Physicists Do What Einstein Thought Impossible

An anonymous reader writes "Einstein worked on Brownian motion (the movement of small particles in a fluid as they collide with the fluid's molecules) in 1905, but said it would be 'impossible' to determine the speed and direction of a single particle during this dance. Now researchers have gone and done it, by suspending a dust-sized glass sphere in air (which slowed down its dance moves, since it had fewer collisions with spaced-out air molecules than it would have had with water molecules). The researchers held the sphere in place with 'laser chopsticks,' and then watched how the glass bead bounced around to determine its direction and speed (abstract)."

Comments

No Fair

[Anonymous Coward]

You changed the outcome by observing it.

Re:Magic words...

[fahrbot-bot]

They would clearly be more "chopsticky".

Impossible for his time

[repepo]

TFA doesn't refute any of Einsteins conclusions about Brownian motion. It only shows that it was something impossible to do at Einsteins time. What a cheap way to grab attention!

This doesn't break the uncertainty principle.

[raving griff]

I've seen a couple of comments (more than one thread or else I would have posted a reply there) that seem to suggest that this breaks quantum physics by accurately predicting the speed and direction of particles, but it should be noted that the Heisenberg Uncertainty Principle states that it is impossible to accurately calculate both the velocity and its position. Speed and angle are components of velocity, therefor the only conclusion of this experiment is that velocity can be calculated under these conditions.

Re:This doesn't break the uncertainty principle.

[jfengel]

That's correct. What's at issue here is a matter of engineering, not physics.

Physicists reserve "impossible" for the truly mathematically unavoidable, while engineers expand it to the wildly impractical. When you say something "is" true, you're speaking in the former sense. When you say you "believe" something to be true, as Einstein did, you're speaking in the latter sense.

So it's not overthrowing any physical principles. It's merely confirming something else Einstein said: the universe is not only stranger than we imagine, it's stranger than we can imagine.

Re:This doesn't break the uncertainty principle.

[MattMattMatt]

Rest assured, Einstein never imagined the mess that is Lost, since it wasn't his saying in the first place.. Quote is attributed to J.B.S. Haldane.

Re:This doesn't break the uncertainty principle.

[agrif]

The Uncertainty Principle only kicks in where the particle (wave packet) size and the wavelength of the observing "light" are approximately of the same order of magnitude. A "dust particle sized" (whatever that means) glass bead is much larger than the wavelength of visible light, else, you couldn't see it.

This is a huge misunderstanding of the uncertainty principle. This isn't about uncertainty in the experiments themselves; this is covered quite well by simple experimental error terms. This isn't even about measuring the position, and somehow knocking the particle out of its old path, though that can happen. This is a fundamental part of the universe: you cannot know perfectly, ever, a particle's position and momentum. No matter how perfect you make the experiments! It's like they're both stored in the same memory space, and getting more precise on one causes a loss in precision in the other as you eat up more space.

Where does this come from? Momentum, and therefore velocity, are related to the particle's wavelength (and yes, even normal matter has a wavelength!) When you measure the momentum, you know that the particle has a specific wavelength, so now its wavefunction spreads out according to that wavelength. This delocalizes the particle: it's likely to appear twice every wavelength, just about equally. You have no idea where! Conversely, if you measure the particle's position, the wavefunction collapses to a single spike right where you measured it to be. This spike is, practically, infinitely large, so the particle loses any well-defined wavelength it once had, so the momentum is now undefined!

Re:This doesn't break the uncertainty principle.

[John Hasler]

However, these dust particles are so large that the Heisenberg uncertainty is small compared to the measurement error. This is a classical system.

Re:Magic words...

[RDW]

'Interesting... How do you suppose laser chopsticks would compare to, say, a laser spanner, or a sonic screwdriver?'

Well, there's a definitive answer to the screwdriver question here:

http://www.youtube.com/watch?v=ue4On8QINxQ

Based on Research by Steven Chu, et. al.

[borroff]

One thing interesting that isn't mentioned specifically: This work, using "optical tweezers", is based on research done by Nobel Laureate Steven Chu's group at Berkeley. Dr. Chu also happens to currently be the US Secretary of Energy.

No job too big, no job too small, Steve Chu does 'em all.

Re:To avoid confusion

[MBGMorden]

Must be regional variation. Around here chow mein is mostly cabbage with onions, celery and your choice of meat cooked in. No noodles at all. Looking at the wiki article on chow mein, that particular dish looks like what is usually called chow mei-fun in the local restaurants.

Re:Keep in mind

[jackchance]

I'm pretty sure that they didn't measure the position and momentum to better than half of Planck's Constant.

Nature of Brownian Motion

[vdorie]

Brownian Motion is a mathematical construct, which, among other things, is nowhere differentiable (almost surely). You can pin a BM down into sets with high probability, but no, you can't really predict it. It is merely used to *model* the movement of a particle in a fluid, it is not actually the process by which the molecules move. Indeed, "such a path represents the motion of a particle that in its wanderings back and forth travels an infinite distance in finite time. [BM] does not in its fine structure represent physical reality." (Billingsley, "Probability and Measure"). At least the science is interesting.

Re:Keep in mind

[Jherico]

Heisenberg was talking about subatomic particles, not specks of dust. Its basically a consequence of not being able to measure both the speed or position of a subatomic particle without affecting the other property. The more accurately you measure one, the less you know about the other. While this principle does apply to macroscopic objects like dust particles, the level of uncertainty about the size and position of something macroscopic (even something as small as a dust particle) is vanishingly small (like on the order of the width of an atom).