Random Science Questions

[Hashi Lebwohl]

Yes. All electromagnetic waves are carried by photons but we can see only a narrow range of wavelengths. A very narrow range, from 400 nm to 700 nm (roughly). Recall that high-energy gamma radiation can have wavelengths under 100 nm and that radio can have wavelengths of over 1 km--that is a range of 12 orders of magnitude.

[peter]

It's an eye-opener to see it all expressed like that Hashi. I've been so locked into the mind-set of equating photons with 'light' that the fact that they are microwaves, and radiowaves, and x-rays and all and every-other-type-of-waves has totally escaped me.

Ok - but aren't black-holes strong emitters of radiation......and if so how is it that those wavelengths of photons can escape - but light can't; is there a threshold wavelength value above which the em wave has sufficient energy to escape the gravitational pull of the black-hole?

[wayfriend]

There's all kinds of crap accreting around black holes, and this crap is under all kinds of stress, and so will emit radiation, since it is outside of the even horizon.

Then there is also Hawking Radiation, which theoretically can be emitted from the black hole itself, and thus making the general rule about black holes and emitting light false. Quantum mechanics allows pairs of particle/anti-particle to spontaneously appear, and if this occurs near the event horizon, instead of crashing into each other and cancelling out, one of the particles can be sucked into black hole, while the other escapes. The escapees are "black body radiation". I guess you can argue that this radiation, too, is from outside the event horizon.

So areas around black holes do emit radiation, even if the black hole itself does not.

[peter]

We talk about the observable universe as being that up to a distance of about 13.5 billion light years away. Beyond this distance the light has not had time enough to reach us because the universe is not old enough to have enough time for the light to get to us at its given speed of passage. The only way we will ever see beyond this distance is by waiting around for the universe to get older, and as it does so the size of the observable universe will get bigger. Have I got this right? If so are we using time to measure distance here - or distance to measure time? And how is it done?

[wayfriend]

The last time I read on this, I remember that the nature of expansion dispersal is: the farther things are from you, the faster they are moving away from you. So things that are so far away that light hasn't reached us yet are also moving away from us so fast that light will NEVER reach us. The bounds of the observable universe are set. There are parts we will never see by direct, traditional observation.

[Fist and Faith]

Yes. All electromagnetic waves are carried by photons...

I've only recently heard this. I don't understand. I always thought electrons carry electricity. Generators don't produce photons, eh? What gives?

The last time I read on this, I remember that the nature of expansion dispersal is: the farther things are from you, the faster they are moving away from you. So things that are so far away that light hasn't reached us yet are also moving away from us so fast that light will NEVER reach us. The bounds of the observable universe are set. There are parts we will never see by direct, traditional observation.

I thought nothing altered the speed of light? If something is moving away from us at less than the speed of light, light should be able to reach us, even if it takes a very long time. In the meantime, things that are closer to us than that, but still so far that we haven't seen them yet, would become visible. Why doesn't it work that way?

[Hashi Lebwohl]

There is a difference between "electromagnetic radiation", like radio waves or ultraviolet light, and "electricity", which is a flow of electrons through a substance. Electricity requires a substance through which it may flow, following the path of least resistance even if that substance is the atmosphere (as in the case of lightning. EM radiation can be transmitted through the mostly-vacuum of space

The two are somewhat related, of course. Any electric circuit, even a simple one with a battery, some wires, and a light bulb, generates radio waves and EM radiation can sometimes induce an electric current but the two are not the same thing.

On the other...consider a circle with a radius of r. Now, let us suppose that I am at .1r from the center but you are at .99r from the center (I am pretty near the middle while you are almost at the edge). Now suppose that the circle is expanding at some rate k. dr/dt (the rate of change of the radius with respect to time) = k so we can integrate to find out our relative positions after a certain time t has passed.
dr/dt = k --> dr = k dt --> r = kt + c (but we will ignore the new constant c here) . Suppose k = .1. After 2 units of t have passed, my new distance from the center is .2 but yours is now 1.98. Originally, we were .89 apart but now we are 1.78 apart so we are (right now) getting farther apart at a rate of (1.78-.89_/2 = .445 units per unit time. The more time passes the farther apart we will get.

That is a boiled-down version of the model of the expanding universe. Objects which are already billions of light years away are moving away from us so quickly (relatively speaking) that they appear to be moving faster than c even though they really aren't. As a different example, it is like the Doppler effect taken to an extreme--the loud noise moving away from you can move away so quickly that the sound it generates stops being heard altogether.

