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Bronze-level articleQuantum mechanics

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Quantum mechanics could use some help. Please research the article's assertions. Whatever is credible should be sourced, and what is not should be removed.

I think I can safely say that nobody understands quantum mechanics.
Richard Feynman, The Character of Physical Law (1965)

Quantum mechanics (QM) is a branch of physics developed to deal with sub-atomic particles and their interactions. Most of the foundations of QM were formulated during the first three decades of the 20th century. Modern quantum physics has advanced considerably, and is used extensively in the study of chemistry and materials, including biological research, and in cosmology and astronomy.


[edit] What QM explains

One of the awesome things quantum mechanics explains is why energy is absorbed or emitted by atoms only in discrete amounts, or "quanta". This in turn explains why hot objects (like the sun) shed light in the specific way that they do. Quantum mechanics also explains a host of other phenomena, like superconductivity (used in MRI machines and some high-speed trains), Hawking radiation (theoretically emitted by black holes), the fine structure of the microwave background cosmic radiation (caused by quantum fluctuations on top of flat spacetime), how magnets work, the biochemical properties of proteins, why metals conduct and plastic doesn't, and more. Its probabilistic nature also explains many every day things like why glass is both reflective and transparent; light has a probability of going through it and another defined probability of being reflected back. In a purely classical system the light would do one or the other because all interactions would be identical and deterministic. It explains why lightbulbs glow and become hot, it explains how electricity travels as quickly as it does; in short, it explains everything.

Well... nearly everything. Physicists have identified electromagnetism, gravitation and the weak and strong interactions as the fundamental forces of nature. As it currently stands, quantum mechanics, via the Standard Model of Particle Physics, can only describe the three nongravitational forces. Furthermore, dark matter, dark energy and neutrino physics remain poorly understood, as are the origin and fate of the Universe.

[edit] The standard model

Since modern quantum mechanics was completed, in around 1927, physicists have sought to construct quantum theories that would describe the fundamental forces of nature - namely gravity, electromagnetism, and the strong and weak nuclear forces - taking into account special relativity. In the 1930s, Enrico Fermi was able to characterize the weak nuclear force, which is responsible for radioactive decay. In the 1940s, Richard Feynman, Julian Schwinger and Sin-Itiro Tomogana developed the quantum-mechanical generalization of Maxwell's equations, quantum electrodynamics. Soon, physicists were able to show that in high energy interactions, electromagnetic and weak interactions are one and the same thing, hence the electroweak force. Building upon these encouraging successes, physicists set their sight on the strong nuclear force. By the 1970s, quantum chromodynamics - note that it has nothing to do with color in the ordinary sense - emerged as the best description of strong interactions available. It then became clear that the behavior of elementary particles can be described with a very high degree of accuracy if one possesses quantum theories of electromagnetism and the nuclear forces. Gravity has invariably been neglected because (1) it is a truly feeble force and (2) no one has ever been able to forge a usable quantum theory of gravity. The quantum theories of non-gravitational interactions is now known as the Standard Model and is considered to be the most accurate physical theory ever created. In July 4th, 2012, the Standard Model made it to the popular press when the Large Hadron Collider at CERN, Geneva, Switzerland, made statistically significant observations and measurements of a particle whose behavior is consistent with the so-called Higgs boson, thought to interact with other particles and give the mass. It is important to note that this does not confirm the existence of the Higgs boson; independent observation is needed.

Together with general relativity, our modern theory of gravity, the Standard Model describes in exquisite detail everything physicists think they know for certain about the Universe. But of course, like all theories of science to-date, there are things not dreamed of in this theory. Besides being unable to successfully incorporate gravity, it has very little to say about neutrino physics, and indeed, nothing to say about dark matter and dark energy, which empirical evidence suggests to comprise the overwhelming majority of our Universe.

[edit] Weird and spooky

Some of the phenomena of quantum mechanics, such as entanglement were described by Albert Einstein as "spooky" because, at the sub-atomic level, physics as we think we know it breaks down and becomes almost incomprehensible.

[edit] Core principles

There are a few basic core principles for understanding quantum mechanics and the supposedly spooky oddness that goes on at the level of atoms. It is very important to remember one key thing: quantum mechanics is not classical mechanics. Quantum theory is mostly a mathematical description of how the world works at the atomic level based on very good evidence. Taking any of the interpretations too literally would be a mistake.

