That sounds a bit off. Nothing is moving faster than light; you are just interrupting the light source.
Is it just me, or is this hard to get your head around for the average person?
That just sounds like we haven’t got a clue yet what is happening beyond the observable reaches of the universe.
Right. The shadow is not a “thing” as such. The point is you can arrange things to observe something moving faster than light speed (in this case the shadow). If you were on some other part of earth (or on the moon) and had no clue what I was doing you’d be surprised to see the shadow zoom across the surface at superluminal speeds. There is no violation here though because the shadow is not substantive, no information can be transmitted at superluminal speeds using this method.
Yes and no. It takes a moment’s thought but you can see how it is working in your head. No single thing is moving at light speed. It is just every part of the universe expanding. It is expanding relatively slowly on a local scale but add up the expansion over huge distances and it all adds up. Something a billion miles away only recedes one inch per year (again, I am making these numbers up for illustration purposes). Something two billion miles away recedes two inches in that year…the first billion expanded an inch and the second billion expanded an inch. Something three billion miles has three inches added between us. And so on. Keep adding that up across the whole universe and the effect is very distant things seem to be moving at near light speed. If you magicked yourself out there everything would seem normal, local space is only expanding at 1 inch every billion miles per year…same as you see from earth.
The effect is things very far away appear to be receding at near light speed (once they recede at over light speed we will no longer see it).
Well, we don’t. We can’t. It is reasonable to presume things work the same there as they do here but once beyond our observable horizon stuff out there is, for all intents and purposes, gone to us. Out of our universe if you like. It cannot effect us in any way and any calculations we do can ignore it. Note this does not mean there is not anything there as in fact there almost certainly is. Just not anything we need be concerned with any longer.
ivan astikov, you might have an easier time understanding the concept of the universe expanding faster than light speed by reading the wikipedia articles about the observable universe which describes the part of the universe we can see because light has had time to reach us, and the article about expanding universe which describes how the distances between things are getting bigger, and the further away two objects are, the faster the distance between them is increasing.
The things aren’t actually “moving away” from each other in the way you’re thinking. The distance between them is just getting bigger, at a rate faster than light can catch up. If two things are far enough away from each other, it’s impossible for any information (including light) to go from one to the other, because the distance between them is increasing faster than light. This doesn’t violate any rules against faster than light travel, because obviously no information is being transmitted faster than light.
It’s not all that hard, actually – just picture two dots on a rubber sheet, dot A and B, then expand that sheet uniformly (i.e. pull on all sides): they’ll ‘move’ apart. Now, picture another point, C, twice as far away from A on the same line that connects A and B, and continue to expand the sheet – C will move away from a twice as fast as B does, because in the time it takes to increase the distance between A and B, the distance between B and C has increased by the same factor, the expansion being uniform and all. Then, picture a third point, a fourth, fifth, etc., each receding from A twice as fast as the one before, and at some point, any given velocity will be exceeded.
So the whole deal looks kinda like this, if you take snapshots of the expansion at fixed intervals:
[-A–B–C-]
[-A----B----C-]
[-A--------B--------C-]
[-A----------------B----------------C-]
And so on.
On preview I see I’ve been beaten to the explanation, however, this:
isn’t quite correct: We do observe, right now, galaxies that are receding faster than the speed of light from us. This seems somewhat paradoxical at first, however, the answer lies directly with the finiteness of the speed of light: if a photon is emitted in a region of space that is receding from us at a velocity greater than c, its total velocity will be directed away from us. Now, let’s call the boundary beyond which things recede from us faster than c the Hubble sphere, because that’s what it’s called. That boundary increases, because it is inversely proportional to the Hubble constant, which is itself inversely proportional to time, if we forget the question of decelrating/accelerating expansion for a moment. Thus, photons originally outside of the Hubble sphere, i.e. photons emitted by objects receding faster than light, can actually enter the Hubble sphere, which means they then are in a region receding with less than c from us, and, themselves still having velocity c in directed towards us, thus are approaching us.
Also, of course, we can see things receding faster than c from us now, because they weren’t receding at superluminal speed at the time they emitted the light we’re seeing now.
