So there’s new news concerning the Hubble; according to this website, “Astronomers at the Space Telescope Science Institute today unveiled the deepest portrait of the visible universe ever achieved by humankind.”
So my roomate found out about this and asked me how a telescope can see back in time. And I tried to explain, because I used to be able to understand this stuff… but I forgot the physics of it! And for the life of me I can’t figure it out.
So, dopers… how can a really far-range telescope like the hubble actually see past events? It has something to do with the speed of light and the movement of the big bang… Any explanations or ideas?
Basically, it’s best to think of a light-year as both a measure of distance and of time. Thus, if you’re looking at a star 4 light-years away, you’re seeing an object that is a billion squillion miles away in distance and 4 years away in time. It takes the light 4 years to get from that star to your eyeball, so you’re looking at 4-year old light.
Telescopes like the human eye see light. The light as you probably know has a speed we like to call this speed c (as the actual number is quite large and difficult to remember). Whenever you or a telescope look at a distant object, you are seeing the light that came from than object some time ago. If the object is a close one the time that the light took to get to you will be short, often too short to measure easily. If you were to look at the sun (not recomended, because it is so bright you can dammage your eyes) you are seeing the light that left the sun a few minutes ago, so you are in a sense seeing backwards in time since the light currently leaving the sun hasn’t got to you yet. Onto much larger distances, when you look at starts in the sky you are seeing light that left those stars sometimes years ago sometimes thousands of years ago, all depending on how far away the star is. With a telescope, you can see stars that are further away than can be seen by the naked eye, you can also see other galazies with a telescope. The better the telescope, the further distance you can see, and the further distance you see, the older the light is that you are seeing.
Hubble being about the best telescope there is, is capable of seeing things so far away, that the light that is reaching Hubble now left the object being looked at many billions of years ago. Hense Hubble is seeing the object as it appeared a long time ago, not as it currently might appear.
Here is a simplified answer, that I believe is basicaly correct, though does not take into account general relativity.
Telescopes like the human eye see light. The light as you probably know has a speed we like to call this speed c (as the actual number is quite large and difficult to remember). Whenever you or a telescope look at a distant object, you are seeing the light that came from than object some time ago. If the object is a close one the time that the light took to get to you will be short, often too short to measure easily. If you were to look at the sun (not recomended, because it is so bright you can dammage your eyes) you are seeing the light that left the sun a few minutes ago, so you are in a sense seeing backwards in time since the light currently leaving the sun hasn’t got to you yet. Onto much larger distances, when you look at starts in the sky you are seeing light that left those stars sometimes years ago sometimes thousands of years ago, all depending on how far away the star is. With a telescope, you can see stars that are further away than can be seen by the naked eye, you can also see other galazies with a telescope. The better the telescope, the further distance you can see, and the further distance you see, the older the light is that you are seeing.
Hubble being about the best telescope there is, is capable of seeing things so far away, that the light that is reaching Hubble now left the object being looked at many billions of years ago. Hense Hubble is seeing the object as it appeared a long time ago, not as it currently might appear.
A simple analogy would be something like this (I apologize if this is too patronizing): Your aunt in Japan writes you a letter and drops it in the mail. The letter takes, say, a month to arrive at your house. You read it. You are not reading what your aunt is doing and thinking that day, rather you are reading what her thoughts and actions were 1 month ago. Things may have changed in the ensuing month, she may have moved, or even died. you won’t know any of that for another month though, until you get her next letter.
The Hubble is the same thing, but on a much grander scale. That object is so distant that the light from it has taken it 12 billion years to get here (at 186,000 miles per second). The Hubble sees it as it was when that light left it 12 billion years ago. If that star instantly changed color or even blew up right now, we wouldn’t know about it until we got the “letter” mailed today (the light) in 12 billion years.
The foregoing are correct, but here’s a short and simple answer: When you look at a star (or other object) fifty light-years away, you’re seeing what it looked like fifty years ago in absolute time. If you could instantly teleport to that star (or other object), there’s no guarantee that it would look the same now as it does from Earth with the fifty-year delay. Extrapolate to distant galaxies millions of light-years away, and you get the picture.
Whenever you look at anything you are seeing it as it was a time ago that is equal to the distance to anything/speed of light in the medium between you and anything.
jeeeez! So simple… I should have figured that one out! :rolleyes: I think i was just complicating the issue when i thought about it. I tend to do that… interferes with my abilty to think through relatively simple ideas.
So thanks a lot, everyone, for the reminder… much appreciated!
And yet, strangely enough, when I look at your post, even though the screen is only a foot away from my eyes, I’m seeing it as you typed it over two and a half hours ago. How weird is that?
Not only “can” they look back in time but they MUST look back in time.
The problem with deep space telescopes, is that we have no idea what the universe looks like NOW.
We can only see what it looked like billions of years ago, and the very distant stars that appear in deep space telescopes today, burned out or exploded long before you were even born.
I think it’s important to point out that there’s nothing special about the Hubble that makes this possible, other than the fact that it’s a very good telescope and in an environment that’s relatively free of light diffraction, as opposed to that present within our atmosphere. This ability to “look back in time” is possible with any telescope, even with something as simple as the human eye. The physics are the same, regardless.
With sufficient magnification, were you to look back at our planet from a platform 65 million light years away you’d see dinosaurs roam the Earth. Do the same thing from a platform 41 light years away and in November you could tell us who killed Kennedy.
But you could never actually do this. Even if you assume a semi-magical transporter that moves at c*, the light reflecting off Kennedy’s exploding neck will still have a 41-odd year head start on you. It will forever be 41 light-years ahead of you, throughout all time (assuming a flat, Cartesian universe, of course).
(If you somehow assume FTL (Faster Than Light) travel, you don’t need to go look at dispersing photons to get an answer: Going faster than light is equivalent to going back in time.)
*c is the speed of light in a vacuum. The bit about it being in a vacuum is important, because otherwise the medium would slow the light down. Light travelling through air is going a substantial fraction of c, but once you get to light going through dense water or thick plexiglass the slowdowns become more apparent (not necessarily to the unaided eye (a fraction of c is still pretty damned fast), but detectable). Another name for c is Einstein’s Constant.
