Q: What’s it like when you travel at the speed of light?

Physicist: From a classical (Newtonian) view point this is a completely solid question.  However, in the context of special relativity the question itself is (unfortunately) non-sense.  For many practical purposes, the speed of light (hereafter I’ll call it “C”) is “infinitely fast”.  If you define infinitely fast as the speed you’ll be going if you accelerate forever, then C is exactly that.

Normally when you want to figure out “the behavior at infinity” you can “take a limit”.  For example; the limit as x goes to infinity of 1/x is 0.  This statement just means that as x gets bigger and bigger 1/x gets closer and closer to zero.  So by looking at the behavior at larger and larger finite values you can talk about what happens at infinity.  C, on the other hand, is fundamentally different from all other speeds.

At a basic level, speed is just distance traveled over time taken (as in “miles per hour”).  Due to the laws of special relativity, movement affects both the relative distances and relative time between two reference frames.

As a quick aside, a “reference frame” is just the set of all things that are moving at the same speed or, equivalently, are stationary with respect to each other.  So if you’re traveling down the highway you’re in the same frame as all the other cars around you (if everyone’s going the same speed), while the repair teams and clean-up crews on the shoulder are in a different reference frame.

It may seem silly to say it, but no matter how fast you move you still see things passing by, and it still takes at least a little time to get where you’re going.  At C however, the distance to your destination is always zero due to length contraction, while the time it takes to get there is also zero due to time dilation.  If you were to calculate your own speed you would say v= \frac{d}{t} = \frac{0}{0} = ?, which makes no damn sense.  I mean, what is that?

The universe: As seen by something traveling slower than C, and something traveling at C.

Also, consider this: at any other speed you can speed up or slow down, but at C you genuinely don’t have time to step on the brakes or the gas.  Literally, “time” and “distance” are phenomena that only make sense if you’re talking about them at speeds slower than C.  Stuff in the universe is divided into two categories: “massive” and “massless”.  Massive objects (anything with mass) always travel slower than C, while massless things must travel at C.

All that being said, you can wave your hands and talk about what life is like for a photon, that can’t exist at sub-light speeds (after all, what speed would you expect light to move at?).  When a photon is generated it immediately takes off at C, and never slows down until it runs into something.  Photons never experience time or distance.  As far as they’re concerned they are emitted and absorbed at the same place and time.  Many of the radio photons hitting you right now (about a third of them), have been traveling for around 15 billion years, but they think that the beginning of the universe just happened (or would, if they could think).

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11 Responses to Q: What’s it like when you travel at the speed of light?

  1. Pingback: Q: Why is the speed of light finite? « Ask a Mathematician / Ask a Physicist

  2. Lamoni says:

    Definitely a fun read, thanks for this.

  3. Joe says:

    This reinforces my feeling the the inside of the universe is older than the outside edge. If one was outside of the universe watching it expand, it would seem that it just started yestersecond ( to coin a word). Of course those of us inside the universe feel it is a bit older than that.

  4. The Physicist The Physicist says:

    It’s important to be careful when talking about the “edge of the universe” or being “outside of the universe”.
    There’s some stuff about this sort of stuff here: Q: How far away is the edge of the universe?

  5. Cass says:

    Don’t consider classical physics at all, it’s not correct ever, only approximately. The time it takes to observe any change in timespace is still relative to each observer. When I hold a hand up, and you recognize that I’ve done so in your reference frame, a sort of “time” has passed, but it’s not really so, only you’ve failed to see it instantly, because it takes “time” to have the photons come into your eyes. The time lag between those events is usually called “elsewhere”, or what’s not yet.

    About the “edge” or other such nonsense, consider the Earth. You go on around it until you come to the same point. If there is no center (as is observed), then it follows that there is no circumference; but the universe is finite in time (as observed), so the spatial edge is here, and we are at the center of all spacetime expansion, as observed. The edge of the observable universe is at 13,74 billion light years, because that’s how long it has taken for it to become our observable universe.

  6. tommyt says:

    Do photons actually slow down when they go through a material other than the vacuum (its not really vacuum I know, but you know what I mean) of space?
    For example when light is passing through the air in my room then goes through the glass of my beaker through the water in my beaker does it actually experience three different speeds and would this effect the mass of the photon?

  7. The Physicist The Physicist says:

    When light travels through a material it is rapidly absorbed and re-emitted by the atoms and molecules of the material. Each time it’s absorbed it’s delayed briefly, but in between it travels at the usual speed.
    The probability that it will be absorbed, and the length of the delay is determined by the electrical properties and density of the material (nasty complicated).
    Also, light doesn’t have mass to worry about!

  8. tommyt says:

    excellent reply… i was thinking something along those lines (about being absorbed and re-emitted). What happens to the speed of light when it is effected largely by a massive object (like when a straight line of photons is bent by a massive star)? If they are going in one direction and then have an acceleration in another direction does that effect their speed, thus changing the speed of light rule (that massless particles always go the speed of light)?

    btw, this is one of the best science sites I have found, ever.

  9. The Physicist The Physicist says:

    Thanks kindly!
    The best (simplest, most self-consistent, and most accurately predicting) model we have of gravity is that gravity is curved spacetime. Locally the photon always believes it’s traveling in a straight line, but globally the effect is that it comes away from the star traveling in a different direction. Intuitively we like to think of the photon as having fallen a little, or of being pulled by the star, but it’s better to think of the photon as just moving in a perfectly straight line through not-perfectly straight space.

  10. brad says:

    having a general interest in science I have a couple of books that talk (briefly) about special and general relativity, yet none of them had ever mentioned what happens when moving at C, only the increase of space-time dilation as C is approached. I concluded on my own that at C, (or from the point of view of the photon itself) the progression of time is zero and space is a 2d plane perpendicular to the direction of propagation (with no change in time the word propagation, distance, and speed all become meaningless) this is the first time I’ve ever read this explanation anywhere!! why is this not presented in EVERY explanation of relativity? Instead they all wasted my time with the stupid twin paradox, hardly a good starting point. Its a very simple logical concussion and I feel fundamental to any actual understanding of the theory. I didn’t think I was so clever as to have arrived at some understanding on relativity that had somehow eluded greater minds then my own for over a century. I suppose I’m disappointed that it is not more commonly presented, and that I never found and read this page 3 years ago when you wrote it.

  11. Xerenarcy says:

    is there any particular reason (lacking a good explanation elsewhere) why light bends due to gravity to a larger degree than the freefall trajectory of ordinary matter, specifically considering matter would deflect less and less with increasing flyby speed?

    furthermore, since c depends on how readily a vacuum can react to / form electric fields, and since light is a wave, there is a time-dependent (or distance-dependent depending on how you look at it) property within light. that isn’t to say time doesn’t stand still at lightspeed, only to point out that to have any appreciation for what to expect at that ‘speed’ you cannot think in terms of how matter would perceive things (interact) since matter relies on the same propagations to stay solid (hence length contraction).

    so technically speaking an inertial frame moving at c would experience no time and no distance, which i imagine is the focus of the original question. whereas light must have some awareness of distance (or time) or it would not exhibit wave-like behavior properly (field phenomena cannot propagate without some dynamic of the field being involved, in this case, a self-sustaining packet of energy manifesting as an oscillation of the field). not certain of this, this explanation just makes the most sense to me from what i understand.

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