Video Transcript: Light Travel Time

There are many profound ideas that we encounter as we study astronomy.   You know, the notion that the elements that are inside my body were once inside stars - that's a profound notion.   Or the idea that every person who's ever lived has lived their entire life on this tiny little speck that is the Earth.   That's profound.   I think one of my favorite profound ideas in all of astronomy is the idea of look-back time.   And this connects, it's woven into this idea that our universe appears old.   There’s really no way around that issue as we talk about this topic of look-back time, but it's so cool that I'm excited to share with you.

Consider for a moment the Moon.  When you look at the Moon, you're looking at light that has traveled from the Moon to the Earth.  I mean, the light doesn't create much of its own light, at least that we can see and so sunlight is bouncing off the Moon like a mirror and coming to the Earth.  Now, the Moon is about 2 light seconds away.  We talked about light years, that's like 6 trillion miles.  Well, a light second could be another unit of distance: the distance that light travels in one second.  

So if you were to imagine shining a flashlight at the Moon and watching that light travel to the Moon, it would look something like this first animation in the gallery.  That light would make its way to the Moon, and then turn off your flashlight and that end of the light would make its way to the Moon and it would take 2 seconds for it to get from here to there.  So imagine that for a second.  You know, when the astronauts were walking on the Moon and they would say, you know, “Houston, the eagle has landed,” you know, that radio signal, which is light, would take 2 seconds to get to the Earth and then 2 seconds for their response.  They’d say, “Great, glad you made it.”  That would take 2 seconds to go back.  So there'll be 4 seconds you’d have to wait.  Even if they responded immediately, there would be a delay.  

And the same thing is true when you talk about the Sun.  We mentioned before that the Sun is 8 light minutes away.  So if the Sun were to just turn off, we wouldn't actually know that for 8 minutes because it takes 8 minutes for the light to get to us.  Light travels really fast but the distances in outer space are so big.  

So now we take that further.  Well, let's say we have an astronaut on Mars someday.  Well, even when Mars is really close to us, it might take, you know, could take 20 minutes for that light to get from Mars to Earth and back.  So imagine if you're an astronaut on Mars, and you're trying to talk to people back on the Earth, and you're like, “Hey, we got a problem here.  What do I do?  Tell me right away,” it takes 20 minutes for that message to get back to Earth and 20 minutes for their response, so you're waiting 40 minutes just for them to maybe say, “Okay, hang on a minute, we'll get back to you as soon as we can.”  It would be really hard to have a conversation like that.  

Now we can take it even further because once we get beyond our solar system, the distances become very large.  In fact, the second picture in the gallery shows the closest star to us besides the Sun.  That closest star to us beyond the Sun is Alpha Centauri.  Actually, there's a Proxima Centauri, it's a tiny bit closer, but it's harder to see.  But Alpha Centauri is here - it's that reddish star on the left side of the picture.  And Alpha Centauri is about 4 light years away.  So when you look at Alpha Centauri, it took 4 years for that light to get to us.  And we can measure that directly through parallax, its distance.  We can do the parallax thing.  We can measure its distance; we know how far away it is.  So that's not really a question.  But we’ve got 4 years for that light to get to us, so if Alpha Centauri turned off, it would take 4 minutes … 4 years for us to know that.  Or if we somehow sent someone there, we would have a conversation… imagine that conversation, waiting 4 years for the other person's response.  It'd be impossible.  And that's the closest star to us.  

So now think about the other stars in the sky.  Now, I should mention that throughout our readings you've been reading about distances and parsecs, and parsecs is what astronomers use.  We also use light years.  Anytime you need to convert, you just multiply the parsecs by about 3.26 or just multiply by 3, and that gives you a sense of how many light years because sometimes it's interesting to think how many years that it takes a light to make that distance. 

I should also say there might be a part of you that saying well maybe light travels at different speeds, like maybe light travels really fast near us but it travels even faster further away.  So maybe it didn't take that long to get to us.  That's a reasonable idea.  I mean, who's to say that light always travels the same speed?  And this is where we get back to the laws of physics and the idea of special relativity, which is what Einstein was first famous for.  

The Theory of Special Relativity, he had the idea, he suggested what if the speed of light is the same everywhere?  For all observers, the speed of light is always the same, then what would be the case?   And it made some rather bizarre predictions; really bizarre predictions.  In fact, a special relativity predicts that if you make that assumption that light is the same everywhere, that when you travel really, really fast, time slows down; that time is kind of relative.  Which is really a crazy idea.  

So scientists took extremely accurate clocks, the world's most accurate clocks, these atomic clocks that are accurate to millions of a second.  They'd have two identical ones, put one on the ground, and fly one around on an airplane going pretty fast.  Then they put them next to each other and they'd see that one of the clocks, the one on the airplane, was actually now behind.  It had fallen behind the one that was on the ground.  So time went more slowly for the one that's flying.  

So here's the crazy thing is through special relativity, and this key assumption that light is the same everywhere, well the speed of light is the same, that it’s the ultimate speed limit and that light always travels at speed, Einstein made some really bizarre predictions.  Those predictions turned out to be true, and that's one of the hallmarks of a really strong theory in science.  It’s when you make a prediction, and if it's a crazy prediction, you made the prediction and it turns out to be true, you must be onto something.  And so time and again, in all the experiments that have been devised ever since that time, special relativity has held up and this idea that light is the same speed has held up, which is really truly astounding.  

