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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49 Responses to Q: Does light experience time?

  1. Riptide says:

    I distinctly remember having a very odd feeling when I was about 4 years old; I was walking from my father’s home-office to the front yard, and I had a flash of realization that *I wasn’t moving at all*. Instead, the whole world was moving *around* me. It seemed quite profound at the time, but my environs were not conducive to inquiry about it, and so I forgot it for the longest time.

    I like to think nowadays that it was my first brush with relativity and, given the appropriate motivation, I could have become a great physicist. Alas, I languished too long, and now mathematics has taken my attention for the foreseeable future.

  2. Terry Hayward says:

    Simple answer.
    Light cannot experience time since time uses light speed as its base, so, at the speed of light, time is at 0.

  3. dørr says:

    But doesn’t light slow down while moving through different materials?
    So wouldn’t that imply light is experiencing time after all?

  4. The Physicist The Physicist says:

    Photons do a neat trick when traveling through a material. If they hit an atom they get absorbed for a moment, and re-emitted a moment later. In between atoms light still moves at the usual speed.
    The whole process happens so many times, so fast, that we don’t see the light starting and stopping, we just see an averaged, slower speed.

  5. A says:

    So what happens to us from the point of view of the photon?

    It sees us passing by at the speed of light? But then it doesn’t experience time either… so what exactly is going on?

  6. The Physicist The Physicist says:

    “Point of view of the photon” questions are a little tricky, because photons can’t really have a point of view (what with not experiencing time). So they don’t see things passing them, or moving at all, because that would require them to have some kind of notion of what distance or time are.
    Forcing the issue though; they see the universe in the direction they’re moving flattened completely. So, rather than seeing us pass by, we’re just another thing in a very flat universe, from the photon’s perspective.

  7. LowOnProzac says:

    “Forcing the issue though; they see the universe in the direction they’re moving flattened completely.” Does this have any relation to the 2-dimensional “Holographic Universe” that I’ve been reading about?
    (Thanks for this wonderful web site!)

  8. joel abel says:

    why is it that when you look at a far away object in a mirror the image is not as clear as if the object was no further away than the mirror?

    being nearsighted and loving shortcuts the idea occurred to me as a child that all i would have to do to see a far away object was to look at the object in a mirror. my reasoning was that the image i was looking at was only as far away as the mirror.

    of course i was disappointed, but i have never really understood why it didn’t work.

  9. Louis says:

    Light “knows” neither time or distance? It has energy but zero mass? Science is doing it’s best, but “how” gets more precise, not “why.” How can massless space-time be curved by anything? Why does light follow curved space-time if neither it nor space-time have mass? . . . I can’t find answers to the most basic of questions . . . Perhaps this is a “dumb” question, but how does light “know” to move? How can it carry energy with zero mass? To us, the light from our sun arrives at Earth 8.5 minutes upon reaching the sun’s surface. It spent a lot of “time” bouncing around inside the sun, banging into other “energies”. But to the wave/particle of radiation we call light, what’s going on? It was “released” during fusion as energy w/no mass, somehow it knows to “run away” at the fastest “permissible” speed in this universe. So the photon was moving around inside the sun at light-speed, too? No, it was slowed, but not drained of energy? It never runs out of “gas” inside the sun, but at the surface of Earth it’s reflected and some is converted to heat “energy” – why is one photo converted to heat and not another? Quantum probabilities? Why does some of it then run out of “gas”? SO. . . For some unknown reason, it knows nothing of time or distance – but it is born, lives, and dies like everything else? I’m sure my lack of “education” (i.e. answers that don’t answer) is showing, and I don’t need a god to make me happy that all’s OK. But the amount not known about the whys in this universe seems to have an infinite slope – more so every . . . Tick-Tock-Tick-Tock . . . Where Am I . . . Is anybody out there? . . .

