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

Is there anyone else out there that just isnt really buying Heisenbergs whole uncertainty principle? Maybe its just because I dont understand it, but was he saying that you cant know an objects location and momentum because it cant be determined experimentaly? If so wouldnt that mean that better techniques would yeild results? Or is he saying that knowing one negates the possibility of knowing the other through some intrinsic property of nature, and if so what?

As I understand it (read that as "someone who knows more than I once told me something and this is how I remember it"), the uncertainty principle is based on the fact that by measuring particles, you change them. Imagine a thermometer. You stick it into your bathtub and a few moments later it tells you that your bath water is 42°C - way too hot to bathe in. Now imagine that you have an Indian-in-the-cupboard kind of room mate, who is lots smaller than you are. Her or his bathtub is also lots smaller than yours. Now, what would happen if you wanted to be nice and check the bath water temperature for your little friend? You take your own thermometer, but when you stick it into his or her bathtub, you would first of all find that your thermometer is too big to be stuck into that tiny bathtub - it is that small. But you insist on being helpful, so you just touch that bathtub with your thermometer. What will happen now is that rather than measuring the temperature of those few drops of water, your thermometer will actually cool down those few drops of water.

So here is a case where measuring something changed the thing you were actually measuring. You might point out that your tiny room mate will likely have her own tiny thermometer to check the bath water temperature with, but the principle remains the same.

When it comes to subatomic particles, you will appreciate that refining your methods of measuring becomes a tad bit difficult. You come to a point when you have to accept that you can either measure velocity/vector/impetus/whatever, or the location of a particle.

Then there are of course those who will say that particles do not even exist properly before you look at them. Now that is an extreme example of influencing a particle: you call it into existence...

Y.

Basically, in order to measure the position and/or velocity of a subatomic particle you have to bounce another subatomic particle off it, and this disturbs the trajectory of the particle being measured. This is also true at the macrolevel, but the disturbance is negligible in relation to the system being measured (the thermometer in the big bathtub will also change the temperatue of the water, but only by a tiny amount.) But when you get down to the world of the very small the disturbance becomes significant. You can't refine your techniques of measurement, because you're "already" using the smallest particles.

What is more controversial is the claim that this lack of measurability means that the particle doesn't *have* a definite location or momentum - but of course this can't be proved one way or the other.

Noggin

I thought that there is a limit to what is knowable in principle when you get to size/time/energy limits related to Planck's constant. The accuracy of the measuring apparatus ceases to matter at this point.