[edit] The Photoelectric Effect

In the late nineteenth century, James Clerk Maxwell formulated a theory of electromagnetism that described a wide range of electrical phenomena, and in particular described light as an electromagnetic wave. Despite the success of this theory, the early twentieth century found it unable to describe certain aspects of the photoelectric effect.

When exposed to light, certain materials release electrons. Studying this effect, researchers found that for a fixed frequency of light, the rate of electron emission is directly proportional to the intensity of the indecent incident light, but if the frequency of the light was below a certain threshold, no electrons would be emitted no matter how intense the light was. It was no longer a matter of how much power was being imparted to the photoelectric material -- a very high-power, but low frequency source was incapable of liberating electrons, while a source with significantly lower power output at a higher frequency would liberate electrons.

This effect was explained by describing the light as a stream of particles, called "photons". Each photon has a small amount of energy which is proportional to its frequency. More intense light has more photons, but each photon has the same energy. The electrons in the photoelectric material only interact with one photon at a time, so if a single photon doesn't have enough energy to liberate an electron, no electron will be liberated no matter how many photons per second are interacting with the material.

Needless to say, this description of light as a series of particles, albeit ones with a "frequency" associated with them, conflicted with Maxwell's description.

[edit] Quantisation of energy

Prior to quantum theory, energy was thought of as necessarily analogue; taking any value indiscriminately and acting as a smooth transition. In the macroscopic world, this observation remains fairly true. Like a hosepipe that can deliver whatever amount of water you like by turning the tap in small amounts.

For bound systems at the quantum level, such as electrons bound to atoms, energy can take on certain discrete values. This is analogous to a car that can only travel at 10, 20, 30 or 40 (and so on) miles per hour, rather than a smooth and seamless increase of speed. If you don't give it enough energy to make the transition between 20 mph and 30 mph, it'll stay at 20 mph. This forms the basis of spectroscopy - and without this quantisation of energy, such analytical tools would be impossible.

To muddy the waters even further, particles need not be in a single energy state at all, but might be in what is called a "superposition of states". Using the previous car as an example, this is analogous to that car travelling at 20, 30, and 50 mph all at once. A particle in this state cannot even really be said to have an energy, although it can be said to have an average energy depending on how much of it is in each state.

This superposition of states is fundamental to the idea of "wavefunction collapse" in the Copenhagen interpretation[1], which states that an observation of energy forces a particle that was in a superposition state into one of the states it is composed of with various probabilities depending on the specifics of the superpostion. So a measurement of the quantum car in a superposition of 20, 30, and 50mph will show the speed as 20, 30, or 50mph, and after the measurement the car will be in the single energy state corresponding to whichever speed you measured.

[edit] Particle-wave duality

Classical mechanics treats particles and waves as different things. A particle is a point, a speck with mass and an exact location. A wave is a little more abstract but it has wavelength - it's spread out, with frequency and speed. In quantum mechanics there is no distinction. Particles can be waves and waves can be particles - although really they're something else entirely with some, but not all, of the properties of both. We've evolved in a macroscopic world where we can see a distinction, but there isn't in the quantum world.

The evidence for this comes from two experiments. Classically, light was treated as a wave - there was no quanta or a concept of individual particles, just waves of energy. This explained Isaac Newton's optics pretty well. However, work done on something called the photoelectric effect, which Albert Einstein won the Nobel Prize for, smashed this interpretation. Einstein noted that the details photoelectric effect - where a metal gave off electrons when exposed to light of discrete kinetic energy - could only be explained if light was a particle. If light consisted as particles then it would explain why the effect was instant (waves would take time to absorb as light waves are hundreds of times larger than atoms), that energy given off was proportional to the frequency and there was a cut-off point where the effect didn't happen below a certain frequency. Each photon carried a discrete amount of energy, proportional to its frequency, and delivered it to the metal. Needless to say, starting to describe light as a particle seriously caused issues with optics and the concept of frequency; waves can have a frequency, but particles cannot.

After this, however, came the double-slit experiment. This experiment fired electrons through two slits. Under classical mechanics, the electron was a particle. The last thing you would expect from a particle being fired through two slits would be an interferrence pattern but this is what was observed. The electrons were exhibiting interference; a properties of waves. The extra "spooky" part was that when the electrons were reduced to the point where only one would flow through the slits at a time the pattern was still seen; not only did the wave of the electron interfere with other electrons, it interfered with itself.