  10. Anthony Rose says:

    @Joel Abel, the *mirror* is closer to you than the object you see reflected in it, but the object itself is still just as far away; and in fac t the distance the light has to travel from the object to the mirror and then off the mirror to you is even greater than the distance it can travel direct from the object to you.
    If you put a fish tank between you and the object, and can see the object through the fish tank, it makes no difference how close the tank is to you. Or if, instead of the fish tank, you put two mirrors almost facing one another, so that the object’s light bounces into the 1st mirror, then into the 2nd mirror, on the way to you, effectively going through the two mirrors just like the fish tank, it’s a similar thing.

  11. Anthony Rose says:

    If light from its own frame of reference experiences no time passing and no distance travelled, does that mean that all of history and all the future are apparent to its ever-present moment, and/or that all photons can be ‘in touch’ with one another regardless of location from our frame of reference?
    (Please excuse/ignore if this is a dumb question)

  12. The Physicist The Physicist says:

    It’s a very profound question!
    What you can say about the light’s “experience” (which is a word that doesn’t really apply) is that all of the events that occurred along its path occur all at once, from the light’s perspective. Events off of its path are unimportant to it.

  13. Anthony Rose says:

    Very helpful – thank you! Mind-bending stuff!

  14. Carl Brooks says:

    It is as i thought. A photon experiences zero time. Its been a problem for me for a couple of years now and i’m still trying to get my head around the consolidation of both perspectives at the same time (the photons and the observer). Take a photon emitted from an atom in a distant star, from the photons perspective its emission and absorption is instant. and yet for us from our relative perspective the time taken for the light to reach us is measured in years. It would lead to suggest that space time has been curved to such an extent as to make the emission atom and the absorption atom exist in the same place at the same time. just a theory i had.

    Also another theory, stemming from the photons have no time idea. What happens when all the atoms in the universe decay into photons (heat death?). If photons experience zero time and zero distance that would lead to suggest that when there ceases to be any atoms in the universe every photon in the universe will be in the same place at the same time… the second big bang or next big bang maybe? Anyone?

  15. Erica says:

    So how do we account for the photon spending thousands of years within the sun and then traveling across the vacuum of space? To us it’s a linear progression of events but to the photon it’s everywhere along that path?

  16. shashank goel says:

    now my comment is. – it’s said that whenever something moves faster than light it will reach into future. okay . but tell me something. neutrinos are substances exceeding light speed . so if perspectives are imagined from nuetrinos point of view,isn’t it sound logical that neutrinos will go into the future.and if they aren’t able to travel to future then why is it so?(please forgive myself if this appears to be a ridiculous stupid and idiotic question)

  17. The Physicist The Physicist says:

    That whole “neutrinos are faster than light thing” was a mistake from a couple months ago. Nothing to worry about, they’re regular old, slower-than-light particles.

  18. christian t says:

    what about the experiment where they made part of the light wave move faster than the rest. how would that work.

  19. Robin Lowenstein says:

    So, in a similar vein, a theoretical space captain who inadvertently set the speed of his spacecraft to within a tiny fraction of the speed of light – say 10 to the power of minus a few trillion – miles per hour less than the speed o light – would have outlived the universe before he was able to lift his finger from the accelerator button.

  20. David Hill says:

    The first answer by the ‘Physicist’ is pretty good. Minkowski space-time is quite different from 4-dimensional Euclidean space, and the qualification that space-time is curved according to a non-Euclidean geometry (Riemannian, based on extension of relativity to cover gravity, or general relativity) is extremely difficult to visualize. Many diagrams or animations that try to explain these concepts are quite misleading. Distances in spacetime can be either real or ‘imaginary’ (space like or time like) for example. A single position in space-time occupied by a photon can appear as a series of positions in space and time, and that photon can appear to be moving, based on the inertial reference frame of the observer. Although energy appears to be inextricably bound to mass in classical physics (e.g., E=(mv^2)/2), the relationship as revealed by Einstein is actually quite different, and mass itself is actually one measure of energy.