[wayfriend]

Yes. All electromagnetic waves are carried by photons...

I've only recently heard this. I don't understand. I always thought electrons carry electricity. Generators don't produce photons, eh? What gives?

Electromagnetic waves are not electricity. Electromagnetic waves are a combination of an electric field and a magnetic field that cyclically re-enforce each other and propagate across space.



An electric field is not an electron. An electron which moves is electricity, and electricity gives off an electric field.

The last time I read on this, I remember that the nature of expansion dispersal is: the farther things are from you, the faster they are moving away from you. So things that are so far away that light hasn't reached us yet are also moving away from us so fast that light will NEVER reach us. The bounds of the observable universe are set. There are parts we will never see by direct, traditional observation.

I thought nothing altered the speed of light? If something is moving away from us at less than the speed of light, light should be able to reach us, even if it takes a very long time. In the meantime, things that are closer to us than that, but still so far that we haven't seen them yet, would become visible. Why doesn't it work that way?

But remember, the farther away it is, the faster it's moving away. (In fact, if you remember that the universe began as a pencil point, you can see that things are farther away BECAUSE they are moving away faster.) Some things are so far away that this means that they are moving away from us so fast that their light will never be able to reach us, ever. Or so I understand. This defies the thoery of relativity somewhere, so my understanding is imperfect. But the concept exists - The observable universe is 45.7 billion light-years across. That's the uttermost limit of what we can observe from lightwaves. See How large is the observable universe?

[peter]

Was going to ask why the observable universe is bigger than twice the radius of the distance that light could have travelled in the time allotted since the b-b, but I think WF's link goes into that so I'll check that out first. I'm sure that mathematically it's not the case, but I almost get the feeling that there is a sort of subtle 'circularity' hidden in the middle of all this (but I know that will just be my failure to get it somewhere along the line ).

[Fist and Faith]

I'll try to follow Hashi's equations, and check out wf's link, when I have a little time. But I just don't understand. No matter how fast something is moving away from us, when it emits a photon, that photon is moving toward us at c. Isn't it? It's not like throwing a ball. Let's say you can throw a ball at 10 mph. If you're in the back of a pickup that's moving away from me at 5 mph, and you throw a ball toward me, you subtract your speed, so the ball is really only moving toward me at 5 mph. (And away from you at 15 mph.)

But light doesn't work the same way. If the ball acted the way light does, it would move toward me at 10, and away from you at 10. Isn't that what I've always heard about light?

Maybe I'll understand this after looking more thoroughly at your posts.

[Fist and Faith]

But here's another question. If we can see a galaxy 14 billion light-years away, that means it was 14 billion light-years away when the light left it 14 billion years ago. But if the Big Bang was 14 billion years ago, how can there have already been anything 14 billion light-years away, and how can galaxies have already formed?

[Hashi Lebwohl]

This is where relativity comes in.

If an object really far away (and thus moving away from us at a high velocity) emits a photon then that photon will travel in our direction at c. Once emitted, the photon is in a different frame of reference than the object which emitted it, which is very unlike being an a moving truck and throwing a baseball behind you. The baseball still carries forward momentum from being in your hand (and you being on the truck); if you are moving left at 10 but throw the ball to the right at 20 then its effective speed is indeed 10 to the right (minus air resistance, etc). A photon, though, does not carry momentum from whatever emitted it--as soon as it is emitted it "forgets" its source and acts on its own, moving at c along its original vector.

Given that very distant objects are moving away from us at relative velocities which exceed c, this is why an object only 14 billion years old can be more than 14 billion light-years away. The distant galaxy really isn't 14 billion years old, of course, because its time dilation is different than ours given its velocity. Still, it appears 14 billions light-years away because the space in between us and it has also been "stretched" during that time; see the case of the inflating balloon or increasing circle I cited above. In the time it took me to write this, the space between you and your monitor increased. Infinitesimally, yes, but it still increased. Remember--as velocity increases time slows down since c is a constant. If I recall, the scale factor is

1/sqrt[1 - (v^2/c^2)]

When v = 0 the scale = 1. As v approaches c, though, the scale approaches infinity. This is the amount by which mass increases, time slows down, and space "stretches".

The idea of "the observable universe" just means that "light goes only so far". The objects farther out that what we can see are so far away and moving away so quickly that we simply won't ever be able to see them. As an analogy, consider that you cannot hear objects moving at supersonic speed; if it weren't for sonic booms we would never be able to hear those objects at all--they are moving too quickly.