Okay, so it's an idea, the idea that maybe light travels at different speeds, but everything we know about physics today tells us that in deep space, light seems to be traveling at the same speed as it does here in the lab on Earth. 

Okay.  Now, here's where we get kind of crazy, is Alpha Centauri is the closest star, but lots of the stars in the sky are further away from us.  So pick just a random star.  Here in Orion, one of the stars in the Belt of Orion is called Mintaka, and it happens to be 920 light years away.  Now think about what that means.  That means that the light left that star 920 years ago, that was in the year, what like 1092 was when this says it was, but like in the in the year 1000 AD, that's crazy.  And we see stars that are 2000 light years away.  We can see stars that the light we're receiving left it 2000 years ago.  If that star disappeared, we wouldn't know about it for thousands of years.  That's amazing.  

But it gets even more amazing than… oh, wait… one thing I like to think about is when you lay out and look at the stars, I like to imagine little dates next to each of the stars.  You know, one of them maybe says 1982.  Another one says 1956.  And other one says 1772.  Another one says the year 581.  And each of those numbers is telling you what year you're seeing that star.  So literally when we look at the sky with just our eyes, we're looking back in time.  We're looking at how those stars looked at those different times.  So, we're not just looking back in time, but we're looking back in time, different amounts back in time.  You know, in one part of the sky, we're looking only 10 years back in time.  Another part we're looking at 1000 years back in time.  This is one of the most profound crazy things about astronomy.  The further we look, the farther back in time we're seeing.  That's amazing.  So let's take this to some extremes.  

The closest galaxy to us, we were just looking at this, the closest galaxy besides, you know we're in the Milky Way, the closest other galaxy is the Andromeda Galaxy.  And remember, we have these really reliable ways of measuring distance like Cepheid variablestars and supernovas.  When we use these, we again and again measure a very accurate distance, about 2.2 million light years, in this case is about what 700,000 parsecs, about 2.2 million light years.  What that means is that we're seeing this galaxy as it was 2 million years ago.  In order for that light to get to us from the galaxy, it needed a lot of time to get to us.  

So we saw, when we talked about white dwarfs, we saw in order for these ashes of these stars to cool off, they needed a lot of time.  Well, in order for the light to get to us from these galaxies, this is the closest galaxy, the further away galaxies are even more millions, and hundreds of millions of light years away.  And so how did this light get to us?  We needed a lot of time.  We needed a lot of time.  And so that's one of those challenges we face.  That's why we say that the universe has the appearance of age.  It looks like it's old.  

Now, could God have created the light in route to us?  Absolutely.  God could do whatever He wants.  So maybe God made the Andromeda Galaxy, and He created the light in route in passage to us, it's already in space on its way to see us.  He could have done that.  But you know, sometimes I wonder.  You know, here's another question.  Could God have created us all 10 minutes ago?  And given us memories of the lives we've lived?  And given us, you know, created photo albums with picture of us of us when we were children?  He absolutely could have done that, too.  He could have created us 10 minutes ago, with this whole world having the appearance of age, but I don't think He did that 10 minutes ago.  And in some ways, I'm tempted to think, you know, maybe, maybe the universe is old, and maybe God's view of time is just so different than our view of time that we have no way of really wrapping our heads around how that all works.  Anyway, it's cool stuff though, thinking about… just looking back in time, we, in a sense, what we can do is we can see back closer to the beginning of the universe.  

And that brings me to the last picture in our gallery, which I believe I've showed you before.  It's the Hubble Ultra Deep Field.  This is the most distant photo ever taken.  It captures galaxies that are the farthest galaxies we can possibly see.  And the furthest galaxies we can see are like 10, 11, 12 billion light years away.  We can't see galaxies that are any further than that.  And this paints a picture, a story that's important because it says there's a limit to how far back in time we can see.  Now think about that with me for a second.  

If the universe has always existed, there would be no limit, we should be able to see back in time, forever, because back in time, there's always a day before the day before the day before.  If the universe has always existed, then we should be able to see back to eternity.  But that's not what we see.  Instead, we see that there's a limit; a beginning.  We see that, we can't see any galaxies before those first galaxies, because that's when those galaxies began.  

And so here's the amazing thing, we can make observations that undeniably show that the universe had a beginning.  That is very significant.  Astronomers prior to these observations, astronomers believed that the universe had always existed, particularly those astronomers who weren't religious.  They saw no reason to think that the universe even had a beginning.  They thought, it's eternal.  It's always been here.  But these kinds of observations and other observations that we'll see, show and really conclusively prove that our universe began.  

And how does our scripture start? It says, “In the beginning, God created”.  It says that the universe had a beginning.  And that is what the universe is telling us as well.  That it had a beginning.  And that's a powerful thing about this idea of look-back time.  Looking back in time, we can see, literally, right up to the edge of the beginning of the universe, and we can start to ask questions about how our universe began and what were those conditions like when the universe was just starting to form?

Okay cool.  Very exciting stuff.  We'll see you next time.