# 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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### 99 Responses to Q: Does light experience time?

1. The Physicist says:

@ Bill S.
Unfortunately there aren’t enough hours in the day to provide a running dialog for each posted comment. Were you wondering about something in particular?

2. Bill S. says:

“Unfortunately there aren’t enough hours in the day to provide a running dialog for each posted comment. ”

I appreciate that, and am grateful that there are experts who are willing to give their time for the benefit of “hitch-hikers” like me.

“Were you wondering about something in particular?”

I wonder about almost everything. 🙂

Also, I had attempted to respond to issues that others had raised, and didn’t like the idea that I might mislead them.

3. The Physicist says:

Looking back I don’t see anything particularly egregious that jumps out.

4. Bill S. says:

Thanks; I take consolation from that. It just seems that two comments such as:

“When something travels at the speed of light it really doesn’t experience any time.”

and

“It seems that the best we can say is that we have no way of knowing if photons experience time, or not.”

appear to be mutually exclusive

5. David says:

What might be some broader implications of this? Entanglement for instance – while the photon appears to us in different places – resulting in ‘spooky action at a distance’, to the photon it’s in the exact same place at the same time, so no surprise a change is affected instantaneously. Might entanglement be really an outcome of how our view of time and distance is different from a particle’s? Do you think a photon wonders the reverse about us?

6. Bill S. says:

Hi David,

I’m not even going to try to comment on entanglement. The chances are you know more about that than I do.

One thing I think we have to keep in mind is that we observe entanglement. In an observer’s frame of reference a photon cannot be in two places at once. It would seem to follow from that that entanglement should be observable only in the F of R of the photon, and as we have already discussed, we cannot establish a F of R for a photon.

I think you raise some interesting points, and I hope others will join in so we can have a good discussion.

7. The Physicist says:

Entanglement is bizarre, but not terribly mysterious. We have found that (despite what popular media overwhelmingly says to the contrary) there is no way for entanglement to be used to send signals faster than light.

8. Bill S. says:

Thanks for those links, Physicist, all I need now is time to read them. 🙂

9. will says:

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.

10. Bill S. says:

Will, I think your reasoning is somewhere between pre & post Einstein physics.

In your first example, if you are talking about motion on the Earth, you can say “this person is moving at 100 kph relative to the surface”, thus establishing that it is that person, not you, who is moving relative to the Earth.

If, on the other hand, you are talking about travelling in empty space, you can reach no such conclusion. All you can say is: “one of us is moving relative to the other”.

Similarly, in your second statement you mention “the person who passed by”. On Earth, that person would admit to moving at 100 kph; but would be able to give your response in space.

I think, what you were saying was that perceived motion depends on the individual’s frame of reference, which is, of course, good sound relativistic thinking.

11. Carmela Chen says:

If time doesn’t pass for light, how is it that light is red-shifted? Since the distance traveled by a photon that gets red-shifted is essentially 0 — even if the space over that distance is expanding from our frame of reference — wouldn’t this mean that the energy at emission and absorption should be the same? How, then, can light be emitted at one energy level, and then absorbed at a different energy level? Is the energy difference observed for a photon traversing expanding (or shrinking, for that matter) space really just an artifact of spacial expansion: something that shouldn’t really be attributed to the photon to begin with?

12. Bill S. says:

In view of the categorical “Nope” of our physicist’s initial reply, hopefully, we can expect him/her to respond to this!

My own (very amateur) feeling is that if light does not “experience” time, any changes we measure must be in our reference frame only. You have probably gathered from my earlier posts that I am not convinced, either way, on this question – still looking for learning opportunities.

13. Stuart says:

I would like to know why light from, say a star a light year away, would take a year to reach us, if it is not subject to a travel time. In other words, why, if the photons are not experiencing time should they not reach us instantaneously?

As an observer, I can see why it might look like it took a year to us, but are we actually seeing an event a year old or one which is taking place now?