From these observations, particle-wave duality was born. At the quantum level, there is no clear distinction between waves and particles. Various interpretations have been put forward to explain this in a way that "makes sense" - however, all suffer from the fact they are trying to interpret quantum mechanics as classical mechanics.

[edit] Uncertainty

Star Trek fan: "How do the Heisenberg compensators work?"
Gene Roddenberry: "They work just fine, thank you."

With the wave-like nature of quantum mechanics established, problems began to arise in figuring out the location of particles. Waves do not have a specific location; they're spread out over an area and aren't described the same way as particles. Thus the "uncertainty principle" was established; in short it means you cannot know the location and momentum of a particle to the same degree of accuracy. This isn't a limit in scientific instruments but a fundamental aspect of physics. Even god can't know the location and velocity of particles at the same time. It is a physical impossibility.

This effect arises from the fact that there is a series of states of a particle that have a definite momentum, and a series of states that have a definite position, but those two series of states are not the same. A state with a definite momentum is a superposition of states with definite position, and vice versa. The uncertainty principle shows that a particle can be in a state that is a superposition of a small range of momenta and a superposition of a small range of positions simultaneously, but the smallness of one of those ranges cannot be made smaller without making the other range larger.

[edit] Interpretations

There are many attempts at intuitive interpretations of quantum mechanics,[2] and a minor industry of physicists coming up with them. These attempt to come up with an intuitive framework to explain the equations - essentially, trying to get them to "make sense" because, frankly, they really don't. Common ones include the Copenhagen and Many Worlds interpretations. Interpretations usually try to translate quantum physics into classical physics terms so our piddly ape brains can grasp it all. (Some claim not to do this and then do it anyway.)

[edit] Copenhagen interpretation

The Copenhagen interpretation, favoured by quantum mechanical pioneer Niels Bohr, envisages that the wavelike behaviour of particles "collapses" upon observation. It proposes that superpositions of states should be taken extremely literally and that a wavefunction is nothing more than an abstract concept that just reflects our uncertainty and lack of knowledge prior to an observation. It's illustrated best by thought experiments such as Schrödinger's cat, whereby a cat is thought to be both dead and alive at the same time until observed (although Schrödinger did initially propose the experiment to show the absurdity of extrapolating the Copenhagen interpretation to macroscopic objects, people still take it literally and that apparently included Nobel laureate Eugene Wigner). Objections to the Copenhagen interpretation evoke its non-deterministic nature.

[edit] Conscious observation

See the main article on this topic: Observer effect

"Observation," in the sense of the Copenhagen interpretation is really just short-hand for any form of interaction with a quantum system. There are some, however, that seem to take it as requiring conscious observation, i.e., observation by a human mind. This is highlighted in the intentional absurdity of Schrödinger's cat experiment, where the cat and the detector itself act as "observers." There are further questions for those who take the conscious observer literally, serious scientists at least tend not to subscribe to the conscious observer idea, but there are/were a few exceptions like Wigner.[3]

  1. Why should a conscious observer be significant?
  2. How can consciousness have effects that unconscious physical processes lack?

There is a parallel here with the claims of psychokinesis:

  1. Psychokinesis assumes that conscious beings cause physical changes by willing them to happen.
  2. The Copenhagen interpretation assumes that conscious beings cause physical changes by observing them to happen.

The idea that consciousness is somehow special appeals to some religions and also to the purveyors of some types of Quantum woo. By contrast p-zombies materialists dislike the idea that there is anything more to consciousness than physical processes within the brain and 19th century block-heads materialists are uncomfortable with the Copenhagen interpretation.

This group of interpretations is known as subjective collapse interpretations, since they hold that the wave function and its collapse are real phenomena, and that collapse is triggered by the conscious non-material mind (as in dualism). The most notable subjective collapse interpretation is the von Neumann-Wigner interpretation.

Well-known physicist and atheist author/apologist Victor Stenger argued against the Copenhagen interpretation in his book The Unconscious Quantum.

[edit] Many worlds

The many worlds interpretation states that the apparent randomness and statistical nature of quantum mechanics is caused, literally, by the universe splitting into different sections each time an observation is made. This interpretation rejects the wavefunction "collapse" of the Copenhagen interpretation. Hugh Everett first suggested many worlds in 1954 [4] but it wasn't until 1970, when Bryce DeWitt (who coined the term "many-worlds") wrote an article for Physics Today, that the general field was first informed of Everett's ideas. [5]

Many worlds is about taking the equations seriously. It assumes that configuration space is reality, where physics actually happens, and physical space is a sort of shadow world. Many people balk at this notion.

