The Universe...

[iranu]

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?

Very.

I didn't know that light could travel faster in a medium (Go towards the light Carol Ann). I recall reading about experiments that slowed light down to "walking pace" in some materials at low temperatures using Bose-Einstein condensates, but I had no idea that light can actually travel faster than the famous "c".

New factoid learnt. Pity the poor bugger that has to listen to it in the pub mind.

I'm always fascinated by this sort of thing. We are such piddly, tiny, insignificant things yet, we as a species, actually have some idea about it all. Thanks to Horizon, Equinox and the Discovery Channel etc, the ordinary bod in the street can gain an understanding too. It's both wonderful and confusing.

[iranu]

Very! It was mentioned at the end of a recent Horizon that since the universe is apparently a 4 dimentional donut, that the end would not be a slow heat death, but the entire universe would be swallowed whole by a strange yellow humanoid creature, mmmmn donuts

Corrected

[Rave]

I can literally get brain-ache from this sort of stuff. I think about all the implications and bam- actual physical pain in the brain.

I certainly think that human interstellar travel is completely impossible. There are more stars out there than there are grains of sand on all the beaches on earth (erm...I think) and yet all we can hope to do is look at them.

In answer to the OPs question, I had exactly the same conceptual gap and couldn't get my head round it. My mum's a physics teacher, and while we were visiting CERN with some of her sixth form students I asked the same question. Her answer was the same as Shaithis', that the universe is not a simple expanding sphere and is in fact shaped so that light can travel round in a long curve and hit us billions of years after it was emitted. She managed to call me a simpleton for thinking that the universe was any kind of shape we could conceive too.

I find that a good way to unwind from my daily stresses is to stare up at the stars and reflect upon the journey across space and time that those light photons have undertaken- in some cases, hundreds or thousands of years, and in all cases, incomprehensible distances- only to end it right there, right at that moment, on my cornea.

[format]

I find that a good way to unwind from my daily stresses is to stare up at the stars and reflect upon the journey across space and time that those light photons have undertaken- in some cases, hundreds or thousands of years, and in all cases, incomprehensible distances- only to end it right there, right at that moment, on my cornea.

That's actually pretty poetic Rave

[Whiternoise]

Very.

I didn't know that light could travel faster in a medium (Go towards the light Carol Ann). I recall reading about experiments that slowed light down to "walking pace" in some materials at low temperatures using Bose-Einstein condensates, but I had no idea that light can actually travel faster than the famous "c".

New factoid learnt. Pity the poor bugger that has to listen to it in the pub mind.

Be very careful what you're saying there.

c is a specific constant, not just "the speed of light". It is the "speed of light in a vacuum" and is 299 792 458 ms-1.

Information cannot travel faster than light's vacuum speed. Light will travel through electromagnetically transparent material (not just visible light, remember that x-rays, gamma, IR, micro and radio are all types of light) at a speed of v = c / n where n is the refractive index of the material. So glass has n = 1.5 and slows light down a fair bit. We typically say that air is 1, although that's not entirely true, it's slightly greater than 1.

The key is "what" is travelling faster than light. Physical objects, i.e. photons, atoms, etc cannot exceed c. What physicists and mathematicians define as "information" cannot be sent faster than c.

Now, what you can do is make a material with a very high value of n - n = 2 halves the speed of light for that material. There have indeed been experiments where light has been slowed down to something ridiculous like 30mph. And of course, particles may indeed travel faster than that - hence Cerenkov Radiation in nuclear power stations.

Whilst you can travel faster than light in some material X, you can never travel faster than light in a vacuum. That said, you can produce an object with a refractive index that is less than one - see metamaterials and superlenses on wikipedia - however, the equation differs and you get that v is v_g = group velocity travelling faster than light which still satisfies the second postulate of special relativity. The group velocity is the speed at which the amplitude of the wave changes, sometimes called an envelope.

Recommended reading: http://math.ucr.edu/home/baez/physic...Light/FTL.html and http://math.ucr.edu/home/baez/physic.../scissors.html


Oh, and Rave? Quite a lot more stars than grains of sand on a beach . Assume a beach is 30 square km, depth of 100m being pretty generous. So 3000000m^3 is the volume of our beach, each grain of sand is perhaps a cube of side length 0.5mm, volume is therefore 1.25*10^-10 m^3 per grain. That gives say 2.5*10^16 grains of sand on a beach. That is a hell of a lot of grains of sand. However.. there are an estimated 10^21 stars in the universe. If we divide it out then you'd have to take 40,000 of our beaches to get close.

As a rough calculation =P


This is why i love physics, you start of with a basic question like "what shape is the universe" and all sorts of other things pop up, from light to gravity, to superlenses, etc

[Rave]

That's actually pretty poetic Rave

Thanks mate. It would be easy to gain a cursory understanding of the state of the universe and then become utterly discouraged by the apparent futility of our lives.

Personally I see it as an encouragement to live for the day, and to screw every last ounce of enjoyment out of this brief time I, and the human race as a whole, have been given.

Think on for a minute, and it's hard to enjoy my very comfortable lifestyle completely when there are people starving who've never had anything like the advantages I've enjoyed just by being born British.

But the human brain seems to be designed for denial. It seems to be a reproductive imperative. And while it works....

In case anyone has had their horizons expanded by this thread- you might want to try taking a psychoactive drug sometime. What you think might be going on in your head is just the start of it. Try thinking about the universe after a joint or two....admittedly there's not much point in it, unless you're going to spend the rest of your life stoned. But to me, the revelation of what's actually going on in my brain all the time without me knowing about it was at least as much of a revelation as what's going on outside the solar system.

[Fraz]

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.

Uh... fraid it doesn't work like that.

If two things are moving away from each other, both going at the speed of light, then they are still only moving away from each other at the speed of light. That's why relativity is so confusing - it doesn't mesh well with our daily experiences. If two cars move away from each other, each moving at 70mph, then the relative speed difference from one car to another is 140mph. Unfortunately as you approach the speed of light, it no-longer works like that.

[Fraz]

Very.

I didn't know that light could travel faster in a medium (Go towards the light Carol Ann). I recall reading about experiments that slowed light down to "walking pace" in some materials at low temperatures using Bose-Einstein condensates, but I had no idea that light can actually travel faster than the famous "c".

Sorry... you are getting the wrong end of the stick here a bit. The speed of light in a vacuum, "c", is the ultimate speed limit. The speed that photons of light travel at when they pass through various materials is lower than "c", but it is possible for other particles to go faster than light can through that material. It is always lower than "c" though, so they haven't broken this ultimate speed limit.

[shaithis]

I certainly think that human interstellar travel is completely impossible.

And humans have the tendency of making the impossible, possible

There are plenty of genetic researchers trying to figure out which gene allows suspended animation. Some creatures have already been released into space to see how they fare and they will sequence the genes of the most hardy.

I beleive mosquitoes are near the front of the pack so far......mosquito genes anyone?

[format]

And humans have the tendency of making the impossible, possible

I was saying something like this in another thread. A lot people have this misguided assumption that once science declares something impossible, we should stop examining it and move on. The sheer scale of the universe makes it very difficult to make blanket statements about it, there are plenty of things that 'should not be' but are. All very fascinating.

[iranu]

Yes I see now.