Photons are one of the few things in the Universe that go at the speed of light. And that makes me wonder a number of things about them. But one thing in particular: If all motion or time stops when something reaches the speed of light, then how do photons give off light? Hasn’t all time essential stopped for them?
Photons don’t give off light: They are light. You perceive their effect when they interact with other particles, especially those in the receptors in your eyes.
As Exapno Mapcase said, photons are packets of light; they do not give off light.
As your velocity increases, time does not slow down; it merely slows down relative to other reference frames. So if you’re speeding along at 99% of the speed of light, from the perspective of a stationary observer, time appears to progress more slowly for you than it does for the observer. However, when you observe yourself and any immediate surroundings that happen to be travelling at the same speed you are (e.g. your spaceship), everything seems perfectly normal. Even in your reference frame, light will still travel at the speed of light relative to your eyes will perceive light in the same way as if you were stationary. Time proceeds normally with the seconds ticking by at the rate to which you’re accustomed. Time does not slow down or stop for you.
So it’s a similar deal with photons; just because they’re moving at the speed of light doesn’t mean that time has ceased to exist for them in their reference frame.
But he brings up a valid point - a photon’s reference frame is pretty much undefined in our reference frame. A sentient photon would perceive that the big bang and the end of the universe happen at the same instant.
A photon would also percieve it’s path as a single point instead of a line, so as far as the photon is concerned it hasn’t moved at all!
Actually, according to Special Relativity, that’s exactly what it means. Photons do not “experience” time, and therefore do not age.
Caldazar is correct for all particles moving at sub-light but relativistic speeds.
But photons do not move at 99% of the speed of light. They move, by definition, at the speed of light.
So Q.E.D. is correct. Photons do not age, do not experience time, and as Sock Munkey says, would not perceive motion. They blink on and blink off, and nothing more.
Sock Munkey brings up a point that I have been wondering about recently.
From the photons point of view, it does not exist within time, and it travels no distance in no time. Now from our refference photons come in many different wavelengths (or equivalently at many different energy levels). What would a photons energy be from the photon’s frame of reference? Or what would be the photons momentum within its own frame of reference?
What is time?
Zero. everything has zero momentum from its own frame of reference. it’s only when you compare it to another point in motion relative to it, that momentum becomes measureable. If you’re in a train with no windows that moves perfectly smoothly at constant velocity, you have no way to measure your own momentum. To you, you appear stationary, and thus have zero momentum.
A sort of nitpick, but not really.
A photon can’t have a reference frame. Light must travel at light speed with respect to all reference frames and light can’t travel at c with respect to light.
Since a photon doesn’t have a reference frame it’s meaningless to talk about what a photon would percieve. However, this doesn’t mean that it isn’t instructive to do so.
Nitpick: although this would be true of a photon that had not been intercepted or absorbed since the Big Bang (and it makes good hyperbole), photons are, in fact, being created and destroyed all the time, as light is absorbed and emitted. Yjere’s no “anti-photon” that lets you posit that there’s only one photon, bouncing back and forth through time (as some have suggested for the electron/positron).
From dictionary.com :
time, n. 1a. A nonspatial continuum in which events occur in apparently irreversible succession from the past through the present to the future.
I took an intro to Astronomy course my last semester of college, because I thought it would be easy and fun and I have always had a healthy interest in space and all things related. I was right, it was easy and fun, but not for the first couple weeks of class when the ditzy professor had to explain to non-science folk things like the electromagnetic spectrum. Yikes, was that boring. The point is this; the one and only note I wrote in my notebook for the first 2-3 weeks was this:
“Time is a measure of change.”
Thanks Q.E.D. I wasn’t thinking, of course it doesn’t have momentum in its own frame of reference.