I also realise that concepts like ‘now’ aren’t necessarily valid but we are often told that light from stars takes a certain time to reach us, so it seems a reasonable question.

14. Bill S. says:

You are right, Stuart, it is a reasonable question, in the reference frame of us hitch-hikers on the journey of scientific discovery.

To some extent you have answered your own question – always a sign of a thinker.

You acknowledge that time and simultaneity may not appear the same in different reference frames, but seem not to trust your own line of reasoning. Time and simultaneity don’t just seem different, they are different. When astronomers look at distant stars they are seeing them as (but not where) they were when the light left them. If (and it is a very big IF) the light doesn’t “experience” time, then the photons would not experience that time, in their F of R, and both would be right.

If we were talking about some futuristic space craft travelling at an appreciable fraction of “c”, it would be possible to calculate the difference in time in each F of R, but once the speed hits “c”, those calculations are no longer valid.

The only answer to your question is that, in our present state of knowledge, light takes one year to travel one light year in our F of R, and that is our reality. If light experiences anything, we have no way of knowing what might be.

Obviously, our Physicist will disagree with that, so I continue to live in hope of a counter argument. Having our mistakes explained is a good, if painful, way of learning. 🙂

15. Stuart says:

#Bill S. Thanks for your answer. Watching the recent news, I guess the fact that data is taking a certain time to reach us from, say, the vicinity of Pluto’s orbit is a good demonstration of the answer to my question.

16. Bill S. says:

It answers half your question, Stuart, in that it demonstrates that EM radiation needs time, in our reference frame, to make the journey, and that when information arrives it is information that was current at the time of the signal’s departure. It still tells us nothing about what you would experience if, as in Einstein’s thought experiment, you were keeping pace with a photon. Some scientists will point out that this is an invalid thought experiment because nothing massive could keep pace with light. True as this is; what’s good enough for Einstein……… 🙂

17. George says:

I say SR does not apply to light itself because from the frame of reference of light, we are traveling at the speed of light too. We all know that leads all kinds of absurdities. What is so puzzling is that light is so special and it is right in front of my eyes, just like a simple life form like a seed or our brains full of miracles all the time. When will we figure these out?

18. Bill S. says:

Hi George.

If we could assign a reference frame to light, your first sentence would be correct, but your statement that “SR does not apply to light” does indicate that we cannot do that.

I would be fascinated to know what the absurdities are that you refer to.

19. George says:

Hi Bill,

I do not have much to say about it. As you know, by SR, in the eye of light, our time is not ticking. Our space is not there. Our mass would be infinite. It almost sounds like we are in a black hole. But I am not convinced that black hole can exist. Does this sound weird enough?

20. Bill S. says:

Hi George,

If you extrapolate the equations of SR to the point where v = c, then you would be right, there would be no time or space in that reference frame. There are experts who maintain that you can do that, and other experts who claim you can’t, because at v = c the equations of SR break down and there is no valid reference frame.

My own feeling is that each of us should look as carefully as we can at the evidence on both sides, then say an honest “don’t know”.

I would be interested to know the line of reasoning that leads you to say: “Our mass would be infinite.” I have a reputation as an “infinity crackpot”, so I’m always looking for people’s thoughts on that subject.

I’d also love to know why you think black holes can’t exist.

21. George says:

Hi Bill,

I see that stationary mass in our space is definitely not zero. So at the speed of light, the mass must be infinite in the eye of light, right?

The main reason for me to say that black hole cannot exist is that it is too strange and it could exist in theory but not in reality. First, even if our current knowledge about basic particles could not provide enough resistance to forming black hole, deeper knowledge may help. If all that fails, I still say that SR will come into play. As we know at the core of the sun, the density is about 150 times of water. That is not a lot. Based on this, I say other stars are similar. Now when it starts to collapse into a little dot first, it will need a lot of matter, at least a size of earth and more. As the density builds, the gravity becomes large such that the falling speed will approach the speed of light in a few thousand kilometers journey. By SR, that will never happen. In the process, matter and energy will build up and we will have a big explosion and no black hole. What do you say?

