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

  1. Steve Cox says:

    Craig had a problem that Andromeda galaxy would get constipated with un-emitted photons.
    David N Roberts responded that Physics had basically found that everything was deterministic.

    I sympathise with Craig, but he should remember that an excited electron can emit it’s quantum of energy to any other electron in the whole of future time and space. This gives a lot of scope for it to “find” its recipient.
    Re determinism: how does that leave our own “free will”? My answer is that 1) Our perceived free will is a mixture of many many quantum events, and is smeared over such a coarse time-frame (milliseconds) that it is effectively “free will”, that is, not externally determined to all intents and purposes (i.e. our own intents and purposes, which are not in possession of the complete knowledge of where every photon’s predetermined course may be). The relevance of (? need for?) an observer to ‘enable’ collapse of the probability function may be worth mentioning here. Lastly, as a religious person myself, I see no harm (and no contradiction) in referencing the concept of an “eternal timeframe” – or a completely independent perspective of “true reality”. From this perspective, everything is indeed, determined (because it has “all happened”, and is spread out like a map to the observer in the eternal timeframe). Nevertheless, for us within the space-time universe, and the electrons around us aligning their advanced and retarded wave roots, there appears genuinely to be choice. Its the old Calvinist predestination v free will chestnut

  2. Steve Cox, you said on 2/9/18:

    1) Since the 1920’s and the work of Lewis, all photons (have) been regarded as transmissions of energy (rather than “little particles”) in a lossless manner between atoms ( or, rather, electrons of atoms)
    2) This means that a photon CANNOT be ‘transmitted’ until both the source AND the destination are, so to speak “in place”

    Here’s a thought experiment with that in mind.
    (Remember too that photons are outside of time)

    Let’s take a star, the Sun. When it shines photons go out in every direction. There is virtually no direction blocked from the light of a star.

    Does this mean that in every direction in space the photon leaving the Sun has a destination? Wouldn’t that connect our Sun with every part of the universe?

    Wouldn’t that make every shining star connect with every other shining star in the universe – with one or the other, or both, being the source and the destination?

    Thus you would have a sort of cosmic photon net that would connect up every dot in the universe, right?

  3. Steve Cox says:

    I think that this is a fascinating thing, Tom. First, however, I think photons are not outside time, they simply do not progress forwards in time (no time passes) from the point of view of the photon. That means that the energy transfer from one point to another simply bypasses the passage of years on its route from one star to another. The absence of time seems to mean that nothing can ‘happen’ to interrupt the energy transfer. There is no mechanism described in Physics for a photon to travel without a destination. Thus it is postulated that every single photon must arrive and transfer it’s energy. The fact that time stops for that process implies that there is a ‘determined end’ in *every* case, because nothing can happen in ‘no time’ to interrupt the process. Thus there is a sort of ‘cosmic photon net’ which ‘has happened/ is happening/ will happen’. Each dot is linked (is part of?) every connected dot. The appearance of spread out time (for us the observers) seems to be completely linked to spread out space (the one reflects/ depends on/ is proportional to the other). All energy transfers in all of time and space seem to have to play out against this background – possibly the background arena is “produced” by matter having mass (conferred by the Higgs boson). mass seems to cause space and time. Possibly this is just a rambling, because I seem to be trying to express something that only maths can describe..

  4. Peter Grimshaw says:

    Wonderful description Steve.
    I must lie in the bath and ponder.
    ‘Time’ as a function of measureable space . . . .
    Maths . . .

  5. Steve Cox, thanks for your thoughtful answer. So let’s have some mental fun:
    Steve Cox said,
    That means that the energy transfer from one point to another simply bypasses the passage of years on its route from one star to another. The absence of time seems to mean that nothing can ‘happen’ to interrupt the energy transfer. There is no mechanism described in Physics for a photon to travel without a destination. Thus it is postulated that every single photon must arrive and transfer it’s energy. The fact that time stops for that process implies that there is a ‘determined end’ in *every* case, because nothing can happen in ‘no time’ to interrupt the process. Thus there is a sort of ‘cosmic photon net’ which ‘has happened/ is happening/ will happen’.

    New question:
    Then can we say that virtually throughout time there is an ongoing, and always present, energy transfer of photons going on throughout virtually every inch of the universe?
    Correct? Then two follow up questions.