This one always got me a bit. While I understand the concept it seems it would have us seeing something apparently close to us that is, in fact, near the edge of our observable space. E.g. Photon was emitted when close in the early universe but due to spatial expansion only now just arrives to us. It would then seem the object is next door (cosmologically speaking) rather than very distant.
I also have a hard time understanding the many distances way of measuring things that gives rather divergent answers. Unfortunately the link I just provided is too technical for my poor mind but I cannot find the web page I once ran across that gave it a much simpler treatment (albeit still rather technical at least approachable to the layman).
So which is it with the above? When we look at the universe and map it do astronomers correct for various motions and time to place galaxies where they are today as opposed to where we see them? (I.e. Galaxies have moved by the time we see them so do astronomers “back them up” accounting for this to place them where they actually are in relation to each other now?)
I clicked on the link, yet… it’s not travelling faster than light, so I don’t see where the violation of physical laws comes in.
It’s receding from us at faster than light speeds, relative to us.
Nope, nothing is moving faster than light, relative to everything around it… locally.
Think if you were to draw circles on a (theoretical balloon), and it were to double in size every minute. It wouldn’t take very long before that
Nope, it’s pretty easy. You just have to pretend that you know nothing about physics at all and approach it with a clean slate. Then reconcile the differing opinions and it’ll be much, much easier.
We pretty much don’t.
Would it be wrong to think of what the astronomers are looking at as being like a Mandelbrot diagram, and that each time they zoom in it just reveals more details? Why can’t they see that bit further? Is there some sort of field or fuzziness to the ‘picture’ they are getting?
The short answer to that is that there are several different ways of measuring distance-like quantities, that, in our everyday experience, agree, but which don’t all agree when we’re talking about cosmology. For instance, if I’m interested in knowing the distance from my apartment to my office, I can measure the size of my office building, and see how big it looks from my apartment, and calculate its distance that way. Or, given the size of my office building, I can look at my apartment from opposite corners of the building, and see how much the angle I’m looking at change, and calculate it that way. Or, I can measure the length of my pace, and count how many paces it takes me to walk up to the office. If I do this for measuring the distance from my apartment to my office, then all of those methods will give me the same answer, so I don’t have to worry about which one is the distance: They all are. But, if I were to use these three methods to measure the distance to a distant quasar, I would get three different answers, because the quasar is rapidly receding away from me, and the space between is curved.
See post #25. by Chronos.
Yes.
The hubble sphere.
The more we zoom in, the more we see, of course. But that only applies to detail. It’s (theoretically) possible to build a large enough telescope to capture only the few photons hitting said telescope from another planet orbiting another star, but that’s just bigger and better telescopes.
You’re always capturing photons. Basically, on the other side of the Hubble sphere, the objects that are emitting photons (or, rather, were emitting the photons) are too far away to see because the cumulative inflation of the universe will never let them reach us.
While no individual section of the universe is accelerating past the speed of light, the change in the difference in our distances is more than the speed of light, therefore their light will never reach us.
That’s true – direct measurements can only yield information about where the light source was when the light was emitted; however, the redshift carries information about the factor the universe has expanded since then (1+z, if z is the redshift), so deriving the ‘true’ distance is pretty straight-forward using this correlation. Of course, since, as Chronos outlines above, the measures for the apparent distance already disagree in some aspects, the obtained ‘now’ distances will similarly; wiki’s article on the cosmic distance ladder goes a bit into the different methods of distance measurement.
The Hubble sphere is not the boundary of the observable universe, it’s merely the boundary beyond which things recede faster than light speed from us; it’s approximately 13.8 billion light years away, and the edge of the observable universe is around something like 46.5 billion light years.
So is he there referring to other stuff, created by other big bangs? Because otherwise I don’t get it.
Originally Posted by Chronos
If the universe is infinite, it most likely has infinite mass
Statistically rare does not equal special. Special is a term that refers to the human emotion of finding something personally meaningful. Specialness does not exist as an actual quality of external things, it exists only as an experience inside the human brain.
Err…huh? Looks like it can be applied here just fine.
I’m pretty sure from the context that the OP meant “special” as in a “meaningful or significant way” and not in a dry “statistically rare” sense. But even in the latter case, it’s still a human determination and not something inherent in the observed.
as promised upthread, here’s a link to a cosmologist’s take on the question in the OP: www.startswithabang.com