22. George says:

If the process that I outlined above is correct, then there are several more things we can say about supernovas. First it will not happen for small stars. Second no matter how big a star we start with, the core ending process is the same, the same explosion process will blow the whole star into bits. Here, of course, due to the property of gravity, a star cannot be infinitely big as outside will drift away, condense and form their own objects. So I do not understand why some people say that if you start with a bigger star, you will get a black hole and if you start with a small one you will get a neutron star in the end.

Also, it is clear that nuclear reaction cannot convert matter into energy completely. In the end, it seems, it may be the converting gravity into energy powers the explosion. I saw some talking about antimatter process in stars. If that is true, we should not have collapsing step. But then I am not clear about antimatter stuff…

23. Bill S. says:

“I see that stationary mass in our space is definitely not zero. So at the speed of light, the mass must be infinite in the eye of light, right?”

I think we may have drifted off topic somewhat by wandering into black holes, but as we are there it is probably worth mentioning that the physical evidence in favour of the existence of black holes is considerable. There’s a lot we don’t know about them beyond mass, charge and spin, but there are things out there that would need a lot of explaining if they are not BHs.

I’m sure there must be threads in which this discussion would be more appropriate, but let me leave you with one thought about gravity.

If gravity creates more gravity, what stops runaway gravity from turning the whole Universe into a giant black hole?

Just a thought.

24. George says:

If I am photon, I consider myself to be stationary and the rest to be moving past me at the speed of light. So I see you as a two dimensional object. From my point of view, your clock is not ticking and your mass is infinite by relativity. How does this sound?

25. Bill S. says:

It sounds like a logical extrapolation of relativity. I think there are plenty of scientists who would agree, at least to some extent. However, as we are using relativity, there are a couple of things to consider.

1. If you are a photon, relativity does not say anything about a frame of reference for you.

2. If you had a F of R you would consider yourself as stationary, but how would you see any other photon that was in your F of R? Think about that; there could be a problem.

26. George says:

I agree with you that it is difficult to establish frame of reference for photon. Now by relativity, to define time and space we have to select a FR first because time and space are all relative to a FR. Since we cannot define a valid FR for light, by relativity, we cannot define meaningful time and space for light itself using relativity. Therefore I say time and space for light are out of the realm of relativity, right?

By the same reasoning, all the massless light traveling particles share this same property. What do all these imply?

27. Bill S. says:

“Therefore I say time and space for light are out of the realm of relativity, right?”

That seems to be the case.

“By the same reasoning, all the massless light traveling particles share this same property. What do all these imply?”

You really need a particle physicist to answer that. Bumbling amateurs like me can get into a mess. 🙂

When I joined this thread I expected a lot of counter arguments to my post; they have not materialised, so I started trying to think of my own objections.

One thing that occurred was that photons do not decay. This might be used as an argument in support of their not “experiencing” time. However, you mention other massless particles. Presumably gluons (for example) travel at c, but I believe they decay. I know a straightforward comparison oversimplifies the situation, but it’s food for thought.

28. George says:

Are you misinformed? The current thinking is that for any particle, if it decays, then it must have mass and as a result, it cannot travel at C. Which particle are you saying that it decays and travels at C? This would be a very interesting case.

29. Bill S. says:

The usual example is the gluon.

