The Universe...

[Fraz]

Say what?

The speed of light in a vacuum is a limit on how fast information or mass can move, nothing else. If mass or information could move faster than the speed of light then it could produce what is know as a "causality violation", where effect could be observed before cause.

During the very early stages of the universe's life, it is hypothesised that it rapidly "inflated" from the size of a proton to about the size of a grapefruit in about 10e-33 seconds, which is about 100000000000000000000000 times faster than it would do if it was expanding at the speed of light.

This inflationary phase doesn't violate causality, as supposedly is just spacetime itself that is expanding faster than the speed of light. The information and mass within the universe is still being mundane enough to move at or below the speed of light.

I'm just regurgitating this crap from having done an astrophysics degree years back. My actual intuitive grasp on the thinking behind it all is pretty minimal to be honest...

[Fraz]

Interestingly, it is very easy to break the speed of light when you are not in a vacuum. If you've ever seen videos of the inside of a nuclear power station, you'll probably have seen big pools of water where they store the uranium rods:



The blue glow is Cherenkov Radiation, which is the speed of light equivalent of breaking the sound barrier. Particles given off by the uranium move through the water faster than the speed of light through water, and the blue glow is the "sonic boom" caused by breaking the light barrier.

Cool, huh?

[Lee H]

Plat - could you go back to drinking on saturdays

Yeah and then we'll have posts on Monday about the big Prang...

Stay sober Plat or saying that watch some late night OU on BBC2

[Virtual Monkey]

Cool, huh?

Very!

It was mentioned at the end of a recent Horizon that since the universe is apparently a 4 dimentional donut (type of icing not specified - yet) light travels through one 'edge' of the universe and comes through the other - so it might theoretically be possible to see light from the Earth from a long long time ago.

Although I suppose not if the actual timespace of the universe (not all the 'matter and information' as Fraz puts it) is constantly expanding at the speed of light too?

[TAKTAK]

Right weird one this but can someone possably explain to me.
Scientists tell us that they can use telliscopes to look back at the early universe, but if the universe started off in the same place and expanded, and nothing can move faster than light then surly the light from the early universe should have overtaken us now and what we see now should be much older no?

Yes my head hurts thinking about it as well.

The universe started out in a very, very, very, very, very, very, very, very, very dense and hot state, this expanded (and it still expanding) into what the universe is today, the quickest way to model the expansion (ish) is with an elastic band...

Grab an elastic band, cut it to make a strip of elastic, draw equally spaced dots on the elastic surface, and then stretch it...

note how the distance between the first 2 dots increases by X amount, and the distance between the first and the third dot is 2X, all the way to the Nth dot which is NX, so dot 6 is moving away from dot 1 alot faster than dot 2 is moving away from dot one (same time to stretch, but bigger distance [Speed=Distance/Time])

So... the universe is expanding, and (using us as an example) the further away from us you are, the faster it is moving away (relatively)...

[Whiternoise]

Right weird one this but can someone possably explain to me.
Scientists tell us that they can use telliscopes to look back at the early universe, but if the universe started off in the same place and expanded, and nothing can move faster than light then surly the light from the early universe should have overtaken us now and what we see now should be much older no?

Yes my head hurts thinking about it as well.

Well here's the thing.. space can travel faster than the speed of light - and has been doing so.

Think of it like a balloon. You draw some dots on the surface representing points in the universe. Now, you living on that balloon cannot travel faster than the speed of light, that is your "speed limit". However, if you inflate the balloon, you are still limited by 3x10^8 ms-1, but the balloon can inflate faster than you can move.

As a sidenote, this sort of thing is actually observed in waves too.

"The group velocity vg is often thought of as the velocity at which energy or information is conveyed along the wave. In most cases this is true, and the group velocity can be thought of as the signal velocity of the waveform. In some unusual circumstances, called cases of anomalous dispersion, the rate of change of the index of refraction with respect to the wavelength changes sign, in which case it is possible for the group velocity to exceed the speed of light (vg > c). Anomalous dispersion occurs, for instance, where the wavelength of the light is close to an absorption resonance of the medium. When the dispersion is anomalous, however, group velocity is no longer an indicator of signal velocity. Instead, a signal travels at the speed of the wavefront, which is c irrespective of the index of refraction.[3] Recently, it has become possible to create gases in which the group velocity is not only larger than the speed of light, but even negative."

In laypeople's speak, the group velocity of some wave (picture a sine wave in your head) is a measure of how fast information is carried (i.e. a signal velocity). Clearly the speed limit of information is the speed of light, however, it's possible to produce group velocities that exceed the vacuum speed of light - though i'm not even going to pretend to understand how or why. Just to be clear, the group velocity my exceed the speed of light, but the information transfer speed is still limited.

[Tzarbo]

Think of it like a balloon.

Yeah... think of it like a balloon... that's also shaped like a donut... and a saddle. It may also be or not be infinite. Or shaped like anything else. Or maybe universe shaped.

It may be the only universe, it may be part of the multiverse, it may be an atom in another universe.

Who knows.

[Whiternoise]

The balloon comment was meant as an illustration of how the speed of light doesn't affect space itself - i didn't mean to say that the universe is balloon shaped (far from it!).

[mediaboy]

Are we now getting in theoretical astrophysics including alternate universes and the multiverse and a million other things here and there?

Wow...

Educational thread. I'm interested.

My teacher failed to explain this epicly. Presumably her degree in chemistry don't cover it.

[Tzarbo]

The balloon comment was meant as an illustration of how the speed of light doesn't affect space itself - i didn't mean to say that the universe is balloon shaped (far from it!).

So, are you saying that the universe is expanding faster than the speed of light?

[mediaboy]

Tzarbo - At least twice the speed of light.

Speed of light away from the centre in two directions is relatively twice the speed of light away from us.

I got that bit. After that I got lost. Someone draw us a life-size map of the universe?

[Tzarbo]

Tzarbo - At least twice the speed of light.

Speed of light away from the centre in two directions is relatively twice the speed of light away from us.

Ah, 2 directions. That would be... left and right, perhaps? Up and down? North and South?

Which 2 directions is the universe expanding in?

[Whiternoise]

So, are you saying that the universe is expanding faster than the speed of light?

Current astrophysical theory suggests that it did after the big bang and maybe now (the expansion is accelerating y'know).

[mediaboy]

2 directions is 2 dimensional.

It's actually extending in as many as there are, theoretically. Of course, theoretically it's also not. That's why Physics is so much fun

[Tzarbo]

And why your answer made no sense.

[format]

Not so much with astrophysical side, but I always find this fascinating -
http://i159.photobucket.com/albums/t...125/spaceb.jpg