Physics for Everyone, Book 4: Photons and Nuclei

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At the time of writing , commercially viable fusion power has not yet been achieved. However, research is underway, specifically at the Culham Centre for Fusion Energy, to bring a fusion reaction under control so that it can be used to generate electricity.


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This would have the advantage of minimal nuclear waste, since the main product would be non-radioactive helium, with some tritium, which has a relatively short year half-life. The fusion of nuclei smaller than Iron releases energy. This is because, if we were to take all the baryons of both the nuclei apart, and then stick them all back together as one, we would do less work that would be required to stick them together as the two separate nuclei.

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The difference in binding energy is the energy that is released by a fusion reaction. This energy might be given to the 'real' particles which are given off, or to a 'virtual' particle such as a photon. What else is produced, apart from energy? What single nucleus does this produce? In this reaction, 1. What difference in binding energy does this correspond to? In order to contain a fusion reaction, electromagnetism may be used. What other force could be used? Why is this not being used for fusion reactors on Earth? To do so, we could measure which path a particle took through the double slits, but only after it has passed through them.

By then, it ought to have "decided" whether to take one path or both. The sheer act of noticing, rather than any physical disturbance caused by measuring, can cause the collapse.

Class 12 Physics chapter 13 - Nuclei revision - blue sky pcm - important questions and topics -

An experiment for doing this was proposed in the s by the American physicist John Wheeler, and this "delayed choice" experiment was performed in the following decade. It uses clever techniques to make measurements on the paths of quantum particles generally, particles of light, called photons after they should have chosen whether to take one path or a superposition of two. It turns out that, just as Bohr confidently predicted, it makes no difference whether we delay the measurement or not.

Atomic nucleus

As long as we measure the photon's path before its arrival at a detector is finally registered, we lose all interference. Whenever, in these experiments, we discover the path of a quantum particle, its cloud of possible routes "collapses" into a single well-defined state. What's more, the delayed-choice experiment implies that the sheer act of noticing, rather than any physical disturbance caused by measuring, can cause the collapse. But does this mean that true collapse has only happened when the result of a measurement impinges on our consciousness?

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That possibility was admitted in the s by the Hungarian physicist Eugene Wigner. Wheeler even entertained the thought that the presence of living beings, which are capable of "noticing", has transformed what was previously a multitude of possible quantum pasts into one concrete history.

In this sense, Wheeler said, we become participants in the evolution of the Universe since its very beginning. In his words, we live in a "participatory universe. To this day, physicists do not agree on the best way to interpret these quantum experiments, and to some extent what you make of them is at the moment up to you. But one way or another, it is hard to avoid the implication that consciousness and quantum mechanics are somehow linked.

Beginning in the s, the British physicist Roger Penrose suggested that the link might work in the other direction. Whether or not consciousness can affect quantum mechanics, he said, perhaps quantum mechanics is involved in consciousness. What if, Penrose asked, there are molecular structures in our brains that are able to alter their state in response to a single quantum event. Could not these structures then adopt a superposition state, just like the particles in the double slit experiment? And might those quantum superpositions then show up in the ways neurons are triggered to communicate via electrical signals?

Maybe, says Penrose, our ability to sustain seemingly incompatible mental states is no quirk of perception, but a real quantum effect. After all, the human brain seems able to handle cognitive processes that still far exceed the capabilities of digital computers. Perhaps we can even carry out computational tasks that are impossible on ordinary computers, which use classical digital logic.


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Penrose first proposed that quantum effects feature in human cognition in his book The Emperor's New Mind. The idea is called Orch-OR, which is short for "orchestrated objective reduction". The phrase "objective reduction" means that, as Penrose believes, the collapse of quantum interference and superposition is a real, physical process, like the bursting of a bubble.

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Orch-OR draws on Penrose's suggestion that gravity is responsible for the fact that everyday objects, such as chairs and planets, do not display quantum effects. Penrose believes that quantum superpositions become impossible for objects much larger than atoms, because their gravitational effects would then force two incompatible versions of space-time to coexist.

Penrose developed this idea further with American physician Stuart Hameroff. In his book Shadows of the Mind , he suggested that the structures involved in this quantum cognition might be protein strands called microtubules. These are found in most of our cells, including the neurons in our brains. Penrose and Hameroff argue that vibrations of microtubules can adopt a quantum superposition. It has been suggested that the idea of quantum superpositions in microtubules is supported by experiments described in , but in fact those studies made no mention of quantum effects.