30. Okay. here is a thought, uniformed though it may be. Is, perhaps, the speed of light what it is because of what the structure of space is? Like, whatever space is made out of is what determines the speed of light, which is why, even though gravity warps space and time, the structure of space is such that light reacts with it, resonates with it, or on it, in it, always the same. Like if you had a piece of fabric, and you bunched it up in some places and dipped it in dye. the osmosis would draw the dye up into the fabric, but at the places where the fabric was bunched, that process would be slowed, so, like that, except, the dye always absorbs at the same rate no matter what the shape or thickness or the fabric. I hope that makes even some sense. So, then, if one wanted to go faster than light, the trick might be figuring out what that property is of space. Like resonance, and determining how to alter the frequency to overcome that resonance, to bypass it. I also wonder about gravity and time. If lots of gravity slows down time, then what about places with very little gravity like out between the galaxies, is time moving like super fast there? or, does time even exist at all if there is no mass for that time to be acting on? No mass, no gravity, no time. Does that make any sense?
Not a physicists. Just an artist. But creative thinker, hopefully.

31. Bruce says:

Maybe I’m missing something but how can a photon be travelling at the speed of light and not experience distance? If it is traveling at a speed, then it is covering distance. If it wasn’t then it wouldn’t have a speed, right? Also, what do you mean by the word experience?
“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.”

32. Bill S. says:

Hi Bruce, your last sentence sums up the position from the point of view of our resident physicist. However, Einstein may have thought differently.
The second postulate of SR says: “The speed of light in vacuum is the same in all inertial reference frames.” If we designate a reference frame in which the photon is stationary, it seems reasonable to argue that we have violated the second postulate.
Because extrapolating time dilation to “c”, and having a RF in which light is stationary in time and space seems so reasonable, intuitively, there will probably always be arguments about interpretation; but in the present state of our knowledge, “don’t know” is the only honest answer.

33. Bill S. says:

Hi Peter, I’vejust realised I missed your post of 11.04. You raise some interesting thoughts, some of which I would like to comment on. As a fellow non-scientist, I shall have to mull it over a bit first, and time is very short, but hopefully “I’ll be back”.

34. Torsh Johansen says:

David Says: “Entanglement for instance – while the photon appears to us in different places – resulting in ‘spooky action at a distance’, to the photon it’s in the exact same place at the same time, so no surprise a change is affected instantaneously.”

This is what I was Googling. Here’s my POV, and tell me where, and more importantly How, I’m wrong:

Light experiences no time. If I’m a photon and I go from the earth to the moon, it’s not that it took a super-duper fraction of a second to me, it’s that it took 0 seconds. To me, a photon, I’m INSTANT. Speed of light, relative to the light, is Instant. Relative to everything else, it is not.

So if I’m that light, and I shoot West 2,000 miles toward something that changes my polarity, and the other photon shoots out East 2,000 miles — my polarity changed when I left. It should be Instant, right? My polarity changed way over there at the same ‘time’ to me as when I left — as when the other photon left in the opposite direction. Time doesn’t elapse for a photon. It’s in all places at once since time (distance) does not elapse.

?

35. Here is another thought/question. So, if you turn on a flash light the light beams out. When you turn it off the light stops. Now the light that came out of the flash light while it was on did not cease to exist. So where did it go? I suppose if you shined the light onto objects and the ground and such, that it was absorbed by all these things, turned into heat, and what didn’t reflected off and away into space or kept bouncing around forever. But that isn’t what happens exactly, I guess. Right? if you light a candle in a deep cave, once the candle goes out you wont see the light still bouncing around. It will dark again. But, what if there was a way to create a reflective surface like a mirror that was always one hundred percent reflective and never absorbed any of the energy or converted it into heat? Might you be able to create a chamber that would perpetually bounce around a photon forever? I always thought it would be fun to build a ball out of that one way mirror glass with the mirror on the inside. I know that light does pass through them, but I would assume at a lower rate than regular glass. If you could build something like that, that would let photons in but not let them out again, what would happen? How many photons could it hold? Would it fill up at some point? Make it with a vacuum inside so there is no mass for the photons to bounce off of

36. Bill S. says:

Peter, you raise the question as to whether time exists in deep space. That’s probably the easiest point to answer. If there were no time there, nothing could happen so we wouldn’t see the stars.

