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Posts Tagged ‘Niels Bohr

“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


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.


“A measurement is not an absolute thing, but only relates one entity to another”*…




Until now, [the mass of the kilogram] has been defined by the granddaddy of all kilos: a golf ball-sized metal cylinder locked in a vault in France [a replica of which is pictured above]. For more than a century, it has been the one true kilogram upon which all others were based…

Made of a corrosion-resistant alloy of 90 percent platinum and 10 percent iridium , the international prototype kilo has rarely seen the light of day. Yet its role has been crucial, as the foundation for the globally accepted system for measuring mass upon which things like international trade depend.

Three different keys, kept in separate locations, are required to unlock the vault where the Grand K and six official copies — collectively known as ‘‘the heir and the spares’’ — are entombed together under glass bell-jars at the International Bureau of Weights and Measures, in Sevres on the western outskirts of Paris.

Founded by 17 nations in 1875 and known by its French initials, the BIPM is the guardian of the seven main units humanity uses to measure its world : the meter for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for the amount of a substance and the candela for luminous intensity.

Of the seven, the kilo is the last still based on a physical artifact, the Grand K. The meter, for example, used to be a meter-long metal bar but is now defined as the length that light travels in a vacuum in 1/299,792,458th of a second…

The metal kilo is being replaced by a definition based on Planck’s constant, which is part of one of the most celebrated equations in physics but also devilishly difficult to explain . Suffice to say that the update should, in time, spare nations the need to occasionally send their kilos back to Sevres for calibration against the Grand K. Scientists instead should be able to accurately calculate an exact kilo, without having to measure one precious lump of metal against another…

More of this weighty story at “The kilogram is changing. Weight, what?

* H.T. Pledge, Science since 1500


As we muse on measurement, we might send well-calibrated birthday greetings to August Kundt; he was born on this date in 1839.  An astronomer-turned-physicist, he developed a method to measure the velocity of sound in gases and solids using a closed glass tube (now known as a Kundt’s Tube).

AugustKundt source

We might also spare a thought for another physicist, Niels Bohr; he died on this date in 1962.  A Danish physicist and philosopher, Bohr was the first to apply quantum theory to the problem of atomic and molecular structure, creating the Bohr model of the atom, in which he proposed that energy levels of electrons are discrete, and that the electrons revolve in stable orbits around the atomic nucleus but can jump from one energy level (or orbit) to another– a model the underlying principles of which remain valid.  And he developed the principle of complementarity: that items could be separately analyzed in terms of contradictory properties, e.g., particles behaving as a wave or a stream. His foundational contributions to understanding atomic structure and quantum theory won him the Nobel Prize in Physics in 1922.



“The missing link in cosmology is the nature of dark matter and dark energy”*…


Familiar visible matter can be thought of as the privileged percent—actually more like 15 percent—of matter. In business and politics, the interacting 1 percent dominates decision making and policy, while the remaining 99 percent of the population provides less widely acknowledged infrastructure and support—maintaining buildings, keeping cities operational, and getting food to people’s tables. Similarly, ordinary matter dominates almost everything we notice, whereas dark matter, in its abundance and ubiquity, helped create clusters and galaxies and facilitated star formation, but has only limited influence on our immediate surroundings today…

The common assumption is that dark matter is the “glue” that holds together galaxies and galaxy clusters, but resides only in amorphous clouds around them. But what if this assumption isn’t true and it is only our prejudice—and ignorance, which is after all the root of most prejudice—that led us down this potentially misleading path?…

Indeed,  Harvard theoretical physicist and cosmologist Lisa Randall asks, “Does dark matter harbor life?

* Stephen Hawking


As we reach reflexively for a flashlight, we might send particular birthday greetings to Abraham Pais; he was born on this date in 1918.  After earning his Ph.D. in physics in Holland five days before a Nazi deadline banning Jews from receiving degrees, he went into hiding– and worked out ideas in quantum electrodynamics (later shared with Niels Bohr) that became the building blocks of the theory of elemental particles.  He was later a colleague of Robert Oppenheimer and Albert Einstein at the Institute for Advanced Studies at Princeton.

Pais was also an widely-respected historian of science.  Among his many works were a biography of Bohr and (the work for which he’s best remembered as a historian) Subtle Is the Lord: The Science and Life of Albert Einstein, which is considered the definitive Einstein biography.



Written by LW

May 19, 2017 at 1:01 am

Monsters of Grok…


From Amorphia Apparel:

Fake band t-shirts for history’s greatest minds:  I don’t know about you, but I think science and philosophy are pretty bad ass, so join me in rocking out with some the most influential thinkers of all time!

More sagacious shirts at Amorphia Apparel


As we opt for ring-spun wisdom, we might wish a thoughtful Happy Birthday to cognitive and computer scientist John McCarthy; he was born on this date in 1927.  A recipient of the Turing Award, McCarthy coined the phrase “Artificial Intelligence” (in a 1955 proposal for a 1956 Dartmouth conference), founded the first A.I. Lab (at MIT in 1958), and created LISP (List Processing Language), the computer language most commonly used in AI research.

In 1961, McCarthy was the first to imagine and propose a future in which computers could be shared by multiple users, and computing could be provided as a utility.  The idea was popular in the late 60s, then waned in the 70s, as it became clear that hardware and software weren’t (yet) up to the task.  But with the new millennium, McCarthy’s concept retook the fore– and in the last few years, rose to The Cloud…



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