  21. tim says:

    photons are light. they exist everywhere at all times. they do not experience time.
    they are the alpha and omega. they provide all life. photons were created before all matter. they can not be distroyed. they can transmit information for eons. when we see the sun we are seeing photons. the sun lights our way and without the sun, we have no life. it strikes me that that is the same defination we use to describe god. is god literally the energy of light. and the absence of god is eternal darkness?

  22. Thatguy says:

    The part I don’t get: light is a form of energy, correct? Then why no mass? E=mc^2?

  23. LDoBe says:


    Photons are force carrier particles. They carry the electromagnetic force. As such only particles of matter have mass. Photons can occupy the same position without knocking into each other since they aren’t matter and have no mass. But when they occupy the same position as matter they do knock into it, are absorbed and either ricochet or deflect when reabsorbed.

  24. Gauss says:

    Saw this on science channel and had some questions…

    A train is traveling close to the speed of light around the earth on a track. And to protect the speed limit, they said that time slows down so you couldn’t break the speed limit by running inside the train..therefore moving faster than light.

    My question is, would the people on earth watching this train see the train moving in slow slow motion? Even though the people on the train are circling the earth 7 times a second?

    If it is why is light not in slow mo, does nature know nothing could be on a photon and “run off” breaking the speed of light.

    What if someone shined a flashlight on this train traveling 99.9% of the speed of light, would time stop?

    The part about photons having no time or distance is breaking my skull.

  25. Mor says:

    If you could somehow hop on a photon you wouldn’t experience time or distance, would you instantly stop existing since that photon will be destroyed at some point. Even if that photon was traveling for billions of years to an outside observer.

  26. [email protected] says:

    I am still confused. Photons do not experience time or space in their direction of travel. However 2 photons can collide and form an electron/positron pair, or simply be absorbed by an electron in an atom. So the photon does experience change. In our frame of reference, we see the photons changing position over time and then change into an electron/positron pair. If there is no time or space in the photons reference frame, what changed?

  27. PETER says:

    Is matter made of Light as David Bohn claimed? and if it is, Doese it not eman that we are Children of the Light?

  28. The Physicist The Physicist says:

    Matter and light are definitely, fundamentally, different things. Wait, did this David Bohm really claim that?

  29. confused says:

    I keep reading that light had no mass but has energy. Can anyone tell me what “energy is”? And I don’t mean describe the properties of energy or how it behaves but what it actually is.

  30. Fabio Di Donato says:

    I can’t help but think that light IS time and vice versa, then…

  31. Roland Org says:

    We have fermions and bosons, but bosons are force-carrier particles. Do they exist in space, or time or space-time? They are not photons, or energy, or have mass; However they somehow travel from fermion to fermion. This is becoming quite metaphysical; maybe they are present in the “ether”. What is curved space-time except a concept that doesn’t quite fit. Maybe we should define a photon as more than a massless particle that transmits energy. Let’s call it the “mystery thing”.

  32. Domikko says:

    E=m(c^2) is an /incomplete/ equation! It is valid only for converting rest mass into energy, a simplification for the purpose of doing nuclear physics.

    The full form is something like;

    E^2 = (m[c^2])^2 + (pc)^2

    That might help clear things up for people wondering about Mass-energy-equivalence, and how massless particles can carry energy. You just put “m” in that equation to 0 and you still get an energy, which is equal to the momentum times the speed of light. (Where momentum is h/lambda.)

  33. Bill S. says:

    It is fascinating that one physicist can say categorically that photons don’t experience time, but another can say the opposite. The way I understand it goes something like this:

    Special relativity provides for an inertial frame for everything that has mass, but it does not cover massless particles such as the photon. Talking of the photon having a frame of reference is, strictly speaking, not scientific. The photon must always be observed as travelling at “c”. It cannot be at rest relative to anything. Of course, one could argue that it must be at rest relative to itself, but that is not a very productive line of reasoning. Science has not actually produced definite proof that the photon cannot be assigned an inertial frame, but to maintain that it does have one is pure speculation, and maintaining that it does not have one seems to be the generally accepted position.

