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

This is an obvious suggestion, but I haven't heard an argument against it yet (please give me one).

If you have a train moving at 70 mph, and a bird is flying at 20 mph inside the train, to someone not moving, the bird appears to be flying at 90mph. Is the bird flying at 90mph?

If it is, why not shoot a beam of light inside the train? That beam will be travelling at 70mph faster than the speed of light. Any objections?

The speed of light is relative to what the light beam is traveling through. light treveling through a vacumn moves much faster than light traveling through glass. Light also moves faster traveling away from the center of the universe than it does towards it. So the speed of light varies.








Clipboard - has just fallen victim to a physics flashback

Surely light travelling towards the centre of the universe would travel faster, because gravity has an effect on light?

nope. the fact hat the universe is expanding pushes light away from the center of the universe






Clipboard-thought he'd clear thing up a bit

There is no centre of the universe (in a spatial sense).

It's a bit like asking where the centre of the surface of the earth is. There isn't one.

Perhaps "origin" would be a better word than "centre". Assuming the Big Bang theory is correct, the centre of the universe can be defined as the place all the matter is travelling away from. Light travelling towards the Origin is slower (relative the to the Origin) than light moving away because the light source itself is moving away. Since the body is moving in the oposite direction to the light travelling in, you have to subtract the velocity of the body from the velocity of light to get the speed of light (travelling inwards)relative the the Origin, but by the same token, you have to ADD the two velocities together to get the speed of the light travelling out.

Should I point out that if the bird is flying at 20mph inside the train going at 70mph, then pretty soon the back inside wall is going to hit that poor sparrow a slap?

But I assume you mean relative to the train's floor. In which case it must be moving at 90mph, no? But it's different if you are walking along the floor, 'cause you stick to it. But if you were to jump really high, the floor would move past underneath. Either that or I'm totally missing the point.


And anyway, who cares?

Thanks, I needed that.
Will get back. Then...

There's neither a centre nor an origin in that sense at all.

Use the classic analogy of the expanding balloon. All the galaxies are drawn on the surface of the balloon. As it's inflated, the galaxies get further apart.

However, they are all moving away from each other. Anyone in their own galaxy sees everyone else flying away from them. So anyone can think that they are the centre of the universe.

Remember that the space itself is expanding. The big bang didn't happen inside an already existing empty space; the big bang created the space as well.

(The only sense in which you could have a 'centre' to the universe is that you could look at the universe as a 4-dimensional hyper-sphere, in which case the 'centre' is the point 15 billion years ago when it all began. Spatially, that 'point' is equivalent to everywhere now, so it's not really a useful concept.)

OK, the problem with your train analogies is that you have included air resistance which just muddies the problem.

If the train was running in a vacuum, and I jumped off the floor, then no matter how high I jumped I would land back down on exactly the same spot from which I jumped (provided I'd jumped straight up, not sideways at all).

Also, the analogy with the speeds breaks down as you get faster. At relatively slow speeds, then if you run at 20mph relative to the train, and the train is running at 70mph relative to the ground, then your speed relative to the ground is very close to 90mph.

However, Einstein showed that this isn't quite true. In fact your speed is very slightly *lower* than 90mph (although it's really not noticeable, except with very sensitive equipment). As you get closer to the speed of light, the difference becomes more and more apparent.

So whatever speed you are travelling at, a beam of light always appears to be travelling at the speed of light. Another observer travelling at a different speed relative to you also sees the *same* beam of light travelling at the speed of light relative to him!

(Actually, this business with the speed of light being the same for all observers is the point at which Einstein started; however, the theory of relativity is fully self-consistent, and there are no paradoxes in it (as far as we know).)

I beg to differ. Space, by definition, is the only thing in the universe that can't actually exist (or fail to exist for that matter, the word simply doesn't do the stuff justice). It's an abscence of anything. Before the big bang, nothing existed EXCEPT space. This is a paradox. If nothing existed, how could there have been anything to go Bang in the first place? This requires some mental gymnastics in which you think of space as a kind of potential energy and matter as a disturbance in this flat rubber-sheet/pond/whatever-you-wish-to-visualise-as-an-analogy thing. Thus matter itself does not truly exist, nor move. It's just kinetic energy propogating through a universe of potential nothing (like ripples in a pond only more chaotic).

This probably doesn't agree with current scientific theory; it's just something I came up with over the years of insomnia. Can't say I'm wrong though, we all have to have a god of some sort.

Modern space ain't what it used to be. Far from being an empty void just waiting for stuff to appear in it, space is now regarded to be a 'thing'. It teems with virtual particles and all kinds of fields. It is, in fact, an expression of the geometry of the universe.

If you try to imagine the big bang as happening within a space that already exists, then none of the maths works out right; that theory doesn't predict a universe anything like the one we actually observe.

'Before the big bang' is a null-statement. There is no 'before'; just as space began at the bang, so did time. (Note that this is a very 'noddy' way of writing it; the real scientific statement is much more subtle (and beautiful) than the way I said it.)

