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According to special relativity, a photon traveling at the speed of light, does not “experience” time. It is traveling into the future at an infinite rate, right?

That would mean that everything that could ever happen (my death, Trump winning the election, death of sun, end of the universe), has already happened (from the frame of a photon). We're just too "slow" to experience this.

Is this correct? Because that would bring some weird implications about free will, and the very nature of our existence. If everything has already happened, then we're just following a predetermined path.

Edit 1: I'm genuinely fascinated by this topic, however, I am also an idiot with very superficial knowledge, gained mostly from youtube videos and Wikipedia. Bear that in mind :)

Edit 2: The nature of my question seems to leave the realm of physics and cross into a philosophical issue. I was seeking physical explanation, to help answer something of philosophical nature, as both these realms seem to coincide for this particular topic.

Some have pointed out that thinking about the frame of a photon doesn't make sense. Instead, one could assume a spaceship traveling at $0.9999c$.

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closed as unclear what you're asking by WillO, glS, Wolpertinger, Jon Custer, David Z Sep 8 '16 at 12:44

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    $\begingroup$ I'll leave the answers to those who can more correctly spell out relativity, but the concept of "not experiencing time" means just that: it has no concept of time. Thus, you cannot say an event "has already happened" because there is no concept of time to order the events with. (Hopefully I got this correct enough that, when the experts weigh in, their answer will agree with this comment) $\endgroup$ – Cort Ammon Sep 8 '16 at 2:01
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    $\begingroup$ "has already happened (from the frame of a photon)" - there is no frame in which the photon is at rest thus, there is no frame of a photon. $\endgroup$ – Alfred Centauri Sep 8 '16 at 2:05
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    $\begingroup$ For a good discussion of this from a philosophical perspective, see the Reitdijk-Putnam argument. One of the main champions of this philosphical position (aka Eternalism or Perdurism) left us in March. RIP Hilary Putnam, a tower of intellectual honesty. $\endgroup$ – WetSavannaAnimal Sep 8 '16 at 5:18
  • $\begingroup$ Very closely related question here. $\endgroup$ – knzhou Feb 1 at 13:59
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This is an idea known as the block universe. The example of a photon is a bit extreme, since photons have no rest frames, but the idea is the same: to a fast-moving observer, our future might lie in their past, suggesting that our future already exists.

However, these are purely philosophical notions. In order to see what physics has to say about it, we need to translate "the future already exists", which is a bunch of words, into an experiment that can actually be performed.

One such experiment would be to attempt to receive signals from the future, such as the result of a coin flip. However, relativistic causality forbids this from happening, and also rules out any and all similar experiments. Physics really has nothing to say about this question.


You might complain, why would we have to experimentally test this? Isn't the future already existing directly baked into the math of special relativity, which we know is true?

It is, but you have to be careful not to mix up features of the mathematics of a theory with the theory's predictions. Showing that the predictions are correct does not show that the mathematical structures used to make the predictions are ontologically real. For example, quantum mechanics has a huge variety of flavors, all of which have different ontologies (wavefunction collapse, many worlds, pilot waves) but identical predictions.

Physics can't distinguish between them. Deciding which interpretation of quantum mechanics is right, or whether Lorentz invariance says anything about free will, is a job for the philosophers.

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    $\begingroup$ One of the main champions of this philosphical position (aka Eternalism or Perdurism) left us in March. RIP Hilary Putnam, a tower of intellectual honesty. $\endgroup$ – WetSavannaAnimal Sep 8 '16 at 5:15
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There's a famous story about Einstein, as a teenager, wondering what it would be like to ride on a light wave, and this line of inquiry developing into special relativity over the next decade. The story as usually told skips over the interesting part: it turns out, nontrivially, that it doesn't make any damn sense to imagine you're riding on a light wave. From the perspective of "an observer" riding alone a light wave, you have regions of static electric field without any source, which doesn't obey Maxwell's equations. The only way it's possible for Maxwell's electrodynamics to work for all observers is if none of those observers can ever "catch up" to an oscillating field, which moves at finite speed.

The worldline of a photon in special relativity is made up of a series of causally connected events: emission, transmission, perhaps reflection, absorption. The absorption of a photon is an event in the future light cone of its emission. And the light cones are not deformed by the Lorentz transformation. There's no pathological coordinate system you can choose to move an event from my "future" to my "elsewhere."

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For the answer, instead of a photon, imagine a fast spaceship moving near light speed. According to the twin paradox, the astronaut could reach far stars and turn back to Earth within his lifetime. However, in the meanwhile, thousand years may have been passed on Earth.

In this example, the astronaut has reached our future, due to the simple fact that he moved near light speed, and due to time dilation. However, time dilation never transports you into the past of somebody, that would entail causality problems.

It is important that time is advancing differently for each particle, and the twin paradox is working also at small scale. For example, a busdriver is aging slower than a prisoner, his clock is going slower. Even if the time difference is astronomically small, the difference is there. Each particle is following its own proper time, and the right hand of a righthanded tennisplayer will see the future of the left hand.

Let us return now to the photon. We might observe a photon from the early universe, which had been emitted billions of lightyears ago, the photon has reached the future of the particles of the early universe of which we are made today. Each particle is following its own worldline and by this the pace of its own clock. Time dilation depends on velocity ($\gamma (v)$) and on gravity.