    Taking the time dilation equation to its ultimate conclusion may seem a logical thing to do, but it is not supported by special relativity because of the lack of mass of the photon which puts it outside the remit of special relativity. It seems that the best we can say is that we have no way of knowing if photons experience time, or not. Nor do we have any scientifically accredited theory that covers this, nor any way to test the idea experimentally, as no massive object can reach the speed of light, and a clock cannot be fixed to a photon.

    To some extent this all seems to be a shame, because if it could be established that photons did not experience time, then it could be reasoned that the speed of light equated to “infinite” speed and several mysteries could be solved. Obviously it would still leave a major problem with the tachyon, because the tachyon can be considered as accelerating away from the speed of light towards “infinite” speed. However, we might console ourselves with the knowledge that the tachyon is only a theoretical particle.

  34. Niko D says:

    So does this mean that photons don’t have a half life? If so, would this mean that the density of photons that reach us from distant galaxies is dependent solely on the angle between us and the source (and any stray object that happens to briefly obstruct the path), rather than on the half-life killing off some photons en-route?

  35. Bill S. says:

    As far as I am aware, photons are not subject to radioactive decay. It is difficult to see how they could be, so the concept of half-life would not apply to them. Once emitted, a photon continues to travel at “c” until it is absorbed. Over time its wavelength may change, it may cease to be visible light, as in the case of the CMB radiation, but it is still a photon.

  36. Scooby Poo says:

    If I were a photon emitted from a source that does not experience time nor distance in my own refererence frame, could I essentially be everywhere in the universe simultaneously?

  37. Bill S. says:

    [quote]If I were a photon emitted from a source that does not experience time nor distance in my own refererence frame, could I essentially be everywhere in the universe simultaneously?[/quote]

    If a photon does not experience time nor distance in its own reference frame, that would seem to be the case. However, since we cannot assign a F of R to a photon, and since we cannot argue scientifically that a photon “experiences” anything; the whole matter is speculation. It may be philosophy, but it isn’t science.

    [quote=Physicist]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.[/quote]

    This is not correct. There is a big difference between investigating something “from an “approaching light speed” way of thinking and the “being at light speed” way of thinking.” Approaching light speed the time dilation equations can be applied, but nothing in physics says that they apply once light speed is achieved. As far as we are aware, the only thing that can travel at “c” is EM radiation, and we have no way of discovering what that would experience, if it were able to experience anything.

  38. Xerenarcy says:

    here is another way to look at the same question, and it is this specifically that i cannot seem to reconcile…

    from the (philosophical) POV of a photon, space and time is compressed to zero ‘length’ along the direction of propagation. however, a photon is equivalently a wavepacket; a propagating disturbance of the electric and magnetic fields. while i understand that a photon’s existence is ‘instantaneous’, i have not been able to describe this ‘propagating’ behavior in the photon’s frame in any meaningful way.

    i guess another way to ask the same question is that given that photons have polarization, they should have a time-dependent (or distance-dependent) phase angle, so how does this angle appear in the photon’s POV? or is phase angle ignored throughout or remain fixed / predetermined for photons?

  39. Bill S. says:

    Xerenarcy, as I understand it, the problem you will always be up against is that relativity does not allow for a F of R for a photon; thus we can never know what anything would look like from the POV of a photon.

    Consider this. In order to describe two photons as being in the same inertial frame we would have to say that each photon was stationary relative to the other; but we would also have to say that each photon observed to other as travelling at “c”.

    How can that make sense?

  40. Mr Mr says:

    I find it hilarious that the author gives such ‘strong’, absolute answers.

    “Nope!” With an exclamation to top it off. Speaking with an air of authority, they continue to explain the intricacies of quantum physics (a largely incomplete branch of science as it is) in layman’s terms, and with absolute certainty.