    Follow Up.
    I have taken a set of two slit experiment boards into the center of an empty place in outer space. Though it is empty there is an ongoing transfer of photons here and virtually everywhere else in the universe (see above). What pattern will the photons make on the 2nd board?

    Remember, the photons are coming from every direction in space, in front of the 2 slit board. What pattern will the photons make on the 2nd board.

  6. David N Roberts says:

    Tom, you need not go to outer space to view what would happen. In a lab place light sources all around your double slit experiments, both 1 and 2, and see what pattern develops, you shouldn’t be surprised at the answer.

  7. Steve Cox says:

    I would like to see that result. Perhaps we need to take a temporary detour and see reality without the time or the space. In this arena, everything is ‘next-door’ and ‘next-moment’ from every connected (quantum) event (that is, one with ends connected by a photonic transfer of energy). The ends might be light-years apart and centuries apart, but are pulled together into a sequence like two widely separated seams of a garment connected by a button/ hole. The trouble is, our instinctive mammalian perception depends on the construct of time/space to ‘order’ the universe into a ‘causative’ story. The ‘cosmic photon net’ idea you have pulls reality into a different (mental) perspective.
    Good luck with the double slits!

  8. El Capitan says:

    The Physicist made some “mistakes” that confused a lot:
    1. if choose a “photon moving at light speed” as reference frame and use SR equations, this do make the photon experience no time and no distance. But, this perspective makes the photon still but everything else to move at light speed!!! This is catastrophic i.e. they have infinite energy/are black holes etc.

    2. Photon is NOT a thing. It’s a quantum as it looks like a probability wave function filled the space-time(like many commentaries beautifully said, but one really should pick up some quantum mechanics to get a picture). Feynman’s famous lecture on QED explained how photon “travels” in quantum theory.

    3. Under holographic principle (gravity in a 3D volume can be described by quantum mechanics on a 2D surface surrounding the volume) people have a vigorous proof how extra dimension by gravity is created by energy density using a quantum theory (that does not include gravity), suggesting space-time is build by entanglement. It’s a hint for someone who want to dig deep “photon travel”.

  9. J. Sebastian says:

    This is an interesting topic which I always enjoy and it is great for theoreticians as it usually generates debate and some consternation but I just want to introduce a slight tangent and discuss the “solution” I see in Steve’s post about the apparent requirement for a photon to arrive and transfer its energy.

    I don’t think this is a correct interpretation of the conservation principle. And I don’t believe it is required for relativity either.

    First some quick background…and this is really what “The Physicist” is referring to, is that most of the “objections” that come up in these kinds of questions, which often lead to “complications”, arise because of our tendency to “intuitiveness”, which is a really just a fancy word for the perfectly understandable human instinct to equivocate or otherwise confuse reference frames and relativistic/non-relativistic measurements. It isn’t “normal” for humans to think of time or distance as being relative, which is what makes this so difficult for most of us to adapt to. But adapt we must.

    However, I suspect you all will see the “solution” as much less confusing, if you will keep in mind some required postulates:

    1. Unemitted photons don’t actually exist. There is only the matter which will eventually emit them. No issue there.
    2. Emitted photons have energy (still no mass though). They may never collide with anything that absorbs or dissipates all of their energy, and that’s fine too.
    3. Everything that has no mass travels at the speed of light, and this is not a problem for relativity or for Newtonian principles. Obviously if you have a zero on one side of the equality, then what’s on the other side? Conservation is built in to the inherent relationship between energy and matter, so in either Newtonian or relativistic systems, we should be okay if only we can keep our limits and frames squarely in mind, which is of course, far easier said than done.

    As long as energy is conserved with regards to the entire system, we should be in fine shape. A potentially confusing issue here is what we define as a system. For the POV of the photon, one can think of the “system” as any space through which the photon travels, which according to relativity, has no dimension that can be measured, because it contracts infinitely (relative to the photon and its frame of reference) and thus is “undefined”. It isn’t really much a “system” anyway…its just a single point, after all, and no time will elapse (from the POV of the photon) in the system.

    From a “phase space” viewpoint, its these null values that are important. Let me explain why that is.

