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”.
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.
Simple answer.
Light cannot experience time since time uses light speed as its base, so, at the speed of light, time is at 0.
But doesn’t light slow down while moving through different materials?
So wouldn’t that imply light is experiencing time after all?
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.
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?
“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.
“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!)
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.
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? . . .
@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.
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)
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.
Very helpful – thank you! Mind-bending stuff!
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?
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?
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)
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.
what about the experiment where they made part of the light wave move faster than the rest. how would that work.
http://www.popsci.com/science/article/2012-05/fed-sluggish-neutrinos-scientists-force-light-move-faster-its-own-speed-limit
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.
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.
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?
The part I don’t get: light is a form of energy, correct? Then why no mass? E=mc^2?
@Thatguy
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.
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.
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.