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

Can anybody explain why space and time behaves the way they do when travelling at close to 300 000 km/s, according to the theory of relativity?

I think it is just the way the universe is.

I seriously doubt that there ever will be an adequate explanation until we can travel at those speeds ourselves and conduct a proper study.

In comparison to some objects we ARE travelling at close to that speed (e.g. Distant Galaxies)

Mr D, do you mean we have to go in spaceships etc? Cosmic rays go almost at lightspeed, at CERN all the particles go almost at lightspeed etc etc. There's plenty of evidence that relativity is correct, although, like Newtonian mechanics, it's not a complete description of how things move (ie, doesn't work in black holes 'n' stuff).

Spaceships would be good, certainly (especially the ones with lasers on them), but what I meant was that we'd probably need to be able to examine more than particles (i.e a large solid object, presumably with billy tons of complicated equipment in it) moving at those speeds before we could even begin to accurately speculate on the whys.

spacetime = gravity, which is on the plate to be studied at CERN in the next five years. So they might actually get an explanation sooner than that.


Actually, I've heard numerous times that some of the particle accelerator experiments that are run create miniature blackholes during the experiment, which leads me to believe they're already probing why spacetime behaves how it does.

Yeah, but there's only so much information we can get from particles before it becomes mathematics, which works great for obscure theories but isn't so hot when it comes to everything else.

Mr D, why are big things better than small things when describing nature, especially when there is no difference that we're aware of in terms of how spacetime affects them (discounting for a second *really* big things like neutron stars etc, which we can also see and take measurements from)?

Why? For the reasons I stated in my last post of course!

And, I read somewhere recently that the microcosm and the macrocosm often don't behave in the same way at all (because of quantum).

Big things vs little things...

well quantum physics?

I don't think QM stuff has much effect on spacetime distortion at the energies involved to test relativity. There's some cosmic ray type thing (whose details entirely escape me) which could not be detected at all if time didn't dilate in the way relativity describes.

I'm still at loss why we have to see macroscopic things up close to be confident of this.

I'll put it this way...
Sure you could zap individual cells with electricity, but it wouldn't give as good an insight as to the effect of high voltage on living tissue as, say, zapping a small child.

Hmm (thinks about calling RSPC). But if you light a match, you know what'll happen if you set fire to a boulder of phosphorus. Simple things (ie without millions of interlocking parts) scale up that bit better.

We know *nothing* about what space and time are in themselves. We only know how they relate to each other when we measure them.

What do we use to measure space?
What do we use to measure time?

And while we're about it, what do we use to measure mass?

Noggin

Time is measured in the electronic waves emitted by a certain atom.
Space is meashured using light, or rather the distance light travels in a specific period of time.

Mass is measured by comparing with a specific lump of metal in Paris.

Indeed and time is measure by measuring the oscillation rate of Caesium atoms

They're currently trying to do away with that lump of metal in France as the kilogramme is the least precisely known of the fundamental units because of it.

It is gradually gaining weight because of it's gradual oxidation and when they take it out every couple of years they measure it's weight gain over a few days and then interpolated the graph back to when they took it out to assess it's mass. The metre and the second are known precisely to an error of about 10^-12, the standard kilo cannot be measured to anywhere near this precision.
From what I've heard from a metrologist they're trying to remove the kilo as a fundamental unit and link it to the speed of light somehow.

It seems that mass & energy being intricately linked (perhaps different expressions of the same fundamental) and space and time being a fabric, rather than separate entities.

On top of that, mass/energy cause spacetime to warp. So they aren't independent of each other at all. Whatever units you use to measure, you must remember that these are all extremely local measures. There is no cartesian coordinate system extending infinitely in our 3 space dimensions while a cesium atom-clock ticks away at all points.

Frankly, I don't know what to think anymore, except to enjoy what science I can on this locally flat, apparently 3 dimensional + time slice of the universe.