    We also know that to an observer, the apparent energy of a photon diminishes as it travels, but only due to red-shift expansion of the universe….so it seems to me that the requirement that we intuitively imagine exists is actually only for the conservation of *relativistic” energy, and in relativistic terms, all of the vectors are interchangeable.

    Thus time, which isn’t defined except by our external frame of reference, is either shifted or is zero (can’t have it both ways simultaneously, so its one or the other depending on your frame), so if apparent (relativistic) energy is diminished, it must be the case that one of the other vectors have been transferred outside of the local system – which for the photon is just that single point, but for us is the entire universe as measured by our reference frame, and that’s fine too.

    (the last poster commented that the problem with the explanation is everything else is moving at the speed of light, but that’s not a correct interpretation of what The Physicist posted: The measurement of that shift is observer dependent. The shift is due to the motion relative to the coordinate frame.)

    Anyway, proceeding on….We know its not mass, since that’s zero at rest, when we measure it. And we know it must also be zero locally in flight as well because it would be infinite, so that doesn’t work either.

    And what about E? Well, once measured at the destination, relativistic E again is equal to zero (since its not moving at all, zero momentum), thus fully conserving that energy across the entire flight of the photon.

    But we DO know that t changes. It vanishes in the local system – goes to zero. But not for us, in our frame. And that is how these vectors are balanced.

    So if you look at it this way, no obvious violation of the conservation principle occurs in this process. And you could say it is counter-intuitive, but like many things in relativity and QED, yes it is and that’s okay! If t=0, (relativistic value), then there can be no energy loss measured anyway. You need to have some interval across which to measure that (at least one Planck interval), and if there isn’t one, then all the other values are zero also. That’s why I said the null values are what is important. It is counter to the human way of thinking but what is most critical to understanding this is what does NOT exist. In the case of these photons that never come to rest, that vector is time – time does not exist.

    For those who care, here is the way I originally came to think of this approach to the “problem”; light is only emitted by some matter that has mass – which loses energy in doing so. That loss in kE is theoretically equivalent to the measured energy (non relativistic) at the destination. So we have conservation. But, IF there is no destination, then all is still well, because t=0 (relativistic measurement) and no energy is truly lost even if the photon never arrives at any point of measurement (destination) – its in transit perpetually (t just goes to infinity externally , which fits in fine with the maths for things like closed time-like curves and singularities), but still within the greater non-local “system” that describes the universe.

    One could characterize this as the postulate that the momentum of a phase space is increased when there is stuff travelling around at the speed of light, while at rest, momentum is decreased (zero for massless objects). But it does really all balance fine with the maths. There isn’t any actual experiment we can do to confirm this on a cosmological scale, but we can model this on a small scale via the theoretical construct of a sealed container with an interior surface of a perfect mirror. The mass of this container (system) does not increase. But its energy and momentum does. And if we could measure the pre-and post-emission energy of the system that produced those photons we should observe the corresponding kE decrease in that system.

    So Steve, if you were trying to think about the need for the proton to deliver energy , via some non relativistic measurement, then I hope this will address the concern you have. Maybe it will even serve to solidify your idea of this cosmic past/present/future photon capture net in a helpful fashion. If it does, I’d love to hear of any refinements you come up with.

    I too, find it very challenging to describe in words what is far more elegant mathematically. But that is unfortunately the best I can do here.

  10. James says:

    “Emitted photons which never collide with anything” do not exist. A photon cannot exist as anything other than a potentiality unless it collides with something.

  11. There are mixed messages in these posts. My question, that I am still not clear on is this:
    Does a photon always have a destination?

  12. Stephen Cox says:

    Hi James,
    I have got to say that it is encouraging to hear from someone who has thought deeply about the issues in this thread. You are quite humble to note how challenging this is to express (mathematically or in English), when you obviously do understand this better than me.

    The subject of the discussion was my assertion that ‘to exist’ a photon needs both emission and absorption (both ends of the journey). You have implied that the law of conservation of momentum does not necessitate the arrival.
    Having read what you say, what comes across to me is that we can look at the Electron-photon-electron system simply from the POV of the photon, or from the POV of the observer (‘whole universe’). You have also wisely and helpfully given 3 postulates which are needed assumptions before accepting your observations.