    The way light travels and its interactions with the rest of the Universe prior, during and upon its arrival to the primitive human eye and the various technology we have constructed in order to detect photons, has only just (and very briefly) touched upon the very basics of such things.

    Ignore anybody who attempts to give you such simple and absolute answers. They are not this simple and certainty not this absolute. The author (if he is any scientist at all) should not state with such certainty that his (and they are indeed his very own) OPINIONS on this matter are cemented in accepted an known science, because, they are not.

    We are only just beginning to explore and unlock our understanding of photons and other particles, and it cannot be stated with any certainty that photons do not experience ‘time’ particularly when the idea of ‘time’ is being called into question as a whole.

    The author (if they wish to be scientific on the matter) needs to seriously review this article.

  41. Jackie says:

    I am not sure anyone answered the question. I understand the question to be the same as i am asking………………..if an object travels near the speed of light, it travels thru time……..so why does the light from a star 1 billion light years away take a billion years to reach earth? Even thru our point of view if it travels to our space time, should we not see the birth of stars instantly?….how can you claim that traveling at the speed of light causes time travel if the very thing you are measuring that speed by, does not time travel. I just dont see what mass has to do with momentum at this level, would a gas react diferent than light at the speed of light?

  42. Bill S. says:

    Hi Jackie. I’m not sure this will answer your question either, but it might help.

    “if an object travels near the speed of light, it travels thru time”

    True, but the measurement of speed is different in different frames of reference.

    “so why does the light from a star 1 billion light years away take a billion years to reach earth?”

    It takes 1 billion years in our frame of reference, but if, for example, you were measuring the time taken be some futuristic space craft to travel 20 light years at 0.8 c, you, on Earth, would measure it as 25 ly. However, someone on the craft would measure the time as 15 ly. I think that is mathematically correct; I’m no mathematician. :)

    It is extrapolation of this time dilation equation to light (travelling at c) that leads to some people; including some scientists; to claim that photons do not experience time.

    The problem with this is that while relativity assigns a frame of reference to any massive object, however close it might come to c; once c is achieved (which no massive object can do) no frame of reference is assigned.

    Light takes 1 year to travel 1 ly, but remember that is in our frame of reference.

    Among the better known scientists who apparently subscribe to the view that photons do not experience time is Brian Greene. He says:


    “……the maximum speed through space is reached when all light-speed motion through time is fully diverted into light-speed motion through space—one way of understanding why it is impossible to go through space at greater than light speed. Light, which always travels at light speed through space, is special in that it always achieves such total diversion.”

    So if you want to say that light doesn’t experience time you will find that you are not totally isolated from the scientific community. However, if you make that claim publicly, you can be sure that you will bring down on your head the disapproval and even scorn of many physicists.

  43. Bill S. says:

    [quote=Mr Mr]I find it hilarious that the author gives such ‘strong’, absolute answers. [/quote]

    I would have to take issue with your hilarity, Mr Mr. Presumably, this site is intended to provide answers to lay people on scientific subjects.

    I am not a scientist, but I have been thinking about scientific matters for some time. A few years ago, from my own reasoning, I reached the conclusion that light could not experience time. Only after considerable research and discussion did I realise that things were not necessarily that simple. Unfortunately, not every “hitch-hiker” goes further than receiving and accepting an answer from an “expert”.

    Giving this sort of “absolute” answer, without qualifying it, or point out that other views exist, is betraying the trust that people put in “experts”.

    Of course it is possible to find similar assertions of the opposite view. These are, also, not funny.

  44. Jackie says:

    Thanks Bill that explains it somewhat. Its that pesky relativity.
    I understand that all things are relative, size, speed, distance etc, are relative to other sizes, speeds, distances etc…….. considering time on earth, it can be easily applied to relativity, 30 minutes is a short time relative to 2 hours. 2 hours is a short time relative to a day…..
    Now the relativity of time in vast values, is what gets me. Does the possibility of warping or jumping thru time, not negate the theory of relativity. As one aspect of the relationship has the ability to overcome what is being related?

