I don’t really understand it, could someone explain in basic terms?
Are we saying if a star is 120 million lightyears away, light from the star would take 120 million years to reach us? Meaning from the pov of time on earth, the light left the star when the earth was still in its Cretaceous period?
In: 292
Which also means that if a civilization is more than about 4 or 500 light years away they probably see our work as an uninteresting rock in space with no intelligent life as we didn’t have any transmissions 500 years ago.
You’ve got it, pretty much. If, today at this moment, an alien on a planet 65 million light-years away had a really good telescope and pointed it at Earth, they would see Tyrannosaurus Rex walking around North America. Because it would have taken 65 million years for the light that bounced off that T-Rex to reach that alien.
If he then kept watching us for 65 million more years, eventually he might see me, today, typing this reddit comment, 65 million years after I’ve died and been buried. Compared to the overall scale of the universe, light travels pretty slowly.
Granted, 65 million light-years is still an ungodly distance. That’s over 650 times the length of our entire galaxy. It’s over 25 times further away than Andromeda, the nearest major galaxy to us. In other words, that alien has to be really, really far away if he wants to see dinosaurs.
You can see the speed of sound on Earth if you pay attention. Dog on the far shore of a lake barks, you see it (eg) a quarter second before you hear it. The dog is a quarter of a sound-second away.
Yes, exactly. Light isn’t instant, it takes time to travel. I compare it to throwing a ball. It takes a few seconds for the ball to reach the person you are throwing it to. If catching the ball was all The information they have then they can tell where you were when you threw it, but not where you are now.
In everyday life we use the same convention: NYC is 6 hours flight away from LA. State A is 2 hours drive away from state B.
We basically use time to measure distance. That sounds a bit strange but we actually implicitly agree on the standard speed. That are in the example plane speed, car speed, respectively. So we no longer need to state explicitly speed.
The same applies to light year and light speed.
Yeah, so I think it’s hard to understand with light because light travels so fast that in our day-to-day life it seems instantaneous. With a [1 trillion frame per second camera, you can capture light traveling through space,](https://youtu.be/EtsXgODHMWk) but it’s not something we can perceive with our eyes/brains.
Although sound is physically different than light, I think it’s a helpful analogy; we can understand sound travel from our own experiences. One example is an echo: if you shout into a canyon, the sound travels into the canyon, bounces off the wall and comes back to you. The sound you are then hearing is you from a second in the past – you’re not currently shouting, but you hear the shout from past you. Another example is when you hear a jet in the east, and look up and don’t see the jet right away because it’s already in the west. The sound you heard is the sound the jet made a second earlier when it _was_ in the east, but that sound didn’t represent it’s _current_ location by the time you perceived it.
Of course light is much, much faster than sound, but it still moves through space over time.
So just like your echo is your sound from a second in the past, the light that reaches us from a star a million light years away is the star’s light from a million years ago.
Big numbers get hard to interpret, so let’s take it down to something easy to understand – a light switch.
The speed of electricity through a wire is about 100 times slower than the speed of light in a vacuum.
If you have a light switch in Los Angeles connected to a bulb in Chicago (about 2000 miles away), it will take approximately one second for the light to turn on after the switch is flipped.
A light-second is therefore 100 times that distance. That’s about the distance to the Moon.
A light-year is ~30 million light seconds.
A photon can travel 13.5 billion light years in 13.billion years. ( the age of the universe roughly) but for the photon no time has passed at all.
Fun fact: like 90+ some percent of the galaxies visible to us now, are already beyond a point where their (currently emitted) light can ever reach us… ever.
This is because all of space is expanding faster than light can travel through it, only defied by celestial objects that are close enough together to keep themselves locally bound.
This means they will fade out to nothingness, never reachable to us in any way (unless we figure out how to cheat spacetime’s limits and wormhole there).
Have you ever seen a lightning strike in a distance. It takes some time for the thunder to reach you. The sound you hear once it does was created the moment you saw the lightning strike, even though you heard it just now. Same way it takes time for the sound to reach you, it also takes time for the light.
Every object you see appears to you in the way it was at the moment the light was emitted or reflected off of it.
From the moon, it takes light 1,3 seconds. So in a way your seeing the moon as it was 1,3 seconds ago. The moon is 1,3 “light seconds” away.
The light from sun takes about 8 minutes to reach us, so if it died out suddenly, we would only know about it 8 minutes after it already happened. The sun is 8 “light minutes” away.
If we looked 13.8 billion light years into the distance, we would theoretically see the Big Bang that created this universe as if it was happening right now, even though the universe has been around for 13.8 billion years
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