Space itself is "dark" to us because the background radiation red-shifted to wavelengths we cannot detect with our natural eyes. If the universe were younger then space would appear to glow yellow, or red, or deep red....which would be both cool and weird at the same time. Of course, then as time went on it would red-shift down into the infrared and then space would also be "warm", which would be a different sort of weird.

[Fist and Faith]

BTW, Hashi, I sent you a pm in November. I believe you would like Greg Egan's Diaspora a whole lot. I gave you a couple pages of it.

[wayfriend]

AFAICT, there are several things going on that need to be accounted for.

1, Some things we see (with light) are so far away that the light is very old. But the light was emitted from where they were, not where they are now. As objects get farther away, the light we see is so old as to have been emitted early in the universe's life. However, early in the universe's life, everything was very close together (and moving apart). At some point, the very oldest light would be coming from very close by. But if it's close by, it would not take that long to reach us. It seems like a contradiction. The reality is, it's a very complicated model, more complicated than it first appears.

2, Yes, relativity says that, at relativistic speeds, how fast the object is moving doesn't matter. An object cannot be moving away from us so fast that it's light can't reach us. (However, there is still the phenomenon of red-shift, which makes it more complicated - it's speed is not affected, but it's frequency is!) However, it's still true that the farther objects are away, the faster they are moving away. In fact, some parts of the universe are moving away from us at speeds faster than the speed of light. Honest. These we cannot see. There are even some galaxies that we can see, but which are moving away from us faster than c, but we can see them because they were not moving so fast at an earlier time, and that's the light we are seeing now. At some point, we won't be able to see them any more. I presume this means that the speed of expansion is accelerating. [link] [link] Therefore, there IS a maximum distance, where things beyond that distance are moving away so fast that we cannot see them.

3, So there is a maximum distance at which anything beyond it would have had to emit light before the universe was created, and there is also a maximum distance at which anything beyond it is moving away too fast to see. However, I am not sure which is closer, the speed barrier or the time barrier. It seems to me that, which ever one is closer, the other one would be therefore undetectable. So maybe they are, for reasons only Einstein could imagine, the same thing in the end.

[I'm Murrin]

I believe it's all to do with inflation, and the fact that before a certain point the universe was too opaque for light to really travel anywhere. The earliest light we can see does not come from the time when the entire universe was a singularity, but from the end of that period of opacity, and the beginning of rapid inflation, when the space between objects expanded so fast that the light is only just now reaching us from some of them.

[peter]

Apologies if this has already beencovered, but it seems to me that it's too big a coincidence that the age of the Universe should be the same in years as the radius of the observable universe in light years, but one should be a fixed figure that will never be exceeded, ie the distance away of the edge of the observable universe, while the other eeps on getting larger .

[I'm Murrin]

The observable universe is not the size of the universe; it's the size of the part that light has reached us from. The furthest we can possibly see is defined by how long the light has had to reach us. I don't think there's any suggestion that the size of the observable universe is constant.

[Zarathustra]

Murrin is correct. Inflation was an early period of expansion during which the universe grew faster than the speed of light. This explains a lot. Until I'd learned this, it had always puzzled me why the light from distant objects would take so long to reach us, since the farther back in time you look, the closer all the objects would have been. At some point in time, the universe would have been smaller than a light-second, and all the light in the universe would have traveled to every other point in the universe in one second.* So why would it have taken 14 billion years for the most distant light to have reached us, when 14 billion years ago it was in our back yard?

The key is inflation. Remember, this is not objects violating the speed of light itself. This is SPACE expanding faster than the speed of light. Objects can't travel through space any faster than c, but that places no limit on the expansion of the universe (space) itself.





*[granted, there wouldn't have been stars then, but you can extrapolate this same logic to the time when the first stars were formed, in a much smaller universe. Also, I imagine the Big Bang created lots of photons itself, right?]

[peter]

So the size of the observable universe will get bigger as the universe gets older, but we won't see additional objects now beyond this limit - just the same objects we see now, but further away (because of the inflation of space)?

(Hi Z. ! )

[peter]

Why, when an alien 10 billion years away moves rapidly away from us is his 'now-slice' of the Universe directed toward the past, but if he moves rapidly toward us it is angled toward the future? If in both cases his motion relative to ours causes his time to slow, shouldn't his now slice be in the same direction irrespective of his direction of motion?