I just took the doggie for the late stroll around athe block. A crisp January night and the stars are really obvious. Since light is photons, I guess the lights from the star are actual photons reaching us, right? Right?
Can they be measured, sampled, quantified?
I suppose it’s a really stupid question, but I can’t wrap my mind around the fact that we’re being hit by particles thousands, millions years old, particles that have travelled from other galaxies to reach us here.
And the bonus Q for cosmologists:
Can space move faster than light? I was reading about the big bang and how space expanded several light years in fractions of a second. I guess the speed of light is limited to matter, but the stuff that went with space during the initial stage of the BB - did that move FTL?
Every picture is a recording of actual photons hitting a medium. Telescopes are just lenses that focus more of the photons into a certain square area. They really are from sources trillions of miles and billions of light years away. Photons are eternal. Unless they interact with something they will just travel forever, unchanging. It’s a measure of how truly empty space is that photons can travel for billions of years and not hit anything. Almost all information we have about anything comes from photons.
Yes, spacetime itself can expand at any speed. The speed of light limits only mass (and information, which is equivalent). You don’t have to go back to the big bang to find matter moving relatively faster than light. Anything that is more than 13.7 billion light years away is doing so right this instant, from our point of view, because of the continuing expansion of space. This doesn’t violate any laws of physics because we can never get any information from this matter. Can’t see it, feel it, detect it. Anybody there would look at us the same way, thinking we’ve disappeared beyond the light barrier. Are we therefore moving faster than light? No. It’s only the relative motion that’s faster. That’s also what happened to matter in space during inflation. The relative motion was faster than light, but individual particles can’t be said to exceed light speed in their own reference frame.
I’m sure the physics buffs will be here to back this up with equations, but that’s the basics in words.
Well, those are the actual photons reaching you. They aren’t old, though, because they travel at c and experience zero passage of time. But a long time has passed on the star they came from, since they left.
Stars you can see aren’t millions of light years away. You only see stars in our own galaxy, and it’s only a few hundred thousand lightyears across. Stars you see with your naked eye are generally relatively nearby. You can see a couple of other galaxies with the naked eye, usually the Great Galaxy in Andromeda and sometimes one in Triangulum. They look like faint fuzzy patches that may be easier to see if you look a little to one side. They are about 3 million lightyears away.
I just wanted to repeat this for emphasis. A photon which hits your eye from your computer monitor has an age of zero. A photon which hits the Hubble Space Telescope from a supernova ten billion lightyears away also has an age of zero. Light is truly ageless.
That’s actually a meaningless question: the rate of expansion of space isn’t measured in units of speed, but of frequency. It is true that a very distant object might be receding from us at greater than the speed of light, but that’s just as true today as it was during the inflationary era (the time of insanely rapid expansion very shortly after the Beginning). The only difference is that now, something that’s receding at faster than c must be very far away, but back then, the distance required (called the cosmological horizon) was much closer.
They also experience maximum contraction of space in the direction of travel, so they don’t experience distance either. Or, that is, they wouldn’t, if they had any time to experience it in.