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
Put a rope or string on the ground. Now wiggle it back and forth. It makes a wave, and it takes some amount of time to get to the other end.
Sound works the same way. Go into a canyon or alley between two large buildings and yell. It takes some amount of time for the echo to reflect back. Just like if you were in a swimming pool and made a wave, it would take some amount of time for the wave to hit the edge of the pool and come back.
Light also works in waves, bit it is much much faster. But things in space are really far apart.
A light year is the amount of SPACE that light can travel in one year. If you stand one light year away and shine a really bright light at the earth it would take one year to reach us, just like the string you wiggle on the floor takes some amount of time for the crest of the wave to hit the other end. If you did this today, people of Earth would see it on Feb. 10, 2023.
Sometimes when you ask someone how far away the next city is, they’ll say something like “Oh it’s about 2 hours away” They’re making a mental shortcut by saying “it’s about 120 miles away and if the speed limit is 60mph, it will take about 2 hours.”
Scientists do the same time when discussing distances to far away stars or galaxies. When they say, “Oh it’s about 100 lightyears away.” what they’re really saying is, “It’s about 946,100,000,000,000,000 kilometers away, and if the speed of light is 9,461,000,000,000,000 km/year, the light will take 100 years to reach us.”
You got it: one lightyear is the distance light travels in one year (assuming the light is traveling through a vacuum). So every time we look at an object in space that is multiple lightyears away, we are seeing light that bounced off of/was emitted by that object some number of years ago (e.g. 100 lightyears = the images we receive are 100 years old).
ELI3 when we say “light yeArs” for a distance, can that distance be expressed in years century etc?
Yes. That’s basically the definition of what a light year is – how far light travels in one year. If it set out from somewhere a light year away, and it’s just arriving, it started out a year ago. 20 light years away, it started out 20 years ago. 120 million light years away, it started out 120 million years ago.
Every time you look up at the stars in the night sky, you are literally looking years back in time.
A light year is the distance light can travels in a year. It doesnt teleport to one place to another, after all. So if you’re looking at an object 1 light year away, you’re seeing light that has traveled 1 year to get to where you are, so in a sense you’re seeing the object as it was last year. Anyone looking at Earth right now from 300 million light years away, is currently seeing the dinosaurs, because the light that was emitted from Earth then is just now reaching them.
Edit: one consequence of this is that, if the sun were to disappear now, we wouldn’t know for 8 minutes. Also, for huge distances, it might take longer than the pure light year distance, because of the expansion of the Universe.
It also means if their was a planet 120 million light years away, if they can now see us they would be seeing our planet 120 million years ago! Blew my mind that did.
What you said is 99% right. Any time someone is having a hard time with light years I make this analogy. If someone asked you how far the next city is from your house you might say “3 hours” as in 3 hours by car on the highway. So “3 car hours” is in fact a measure of distance. It takes a car 3 hours to go that far, but something faster or slower would take less or more time to go the same distance.
Continuing this analogy, if you took a picture of a hole your dog was digging when you left, when you got to the next city and showed it to a friend, they would be seeing the hole as it was 3 hours ago, not now. That is how light mostly works.
The 1% is barely worth mentioning. Due to inflation, the distance between distant objects increases as the light travels here. So the object is farther away than when the light first set out, and it took longer to get here than the distance when the light first started it’s journey would have been in a static universe. But this really just complicates things and is barely worth mentioning as it doesn’t change the fundamental understanding of the speed of light.
eventually light from earth haven’t spread far away yet, so technically if you theoretically will be able to appear far away from earth with kinda big telescope-you will be able to see the past. or even god creating us. or not.
Every time you jump, the light from the sun moves towards you faster than the speed of light
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