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

Just to clear up some confusion here.......
a. Light travels at the speed of light because photons (light particles) have no mass, and so according to Special Relativity can only exist if they travel at the speed of light.
b. Matter (i.e. stuff with mass, like cups of tea etc.) cannot travel faster than light as this means having a mass involving the square root of -1, which is called an imaginary number and doesn't fit very well into engineering (though it is OK in physics).
c. As you approach the speed of light, your mass (NOT weight, that is due to the application of the gravitational force on matter) approaches infinity. Time, as perceived by an outside observer. also slows down, so you get things like the twin paradox. This is complicated and makes your brain hurt.
d. The speed of light is constant (at 299792458 m/s), but your perception of it depends on your frame of reference....for more details read a book on relativity.
e. Vacuum is a relative term - deep space contains about 15 atoms per cubic metre, earth's atmosphere about 7*10^24 atoms per cubic metre. So there is friction in outer space, just not very much, but when you are travelling at close to the speed of light it is a significant factor.
f. Physics is like building a house - you have to start at the bottom and work your way up - missing out a few floors in between is not an option. So if in doubt ask a physicist and he/she will confuse you into immobility and then you can get on with more important things, like life.
g. I have a PhD in the subject, so I am just trying to be kind......lots of love

Well, now that's clear...

Errrm. Speed of light is constant only in the same medium. The 3x10^8 figure you quote is only true in a vacuum. I think you'll find that it takes a slightly more relaxed approach when travelling through, say, glass, water, or the Italian town of Vinci.

This is why refraction happens. Isn't physics fun?

Sean.

p.s. Just to be clear - while I concur that photons always travel at c, waves of light aren't always so lucky

Thanks, Sean - you are quite correct. I should have said that the upper limit of c is 299792458 m/s. One could argue that the interaction of light with matter is what slows it down, so it does depend on your frame of reference, but that is splitting hairs.
And yes, physics is fun.................

Jan -

You've made my day Anywhere else on the internet I'm pretty sure my reply would have unintentionally triggered a flaming argument.

Instead, we end up not only agreeing, but sharing gems with the rest of the guide. I keep seeing this sort of thing happening, and it reassures my hopes that h2g2 really is the start of something special

Best,

Sean.

If Light has no mass then how is it sucked into black holes by gravity?
Also how do you explain it defecating?

Light just travels through space. When the space is bent, it doesn't know. It only defecates when it knows its hit something!

Sean- just shows what we can do if we try!!
wtg
Jan*

Black holes have an awfully deep graviational field - they are the ultimate suck (ok, ok, i know....). Even light is sucked.
The only defecation involved is that of the people being sucked into the black hole, but by that time they are too dead to worry.

Also, although light has little (not *exactly* none) mass, it has plenty of energy. From E=Mc(squared) we conclude that the two are more or less equivalent, matter being kind of a "frozen" energy. Hence, the effect of black holes (also due to the fact that under Einstein's framework, gravity is not treated as a force but as a curvature in spacetime, sort of like the valley formed by a bowling ball resting on a suspended rubber mat). Sorry, had to get all that out. It's good for the circulation.

A former physics (now linguistics, of all things) major

Yes, sort of (ever tried teaching advanced physics over the net?)
The full equation is E^2 - p^2*c^2 = m0^2*c^4
where E= total energy, p= kinetic energy, and m0= rest mass.
So, rest mass is like a potential energy and is affected by a gravitational field, hence the rubber sheet analogy.
Personally I prefer watching the water spiral down the plug-hole.
Anyway, you can reduce this equation to E=mc^2 by defining m (perceived mass) as m = m0/(1-v^2/c^2),
where the factor 1/(1-v^2/c^2) is called gamma^2.
Please don't ask why it is squared.
So, as v (your velocity) tends to c (the speed of light), gamma goes to infinity, so your perceived mass goes to infinity and you have problems. If v>c, gamma becomes imaginary (square root of minus 1 in here folks) and you annoy engineers a lot, cos they don't like it.
And the rest is General Relativity crossed with Science Fiction.

If you want an entry in The Guide on this, then ask me, but otherwise address your questions to Dr. Hawking, Gonville and Caius College, Cambridge. (or don't if you have any sense, as he is a busy man).
lol

P.S. I think you were remembering that neutrinos have very little (or no - who knows) rest mass. Since they abolished the quest for the speed of light this has become the main occupation of the anorak wearing contingent of physicists. Hey! - they have a right to a job too you know!

Check my post under Probes Vs People, it should clear things up.

Yup, you're right. I was also confused, I think, by the photoelectric effect(?), which I used to think was caused by light having a small amount of mass, but I've since been corrected on that. Great discussion. I love this thing.

oops, sorry, before anyone notices, p=momentum, not kinetic energy (it may not matter to you but it does to me!)

If light is a wavicle then its wavefronts can travel faster than light, if shone through solid? matter. Or something like that. Einstien proved that light can't travel at the speed of light. This does not exclude it from traveling faster than light, it just can't go through the speed of light. Hmmmm! If something was travelling faster than light we wouldn't be able to detect it as all our instruments use light. I haven't majored in anything but I love the ideas.

Yes, get your teeth in order Agrajag. Things that travel at speeds greater than c are called trachyons. We can't detect them because their masses (if any) are imaginary, in the mathematical sense. Also we tend to detect things by electromagnetic waves, which can't get above c without sub-space thrusters or Hollywood special effects.
Nice try though. LOL

P.S. the reason c is called the speed of light is that light travels at that speed (in a vacuum, Sean).

But seriously, this is a fascinating subject. If you want to understand more about it, do a degree in Physics or (quicker) read a book. Although, with all best wishes to Prof. Hawking, read something else before you tackle his book and you will understand 'Brief History of Time' better.

> Things that travel at speeds greater than c are called trachyons

Or even tachyons. (Sorry Jan).

Sean-the-pedant.

p.s.

There was a young lady named Bright,
Whose speed was far faster than light.
She went out one day,
In a relative way,
And returned the previous night!

(Reginald Buller).