How is it possible that astronomers can see light emitted from objects that supposedly formed “shortly” after the big bang? Wouldn’t that light be long gone from our perspective?
The Sun is the star closest to us. When we see the Sun, we are looking back in time about eight minutes. When we look at Proxima Centauri we are looking back in time about 4.2 years. Astronomers see back in time as far as they do by looking farther away in distance.
The farther you look out, the more you’re “looking into the past”. Just as Johnny said, when you see something X light years away, you know it took X years for the light from that something to reach your eyes.
If you see a galaxy that is 13 billion light years away, it took 13 billion years for its light to reach you (in other words, the light from that galaxy left 13 billion years ago). Since 13 billion years ago is relatively “shortly” after the bang, you’re seeing that galaxy as it was “shortly” after the big bang.
Basically the further away an object is the longer it takes light to reach us. So for something 100 million light years away, when we look at it we’re seeing photons that are 100 million years old (discount the fact that photons don’t experience time).
Gone where?
Gone here - that’s the point.
Imagine light is a bullet. During the big bang, objects were “shooting” light toward us. The objects may be gone but the light is just now getting here.
(just a metaphor, dont pick it apart.)
Photons last forever… until they hit something. So any photons that were created at any time in the history of the universe and haven’t run into another particle are still flying around.
If you then argue, “but that’s so long ago, they must have hit something by now” I refer you to the late eminent authority Douglas Adams who wrote, correctly, “Space is big. You just won’t believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”
I believe the op understands this. I think his question is about the fact that for us to see something, light has to reach us. If we see light from near the time of the big bang, that means somehow we got here (we were part of the big bang) before the light did. It would seem all the light emitted at the time of the big bang should be further away from where it happened than we are. I believe the answer involves the expansion of the universe, but I will let someone more informed answer.
You contradict yourself in these two sentences. It’s true that every part of the visible universe was part of the big bang. Therefore it’s also not true that any part is farther away from where it happened than any other. It happened everywhere all at once.
Therefore, any photons that were emitted as part of the big bang, close to 13.7 billion years ago, will reach us after 13.7 billion years. Yes, space has expanded during that time. That part of it we are seeing has expanded to 13.7 billion light years. But that exactly equals the distance a photon would travel in 13.7 billion years, since it travels at the speed of light. It all comes together.
Off-topic: why exactly do I sometimes see this “"” stuff? Is it because the original poster was using (I think) HTML codes, which the Dope board does not parse?
Ah crap. For me, it’s because I often use a forum browsing app on my iphone instead of a normal web browser. It works in stripped down plain text and is extremely fast but has a bug that causes it to convert quotation marks into what you see there. If you notice, I was careful to spell “dont” without the apostrophe but I forgot to leave the quotes off the other word. Boards, blogs and other web software often convert special symbols for various reasons, but that one was totally the fault of the app I was using.
It occurs to me that a potential science fiction plot might involve somehow transporting yourself to another part of the galaxy and looking back at Earth in order to answer some questions about the past.
Something I’ve always wondered about, related to the topic of the thread. If I’m seeing things 13.7 billion light years away, they’re 13.7 years old, meaning they’re almost as old as the big bang itself. Does this mean that all these objects were much closer together when they emitted the light than they appear now? Is this in turn reflected in their observed behavior? For example, do they appear to be gravitationally attracted to each other differently than they “should” be given their apparent distance from each other? (I.e., because they’re actual distance from each other is much smaller than their apparent distance from each other?)
The expansion of the universe really is the key to understanding this. If you think of “Newtonian” explosions, then every object in the explosion has to be moving slower than light and the light from the original explosion will always be heading away from you - some in the opposite direction and some in front moving faster than you.
But the Big Bang was a “non-Newtonian” explosion. Space itself was (and is) expanding, and it’s not just inertia that moves things apart from each other. Because space expanded so quickly, we can still see things that we shouldn’t see from a purely Newtonian view. The universe started off with an extremely high rate of expansion that then slowed down, but apparently the expansion rate is now increasing again. (And, of course, expansion has always been proportional to distance).
So, we have this sort of scenario, where G is a faraway Galaxy, U is Us P is a photon, with dots to represent empty units of distance:
GpU (instants after Big Bang
G...p..U (some time passes; things expand very, very quickly)
G.....p.U (more time passes; things expand somewhat slowly)
G.......pU (13 billion years pass; things expanding somewhat quickly)
G.........U (13.7 billion years pass; light finally arrives)
Maybe that’s a lousy picture, but I’m working in text, here! The point is that the expansion between G and U has turned what would once have been a 1 light year journey into a 13 billion light year journey.
Most of our current knowledge of cosmology comes from observing the patterns in the cosmic microwave background, and those patterns originated from the ways the matter in those parts of the Universe were interacting with each other. So to answer your question, yes, those interactions are relevant.
I think Dracoi’s explanation captures what seems to be the correct answer concisely and logically. But I’m baffled why the answer - even the question itself - has been so hard to come by. My college-level astronomy textbook makes no mention of what I beleive is a fundamental concept.
So given this answer, there is an absolute limit to how far back in time we can see. We’ll never be able to see the big bang. I’m sure astronomers have calculated the earliest possible object/time that can be theorectically observed from Earth.
I’ve seen several examples of this in SF - one story I read recently involved the construction of an enormously powerful telescope in space at just the right point to observe a planet circling the star that blew up to form the Crab Nebula (also using gravitational lensing from a black hole in between and resolving enough detail to observe the lives of the inhabitants.
(FTL travel was possible in this story, a gift from an alien race that had also told us about the existence of the planet to be observed - and I expect the resolving power of the telescope was probably a bit fanciful too).
In the story, the project hoped to poignantly preserve a record of the art and culture of the civilisation that existed before the star blew it all to vapour, but as it turned out… The whole planet was oblivious to its fate and was torn, engaged in a bloody and cruel war over some pointless thing or other
The earliest we can see is the cosmic microwave background radiation. When we look at it we’re actually looking at the “hot fog” that filled the universe for the first several hundred thousand years of its existence. During that era the universe was opaque, so we can’t see any further back.
This scenario is impossible without actual time travel. (I know that relativity says that FTL travel is equivalent to time travel, but sf writers almost always separate the two.)
Think of looking at the sun. We are always looking at the sun as it was 8 minutes ago. If we move a telescope closer, we could see the sun as it was 5 minutes earlier or 2 minutes earlier. But we can never see the sun as it was last year. You’d have to move in time as well as in space to accomplish that.
Moving a camera in our time closer to a supernova that took place 1000 years ago wouldn’t capture any images from before the supernova, no matter how fast you got the camera closer.
Though it’s a cool idea for a story, please never, ever believe the “science” in a science fiction story. It’s either dead wrong or manipulated to make the story work.
Additionally, we need to keep in mind the possible resolution from far distant objects even when they can be seen. We might be able to see a star, but even with infinite resolution, we could never see people on a planet circling htat star; there just aren’t enough photons coming to us to build a picture at that level of detail.
…unless, of course, you have CSI: Miami enhancement software available to you.