    The first postulate “Un-emitted photons don’t actually exist..” is important, because my comments relate to the existence of a photon (or not). When drawing a Minkowski diagram of the electron-photon-electron interaction, there will be places on the diagram where the photon does not ‘exist’, and then it will exist, and then stop existing again, as one progresses up the time axis. Now, it may be a philosophical point, but as a Human I would say that even if the photon does not exist at certain points on the chart, it some ways is still right to say the photon “exists” throughout all space and time (if only because I remember it existing when it stimulated my optic nerve). In the same way, it may not be quite correct to say that un-emitted photons don’t exist, when if you are looking at the whole of space and time (at once, in Einsteins 4D universe), the photon does “exist” – just in a different position from me (say, in “my” future). This is to say that I tend to look at the universe as having ‘persistent existence’. Once something has happened it’s there for ever. Also just because I don’t know it is going to happen does not alter the photon’s “future” existence. This may seem quite a layman’s POV, and I apologise.

    Previously (and I find this unsettling), I have read that when looked at on a quantum scale, the deterministic nature of the universe becomes much more obvious. It is almost as if we had a universal Minkowski diagram, in which all the lines were in place, and nothing could vary from it’s pre-written course. If this is the true nature of existence, then when you say “They may never collide with anything that absorbs or dissipates all of their energy..” would that be written down on the Minkowski diagram as an unending line going off the map at a 45 degree angle?

    I won’t disagree with your interpretation of the law conservation of momentum, both from the POV of the photon system and the phase space system, both with Energy and Relativistic energy equations. I think what you wrote is illuminating. What you are saying about this lack of need for a receptor is that the energy lost in these “homeless” photons, is still there in the whole Universe, so there is non-relativistic conservation of momentum.
    I have 2 questions then
    1) Is there any way you could prove these photons (ones which don’t “arrive”) exist? The reason I ask is that I suspect that you can’t detect an individual photon without it interacting with an electron in your apparatus (thus invalidating it as a member of the group of photons we are talking about).
    2) If it is in fact impossible to prove these photons exist, then do they not belong in a category very similar to those in your first postulate (“un-emitted photons”)?

    I don’t think that my original assertion was made to satisfy just the law of conservation of Momentum.
    It is not mine, BTW! : I attach the quote from Gilbert Lewis in 1926 at the end of this note).

    Thanks for receiving my pretty ignorant reply, James. It is a privilege to hear what you said.

    The Nature of Light
    Gilbert. N. Lewis PROC. N. A. S.
    PHYSICS VOL. 12, 1926 “I am going to make the contrary assumption that an atom never emits light except to another atom, and to claim that it is as absurd to think of light emitted by one atom regardless of the existence of a receiving atom as it would be to think of an atom absorbing light without the existence of light to be absorbed. I propose to eliminate the idea of mere emission of light and substitute the idea of transmission, or a process of exchange of energy between two definite atoms or molecules. Now, if the process be regarded as a mere exchange, the law of entire equilibrium, which I have recently advanced,6 requires us to consider the process as a perfectly symmetrical one, so that we can no longer regard one atom as an active agent and the other as an accidental and passive recipient, but both atoms must play coordinate and symmetrical parts in the process of exchange…”

  13. Stephen Cox says:

    I still think that the answer is ‘yes’, if you look at the whole system (past present and future).
    James has shown that the law of conservation of momentum allows for a photon to ‘be in a state of emission but not reception’, from the POV of Phase-space. I agree with that, but would maintain that from the POV of the photon, no time passes, therefore there can be no “causal division” between emission and reception. That is, emission cannot happen without reception. But because I’m not a post-doctoral level mathematician/ physicist, my support for this has to be on a philosophical level, with reference to those more knowledgeable.

    Hope this brings the thread back on topic.

  14. Good questions Steve.
    I am also interested in any replies.
    I remind readers that our Sun, for example, is emitting photons in all directions,
    So is it connecting up with billions of destinations?

  15. James Becker says:

    @Stephen Cox,

    I agree with your last two posts, and enjoy the succinctness the quote you shared from Gilbert N. Lewis.

    Regarding what you said here: “If this is the true nature of existence, then when you say “They may never collide with anything that absorbs or dissipates all of their energy..” would that be written down on the Minkowski diagram as an unending line going off the map at a 45 degree angle?”