Religious people are likely to be uncomfortable with many worlds. It is troublesome for those who believe in souls because when worlds branch or split souls must branch with the worlds.

It's not hard to see why so many people find these ideas disturbing. For if they are correct, they have profound implications for our understanding of the nature of the Soul, because the Soul (if there is such a thing) must branch along with the worlds that contain it. It would appear that the writings on which many contemporary religions are based make no mention of such an idea.[6]

Like any subject related to human consciousness, many worlds is a fruitful source of philosophical whaargarbl.

It is also worth noting that the Many-Worlds Interpretation relies on the notion of unitary evolution of a wavefunction which behaves linearly. However, if, as some theorists such as T.P. Singh have pointed out, gravity when quantized must behave nonlinearly, one of the crucial assumptions of the Many-Worlds Interpretation — linearity — breaks down.[7] Wavefunction collapse also re-enters the picture as a real, dynamically explicable phenomenon.

[edit] Problems with interpretations

The problem with interpretations of quantum mechanics is that it's the same mathematical theory, and these interpretations are experimentally indistinguishable. This leads to attempts to discriminate between them on subjective piddly ape brain criteria such as intuitive beauty (unreasonable), invocation of Occam's razor (now that’s reasonable), thought experiments (sometimes useful but not the physical kind) and so forth. This means that even an intellectually honest interpreter ends up writing pseudo-philosophy which is not scientifically rigorous, objective or testable. If you work on the interpretations right, you can get QM to say whatever on earth you first thought of before starting, that leads to more quantum woo.

While interesting, the interpretations are the bane of a philosophy of science known as instrumentalism, which states that theories be judged entirely on predictive qualities, not their ability to make sense to our particular brains. Basically, cut it with the visualisations — they're not making any predictive difference — and just do the numbers.

[edit] Quantum woo

See the main article on this topic: Quantum woo

Quantum physics is a difficult subject and people without science degrees are rarely expected to understand it — even those with the degrees are usually expected to have a working knowledge and not a full appreciation of every aspect of it. Its difficulty is further increased by the fact that, in many cases, there really are no decent lay explanations of how it works, so more accurate and nuanced explanations are lacking in popular science. Given the level of both complexity and counter-intuitive nature of quantum theory (and perhaps because of the quantum-based technobabble frequently employed in science fiction like Star Trek) woo-meisters can always call their wares "quantum" something or other and people are likely to expect it. Then ordinary people sometimes react with, "Well it doesn’t make sense to me but I suppose the scientists understand it." There is some psychological evidence to suggest people are more likely to believe explanations that are wrong if they are dressed up with science like terms — indeed, advertisers have exploited this for many years, notably cosmetics commercials that border on self-parody. This all combines to make quantum woo a very attractive pseudoscience for people to engage in.

[edit] Quantum consciousness

See the main article on this topic: Quantum consciousness

Scientists have some partial understanding of quantum physics but frequently disagree with each other while ordinary people are regularly mystified. Similarly the reason for consciousness is, given current scientific knowledge, impossible to understand. (Of course various religions and woo advocates convince their dupes acolytes that they know the answer to consciousness.) Its reasoning is as follows:

  • Quantum mechanics is weird, spooky and I can’t understand it.
  • Consciousness is weird, spooky and I can’t understand what causes it.
  • Therefore perhaps the two are connected.

Quantum consciousness is just one example where the woo-meisters can make it big time.

[edit] See also

[edit] External links

[edit] Footnotes

  1. Other interpretations of quantum mechanical observation that don't involve wavefunction collapse, such as the many-worlds hypothesis, are postulated, but the wavefunction collapse interpretation is one of the oldest and easiest to describe.
  2. Wikipedia has an entire category devoted to these interpretations.
  3. Consciousness and Quantum Physics The author claims to have a PhD and be a former professor. The text is difficult to understand but if you read carefully there is the unproved assumption that consciousness creates reality. Take that assumption away and all the nice spirituality fails.
  4. Do parallel universes really exist?
  6. Douglas S. Jones. "The Many-Worlds Interpretation of Quantum Mechanics." 2001 September 11.
  7. T. P. Singh. "The inevitable nonlinearity of quantum gravity falsifies the many-worlds interpretation of quantum mechanics." 2007 May 16.
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