if the universe is infinite which I always thought it was as if there isn't a universe there isn't anything why..

[Ormirian]

Prof Cox insists on talking about hundreds of billions of galaxies. Because even a 1000000000 is a number and it isn't infinity.

He keeps using these BIG numbers but surely all he needs to say is 'an infinite number' or even 'lots and lots'. Both of which are more correct.

AIBU?

cumfy: ah, the anthropic principle. The universe is as it is, because we wouldn't be around to observe it if it was any other way.

The fact that we have an inverse-square law is a very good argument for there not being more than 4 macroscopic dimensions, although of course there could be other weeny dimensions all wrapped on top of each other a la string theory.

But it doesn't explain why there are only 4, only that if there were more (or fewer) than 4, then the universe would not exist in it's current form.

Yes, I know all about the wave particle duality of light, but obviously I'm no expert!

I know each of the other forces have defining particles (photons etc). But why do you need a graviton? I don't get why it can't be a bi-product of a different force? Do you really need a carrier partcile in this case? Then again, I suppose that just because not everyone understood why the photon must exist a hundred years ago, it didn't mean it didn't.

Another thing that bugs me: Everyone knows that travelling at high speeds has dilative effect on time. That's fine.

But why doesn't this effect work on the quantum level? Why doesn't a particle that reaches relativistic speeds suffer from dilation? Or does it?

Going down the dimensional route does start to bring up a few philosophic arguements, doesn't it?

What about many worlds theory? That would say that there also exists a large number of other universes where the universe failed, and somehow we are in one of the succsessfully created ones. All the other probabilities and all that.

I don't like that theory, though. I don't get why we are in this particular one, and not the other ones. Unless our position in this one is just an illusion, but frankly that just doesn't compute.

AtomicBun: reaching the limit of my undergraduate knowledge here I'm afraid. Of the 4 forces we know of, 3 so far can be described in terms of QM and carrier particles. We therefore expect the 4th one to be too. Or we would like to demonstrate that it can't be. And then we would like to know why it's an exception.

We theorise that all forces were initially one during the early universe, and that as the universe cooled they "froze out" into the individual forces we see today.

The idea of unifying forces is not new. Maxwell unified electricity and magnetism back in the 19th century, previously they were thought to be unrelated but he showed that they were simply aspects of the same force, electromagnetism. This development allowed the development of modern electric motors and generators.

Then in the 1950s, Richard Feynmann demonstrated that electomagnetism and the nuclear weak force were actually two aspects of the same force, the imaginatively named electroweak force.

Regarding time dilation for relativistic particles - this absolutely happens. In fact when calculating the half-life of unstable particles we need to factor this in, as the half-life will appear greater the faster the particle is travelling.

Hogsback, thanks for that. I thought gravity could be affected by certain frequencies of electromagnetism? In that case, why could it be another of the same force?

With regards to the dilation of relativistic particles, that does make sense and must be a bugger to work out since their speeds are never consistent, yes?

But how can photons move at the speeds of light, then? At that point, time dilation reaches infinity, and that's why people say that you cannot reach that speed (along with the fact that apparently your molecules would calapse because you are moving faster than the absolute max that the photons can move at, so the atoms you are made of couldn't hold themselves together).

Oops, sorry. The third sentance should say:

In that case, why couldn't it be another aspect of the same force?

TheAtomicBum - photos travel at the speed of light because they are massless.

Particles with mass cannot reach the speed of light, because another effect of relativity is that mass increases with increasing velocity, with mass approaching infinity as the particle's speed approaches the speed of light.

Not sure what you mean by gravity affected by certain frequencies of EM I'm afraid. Gravity appears to affect electromagnetic radiation (bending light etc) but actually it's bending space-time and everything that moves in it.

Photons can move at the speed of light as they have no mass. It is only massive particles that cannot move at the speed of light.

The reason you cannot have a massive particle moving at the speed of light is not to do with time dilation, but due to the fact that it would require an infinite amount of energy to get it to go this fast.

Time for a photon effectively stands still. Which makes for an interesting question: how do we know whether the photon is a stable particle or not? We will never see one decay, as time stands still for them.

So the photon doesn't need to generate the infinite energy because it doesn't need that amount of energy to propel it as it has no mass?

As for the effects of EM on gravity, I read it in New Scientist a while ago. Something to do with the interaction of higher dimensions, and gravity being a force that somehow relates to others. It was a long time ago I read it.

Ah, just looked it up. It's a theory to do with anti-gravity that's not widely accepted, although some theories are still being given consideration. Appratently, it's more a sci-fi thing at this point.

But if a photon has no mass, how can gravitational forces affect it? Black holes being the thing that springs to mind.

Hogsback....yes, you make some very good points.
Sorry if I seemed dismissive.
I will tell you who I do rate highly Dr Jonathan Miller.(I know he is a medical Dr)
But he seems to have stopped appearing on radio and TV. Gone to produce opera or something. What a waste of a brilliant brain.

He say things like. " I can tell you how the human body works. But I cannot explain consciousness; and I dont thing anyone ever will. "

I like his modesty...but never is a piggin long time.

Sorry about the diversion

TheAtomicBum: very good question about how gravity affects objects with no mass. The answer is that while mass creates gravity, you do not need to have mass to be subject to gravity.

This is because gravity is not a force, although it's effects look like a force to us. Gravity is actually a curving of space-time, the effects of which we perceive as a force proportional to the mass of an object. However, this is something of an illusion, and the curvature of space-time affects even objects that have no mass, such as photons.

But if that's true, and it's not a force in it's own right, then why would it be necessary for gravitons to exist? What if it's just a by product of another force? You know, like inertia being the by product of kenetic energy. I've always thought that the two were linked (inertia and gravity) somehow. Not inertia (because that only happens during acceleration), but the effect of increased mass at high speed.

I guess they're linked not because whilst increasing speed increases your mass, high gravity increases only your weight, but not your mass. Unless I've got my wire's crossed again?

It's been many years since I really read up on a lot of this stuff, so my memories a bit fuzzy. Hence, sorry for all my inaccuracies (particularly about mixing WIMPS up with God Partciles.)

TheAtomicBum: in a QM formulation of gravity, it would be a force, well a field actually, and would require a carrier particle! However, we haven't managed to come up with a QM theory of gravity that works - hence sticking with general relativity, in which gravity is not a force.

Intertia isn't really a by product of kinetic energy - inertia is a function of mass, actually it is the mass. Think of a stationary train carriage. It has no kinetic energy (not moving) but massive intertia (really hard to get moving).

Yes, gravity could be the by-product of another force (in much the same way that magnetism is the by-product of the electric force) but it's hard to see how this could be. Gravity interacts with matter in ways that none of the 3 other forces do.

Of course, it could be that gravity is actually the manifestation of some other, as yet unknown force, but for the sake of argument, we might as well call that unknown force "gravity" :)

I'm the same as you: dredging up a 20 year-old physics degree :(

A simplistic way to think of inertia is the tendency of an object to resist changes in its motion - which , as Hogsback says, is essentially its mass.