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
Yes. Light travels at approx. 300,000kps. 1 light year is the distance light travels in 1 year. 120 million light years takes light 120 million years to travel.
And it’s from the point of view of the Earth because for the light the journey was instantaneous.
Yeah we’re not looking at the current date of those stars, they might have exploded already but we don’t know because the light from the explosion haven’t reached us
That’s exactly right, that’s why if we look further away, we’re also “looking further into the past” as well
When people invented miles they were to talk about things on earth scales. Something can be 5 miles away or 100 miles away and that is a number that makes sense. The nearest start is 55 trillion miles away 55,000,000,000,000. No one wants to write 55,000,000,000,000 miles over and over, so they decided to go with something good for space.
A light year is the number of miles light can travel in one year. The nearest star is 4.3 of those, and it’s back to a scale people can talk about easily.
Others have explained already, but here’s something to add: Earth is 500 light seconds away from the Sun, so the light you’re seeing right now left the Sun roughly 8 minutes ago
For every-day understanding, yes. Light takes time to travel so we call a light year the distance light travels in a year. Proxima Centauri is 4.2 light years away, meaning the light we are seeing is 4.2 years old.
There’s more to it but it’s way over an eli5 level. It requires General Relativity and doesn’t really have a big effect until we start considering VERY distant objects. Like in the billions of light years away.
Side note: that “more to it” is why the observable universe has a radius of 46.5 BILLION lightyears. Without that complex stuff, it would suggest that the furthest objects released their light 46.5 billion years ago, before the big bang, but that is of course impossible.
If a star is lets say 65 million light years away, when you look at that star that night, bear in mind that what you see happened when the dinosaurs went extinct.
Basically all you see above is the past
If you could teleport to another planet, which is 3 light years away, and if you had a telescope which could see the Earth from there, you would see yourself gazing up in your garden, as the light is still the past, from three years ago.
Look up the deep space picture from Hubble, with the furthest galaxies, which are not yet properly formed yet. We are seeing in this picture the past, how it looked like 6-10 billion years ago. We dont really know if that galaxy is actually there, it was there 10 billion years ago, but where could it be now ? We dont know
Here on Earth, if you see your neighbour driving on the street, it is still the past, the difference between that and the present is negligible… It is less than 0.000000000000001 seconds, thus you percieve that as present, if we go further how do you define the present.
It never really exists if you think about it…
So it is really fascinating and mind blowing
Answer: It is tricky.
When we look out into space, it takes time for light to reach us. 8 minutes for our dear Sol to reach Tellus (Earth).
So as we pan out we learn that our galaxy is pretty big. About 150 thousand lightyears across. Oh.
Means that most stars emitted their light before we could catch them. The stars are not there anymore. But gravity is also travelling at the speed of consequence, so in that sense they are there. We feel them as they was there now.
And it hits a border. We have learned that the oldest light hit us about 13.7 billion light years ago.
This is called the big bang. It was very smol and it didn’t bang.
One way to understand this is to realise that our universe is expanding. And it is expanding everywhere. There is no center.
So our border is 13.7 billion years away. Now if you could hop n’ skip to that border, it would be 46 billion light years away as spacetime has been expanding during all that time.
And if you could magically teleport there and look back at our home; it would look the same. A 13.7 billion year barrier border that is background radiation. Because our Sol didn’t happen yet. Or it did, but it will take 13.7 billion years to even notice the beginning and end of the star that Sol was made from. It took a while.
This is our observable universe and it must be a lot smaller than the actual universe. Or so we like to think.
So what is outside of our observable universe? The cop out answer is more stuff. In the same way we see galaxys all over the place, you should see the same wherever you are. It could be infinite as far as we know. But we don’t know.
Yet..
If you were able to look at a magical mirror that was 1 light year away, you would see yourself from two years ago. One year for the original image to travel to the mirror, and another year for that mirror image to come back to you
Yep. The really mindblowing thing is that we can never see anything – ever – truly in the present. Even someone only inches away from you could (hypothetically) no longer really be there, because you’re seeing the light that bounced off of them and landed on your retinas. In that light’s travel time, they could have been whisked off by aliens (or something).
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