On the subject of what “really exists”, Sascher Vongher has some interesting thoughts about the existence of light. He is, obviously, in the “light doesn’t experience time” camp.

http://www.science20.com/alpha_meme/fundamental_nature_light-75861

37. Bill S. says:

Hi Torsh, there are probably plenty of people who would tell you how and where they think you are wrong, but the reality is that as far as the original question is concerned, the answer is still “don’t know”. Sascher’s comments are well worth reading, but he doesn’t really cover the problem of assigning a IRF to a photon.

I’m certainly not qualified to comment on entanglement, but remember that the apparently instantanious action at a distance is not restricted to photons.

38. Bill S. says:

Peter; your thought: “No mass, no gravity, no time” raises some questions that may not be strictly on topic, but are interesting. By “no mass” do you mean no mass/energy; if so, are you actually talking about “nothing”? We probably shouldn’t go there in this thread, that’s a minefield all of its own.

A thought that arises from that relates to the apparent slowing of light as it passes through any medium denser than a vacuum.
Manifestly, light slows down when passing through any medium denser than a vacuum, but how does it do that? One explanation goes something like this: Consider a single photon passing through a block of glass; for simplicity, have the glass surrounded by a vacuum. It approaches the glass at “c”. Once it enters the glass, it is absorbed by an atom, and then re-emitted. This process is repeated as it continues to pass through the glass. In fact, it is not a single photon that travels through the glass; it is a succession of new photons, created at each new emission. Any travelling done by the photon within the glass is at “c”; the apparent slowing results from a succession of minute instants during which the photon does not exist. We have, therefore, to regard it as travelling at “c” whatever medium it is travelling through.

Would that explanations were that simple, but of course they are not. It turns out that if this were what actually happened, then the absorption spectrum would be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured frequencies. In fact, the refractive index (which reflects the speed of light through that medium) varies continuously, rather than abruptly.

The reason for this is that a solid is composed of a network of ions and electrons fixed in a “lattice”. Because of this, they have what is known as “collective vibrational modes”, sometimes called phonons. These are quanta of lattice vibrations, and it is these vibrational modes that can absorb a photon. So when a photon enters a solid, and it can interact with these phonons, it can be absorbed by the solid and then converted to heat. The solid is then opaque to this particular photon (i.e. at that frequency). Unlike the atomic orbitals which are discrete, the phonon spectrum can be broad and continuous over a large frequency range.

If a photon has an energy beyond the phonon spectrum, the solid cannot sustain this vibration, because the phonon mode is not available. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This appears to be the origin of the apparent slowdown of the light speed in that particular medium. The emitted photon may encounter other lattice ions as it makes its way through the material and the resulting reactions accumulate to cause the delay.

So far, so good, but what does this have to do with light being slowed by gravity? Does gravity have a “lattice”, “collective vibrational modes” or “quanta of lattice vibrations”? If not, what slows the passage of light? Could gravitons be involved?

We hitch-hikers really need some expert guidance.

39. That was very informative, even if I do not comprehend it in total. of course what you describe there would be related to how we perceive color, or at least which colors we perceive, do to the fact that some of the spectrum of light is absorbed by objects and others reflected. Thouh you raise the very interesting question I never thought about before, which is how does light pass through glass? What you there describe reminds of the way electricity flows through wires. I read in one of the very nice Asimov books on science about how electricity does not flow through wire like the way we think of water flowing through a pips, but rather that the electrons are pushed, one after another, from the material itself, sort of like the wave at a sporting event. Maybe a good analogy is something like – a man in a doorway, another man runs into the doorway, he gets there and stops and sends the first man running from the doorway. or something like that.

My other idea was imaging the complete opposite of a black hole. In a black hole we have massive amounts of mass all squished together in one place, this causes super massive amounts of gravity, or space curvature, if that is all that gravity is, I son’t know, and all that gravity causes time to move slower, at least relative to time elsewhere. So what would be the most opposite of this scenario that you could get? Somewhere with the least possible amount of mass, or no mass at all, like the deep void of space, out in between galaxies, for instance. if you consider a place like that, what effect does the complete absence of mass have on time and on gravity and on the curvature of space? It seems to me that the rate of time flow, if it can be slowed by gravity must therefor also be speed up by absolutely no gravity.