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The basic assumption of this question is wrong. It is wrong to assume that no time passes for a photon because its velocity along time axis is zero, because the dimension of time and the arrow of time are two different concepts.

Einstein wondered what would happen if a moving car shines its headlights, since it intuitively suggested faster than light velocities. But this line of thought was later developed into the Special Theory of Relativity according to which, it is not possible to attach a frame of reference to a photon where it can be treated to be at rest. Basically, there is no universal frame of reference, since everything is in relative motion with respect to other. Thus saying that no time passes for a photon is fundamentally wrong, although for a (so called) frame attached to a photon, any events occur simultaneously.

It should be beneficial to understand the difference between an absolute future and a relative future. Let's take example of a supernova occurring at this instant in a galaxy 2 light years away. The photons emitted from this galaxy will reach the Hubble Space telescope 2 years from now. Now this event, will occur 2 years in the future for us, whereas for the photons of the supernova remnant, it had already occurred 2 years ago. This is a case of relative futures, the future of one event differs from the future of other.

However, an absolute future is not likely to be true, since at the smallest of scales, the events are completely random, and these uncertainties cause a butterfly effect on the probable future. Thus no matter how much information you have, the future will be random.

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A photon doesn't experience time means it has transcended time. There is no concept of past, present or future in its experience. Pondering whether a photon experiences future is like pondering whether a stone is happy or sad. As there is no concept of happiness or sadness for a stone, no concept of time for photon.

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If the moving clock runs slow (more slowly than the stationary one), then the future already exists, in the sense explained here:

Thibault Damour, in http://www.bourbaphy.fr/damourtemps.pdf:

The paradigm of the special relativistic upheaval of the usual concept of time is the twin paradox. Let us emphasize that this striking example of time dilation proves that time travel (towards the future) is possible. As a gedanken experiment (if we neglect practicalities such as the technology needed for reaching velocities comparable to the velocity of light, the cost of the fuel and the capacity of the traveller to sustain high accelerations), it shows that a sentient being can jump, "within a minute" (of his experienced time) arbitrarily far in the future, say sixty million years ahead, and see, and be part of, what (will) happen then on Earth. This is a clear way of realizing that the future "already exists" (as we can experience it "in a minute").

That the moving clock runs slow is a conclusion in Einstein's 1905 article:

http://www.fourmilab.ch/etexts/einstein/specrel/www/ ON THE ELECTRODYNAMICS OF MOVING BODIES, A. Einstein, 1905:

From this there ensues the following peculiar consequence. If at the points A and B of K there are stationary clocks which, viewed in the stationary system, are synchronous; and if the clock at A is moved with the velocity v along the line AB to B, then on its arrival at B the two clocks no longer synchronize, but the clock moved from A to B lags behind the other which has remained at B by tv^2/2c^2 (up to magnitudes of fourth and higher order), t being the time occupied in the journey from A to B."

Why is the moving clock slow and the stationary one fast? No such asymmetry follows from Einstein's 1905 postulates. What validly follows is that the moving clock is slow as judged from the stationary system, and the stationary clock is slow as judged from the moving system. Einstein's conclusion above (the moving clock "lags behind" the stationary one) is invalid - it does not follow from the postulates. In other words, the assumption that Einstein's 1905 constant-speed-of-light postulate is true (actually it is false) does not entail the conclusion that one of the clocks "lags behind" the other.

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    $\begingroup$ That last sentence contradicts all observation. Can you name an experiment where a non-constant speed of light was measured? $\endgroup$ – Jens Sep 8 '16 at 12:17
  • $\begingroup$ "Can you name an experiment where a non-constant speed of light was measured?" Yes, Doppler shift measurements. There is no space here to explain so let me refer you to this: network54.com/Forum/304711/thread/1468484013/last-1469661992/… $\endgroup$ – Pentcho Valev Sep 8 '16 at 15:10
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According to special relativity, a photon traveling at the speed of light, does not “experience” time.

Nor would you if you were travelling at the speed of light. (Whilst you can't actually travel at the speed of light, you can in theory travel so close to the speed of light that we couldn't tell the difference). When you're travelling at (nearly) the speed of light you're subjected to time dilation, to such a degree that you're a "popsicle". You're effectively frozen, you can't see or hear or think. You don't experience anything, including time. However this doesn't mean that you travel in some instant fashion. With advance knowledge of your course I can lug an asteroid into your path. BLAM! And that's the end of you.

It is traveling into the future at an infinite rate, right?

No it isn't. It's travelling through space, not through time. The ideo of travelling to the future is just a figure of speech. You don't really move through time. When you suffer time dilation it's like you're in slow-motion mode, that's all. An extreme example of this is offered by the stasis box. Like time travel, it's science fiction, but IMHO it gets to the heart of the matter. No motion of any kind occurs in the stasis box. Light doesn't move, electrochemical signals don't move, nothing moves. So when I shut you inside the stasis box for five years then open the door, you think I opened it immediately. You "travelled to the future" by not moving at all while everything else did.

That would mean that everything that could ever happen (my death, trump winning the election, death of sun, end of the universe), has already happened (from the frame of a photon). We're just too "slow" to experience this.

Not so. The photon has no frame, and those things have not happened. If you were travelling at nearly the speed of light, that incident with the asteroid hasn't already happened.

Is this correct? Because that would bring some weird implications about free will, and the very nature of our existence. If everything has already happened, then we're just following a predetermined path.

It isn't correct. Sorry. But it's good that you think about this sort of thing.

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