h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Well, if you don't define what it is, you can't define if it is travelling faster than the speed of light or not, now can you.

The map of the background radiation is just that: a two dimensional representation of the three-dimensional skies around us, so there are no borders.

most cosmologists now beleive that space is finite but unbounded reasons;
The microwave background radiation: this appears from every part of the sky this indicates that there is no edge of the universe(there is no break in it) so the universe is unbounded. The is also homgenity of the universe, it appears the same in all directions a bounded universe would be hterogenous at the edges.

Basic phyics(and some complex quantum physics) tells us that if you have aa closed system(the universe) expanding at a certain rate from the start point of a singularity then it cannot be infinite.

conclusion the universe is finite but unbounded, there is enough matter to create a closed universe due to the fact that scientists believe there is vast amounts of dark matter floating round in the universe, hence the reasons for all projects trying to detect neutrinos.

From ANY given point the universe may be seen to be expanding in every direction at speeds that come close to, but don't exceed the speed of light. Therefore parts of the universe are moving away from each other at faster than the speed of light.

Noggin

Noggin has got it right.

The Universe is expanding but the crucial factor is the point of reference (this is also crucial in Relativity and Quantum physics).

If an observer sets out to measure the rate of expansion of the Universe, s/he (not wanting to be sexist here! ) starts in her/his back yard where the rate of expansion relative to the observer is, to all intents and purposes, nil. Even within our Milky Way galaxy expansion is cancelled out by gravity for the most part, and the same can be said for our local group of galaxies (M31 in Andromeda being the most notable member).

Beyond this point the rate of expansion becomes measurable. The observer would see a red shifting of electromagnetic waves (see A636897, A827372 and A623279 for details). This applies regardless of the direction the observer is looking in at the time.

At some point the recession velocity of an object caused by the expansion of space reaches the speed of light. The object is moving away as fast as the light emitted from it (or reflected by it) is travelling in the direction of the observer. The two cancel each other out and the object disappears from view with reference to THAT observer only.

Another observer half way between the object and the first observer would be able to see the first observer in one direction and the object in the opposite direction, equally red shifted with reference to their rest position.

As has been stated above the Universe is thought to be finite but unbounded. It should be noted that this is one of the better theories explaining all the available data and observations so far obtained.

The reasons for thinking that it is finite but unbounded are to do with its assumed age – between 12 and 20 billion years old, the 12*10^9 being measured and the 20*10^9 being another interpretation of the measures which allows for stars in our Milky Way at 15 billion years old being older than the Universe itself (patently ridiculous – but equally open to challenge in the future). From a singularity starting point (the Big Bang) to the present would not allow enough time for the Universe to become truly infinite in extent.

The unbounded bit is a consequence of the curvature of space inherent in Einstein’s theories of General and Special Relativity. The consequence of this is that light or any object will eventually return to its starting point. This means that in effect there is no boundary to space.

The rate of expansion of the universe is governed entirely by gravity. If there is insufficient mass (which exerts a gravitational pull) in the Universe (an omega of less than one) it will continue to expand forever and achieve a uniform, entropic heat death. If the mass is just right (an omega of one) it will slow down as time goes on and eventually halt. If omega; is greater than one it will stop relatively soon (we are not talking next week BTW) and re-collapse.

Omega is the ratio of mass to the size of the Universe. The most reliable recent measures of omega peg it at around 0.3 (taking only visible matter into account), way too small to ever halt the expansion. Dark matter is still a contentious theory although there is strong evidence to suggest that it is present. All observable clusters of galaxies would not behave gravitationally the way the do if they were only made up of the visible matter within them. Even taking into account the dark matter omega is no higher that 0.7 at best, still insufficient to halt the expansion.

Nothing in the Universe can travel faster than the speed of light and only massless particles can travel at the speed of light. Any object travelling at relativistic speeds requires exponentially higher amounts of energy to achieve smaller and smaller increases in speed. An infinite amount of energy would be required to achieve the speed of light for an object with mass and its mass would become infinite (= the mass of the Universe). This is a consequence of E=mc^2.

Hope all that was clear. If not go talk to Stephen Hawkings! or here and ask the others. A413876

turvy

PS this might end up as a guide entry!

Noggin has got it wrong.

If I see Joe heading away from me to the North at three quarters the speed of light, and Pat heading south from me at three quarters the speed of light, Joe will see Pat travelling at less than the speed of light. That's what EInstein's theory says.

So the edge of the visible universe is the edge of the actual universe, Gnomon?

Alternatively the edge of Joe's visible universe will be somewhere between me and Pat. Don't know about Elnstein's theory, but that's what Einstein's theory says.

Noggin

Noggin, I wasn't talking about boundaries or anything else. What I was objecting to was this:

>>From ANY given point the universe may be seen to be expanding in every direction at speeds that come close to, but don't exceed the speed of light. Therefore parts of the universe are moving away from each other at faster than the speed of light.

That's wrong. Any elementary book on Relativity will explain why.

My attempted explanation with Joe and Pat was intended to show that you can't just add relative speeds. If I'm travelling at half c relative to you and you're travelling at half c relative to a third person, all in a straight line, then the third person is travelling at three quarters c relative to me.

