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Posts Tagged ‘quantum physics

“The ‘paradox’ is only a conflict between reality and your feeling of what reality ‘ought to be’”*…

One of the most bizarre aspects of quantum physics is that the fundamental entities that make up the Universe, what we know as the indivisible quanta of reality, behave as both a wave and a particle. We can do certain experiments, like firing photons at a sheet of metal, where they act like particles, interacting with the electrons and kicking them off only if they individually have enough energy. Other experiments, like firing photons at small thin objects — whether slits, hairs, holes, spheres, or even DVDs — give patterned results that show exclusively wave-like behavior. What we observe appears to depend on which observations we make, which is frustrating, to say the least. Is there some way to tell, fundamentally, what the nature of a quanta is, and whether it’s wave-like or particle-like at its core?

That’s what Sandra Marin wants to know, asking:

“I wonder if you could help me to understand John Wheeler – the delayed choice experiment and write an article about this.”

John Wheeler was one of the most brilliant minds in physics in the 20th century, responsible for enormous advances in quantum field theory, General Relativity, black holes, and even quantum computing. Yet the idea about the delayed choice experiment hearkens all the way back to perhaps our first experience with the wave-particle duality of quantum physics: the double-slit experiment…

Although Einstein definitively wanted us to have a completely comprehensible reality, where everything that occurred obeyed our notions of cause-and-effect without any retrocausality, it was his great rival Bohr who turned out to be correct on this point. In Bohr’s own words:

“…it…can make no difference, as regards observable effects obtainable by a definite experimental arrangement, whether our plans for constructing or handling the instruments are fixed beforehand or whether we prefer to postpone the completion of our planning until a later moment when the particle is already on its way from one instrument to another.”

As far as we can tell, there is no one true objective, deterministic reality that exists independently of observers or interactions. In this Universe, you really to have to observe in order to find out what you get.

The history and the results of John Wheeler‘s famous “delayed choice” experiments: “Is Light Fundamentally A Wave Or A Particle?

* Richard Feynman

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As we reconsider categories, we might recall that it was on this date in 1404 that King Henry IV signed the “Act Against Multipliers,” stipulating that “None from hereafter shall use to multiply gold or silver, or use the craft of multiplication; and if any the same do, they incur the pain of felony.” Great alarm was felt at that time lest any alchemist should succeed in “transmutation” (the conversion of a base metal into gold or silver), thus undermining the sanctity of the Royal currency and/or possibly financing rebellious uprisings. Alchemy, which had flourished since the time of Bacon, effectively became illegal.

The Act was repealed in 1689, when Robert Boyle, the father of modern chemistry, and other members of the vanguard of the scientific revolution lobbied for its repeal.

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“Oh how wrong we were to think immortality meant never dying”*…

 

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Quantum simulation (Verresen et al., Nature Physics, 2019)

 

Further (in a fashion) to yesterday’s post…

Nothing lasts forever. Humans, planets, stars, galaxies, maybe even the Universe itself, everything has an expiration date. But things in the quantum realm don’t always follow the rules. Scientists have found that quasiparticles in quantum systems could be effectively immortal.

That doesn’t mean they don’t decay, which is reassuring. But once these quasiparticles have decayed, they are able to reorganise themselves back into existence, possibly ad infinitum.

This seemingly flies right in the face of the second law of thermodynamics, which asserts that entropy in an isolated system can only move in an increasing direction: things can only break down, not build back up again.

Of course, quantum physics can get weird with the rules; but even quantum scientists didn’t know quasiparticles were weird in this particular manner…

Maybe some things are forever.  More at “Scientists Find Evidence a Strange Group of Quantum Particles Are Basically Immortal.”

Read the underlying Nature Physics article, by physicist Ruben Verresen and his team at the Technical University of Munich and the Max Planck Institute for the Physics of Complex Systems, here.

* Gerard Way

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As we ponder perpetuity, we might send carefully-deduced birthday greetings to Richard Bevan Braithwaite; he was born on this date in 1900.  A Cambridge don who specialized in the philosophy of science, he focused on the logical features common to all sciences.  Braithwaite was concerned with the impact of science on our beliefs about the world and the appropriate responses to that impact.  He was especially interested in probability (and its applications in decision theory and games theory) and in the statistical sciences.  He was president of the Aristotelian Society from 1946 to 1947, and was a Fellow of the British Academy.

It was Braithwaite’s poker that Ludwig Wittgenstein reportedly brandished at Karl Popper during their confrontation at a Moral Sciences Club meeting in Braithwaite’s rooms in King’s College. The implement subsequently disappeared. (See here.)

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“Aside from velcro, time is the most mysterious substance in the universe”*…

 

Time

Detail from Salvador Dali’s Persistence of Memory

 

In normal life, you open the car door before getting into the car. Operation A happens before operation B. That’s the causal order of things. But a new quantum switch weirdly enables two operations to happen simultaneously. From Science News:

The device, known as a quantum switch, works by putting particles of light through a series of two operations — labeled A and B — that alter the shape of the light. These photons can travel along two separate paths to A and B. Along one path, A happens before B, and on the other, B happens before A.

Which path the photon takes is determined by its polarization, the direction in which its electromagnetic waves wiggle — up and down or side to side. Photons that have horizontal polarization experience operation A first, and those with vertical polarization experience B first.

But, thanks to the counterintuitive quantum property of superposition, the photon can be both horizontally and vertically polarized at once. In that case, the light experiences both A before B, and B before A, Romero and colleagues report.

While this is deeply weird and amazing, it unfortunately doesn’t occur at the human scale but rather in the quantum realm where measurements are in the nanometers. Still, quantum switches do have clear applications in future communications and computation systems.

Indefinite Causal Order in a Quantum Switch” (Physical Review Letters)

From the ever-illuminating David Pescovitz at Boing Boing: “Weird time-jumbling quantum device defies ‘before’ and ‘after’.”

* Dave Barry

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As we check our watches, we might send timely birthday greetings to Louis Essen; he was born on this date in 1908.  A physicist, he drew on his World War II work on radar to develop the first generally-accepted scientific measurement of the speed of light (one that has held up well as measurement techniques have advanced.).

But Essen is probably better remembered as the father of the atomic clock: in 1955, in collaboration with Jack Parry, he developed the first practical atomic clock by integrating the caesium atomic standard with conventional quartz crystal oscillators to allow calibration of existing time-keeping.

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Louis Essen (right) and Jack Parry (left) standing next to the world’s first caesium-133 atomic clock

 

Written by LW

September 6, 2018 at 1:01 am

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