    First, there is no edge of the diagram to go off of, because the diagram necessarily extends forever. A photon wave’s scope of potentiality in space extends to infinity, especially because the edges of the universe are likely expanding faster than the speed of light according to the freedom permitted by general relativity. Furthermore, such a photon does not even have a vector property unless it is observed or interacts with matter. Therefore there is no line to be drawn, for a line would indicate a vector property.

    @Tom Hendricks
    The Sun is not emitting photons themselves; rather, it is emitting photon wave potentialities in all directions. Where-when matter is present, these potentialities collapse into bi-directional photon vectors with null lifetimes frozen in space-time. And yes, there are certainly billions of these destinations.

  16. Stephen Cox says:

    Dear James,
    thanks for your reply. This is very interesting to me (although maybe old hat to bright sparks!). In view of the suspicion that there are 3 classes of photons: a) Complete (emitted and received), Incomplete (emitted but not received) and Possible (a photon wave potentiality not emitted yet, but .. waiting), 3 topics arise from this middle class of photons (Incomplete ones). I am going to assume that Einstein’s 4D universe is the map of all possible places and times.
    1) Collapse of Wavefunction.
    2) Where does that energy go?
    3) To Be or not to Be (discussion of potentialities that never get realized).

    1) The collapse of the wave function on detection/ reception of the photon is well known.
    I have assumed that “Collapsed wave function” = Complete photon.
    The “Un-collapsed wave function” should = Possible photon, as the probability is not definite (yet). The photon could be ending up in any place or time (within the Minkowski diagram 45 degree cone) in the universe. This class of photons still “exists” because Einstein’s 4D universe – all of time as the extension of all of space – contains them as completed ‘lines’ with emission and reception at either end. An un-collapsed wave function is perfectly compatible with a Deterministic universe. It’s just that the non-collapse of the wave-function is only non-collapsed to the observer who is in a particular time and place.
    Where does the Incomplete photon fit in to this scheme? Has it’s wave function collapsed? Or is it still indefinite? Is it on the perfect cusp of collapse – fence sitting through the wilds of space? Maybe it’s like one of those exploding buildings, just hanging there in the air going to fall (definitely). But this implies that the probability function has already collapsed (because we *know* it is destined to collapse). Or possibly I have mixed up two separate concepts leading to confusion. Perhaps the collapse of the wave function does not = reception/detection? But if it does equate, then this class of photons by definition will Never collapse their wave functions because we have stated “they are emitted, but will never be received”.
    To summarize: am I correct in saying that Incomplete and Possible photons are fundamentally different beasts, in that one is not received and the other *will* be received?
    2) If there is a class of billions of photons which at no place and time in the whole of the 4D universe get received, then as they take a small quantum of energy with them (as you said), is there in fact, a drain of energy, which although technically is floating around out there, can never be received into an electron (by definition). You said “the edges of the universe are likely expanding faster than the speed of light”. Is this, in fact contributing to the eventual heat death of the universe? Is it one way in which energy actually (because it is effectively irrelevant to the universe now) is destroyed?

    3) You said that “A photon wave’s scope of potentiality in space extends to infinity”. You also made your first postulate “Un-emitted photons don’t actually exist.” I think that the nature of Being and Existence and philosophical questions become pertinent here. A photon which WILL exist is in fact a “real” thing in the 4D universe. That is the nature of Determinism. What will be, will be (to coin a phrase)!

    Now, an electron pregnant with a photon and seeking a good future home for it(with ‘potential in space extending to infinity’) may emit the photon (at a particular time and place). But it’s un-predictable (as is radioactive emission). Now, if a suitable home is not found (in all of future available time and space) does that emission go ahead anyway, at random, and what causes that emission to take place? I think that Lewis was expressing the thought that if reception *always occurred*, then we would be in principle able to determine why a photon is emitted to one recipient (in all of available time and space) rather that to another one. As it is, to have an ‘incomplete photon’ class rather makes it difficult to say the photon “exists” at all.