For some more "official" info from latest discoveries:

http://news.bbc.co.uk/1/hi/sci/tech/2814947.stm

The newest pictures of the background radiation.

And what if I'm travelling at .99 c relative to you, and you're travelling at .99 c relative to a third person, all in a straight line? If myself and the third person are still moving apart at less than c, then the visible universe is coextensive with the actual universe. And the cosmologists say it's not.

Explain, please.

Noggin

Noggin, I fear you are thinking Newtonic mechanics, where you need special relativity.

As Gnomon said, get a "Relativity for dummies" as it is a bit too complicated to explain it all here, including all the horrid mathemathics.

There's also the wormhole cheat.
For this, you simplify it b(since humans tend not to be able to think in 4+ dimensions) ny visualising spacetime as a peice of paper with two points A and B. Normally, the shortest distance is a straight line drawn on the paper. However, if you bend/fold the peice of paper--like warping spacetime--you can make a shorter straight line between the peices of paper. This is equivalent to a theoretical hole in spacetime that could connect to distant points in space (e.g. Earth and a planet in the Andromeda galaxy) or even time.

For a diagram to help explain this, please see

http://kingal86.tripod.com/spacetime.gif

Note: if anyone tries viewing the image through Netscape, and can't see it, try copying an pasting the address, and then reloading a few times. Or use Internet Explorer if you have it.

Right, gotcha. Sorry, Gnomon.

Although I see Joe and Pat apparently moving away from each other at faster than the speed of light, they don't see it that way.

Noggin

Some time ago in this thread, someone made a reference to the Mr. Thomkins series. The author was George Gamov.

Now, as to the question of the Universe, time and space only have meaning in relation to dynamical (measurable) phenomena. "The Universe is expanding" means, as was pointed out, that particles that make up the universe are getting farther apart. They cannot be observed to be getting farther apart from us (the observer) any faster than the speed of light because they would become infinitely red-shifted and therefore invisible. It makes no sense, cosmologically speaking, to talk about things that are not observable. Such phenomena, as was said by Kip Thorne (the father of the wormhole) travel at the speed of thought.

Who said light was the fastest thing there is?
Maybe that's half your problem guys!

Remember 'light' is just one form of energy, one we hairy apes can see and so in our terribly egocentric universe we assume it must be just about the fastest thing there is. But 'the Universe' we thought we knew turns out to be about only 4% of 'Everything'.

And besides the possibility that faster things exist, I seem to remember seeing on TV recently some particle accelerator type person revealing that he has observed that when 'light' collides with 'light' it goes off in a tangent at twice the speed of light and if this should 'collide' it would double again. Sounds like a workable interstellar engine to me!


~jwf~

Lightspeed (c) is supposed to be the fastest speed, and impossible for things with mass to reach because it takes an infininite amount of energy to reach c. Also, time gets slower and slower the faster you go (i.e. your time slows down), under Einstein's theories, so at c time stops (time dilation).

However, this does mean it is possible, by going really near c, to go anywhere in the Universe in what seems like a few seconds to you--although it would seem to be years, millenia or even longer to people back on Earth.

Some people think there could be faster-than-light particles, tachyons, although I think it's supposed to be impossible for tachyons to go at c or slower.

Maybe I misunderstood what light was.
I thought it was a form of energy radiation, like gamma radiation, radio waves and dozens of others we are now able to detect. My question then is, are all forms of radiant energy called 'light', and do they all travel at the same speed.

I doubt it somehow.
And at the rate we have been detecting and discovering new energy radiations (x-rays, UHF, sub-atomic particles, etc) I also doubt we are able yet to detect most of the energy radiations out there.

What sort of medium do energy waves travel thru.
Sound energy, which depends upon creating a molecular wave in a relatively dense medium, is considerably slower than light. But since all forms of energy are percieved as waves wouldn't they all by definition course thru some medium, be it air, water, copper, iron, dark matter or perhaps even another energy form or some kind of cosmic plasma or dust.

And finally, do all the colours of the light spectrum travel at the same speed or is purple slower than green as one might imagine?


~jwf~

Maybe I misunderstood what light was.
I thought it was a form of energy radiation, like gamma radiation, radio waves and dozens of others we are now able to detect. My question then is, are all forms of radiant energy called 'light', and do they all travel at the same speed.

I doubt it somehow.
And at the rate we have been detecting and discovering new energy radiations (x-rays, UHF, sub-atomic particles, etc) I also doubt we are able yet to detect most of the energy radiations out there.

What sort of medium do energy waves travel thru.
Sound energy, which depends upon creating a molecular wave in a relatively dense medium, is considerably slower than light. But since all forms of energy are percieved as waves wouldn't they all by definition course thru some medium, be it air, water, copper, iron, dark matter or perhaps even another energy form or some kind of cosmic plasma or dust.

And finally, do all the colours of the light spectrum travel at the same speed or is purple slower than green as one might imagine?


~jwf~

Omigod Einstein was right, there I am back again and I hadn't left yet.

~jwf~