some ( a human) places an opaque object infront of the photon detector.

    Here is where I dont get it;
    1.The photon doesnt experience time,hence it take a “snapshot” of the universe as it leaves the star for the photon detector.
    2.We know for a fact that it travles at a finite speed (speed of light) and that it doesn’t reach the detecor instantenously.
    3.We place the opaque object infront of the detector AFTER the photon has left the star (its quite a distance away,so it would take several minutes for it to reach the detecor anyway),hence blocking it’s path to the detector.

    Now the weird thing;
    The photon doesn’t experience time,hence for it,nothing should have (could have?) changed since it left the star and continued towards the detecor,so the path to the detector should remain unobstructed (as we couldn’t have place the opaque object infront of the detecor,since that action would take time to accomplish).

    From our perspective however,the photon travels at a finite speed,so there was plenty of time to block its path.

    So how do objects traveling at the speed of light experience (outside) changes that take time,given that they don’t experience tiem themselves?

  18. Enn Norak says:

    Anything that moves, whether in the form of a wave or particle must experience time because velocity is distance divided by time i.e. time is distance divided by velocity.

    It is known that the speed of light is invariable and it is known that the Doppler effect shifts the frequency of light as seen by an observer in motion relative to the source.

    Length contraction and time dilation are mere illusions only to be outdone by the concept of spooky entanglement. Create the right mathematical formula and one can claim that we do not really exist. And then the creator of such formula will say “my formula has the best fit to what we know from empirical evidence”.

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