It is possible to imagine matter as a kind of wobble in some fabric. However, it would be illogical to conclude that therefore "matter does not truly exist". If that's the nature of matter, then so be it; that's the nature of its existence.

"If nothing existed, how could there have been anything to go Bang in the first place?" - I don't know. There is a school of thought in physics that if you add up the amount of energy (kinetic, mass etc.) and the 'binding' energy (the negative energy associated with forces), then the total comes to zero. That makes it easier to understand where the energy came from (i.e. there isn't any!). However, this is a hot topic!

As to why the universe should behave in all these bizarre ways... I haven't the faintest idea. Bloody weird, innit?

Modern space ain't what it used to be. Far from being an empty void just waiting for stuff to appear in it, space is now regarded to be a 'thing'. It teems with virtual particles and all kinds of fields. It is, in fact, an expression of the geometry of the universe.

If you try to imagine the big bang as happening within a space that already exists, then none of the maths works out right; that theory doesn't predict a universe anything like the one we actually observe.

'Before the big bang' is a null-statement. There is no 'before'; just as space began at the bang, so did time. (Note that this is a very 'noddy' way of writing it; the real scientific statement is much more subtle (and beautiful) than the way I said it.)

It is possible to imagine matter as a kind of wobble in some fabric. However, it would be illogical to conclude that therefore "matter does not truly exist". If that's the nature of matter, then so be it; that's the nature of its existence.

"If nothing existed, how could there have been anything to go Bang in the first place?" - I don't know. There is a school of thought in physics that if you add up the amount of energy (kinetic, mass etc.) and the 'binding' energy (the negative energy associated with forces), then the total comes to zero. That makes it easier to understand where the energy came from (i.e. there isn't any!). However, this is a hot topic!

As to why the universe should behave in all these bizarre ways... I haven't the faintest idea. Bloody weird, innit?

OK, it was really really important to get that posted twice!

What does it smell like? And is that smell 'relative'? And if so what to?

Depends on the age of the relative. The really old ones can sometimes smell of wee.

I love that thing about everything adding up to zero. Brilliant. I must have been getting too much sleep to have not thought of that one.

It would indeed be illogical do deduce that matter does not exist, but we're dealing on a quantum level here, where the word sort of starts to lose its meaning.

Time is just a concept invented by sentient beings to cope with the fact that we can remember a past and predict a possible future. without it we'd go mad. It certainly doesn't appear to be a dimension except in a philosophical sense. The other three dimensions we all know and love (and I'm aware current thinking dictates more than three) are all measured in the same units, are tangible and define physical space. The vectors created by these three dimensions can also be travelled bidirectionally, whereas time is a one-way street, measured in a completely unrelated set of units.

This is to say that time does not PHYSICALLY exist, in that we cannot control our place in it. It exists in the same way that unreal numbers exist in mathematics (the magical number "j", the square root of -1).

I'm really not expressing myself very well. I think perhaps Jim and I are more in agreement that not in most cases; it's mostly a matter of interpretation.

Keeps the conversation going though dunnit.

Existence is a very flaky concept, I think. I heard something by Marvin Minsky when I was about 14; it stuck in my mind, and has dominated my philosophy of 'existence' since then.

He said (paraphrasing) that the universe doesn't actually *have* to exist; it just has to be *possible*.

Now don't get me wrong; I'm not claiming that the world doesn't exist! It's just that what the word really means is probably a lot less firm than the way it seems.

I think time is a dimension in every useful sense of the word. However, as you say, we probably don't disagree deeply. I do understand what you're saying!

(Maybe a bit like two mathematicians arguing whether factorial 0 (0!) should be 1 or not. It makes sense pragmatically to *define* it as 1, because then all the combination/permutation formulae work nicely. However, if you start with the classical definition of factorial n*(n-1)*...*2*1 then it can't really be justified. Or maybe I'm just too tired today.)

It also depends what problem you're trying to solve. In most simple physics, you don't regard time as a dimension as such. There is a 3d space which evolves with respect to time. (i.e. in the maths, time looks like a dimension, but that's not how you're treating it because it happens not to be a useful view).

But at the risk of being boring, I'll repeat that I have no idea why the universe should behave in the way it does. Our theories just happen to be good at describing it.

IT's all because of Quantum or something.

I'm amazes that somebody actually understands what I'm saying. Half the time I'm not really convinced I understand it myself.

Here's a thought: consider the phrase "I think, therefore I am". Does this mean that a rock doesn't think therefore it's not?

Or maybe it means that because a rock is, then it must think!

Seriously, "I think, therefore I am" means "I think, therefore I know that there is at least one thing that exists".

In other words, the fact that I think proves that 'I' exist.

In logic, "A implies B" does not imply "B implies A". So you can't say "I am, therefore I think".

However, logic also says "A implies B" does imply "not B implies not A". Therefore, you *can* say "I am not, therefore I do not think".

(Yeah, logic's specialist subject is the bleedin' obvious! )

Back on the topic of faster-than-light, Nature has published the interesting article that we've all been waiting for. You can see it here: http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v406/n6793/full/406277a0_fs.html