If that is so, it would also seem to me that there must an absolute limit as to how fast and how slow time can move, like a spectrum, or a wave frequency. I don’t really know what this means, but it seems like it might be important. I also have no idea how you could test it. if you could send a probe into the expanse between the galaxies, for instance, the probe itself would have mass and change the situation. But I bet someone who is handy with math could work out something.

Here is another thought. Sort of a thought experiment, or question, dealing with light and all the mass and objects in the universe. So we do not know if the universe is in fact infinite or finite. If it is finite, it is impossible to comprehend what the edge of it would be or mean. That is also like trying to understand what true and real nothingness is, like the moment before the big bang. For as I understand it, the big bang was not a singularity exploding out into some already waiting infinite and empty space, but that space itself is a product of the big bang.

So the question is this. if you think about all the light shining from all the stars in the whole entire universe. Does all that light eventually terminate somewhere or upon something. Like, the light that hits the earth, that light stops here, well, except for what is reflected, but much of it is absorbed by the earth and used. but the spot where the earth was a week or month ago, the light passing through that vector (or whatever you call it) keeps on going past the earth. Now it might hit mars or pluto, or the oort cloud, but judging by all the stars we can see the light from stars makes it well out past the solar systems, generally speaking.it would seem like there is a lot more open space in the universe than there are things floating around in it. So conceivably some beams of light might just go on forever without ever hotting a planet, or floating rock, or whatever. Or do they? maybe every photon does in fact eventually hit something and if not where then does it go? if the universe is finite what happens when it reaches the end?

I do ever so much wish that I was handy with engineering and electronics and such. or had ridiculous amounts of money to hire people that are. I would love to attempt to build some of those so called anti gravity machines on youtube, or even just build weird stuff out of magnets. Magnets are really cool.

40. Bill S. says:

The opposite of a black hole would be a white hole. It might be worth googling that.

One thing that strikes me is that you seem to be looking for a situation in which you can somehow concentrate lack of mass in the same way that mass can be concentrated. My initial reaction is that it would not work. Consider the process: starting with highly concentrated mass, gradually reduce the concentration. How far can you go? In principle, you can reduce it until you have nothing, but that is as far as you can go. You cannot concentrate nothing.

I’m out of time, now, but there are other points I would like to come back to.

41. Not a concentration of nothing. I am afraid you misinterpret my point. What I am concerned with is the effect on the rate and flow of time in places where there is nothing. In a black hole time stops or moves much much slower than outside of the black hole. As we know time moves faster where the satellites are above the earth than on the surface if the earth. the closer you are to a center of gravity the slower time moves. So at points that are the furthest away from gravity time must move faster. But how fast? It seems that time must have a quantifiable range of relative speeds, just like wave frequencies. I guess we have no idea why gravity affects time. Well, that is not completely right, it has to do with the curvature of spacetime, But i am pretty sure that our complete grasp of what that means exactly is somewhat sparse still.

42. it occurs to me that the very idea of “time” though, may be a woefully inadequate way to describe the reality of whatever the truth is of what we perceive as time. Time is really only the process of change of objects in terms of motions and systems. Right? Like the tiny bits flying in circles inside atoms or the vibration of the strings. Time is motion of objects. Without objects/energy time has no meaning. So when we say that time moves slower in the more gravity we have what we really mean is that the process of change, the motion of atoms, the vibration of the strings moves slower relative to elsewhere. Errgg. I wish I understood all this stuff better.

43. Bill S. says:

Peter, I think I detect an evolution in your thoughts that is very similar to my own.