    My thought experiment:

    Bill you are a time traveler, and I am average Joe, following the path of time. It is 1pm at the local baseball park we are both at. I say I would like to know who wins the world series this year. So you make a leap ahead 6 months and see the last game of the world series.
    I see you there ,and say hello, as I know what has happened, and true enough your jump to 6 months ahead, took you 1 second but took me 6 months.
    So your past self returns to the past, to 1pm at our local baseball park, before your initial time jump. Technically at that point in my time frame, you never left. But it took you in your time frame, 1 second per jump plus the time spent in the future. So relatively your time jump took more time than the non jumper.
    As I repeat wanting to know who will win the world series this year, you already know. Now say you jumped to one second after I asked on the return jump, it took me one second, and you the above period of time.
    So light starting from a billion light years away, traveling at speeds presumed to be capable of stopping or warping time, why is it that light isn’t instant relative to our time.

    Or maybe even the light travels so far thru time we will never see it until we catch up with it in time………………………as opposed to the light catching us by closing in the distance………………………………………………just seems like anything with the ability to time travel can overcome many constraints, even light overcoming the expansion of space…. thanks bill! And sorry I’m a radio personality not a physicist lol but I wanna know!!

  45. Bill S. says:

    Jackie, you have obviously done some thinking about time travel and its complications.
    Let’s see if we can have a go at tackling some of the points you raise.

    “30 minutes is a short time relative to 2 hours. 2 hours is a short time relative to a day…..”

    While that is true, it doesn’t, perhaps, quite capture the essence of relativity. Relativity goes somewhat further and says, for example, that 30 minutes in your frame of reference will not necessarily be measured as being the same as 30 minutes in mine, depending on how each of us is moving relative to the other, and who is doing the measuring.

    I like your baseball example; it’s the sort of thing one often finds in science fiction, which can appear quite reasonable.

    I am not a scientist, so you will need to keep that in mind when evaluating anything I say.

    There is fairly good scientific evidence that future directed time travel is possible, and there are two ways of looking at that.
    1. We are all travelling through time at the rate of 1 sec. per sec.
    2. Astronauts who spend time travelling at great speeds relative to those of us who are earthbound, end up younger than us; but only by tiny fractions of a second.

    There are a few scientists (possibly the most prominent of these being J Richard Gott) who believe that past directed time travel is possible, but most scientists believe that if you were able to travel into the future, relative to the rest of the world, there would be no coming back. Although not a scientist, I have given a lot of thought to the ideas of time travel, and am firmly in the latter camp.

    Your example seems to require that I am with you at a particular point in time and space – let’s call it point X. I travel into the future, acquire some information, then return to point X with that information.

    What we need to keep in mind here is that point X is a spacetime event, and every spacetime event is unique. I cannot be at point X without, then with, the information about the outcome of the series.

    “But it took you in your time frame, 1 second per jump plus the time spent in the future. So relatively your time jump took more time than the non jumper.”

    That would be right if my trip to the future had been the result of some “magical” act. Scientifically, the only ways to travel to the future relative to the rest of the world would be to go on a journey at an appreciable fraction of the speed of light, in which case time dilation would mess up you reasoning; or to be influenced by a very strong gravitational force to which others were not exposed, which would have the same end result.

    “So light starting from a billion light years away, traveling at speeds presumed to be capable of stopping or warping time, why is it that light isn’t instant relative to our time.”

    Stopping time and warping time are two different things and involve a lot of complications.

    It seems you are bringing us back to the original question of whether or not light experiences time. The very question of what light might “experience” anything takes us into metaphysics.

    Light is measured as moving at “c” in our frame of reference. This tells us nothing about what light might experience, if it could experience anything, in its frame of reference, if it could be said to have a frame of reference.

    I think that’s as much as we can reasonably cover in one post, but I’m quite happy to try taking things further if you like.