Besides, most researchers think that the Orch-OR idea was ruled out by a study published in Physicist Max Tegmark calculated that quantum superpositions of the molecules involved in neural signaling could not survive for even a fraction of the time needed for such a signal to get anywhere. Quantum effects such as superposition are easily destroyed, because of a process called decoherence.

This is caused by the interactions of a quantum object with its surrounding environment, through which the "quantumness" leaks away. Nerve signals are electrical pulses, caused by the passage of electrically-charged atoms across the walls of nerve cells. If one of these atoms was in a superposition and then collided with a neuron, Tegmark showed that the superposition should decay in less than one billion billionth of a second.

It takes at least ten thousand trillion times as long for a neuron to discharge a signal. However, Penrose is unmoved by those arguments and stands by the Orch-OR hypothesis. And despite Tegmark's prediction of ultra-fast decoherence in cells, other researchers have found evidence for quantum effects in living beings. Some argue that quantum mechanics is harnessed by migratory birds that use magnetic navigation , and by green plants when they use sunlight to make sugars in photosynthesis.

Besides, the idea that the brain might employ quantum tricks shows no sign of going away. For there is now another, quite different argument for it.


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  8. View image of Could phosphorus sustain a quantum state? In a study published in , physicist Matthew Fisher of the University of California at Santa Barbara argued that the brain might contain molecules capable of sustaining more robust quantum superpositions. Specifically, he thinks that the nuclei of phosphorus atoms may have this ability. Phosphorus atoms are everywhere in living cells. They often take the form of phosphate ions, in which one phosphorus atom joins up with four oxygen atoms.

    Such ions are the basic unit of energy within cells. Much of the cell's energy is stored in molecules called ATP, which contain a string of three phosphate groups joined to an organic molecule. When one of the phosphates is cut free, energy is released for the cell to use. This media cannot be played on your device. Cells have molecular machinery for assembling phosphate ions into groups and cleaving them off again.

    Fisher suggested a scheme in which two phosphate ions might be placed in a special kind of superposition called an "entangled state". The phosphorus nuclei have a quantum property called spin, which makes them rather like little magnets with poles pointing in particular directions. In an entangled state, the spin of one phosphorus nucleus depends on that of the other. Put another way, entangled states are really superposition states involving more than one quantum particle.

    Fisher says that the quantum-mechanical behaviour of these nuclear spins could plausibly resist decoherence on human timescales. He agrees with Tegmark that quantum vibrations, like those postulated by Penrose and Hameroff, will be strongly affected by their surroundings "and will decohere almost immediately". But nuclear spins do not interact very strongly with their surroundings. All the same, quantum behaviour in the phosphorus nuclear spins would have to be "protected" from decoherence.

    This might happen, Fisher says, if the phosphorus atoms are incorporated into larger objects called "Posner molecules". These are clusters of six phosphate ions, combined with nine calcium ions. There is some evidence that they can exist in living cells, though this is currently far from conclusive.

    I decided In Posner molecules, Fisher argues, phosphorus spins could resist decoherence for a day or so, even in living cells. That means they could influence how the brain works. The idea is that Posner molecules can be swallowed up by neurons. Once inside, the Posner molecules could trigger the firing of a signal to another neuron, by falling apart and releasing their calcium ions.

    Because of entanglement in Posner molecules, two such signals might thus in turn become entangled: a kind of quantum superposition of a "thought", you might say. Lithium drugs are widely used for treating bipolar disorder. They work , but nobody really knows how.

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    But then he came across a paper reporting that lithium drugs had different effects on the behaviour of rats, depending on what form — or "isotope" — of lithium was used. On the face of it, that was extremely puzzling. In chemical terms, different isotopes behave almost identically, so if the lithium worked like a conventional drug the isotopes should all have had the same effect. But Fisher realised that the nuclei of the atoms of different lithium isotopes can have different spins. This quantum property might affect the way lithium drugs act.

    For example, if lithium substitutes for calcium in Posner molecules, the lithium spins might "feel" and influence those of phosphorus atoms, and so interfere with their entanglement. At this point, Fisher's proposal is no more than an intriguing idea.