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Duration:03:40
Uploaded:2011-02-04
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In which Hank answers a very commonly asked question in the comments of the JWST video.
Hello Nerdfighters, this is my second channel Hank's Channel. And there were some questions about what I said about the James Webb Space Telescope and the number one, most important, most asked question was "How can a telescope see into the past?".

That, obviously, is a somewhat counter-intuitive idea that something can see into the past, we cannot travel in time. How can we see back in time? It's actually a pretty easy thing to understand.

So, let's start with sound. Sound travels at a speed. You know this because you see people far away making a sound, and then the sound gets to you. You can be shot by a bullet before you hear the gun fire.

Fascinating. True. Does not seem necessarily intuitive at first, but it is true. Sound is a wave that goes thrugh the air and you hear it after the sound is created. After the thing that makes the sound makes the sound.

This is also true for light, though light is much much much, much much much, much much much faster than sound. Much faster, in fact, than anything. It is impossible for any thing to go faster than light, unless you are talking about tachyons, which we're not going to because I don't understand them at all.

But light has a speed, so the Sun is very far away from the Earth, by Earth standards, very far away from the Earth. And so far away, in fact, that the light the Sun creates doesn't get to us for 8 minutes because its moving at a speed. So, the way that the Sun looks when we look at it right now is, in fact, the way that the Sun looked 8 minutes ago.

The Sun is 8 light minutes away from us. And so, the Sun could not be there. It is possible, though very unlikely, that the Sun has winked out of existence and we won't know about it for 8 minutes or so. Now that's unlikely, thank goodness, but it is possible.

A star inside of our galaxy might be 100 light years away. We would see that star as it was 100 years ago. Now with the Webb telescope, we're talking about a lot longer than 100 years, a lot further away than 100 light years. 

13.4 billion light years away. So that light was created 13.4 billion years ago. So what we're going to be seeing is what that looked like 13.4 billion years ago. It's not what it is like now. Now it's completely different, it's probably very similar to what we see around us, nearby.

Somebody asked me "Why don't we just look back 13.7 billion years and see the beginning of the universe?". Well, the reason that we can't see that is because there was nothing there, as far as we know.

That is why we know there is an age to the universe, because there is nothing that we can see beyond 13.7, nothing that we can even detect, we're not talking about see, like visibly see. Even when we're just talking about the detection of the background radiation of the universe, there is nothing that happened, that we can detect, there is nothing there beyond 13.7 billion years ago.

So that is, if we looked beyond that, there would be... there is nothing there, which is why we know that the universe has an age. Um, though we don't know what happened before that, and no one is supposing that it was nothing, it's just supposed that we don't know.

So, that is the answer to the question of "How can we look back in time?", we look back in time because light has a speed and we're looking so far away that the light from those objects has been traveling to us for 13.4 billion years.

Fascinating! A little bit difficult to get your mind around, but, in fact, it is the case.

Thank you, hopefully that has answered the question for you. There were some other questions, but that was certainly the biggest one, um, and I hope that that helped. Alright, goodbye!