Category Archives: Physics

Braaiiins

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Darpa: Heat + Energy = Brains. Now Make Us Some.

The idea behind Darpa’s latest venture, called “Physical Intelligence” (PI) is to prove, mathematically, that the human mind is nothing more than parts and energy. In other words, all brain activities — reasoning, emoting, processing sights and smells — derive from physical mechanisms at work, acting according to the principles of “thermodynamics in open systems.” Thermodynamics is founded on the conversion of energy into work and heat within a system (which could be anything from a test-tube solution to a planet). The processes can be summed up in formalized equations and laws, which are then used to describe how systems react to changes in their surroundings.

Sorry, Wrong Model

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Extreme Ultraviolet Laser Challenges Einstein

No, not really. (Any headline that implies that Einstein might be wrong is invariably incorrect — these are things that have been tested for 100 years)

In the new study, the physicists shot xenon atoms with FLASH, an x-ray laser that uses intense photons in the extreme ultraviolet energy range, about forty times the energy of visible light. The xenon atoms lost a whopping 21 electrons at once, which indicates that it was hit by 50 photons simultaneously. Not only that, but the first electrons to pop off were from an inner region of the atom, like if you peeled an onion starting with the second layer.

Here’s the thing: there are situations where you look at E&M interactions classically. If you put a large electric field around a material, you can ionize it; even though E&M interactions are explained by virtual photons, this is a case where classical physics works out just fine, and a high-intensity laser has a large electric field. Another case is a FORT (far off-resonant dipole force trap), where the intensity profile of a focused laser gives an electric field gradient.

So ionizing 21 electrons is pretty cool, but one needs to be careful in how one phrases these “challenge to Einstein” headlines. You have models of light that are wave-like and particle-like, and you use the model that works. The lesson of the photoelectric effect is NOT that light always exhibits particle properties.

What an Entangled Web We Weave

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Even if we don’t practice to deceive.

zapperz has a post up which points to an article in the WSJ on quantum entanglement: Science, Spirituality, and Some Mismatched Socks

zz marks it as a good layman’s review, but I don’t agree.

Stranger still is entanglement. When two photons get “entangled” they behave like a joint entity. Even when they’re miles apart, if the spin of one particle is changed, the spin of the other instantly changes, too. This direct influence of one object on another distant one is called non-locality.

This is a common summary of entanglement, and it’s wrong. The entangled particles are in indeterminate states — the only thing you know is that the states have a particular relationship, e.g. one is spin up and one is spin down, or the polarizations are perpendicular, depending on how you entangled them. But the notion that one of the particles has a definite state before it’s measured is a classical interpretation, not a quantum mechanical one, and that’s where the analogies that are often used fail to work. That is, a particle prepared in this fashion does not have a state until it is measured — the state of the particle does is not “hidden.”

So if you don’t know what the state of the particle is, you can’t say that it has changed. What you can say is that when you measure the state of one particle, you instantly know the state of its entangled partner, but at the instant you do this measurement, the particles are no longer entangled. Further interactions affecting that attribute will result in no effect on the other particle. And I think this is where the quantum wheels come off the wagon, because this classical misconception has not been dispelled. There’s still this idea that the two particles communicate, and do so instantly. The description given gives the implication that this is so, and then you have the contradiction when you are told that faster-than-light communication isn’t possible with entanglement.

The amusing story of Bertlmann’s socks harms the explanation.

Mr. Bell noted that if he saw one of Mr. Bertlmann’s feet coming around the corner and it had a pink sock, he would instantly know, without seeing the other foot, that the second sock wouldn’t be pink. To the casual observer that may seem magical, or controlled by “hidden variables,” but it was no mystery to Mr. Bell because he knew that Mr. Bertlmann liked to wear mismatched socks.

For the story to work properly you have to also include the notion that which foot was sporting the pink sock wasn’t known until you measured it. All you knew was that one foot had a pink sock, and that if you measured it on Mr. Bertlmann’s left foot, you cannot say that it was on his left foot at any previous point. Thus is the weirdness of quantum mechanics and entanglement.

My take is that any article that puts forth such a basic misconception can’t be a good layman’s guide.

A better treatment

Keeping With the Theme

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More childrens’ stories physics.

Built on Facts: The Physics of Rapunzel

[T]he plot revolves around her letting down her hair. Hair has weight, and so she’s going to have to have some strength to hold up the weight of all that hair.

I have a cartoon for this, too, though not about the tensile strength of hair.

rapunzel

Let's Violate Causality, Too

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Chad points out the physics problem with Goldilocks in The Faulty Thermodynamics of Children’s Stories

The description provided of the other two bowls, though, is not consistent with known physics. The Mama Bear, as the other adult, ought to have the second-largest bowl of porridge, which, in turn, ought to be the second-warmest bowl of porridge (assuming that equilibrium has not been reached). But the story says that this bowl is too cold! Meanwhile, the Baby Bear, who ought to have the smallest portion of porridge, has a bowl that is “just right,” neither too not nor too cold. As the smallest bowl, though, the Baby Bear’s porridge ought to be the coldest of the three (until equilibrium is reached, of course). There is no way for the bowls as described to have the temperatures described, while being consistent with the known laws of thermodynamics.

So I decided to travel back in time to draw a cartoon depicting the problem.

goldilocks

The Rogue Nano Rumba-Line Dancer

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Nobody can see the one at the end of the line, and (s)he’s doing something different.

Nanophysicists find unexpected magnetic effect

In new research appearing this week in the journal Nature, physicists at Spain’s University of Alicante and at Rice University in Houston have found that single-atom contacts made of ferromagnetic metals like iron, cobalt and nickel behave very differently than do slightly larger versions that are on the order of the devices used in today’s electronic gadgets.

“We’ve found that the last atom in the line, the one out there on the very end, doesn’t want to align itself and behave like we expect it to,” said study co-author Doug Natelson, associate professor of physics and astronomy at Rice. “What this shows is that you can really alter what you think of as a defining property of these metals just by reducing their size.”

Doug tells the story of the collaboration and promises a post on the science.

Update: And has posted it