Monday, April 27, 2009

Quantum mechanics is best understood when you're blind

Occasionally I go through some threads of newsgroup sci.physics.foundations. This moderated newsgroup is dedicated to my main subject of interest and one is bothered neither by flames or spam as in sci.physics nor with biased moderation. So for everyone with an interest in physics foundations, this is a good place to be.

Recently there have been discussions on the difference between classical and quantum. I appreciate the description given by Charles Francis, which I quote hereafter:

Classical mechanics is the mechanics of bodies whose position can be continuously known up to the limit of experimental accuracy.

Quantum mechanics is the mechanics of particles whose position cannot be known between observations even in principle.

I like the fact that this definition goes beyond the historical bias that ordinary objects behave classically and submicroscopic particles behave quantum-mechanically. Charles Francis's definition focuses on the way we describe the behavior of objects, not on the way the objects behave. In other words, this definition doesn't assume a boundary between classical vs. quantum bodies. It merely assumes that we may choose between two descriptions. It assumes we have bodies and particles and we can choose to describe them either classically or quantum-mechanically. This point is very important: it empowers us to go beyond the historical bias. We just have to set up a suitable experimental procedure where:
  • positions cannot be known between observations even in principle --> use quantum mechanics.
  • positions can continuously be known --> use classical mechanics.

So a body is neither classical nor quantum, it just is. Depending on our purpose, we choose to describe it classically or quantum-mechanically.

As an illustration, we may refer to the perception of blind people. A blind man relies on his blind stick to analyze the behavior of objects surrounding him. He cannot know the position of objects between two "blind stick observations" even in principle. His blind stick "scans" the objects quantum-mechanically, with intrinsic uncertainties, intrinsic probabilities. At each hit of his stick against an object, the "state" of that object collapses at the location of interaction. It had some probability to be detected at any other place depending on the environment, on the boundary conditions and on the respective angles (phases) of the stick and the geometry of the detected object, but the fact of detecting the object collapsed the body at a specific location. It instantly pops up at that location.

I guess a blind man will intuitively choose a quantum-mechanical description with states whose phase varies with time and location, with probabilities, with uncertainties, with interference patterns, etc. Teaching him to describe the world classically would be counter-intuitive because he cannot continuously know the position of the objects. He has to infer it with square state (blind stick projected on detected object) probabilities.

Paradoxically, we could say that quantum mechanics is best understood when you're blind...

Saturday, April 25, 2009

Physics Quote of the Day (April 19 - April 25)

"I have done a terrible thing, I have postulated a particle that cannot be detected." Wolfgang Pauli, born 25 April 1900.

"In principio it is impossible to prove from experiments that something is non-existent." Felix Ehrenhaft, born 24 April 1879.

"A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it." Max Planck, born 23 April 1858.

"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any question, to doubt any assertion, to seek for any evidence, to correct any errors." Robert Oppenheimer, born 22 April 1904.

"Divided minds, getting lost on different paths, are losing the huge advantage that would result from their combined forces." Jean-Baptiste Biot, born 21 April 1774.

"As for the Internet, I tend to have profound doubts about the value of this communication advance to science. I wonder if, in an era of the Internet, we can have somebody like Eugene Wigner. Eugene Wigner's genius manifested itself in his ability to concentrate for a long time on a single idea. If you are constantly beset by outside ideas, can you really get to the true heart of the matter? It's a very different way of doing science." Alvin Weinberg, born 20 April 1915.

"There is a beauty in discovery. There is mathematics in music, a kinship of science and poetry in the description of nature, and exquisite form in a molecule. Attempts to place different disciplines in different camps are revealed as artificial in the face of the unity of knowledge. All literate men are sustained by the philosopher, the historian, the political analyst, the economist, the scientist, the poet, the artisan and the musician." Glenn Seaborg, born 19 April 1912.

Saturday, April 18, 2009

Physics Quote of the Day (April 12 - April 18)

"They get the theory of nuclear physics thrown at them; sometimes before they ever know there is a phenomenon they have the complete theory of it. The phenomena are not sufficiently emphasized, I think, in teaching today." Maurice Goldhaber, born 18 April 1911.

"Quantum mechanics is the Disney World for adults!" Jan Zaanen, born 17 April 1957.

"There are no limits to what science can explore." Ernest Solvay, born 16 April 1838.

"Although to penetrate into the intimate mysteries of nature and thence to learn the true causes of phenomena is not allowed to us, nevertheless it can happen that a certain fictive hypothesis may suffice for explaining many phenomena." Leonhard Euler, born 15 April 1707.

"What a wonderful and amazing Scheme have we here of the magnificent Vastness of the Universe! So many Suns, so many Earths, and every one of them stock’d with so many Herbs, Trees and Animals, and adorn’d with so many Seas and Mountains!" Christiaan Huygens, born 14 April 1629.

"The mathematicians know a great deal about very little and the physicists very little about a great deal." Stanisław Ulam, born 13 April 1909.

"If we look backward, we seem to discern clear signs of progress; if we look forward, we discern nothing but the veil. Science is but organized experience, and experience of the future we have none." Edward Maunder, born 12 April 1851.

Monday, April 13, 2009

Physics Quote of the Day (April 5 - April 11)

"Always try the problem that matters most to you." Andrew Wiles, born 11 April 1953.

"Science that abdicates its cultural values risks being perceived as an extension of technology, an instrument in the hands of political or economic power. Humanity that disavows science risks falling into the hands of superstition." Nicola Cabibbo, born 10 April 1935.

"When it comes to scientific matters the ready talkers simply run riot. There are a lot of pseudo-scientists who with a little technical jargon to spatter through their talk are always getting in the limelight by making startling predictions of what the future has in store, using as their text the most recent discovery or invention." Charles Steinmetz, born 9 April 1865.

"The direct tendency of [Astronomy] is to dilate the heart with universal benevolence, and to enlarge its views." David Rittenhouse, born 8 April 1732.

"There is no promised road leading to definite results. What's important is how to keep open as many options as possible." Makoto Kobayashi, born 7 April 1944.

"It’s the boundaries where the excitement is and where we will be in the future." Horst Störmer, born 6 April 1949.

"One should not make things complicated when a simple explanation will do." Ivar Giaever, born 5 April 1929.

Sunday, April 5, 2009

Physics Quote of the Day (March 29 - April 4)

"Physics without interpretation is only part of the story, and ... theories like quantum mechanics need careful foundational reflection." Harvey Brown, born 4 April 1950.

"The only effectual method of impressing abstract formulae and rules upon the memory, and, indeed, of making them fully and clearly apprehended by the understanding, is by examples of their practical application." Dionysius Lardner, born 3 April 1793.

"Light propagates and spreads not only directly, through refraction, and reflection, but also by a fourth mode, diffraction. ... Occasionally, light added to itself may give obscure surfaces on a body that has already received light." Francesco Maria Grimaldi, born 2 April 1618.

"Science is the future of mankind." Claude Cohen-Tannoudji, born 1 April 1933.

"Nature was not satisfied by a simple point charge but required a charge with spin." Sin-Itiro Tomonaga, born 31 March 1906.

"Over the past fifty years or so, scientists have allowed the conventions of expression available to them to become entirely too confining." David Mermin, born 30 March 1935.

"Publish an invention freely, and it will almost surely die from lack of interest in its development." Elihu Thomson, born 29 March 1853.