From a philosophical standpoint there are two major schools of thought regarding the nature of time. These turn up under various titles, but, perhaps most frequently as the “tensed” and “tensless” interpretations of time. Much of the discussion around these two viewpoints is philosophical, rather than scientific, and seems to be largely a matter of semantics. However, I thought it was worth looking at the essentials of both positions, chiefly because some physicists have been known to take sides in this sort of discussion.

John McTaggart (1908) produced a lengthy paper on the subject of this kind of division in the way philosophers consider time. It’s dated and heavy going; I don’t really recommend it, but his “A Series” roughly equates to the tensed view of time; while his “B Series” roughly equates to the tensless view.

Tensed time is “now relative”; it assumes an ever moving now, which progresses towards the future, always leaving more past behind it. Of course, it is not only “now” that is moving. Anything defined in terms of “now”, such as an hour ago, or two days time, must move along with it. A railway is frequently used as an analogy for the passage of time. Using that analogy, if “now” is taken to be a passenger somewhere around the middle of the train, then the engine at the front and the guard’s van at the rear equate to “now” related points in the future and past respectively; obviously, they all move along together.

Tensless time relates to clock time, dates etc. and is regarded as being static. For example, 11.30 (GMT) on the 25th October 2016 is a tensless time; in relativistic terms it is an unchanging spacetime event. In this view there is no objective passage of time. To return to the railway analogy; tensless times would be like stations, or landmarks along the way, each would remain in its allotted place.

We must think think relativistically, so we observe that the distinction, between moving and static time is in fact quite arbitrary. One could, with equal validity, regard tensed time as being static, and tensless time as moving. In the railway analogy, this would be like regarding the train as being stationary, and the track moving; absurd as that might seem in the “real” world of our everyday experience, relativity tells us that that interpretation is as valid as having the train moving on a static track. There is story, which I suspect is apocryphal, in which Einstein was travelling by train from London to Oxford, and asked the ticket inspector: “Does Oxford stop at this train?”. One can but wonder if the ticket inspector managed to think of an appropriately clever response.

I think, what I am getting to with all this is that if we regard time as a static concept (as you suggest: just a way of recording change) some of the problems may go away. Undoubtedly they will be replaced by others, but that’s part of the fun of scientific “dabbling”.

44. Movement of any sort can only happen because of time. if there was no time, then everything would have to be frozen in place. No process, no change. The train is moving or the track is moving. We can know in fact that the train and the track are both moving. the track is on the earth, the earth is moving, there is in fact nothing which is static. Everything is always moving. Some things do not move in relationship to other things. for instance, the stuff sitting on your desk seems to be still. They do not change their spatial relationship to one another. But since we know that the whole earth is moving, and we know that on a subatomic level there is also motion, the notion of anything being static is somewhat absurd.

Time is described as being a dimension, like space, but I feel like that is wrong ish. Because, as has been stated, we, to some extent anyway, can move freely about through space, but not through time, in fact the moving through space requires time. Time itself is only measurable or perceivable because of motion and change of mass and energy. So one question that arises, to me, is this. if you create a perfect vacuum inside a chamber. There is then nothing inside there. or is there? I am not sure about that, I don’t know if a complete and total vacuum is possible anywhere. Outer Space is still full of dust and bits and then there are the neutrinos flying around everywhere, and I have read that empty space has this energy change, and that even in space particles sometimes can blink into existence out of nowhere and then disappear again, but let us suppose you could create a closed chamber and none of that stuff was going on there and you make a perfect vacuum. With no mass or energy inside that space, nothing to undergo change or motion, is time happening there?

Now I rather think that space is actually something. But we don’t know anything about that/ beings and mass and energy are to space as fish are to water.
Then there is light and the speed of light. I think that maybe light, photons, are not something that moves through space, but something that happens to space. The energy from stars and the reactions therein act upon space like a vibration. That the nature of light is an interaction between that energy and whatever space is made out of. I don’t know.

