ELi5: How does the whole light year thing work? How does something happen a long time ago that we know of yet we can still see it and have to wait for it to happen again (like those exploding star things on tt)
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Hi /u/chimmy_w!
The speed at which light (and information) can travel is finite. In fact, on cosmic scales, the speed of light is fairly slow. Thus, when a star goes supernova very far away, it takes the light emitted in this event considerable time to reach us.
If the star is, say 100 light years away from us, it takes the light 100 years to reach us. If it is 1000 light years away, it takes 1000 years to reach us and so on.
Light years are a measure of distance; how far something traveling at the speed of light would travel in a year. Because light is how we see, if something happens 1 light year away, it would take 1 year for the light to travel from where it happened to our eye. So if a star exploded 1000 light years away, the star would still be visible in the night sky here on earth for 1000 years even though it isn’t actually there anymore.
Imagine you and I live a few towns apart. It takes a letter three days to travel from me to you.
On January 11, I stub my toe. It pisses me off, so I write you a letter. On January 12, that letter is still in the mail. Me stubbing my toe hasn’t reached you yet – as far as you can observe, my toe is still un-stubbed. It’s only a couple days later, on January 14, that news of my toe-stubbing reaches you.
In this analogy, you live 3 “letter-days” away from me. So the fastest you can know about anything that happens in my town is 3 days, when the first letters from that event arrive.
Light is much faster than a letter, but the distances to the stars are incredibly vast. So it takes light years to reach us from stars. When something happens far away from us, we only know about it when the light (and other signals) from that event reach us; since other signals can never move faster than light, the light always gets here first.
When we say “speed of light” we are actually referring to the speed at which information about one part of the universe can get to another, so it’s actually the “speed of causality.” Other things like gravity and electromagnetism also propagate out at this speed. Even light is only going “light speed” in a vacuum. Things like air, water, or glass actually slows it down.
There are two different things at play here.
A “light year” is a unit of distance (not time), defined as the distance light travels in a year. So it doesn’t “work” or do anything or explain anything by itself, it’s just a measure of a certain distance (about 5.88 trillion miles).
A few other commenters have done a great job of answering your real question, I just wanted to make that part clear.
A light year is the distance light will travel in a year, so the imagine you see an object a light year away it will take light 1 year to reach your eye so you are seeing the object as it was one year ago. https://youtu.be/35kQspMO2Jg
Light is very fast, literally the fastest thing in the universe. It seems instant to us tiny humans but space is so big that light actually takes a long time to get around. So a light year is just the distance that light travels in a year. That’s why when you look at the stars in the sky you’re not actually seeing them in “real time” per se. You’re actually seeing how they were hundreds, thousands, or even millions of years ago because of how long the light from the star takes to reach Earth. In fact, you can actually tell how delayed what you’re seeing is if you know how many light years away something is. For example, if you’re looking at a star 1000 light years away, that means you’re actually seeing it as it was 1000 years ago, because the light took 1000 years to go the distance from the star to Earth.
Light travels at a certain speed in a vacuum (like space). A lightyear is the distance that light in a vacuum (space) travels in the time it takes the Earth to make a lap around the sun (a year). That turns out to be 9.46 x 10^(12) km or 5.88 trillion miles.
So, if something is really far away, like a star exploding 10 lightyears away, it takes 10 years for the light to reach us. If we “see” the star explode, it’s because we see the light from the explosion, which takes 10 years to get to us. We’re used to these sorts of delays in communications. If we send a letter, it takes days to arrive. If we watch an interview on the news with a reporter on the other side of the planet, we see a few seconds delay as we wait for the radio signals to move back and forth. This is the same thing, a delay while we wait for the message (the light) to travel some distance.
You would have to be going at the speed of light to get to Proxima Centari in about 4 years.
They’re a measurement of distance. A light years is how fast light would take to travel in a year. Do some crazy math and you arrive at a distance of 5.88 trillion miles/9,461,000,000,000 kilometres. Light will take time to reach us, that’s why it’s referred to as the speed of information and causality. It’ll take 4 yrs to reach Proxima Centauri. The light that left it on Jan 1,2019 was seen on Jan 1,2023.(Well, not exactly but just an example.)
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