    Thanks for your patience (as the Red Queen said to Alice)

  17. Recently, I asked the guys that run this site this very question: does a photon always have a destination. Here was the Physicists reply. I hope it is OK to reprint. If not please feel free to take it down:

    Probably not.
    This idea comes out of the “Transactional Interpretation” of quantum
    theory, which says that when light is emitted there is a corresponding
    retarded wave function, that propagates forward in time normally, and
    when it is absorbed there is a corresponding advanced wave function,
    that propagates backward in time. When these waves line up exactly
    the interaction is allowed to exist as a “standing wave between the
    emission and absorption”.
    I think this is silly for two big reasons:
    First, there is no experimental evidence to support the idea.
    Second, it is “non-local” meaning that it requires effects to bounce
    around faster than light as well as forward and backward in time.
    Some variants of the theory involve “hierarchies of time” which is…
    ugly and unnecessary.

  18. J. Sebastian says:

    Hi again, Stephen Cox. You keep coming up with great questions! BTW I think you may have confused myself and James Becker in your earlier post and portions of your question 3. But the answers get even more mathematically complicated and thus even harder to describe in writing! But there are some responses, some less satisfying than others.

    I agree with answer The Physicist gave Tom H. for the most part. But I don’t agree that the interpolation theory is all that silly, its just not a “real” solution in the sense that we expect real solutions to be a physical solution. I like to think of a photon without a destination like the potential debit when you have gotten a small check from your grandmother and never gotten around to cashing it. Its out there, potentially, but until you cash that check, there is only an accounting entry in Grandmother’s ledger. She still has that $5. Not a perfect analogy, since a bank will generally only honor a check for six months after the date it was written, but you get the gist, I am sure.

    I used to not like the backwards-in-time solutions myself either…I felt them to be inelegant in the sense of providing a better understanding of the real nature of the universe, which of course is likely stranger than we know or can imagine. You can think of them as non-physical solutions, but that might not be truly accurate either. They are of course mathematically valid and implied by the negative energy solutions to the Dirac equation.

    Stueckelberg first suggested that positron were electrons moving backwards in time. So this antiparticle idea was later explored by Feynman and Wheeler and that’s produced some interesting theories that speak to these kinds of infinite world-lines or possibilities that keep cropping up in various investigations into “reality” , where similar constructs are artifacts of higher dimensions in string-theory, and in multiverse probability functions, and even for dark matter and Hawking radiation, and other such oddities. Probably the weirdest conjecture this spawned is that Wheeler once had the idea that any given moment in 4D space-time is actually just a slice across this reverberating particle/antiparticle wave function and all the world-lines it produces, going in every conceivable direction “simultaneously” (simultaneity of course is an illusory construct and only used for the purposes of visualization of the idea) , but that all these paths or potential paths were traced by just a single particle! As the story was told, this idea smacked Wheeler between the eyes late one night and he had to call and wake Feynman up to tell him about it. Pretty strange stuff, right?

    But Feynman was just trying to get the maths to come out right for QED, and of course its a bold idea even that there is a wave that is backwards in time. Its not like we can observe that! But it does all work out in the math when you try to account for these waves or particles that arrive late (the time retarded functions)

    I came up with an analogy of sorts that I find helpful ….a backwards in time antiparticle or wave as a mathematical, Newtonian interpretation of proton wave “pressure”, like a quantum reactive force. Because the proton leaves with energy, but no momentum, and arrives with energy but no momentum but somewhere in the intervening period (which is locally equal to zero), it gains momentum. How in the world can this happen when the departure and arrival are instantaneous for the proton’s reference frame? It doesn’t seem to be possible, based on what we know about photons.

    Well, its because if you have a wave that is backwards in time, you get an equal period of this “negative time” spacetime where you should have an equal, but also negative momentum. This is the part that The Physicist thinks is silly. But that’s the other implication of the path integral solutions Feynman created, and I think that might be what you’re referring to with these diagrams that would go off into nowhere (and on that metric its a scalar not a vector so its not actually possible to map that using that device – so don’t bother yourself about the idea too much – it would indeed “go off” if it were represented that way) .

    There are some space vectors, but exactly can we do with these ephemeral vectors? Well the real answer, which is after all what we are interested in, is we can’t manipulate a photon in flight, where it has any momentum at all, because to do so requires interfering with the wave (collapse of the R function, loss of momentum, gaining knowledge of its position), so we just have to take the existence of its temporary momentum as a matter of fact that arises from the math. Conversely we can take the negative temporary momentum of this anti-particle as a fact as well, but certainly we can no more measure this backwards-in-time system ourselves, than we can travel back in time…. since we cannot observe things that happened before we knew about them – since prior knowledge is required in order to begin a measurement.