It does seem to me that basically we need to work out more and more different analogies and conceptions of all these things, because in so doing we create different ways of looking at and understanding the problems to be solved, and in so doing we can then formulate tests and hypothesis to reject or accept.

45. Bill S. says:

Hi Peter, I’ve been extra short of time of late, so just a couple of comments to let you know there’s still life out there.

You say: “Movement of any sort can only happen because of time.”

As you point out, you could turn that round and say that time is a measure of change, so without change there is no time.

You say that without time, everything would be static. It’s not as simple as that, though. If everything is static, this implies that it remains static. What can it mean to say that something “remains” static if there is no duration within which it can remain?
You rightly point out that everything can be defined as moving relative to something, but in using a railway as an analogy for the passage of/through time, only the relative movement of train and track is actually relevant. Other movements just complicate the analogy unnecessarily.

You raise some other interesting points, but I’ll have to find some time (no pun intended) even to think about them. Hopefully someone else might ship in.

46. David Martin says:

I should say that I’ve found some excellent answers on this site, and some very good articles. My estimation of the guy who wrote them has steadily gone up over time. And I think he’s also a good writer, from quite a few of the articles I’ve read.

But this idea that light doesn’t experience time (or distance) is simply bad physics. There’s a list of reasons to think that we can’t apply the rules for matter to light. What the guy is doing here is taking light to be matter moving at c. But in physics, we only apply rules if we know we’re in a position to apply them. (Certainly if we’re answering questions from [possibly stoned] physics students.)

Light and matter are different, and the rules for matter can’t be applied to light in some situations, therefore we can’t depend on applying them to light in any situation. Let’s give one example, that should be enough to make the point that we can’t rely on this approach. Light has energy, but no mass. But with matter of the kind that experiences time dilation, energy translates to mass. You get relativistic energy and relativistic mass, and at c those would be infinite. So does the physicist think that light’s energy becomes infinite because it travels at c? of course not. So he’s picking and choosing out of which rules for matter he chooses to apply to light, and which ones he doesn’t. And he’s chosen one that allows some weird talk about some concepts that we haven’t yet interpreted. There may or may not be an interpretation out there for these concepts, but either way, we don’t have one now. So we simply can’t talk in that way, or make the assumptions that are implied on this page.

47. Bill S. says:

Hi David,
From your comments about students, it sounds as though you may teach physics. I am not a scientist, and am hear to learn.

In case you have not waded through this extensive thread, I would like to call your attention to my first (of many) contribution on Jan 15 2014, in which I express the opinion that:

“It seems that the best we can say is that we have no way of knowing if photons experience time, or not.”

I would appreciate your comments on this, and the reasoning in my post.

Thanks, in anticipation,

Bill.

48. John Smith says:

Actually light experience time while passing through mass (for example through water) because it’s speed in them is less than the light speed.

49. Bill S. says:

It’s not quite as simple as that, John, a solid is composed of a network of ions and electrons fixed in a “lattice”. (This doesn’t apply exactly to water, but that’s not important, here) Because of this, they have what is known as “collective vibrational modes”, sometimes called phonons. These are quanta of lattice vibrations, and it is these vibrational modes that can absorb a photon. So when a photon enters a solid, and it can interact with these phonons, it can be absorbed by the solid and then converted to heat. The solid then becomes opaque to this particular photon (i.e. at that frequency). Unlike the atomic orbitals which are discrete, the phonon spectrum can be broad and continuous over a large frequency range.

If a photon has an energy beyond the phonon spectrum, the solid cannot sustain this vibration, because the phonon mode is not available. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This appears to be the origin of the apparent slowdown of the light speed in that particular medium. The emitted photon may encounter other lattice ions as it makes its way through the material and the resulting reactions accumulate to cause the delay.

So, it is reasonable to argue that while the photon is interacting with the phonon, it is not actually light, thus only the travel time between lattice ions counts as light’s travel time, thus, it remains at c, but for practical purposes, it is delayed. This allows things like lenses to work.