  46. Llia Lynn Olsen says:

    Is light actually directly affected by gravity? Or is it being indirectly affected, and is simply steered in circles by the particles reflecting or absorbing it, already trapped by the intense gravity .

    Can light exist without gravity?
    Or is Gravity the Aether in which light can reveal an observable trace of itself on it’s path.

    Since light had no mass, is light, light? or only the wake of a ship, footprints of a traveler we cannot measure or observe directly?

  47. Llia Lynn Olsen says:

    Super fluidity, slowing light.

    I read that super fluids can Appear to defy gravity, but in the experiments they were using the fluids in a container to slow light.

    Well to slow something down with a substance, that substance has to absorb the force of what you want it to slow, and in so doing the force is transferred from the light to the liquid.

    Since the fluid allows the light to travel in it, I believe it forms a skin, a fiber optic like skin that thins itself out as light bounces around inside it. Since the fluid is contained it has no choice but to climb the walls.

  48. Bill S. says:

    Hi Llia, time’s short so let’s take one post at a time.

    “Is light actually directly affected by gravity?”

    It would seem so, because the stronger the gravitational field, the more the path of light is deflected.

    “Can light exist without gravity?”

    If the range of gravity is “infinite”, as seems to be the case, we cannot test this idea in a state of absolutely zero gravity. However, we do know that light travels through intergalactic space, where the influence of gravity is minimal. Do you know of any experiment that differentiates between the behaviour of light in varying gravitational fields, apart from diversion?

    “Or is Gravity the Aether in which light can reveal an observable trace of itself on it’s path.”

    An interesting thought, but I suspect we would have to observe a greater difference between the propagation of light in differing gravitational conditions in order to establish this. It is possible to find scientists who claim that gravity is a force, and scientists who claim it is not. That’s confusing enough for us amateurs, so let’s go with William of Ockham and not add more possibilities until there is more evidence.

    “Since light had no mass, is light, light? or only the wake of a ship, footprints of a traveler we cannot measure or observe directly?”

    I’m not sure what you are saying here.

  49. Bill S. says:

    “I read that super fluids can Appear to defy gravity, but in the experiments they were using the fluids in a container to slow light.”

    Was that on line? If so, perhaps you could give us a link.

    I don’t know much about superfluids. The only example I know of is helium, cooled to a temperature close to absolute zero (2.18 K). I understand it has very low viscosity which enables it to flow almost without friction, very high thermal conductivity, high capillary activity and an ability to flow uphill, which I have always assumed was linked to the capillary activity.

    “Well to slow something down with a substance, that substance has to absorb the force of what you want it to slow, and in so doing the force is transferred from the light to the liquid.”

    It is probably safer to talk of a transfer of energy, rather than force.

    In pop science books one often finds an explanation for the slowing of light in a medium, which involves photons being absorbed by atoms in the medium, then re-emitted, as – for example – when an electron absorbs a photon, jumps to a higher energy level, then drops back to its original level and emits a photon. Intuitively, this seems like a good explanation, but consider a photon passing through a block of glass. If the above explanation were correct, not only would a different photon emerge from the glass; its frequency would have changed. Normally, no such change is observed.

    What happens is more like this: A solid is composed of a network of ions and electrons fixed in a “lattice”. They have what is known as “collective vibrational modes”, sometimes called phonons. These are quanta of lattice vibrations, and it is these phonons with which the photons interact. Unlike atomic orbitals which are discrete, the phonon spectrum can be broad and continuous over a large frequency range. If a photon’s energy is beyond the phonon spectrum, the solid cannot sustain this vibration; so the lattice does not absorb this photon and it is re-emitted but with a very slight delay.

    “Since the fluid allows the light to travel in it, I believe it forms a skin, a fiber optic like skin that thins itself out as light bounces around inside it.”

    Maybe I’ve missed something, but I don’t get the bit about the “skin” formation.

    “Since the fluid is contained it has no choice but to climb the walls.”

    Are you saying that the energy from the absorbed light is pushing the fluid up the sides of the container?

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