    However, out of these backwards in time solutions, which honor symmetry and orthogonality and conserve the components we need, came some truly bizarre implications, some of which Feynman shared in his QED work. They just don’t satisfy our urge for a “real” solution.

    They do however, have some validity, because some of the implications Dirac found which were later explored by Stueckelberg, and incorporated into Feynman’s model, have been empirically validated, such as the Higgs boson – the only known scalar particle. But, who is to say a photon we never measure or observe is not the same kind of particle? If the photon is the force carrier of the quantized electromagnetic field, as gauge field theory has shown, this scalar identity is exactly what we should expect as a transitional state; the consensus or general view is that this pressure is exerted by a “virtual” photon. And as you work through all the maths of gauge invariance, you do see a scalar photon. So it might be the case that such photons are “temporarily” (a term that has no real meaning in this context) in a scalar state, like the Higgs boson. Its a very interesting idea. But I think the situation is that nobody knows right now. And everyone agrees that photons aren’t scalar, at least not when we observe them, anyway.

    So to recap and answer all the questions from your prior two posts in one place:

    “1) Is there any way you could prove these photons (ones which don’t “arrive”) exist?”

    Not directly. They are virtual or potential. There’s some maths that are used to calculate the probability density for certain regions. It might be possible to measure the energy of the system that emitted them before an after a photon has been emitted, and by deduction ascertain when a photon has left but the problem there is you’re adding energy to the system just to make the measurement. Barring that it might be possible to make a measurement of a photon entangled with the emitted one. But the problem is that even if it does have a destination, you might not know about it for so long that the experiment never finishes. Because when T is very large its indistinguishable from infinity or asymptotic to infinity which of course is the same answer – and for all practical purposes the experiment never concludes.

    “The reason I ask is that I suspect that you can’t detect an individual photon without it interacting with an electron in your apparatus (thus invalidating it as a member of the group of photons we are talking about).”

    CORRECT. Except for the possibility of entangled pairs, which might be useful. This is now an actual thing in the lab.

    “2) If it is in fact impossible to prove these photons exist, then do they not belong in a category very similar to those in your first postulate (“un-emitted photons”)?”

    YES, I believe that’s fair. That’s really what is meant by “virtual”. They are implied by gauge field theory.

    “1) Collapse of Wavefunction.”
    YES. I think your thinking is correct on that – the unemitted and unreceived photon are different in terms of the probability density in a given region. One is of course zero, and the other is some non-zero value. But beyond that, defining the differences requires some complex QFT math and its not possible to calculate it like other particles. (Square the wave function etc). Doesn’t work because of the relativistic issues.

    2) Where does that energy go?

    Well, that’s the million dollar question, isn’t it? You answer that and you get a Nobel. Its the force carrier in EM, its possible that backwards in time/forwards in time pair entanglement/annihilation is responsible for dark matter, or some of it, its possible that parity and symmetry “violations” account for quantum flux and chaos, there’s all sorts of potential paradigm-shifting discoveries that could result from being able to definitively answer this question. The conventional view though, is still that the energy doesn’t truly “go” anywhere, as it is still conserved. How it manifests while its in these unmeasurable (currently) states – we know very little about that.

    3) To Be or not to Be (discussion of potentialities that never get realized).

    This is the question in which have mixed up my response and that of James Becker…but let me just chime in real quick on this. This is by far the most thorny question you asked. It greatly bothered Einstein. (the “God does not play dice with the Universe” quote). What you’re referring to here is a “probability field” in QFT. So its not deterministic, at all. Quantum flux appears random. And there’s a lot of weirdness in our understanding of these probabilities. Without bogging down in all the details, there are some contradictory ideas about it. There’s some Bayesian maths that are helpful. But, there is also Bell’s Theorem; there are no hidden local variables to help us get more information about the state of that system, and once observed (R function collapse), all of the other information about the portion we didn’t observe is lost. This is why its not deterministic.

    Great questions, btw.

