Evolutionists and credentialism

[sinan90]

Yes, it is found in classical physics, because there's no such thing as a perfect experiment. Limits of our experimental uncertainties usually come into play long, long before limits of quantum mechanics. Until, of course, we get down to the quantum world.

The whole point of classical physics is that it is possible to have a "perfect" experiment. Quantum states that it is impossible with the uncertainty principle. We can't know something's position and velocity no matter how good the equipment is or how we measure the particle. In classical theory, if you had the right tools you would be able to measure both. Fact is reality doesn't allow it.

 

[MasterOfKrikkit]

The whole point of classical physics is that it is possible to have a "perfect" experiment. Quantum states that it is impossible with the uncertainty principle. We can't know something's position and velocity no matter how good the equipment is or how we measure the particle. In classical theory, if you had the right tools you would be able to measure both. Fact is reality doesn't allow it.

OK, so the the derail has been derailed. Yay, this derail is more interesting anyway!

I agree that there's a difference in the limits of measurement in classical vs quantum physics, but I think the difference is a matter of asymptotic vs finite limitation. That is, I don't know if I'd say you can actually achieve a perfect experiment in classical physics, but you can perhaps approach such a theoretical construct asymptotically. Or, to put it a third way, while 0 error may not be possible, error < epsilon is, with enough experimental cleverness. The fundamental difference with quantum is that there's a finite limit on the error.

Now, as a slight tangent, whether even a theoretical perfection can exist in classical physics leads into some brain-bending philosophy (I think). Eg, can I build a right triangle that asymptotically approaches (as my instrumentation gets better) a theoretical 1-1-sqrt(2) triangle? Or even just a perfect circle? The problem being that of irrationality -- what does it even mean for a length to be irrational? Perhaps I'm approaching the "correct" measurement asymptotically, but does the theoretical construct I'm supposedly approaching even make physical sense? Can it even exist? Or is the fault with the mathematics? In which case, shouldn't it all fall to pieces? This really weirds me out! (And, perversely, makes me grateful for Heisenberg!)

 

[Tomk80]

The whole point of classical physics is that it is possible to have a "perfect" experiment. Quantum states that it is impossible with the uncertainty principle. We can't know something's position and velocity no matter how good the equipment is or how we measure the particle. In classical theory, if you had the right tools you would be able to measure both. Fact is reality doesn't allow it.

But in the realm of classical physics, namely the macro-world, we can measure both. We can measure the speed of a race horse at the exact same time that we can measure it's place on a race track. The point with quantum physics is that a measurement has a direct impact on what happens with the quantum states, so that this becomes impossible. In the macro world, such a direct impact is not present. The macro-world does not obey quantum rules and vice versa.

 

[MasterOfKrikkit]

But in the realm of classical physics, namely the macro-world, we can measure both. We can measure the speed of a race horse at the exact same time that we can measure it's place on a race track

But not exactly, unless we're still using classical theory. Classically, we can measure both perfectly (or, at least, as accurately as we can be bothered fine-tuning our equipment). Bu in reality, the measurement would still be subject to Heisenberg. Practically, we don't have the precision to determine that at a macroscopic level, but the limitation is still there. (Assuming, of course, that I understand quantum physics. Big assumption!)

The point with quantum physics is that a measurement has a direct impact on what happens with the quantum states, so that this becomes impossible. In the macro world, such a direct impact is not present. The macro-world does not obey quantum rules and vice versa.

Again, I believe the idea is that they do both obey the quantum rules, but we never see the weird effects because we see statistical ensembles at the macro level. So technically measuring the speed of the racehorse does affect it, but at such tiny levels that we don't notice it. It's like me being length contracted as I walk; it happens but the effect is a grillion orders of magnitude too small to notice.

 

[Naraoia]

But not exactly, unless we're still using classical theory. Classically, we can measure both perfectly (or, at least, as accurately as we can be bothered fine-tuning our equipment). Bu in reality, the measurement would still be subject to Heisenberg. Practically, we don't have the precision to determine that at a macroscopic level, but the limitation is still there. (Assuming, of course, that I understand quantum physics. Big assumption!)

Was it Feynman who said anyone who thinks he understands quantum physics doesn't?

Again, I believe the idea is that they do both obey the quantum rules, but we never see the weird effects because we see statistical ensembles at the macro level. So technically measuring the speed of the racehorse does affect it, but at such tiny levels that we don't notice it. It's like me being length contracted as I walk; it happens but the effect is a grillion orders of magnitude too small to notice.

That's my understanding too, but I know all I know about quantum physics from John Gribbin's books

 

[Chalnoth]

The whole point of classical physics is that it is possible to have a "perfect" experiment. Quantum states that it is impossible with the uncertainty principle. We can't know something's position and velocity no matter how good the equipment is or how we measure the particle. In classical theory, if you had the right tools you would be able to measure both. Fact is reality doesn't allow it.

In a way this is sort of true. But thermodynamics screws it up. Basically, any experiment performed at a finite temperature ensures that simple Brownian motion prevents any such "perfect" measurement. Brownian motion requires no quantum mechanics.

 

[Chalnoth]

But in the realm of classical physics, namely the macro-world, we can measure both. We can measure the speed of a race horse at the exact same time that we can measure it's place on a race track. The point with quantum physics is that a measurement has a direct impact on what happens with the quantum states, so that this becomes impossible. In the macro world, such a direct impact is not present. The macro-world does not obey quantum rules and vice versa.

Well, not quite. Quantum mechanics gives the proper predictions for the macroscopic world, because it simply doesn't differ in its predictions from pre-quantum physics for the macroscopic world, at least not in any measurable way for most systems. Classical mechanics is an approximation to quantum physics that works until you get to the very small or to very low temperatures.

 

[sinan90]

Was it Feynman who said anyone who thinks he understands quantum physics doesn't?

The best thing ever said by a scientist in my opinion, and the most true.



I'm not 100% of all the workings of quantum theory but as far as I understand it the uncertainty is still there on a macro scale but its next unnoticeable, much like the de Broglie wavelength of someone walking along. From a physics lesson earlier this year it was calculated at something like 10^-35m - 10^-40m. It's there but no instrumentation of current technology would be able to pick it up.

 

[Chalnoth]

The best thing ever said by a scientist in my opinion, and the most true.

Perhaps, but completely pointless without this as well:

Just to give you an idea of how the theory has been put through the wringer, I'll give you some recent numbers: experiments have Dirac's number at 1.00115965221 (with an uncertainty of about 4 in the last digit); the theory puts it at 1.00115965246 (with an uncertainty of about five times as much). To give you a feeling for the accuracy of these numbers, it comes out something like this: If you were to measure the distance from Los Angeles to New York to this accuracy, it would be exact to the thickness of a human hair. That's how delicately quantum electrodynamics has, in the past fifty years, been checked-both theoretically and experimentally.

Quantum electrodynamics, in particular, is the most precise theory which we have yet produced. And bear in mind that Feynmann gave this lecture back in 1986: the precision of the above result has only increased since then (by a factor of about 100,000), with no detected deviation.

When Feynmann was speaking of understanding, he meant intuitive understanding. We have no problem describing how the world works. But understanding why it works that way, or intuitively grasping what is going on are beyond us.

You can read the whole lecture here:
http://press.princeton.edu/chapters/i2352.html

It's quite a good read.