  19. Stephen Cox says:

    Hi there. It’s very good to hear from someone who understands the questions asked. I worry that, like a child, it is possible to ask impossible things easily, not understanding that they are like asking “how high is blue?”
    I have to read your reply carefully and use it search for better answers!
    BTW, download a utube video of cramer discussing the transactional interpretation of quantum theory..
    Best wishes, Steve

  20. J. Sebastian says:

    Oh, yah. Well, to be honest I’d completely forgotten about this “handshake” idea in Transactional Theory. I now agree with The Physicist…that is even less appealing than the standard antiparticle/retro-time theory and unworkable because it assumes this degree of entanglement that he mentions. If the waves left at the same moment (of course, given the problems with observer-dependent simultaneity that might be hard to establish), like in the standard Feynman-Stueckelberg model, then you don’t get all tied up in this requirement for a receptor before a proton can be emitted.

    Again I don’t think there is any REASON to create these kinds of dependencies. It isn’t necessary for the purposes of conservation of energy or anything like that. It seems to me like a solution looking for a problem rather than the other way around.

    Causality is the least of the issues I have with it though. That part doesn’t really bother me very much. I also don’t like his conception of Quantum Computing futures. He’s just justifying high-energy Physics department budgets. But that’s a political objection, not a scientific one.

    That conversation goes off the rails rather rapidly after the Quantum Computing comment. He makes some disturbing comments about modifying past probabiities (which are 100%), FTL, time travel, and moving wormholes around…it is very concerning.

  21. Stephen Cox says:

    Its good to have your views on this version of quantum theory: by myself I cannot triangulate effectively..

  22. Geert says:

    I am a very newbie to this matter, but i have been thinking about the same thing: does a photon even travel ? And does it even have a direction ? I don’t really like to call it photon either, as this sounds like it is a particle or something. Better to call it “light energy quanta” or something like that.
    Indeed, i think a light quanta doesn’t experience time at all and consequently, it doesn’t experience space at all either. I also think it doesn’t experience a direction either. Any direction (up/down/left/right/back/forth) implies a reference. I think a light quanta “travelling” in absolute vacuum with -no- interaction at all with any other particles, all alone, well, it doesn’t have any notion of space or movement or direction. How do you know you are even moving if there is nothing to move against ?
    As a consequence, therefore, i think that a single light quanta that has no interaction at all with any other particles, actually exists in all space and in all times, in all directions. In fact, it is nothing but a flat probability function. It can appear anywhere in any time. When it is given a “direction” at creation time (for example in a laser), the probability function is just conditioned (not collapsed but changed) into a probability function that describes linear motion in time. For motion along the x-axis, it just means that the probability for any y and z changes to 0, the probability along the x axis is 1 for position xt at time t only. Or if you want to look at it as a wave, a highly directional wave. It must be directional, since the probability that you detect a photon outside of a laser bundle during its lifetime is very very low.
    Now -anything- put in the way of the light quanta changes this probability function. For example, a change in density (refraction) changes the direction of the probability function. Any mass particle closeby will also change or bend the probability function away from the mass. Introducing a mass or particle in space that collides with a quanta ( a brick wall), will collapse the quanta into existence (a collision or a detection or measurement is in fact also a collision) will end the probability function and transfer the quanta to something else (electron, heat ….).
    This enables entanglement. Entanglement would just mean that you change the probability function in such a way that it now has two peaks of probability 1 in time and space. It is still one “thing” and therefore can collapse without locality as it now seems to be for quantum effects.
    The only really strange thing is still the double-split experiment. There must be a difference between just travelling through the splits and eventually “colliding” on the screen into an interference pattern and between just colliding on a detector of a split. This can only be explained by the fact that the quanta can hold one bit of information. The collision on the detector would read out this bit of information. The collision on the screen in the back would not read out this information. And, what is really strange, since the wave function is a function in space and time, it seems that the quanta knows from the beginning if this bit of information is going to be read out or not eventually. This is a consequence of the delayed-choice quatum eraser experiment. What would really be interesting to know is, in the delayed-choice quantum experiment, if you do not look at the interference patterns at all in the end, but would just try to determine the spin of the entangled photons at the prisms, i believe you would never get an interference pattern. So besides the energy, the quanta contains also something else, “knowledge” or “information” and by observing this, we can change reality in present, past and future. oh whatever…..

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