GRE Question of the Day (March 2)

Verbal
Subatomic particles can be divided into two classes: fermions and bosons, terms coined by physicist Paul Dirac in honor of his peers Enrico Fermi and Satyendra Bose. Fermions, which include electrons, protons, and neutrons, obey the Pauli exclusion principle, according to which no two particles can inhabit the same fundamental state. For example, electrons cannot circle the nuclei of atoms in precisely the same orbits, loosely speaking, and thus must occupy more and more distant locations, like [b]a crowd filling seats in a stadium.[/b] The constituents of ordinary matter are fermions; indeed, the fact that fermions are in some sense mutually exclusive is the most salient reason why two things composed of ordinary matter cannot be in the same place at the same time. Conversely, bosons, which include photons (particles of light) and the hitherto elusive Higgs boson, do not obey the Pauli principle and in fact tend to bunch together in exactly the same fundamental state, as in lasers, in which each photon proceeds in perfect lockstep with all the others. Interestingly, the seemingly stark division between fermionic and bosonic behavior can be bridged. All particles possess “spin,” a characteristic vaguely analogous to that of a spinning ball; boson spins are measured in integers, such as 0 and 1, while fermion spins are always half- integral, such as \({\frac{{1}}{{2}}}\) and \({1*}\)\({\frac{{1}}{{2}}}\). As a result, whenever an even number of fermions group together, that group of fermions, with its whole-number total spin, effectively becomes a giant boson. Within certain metals chilled to near absolute zero, for instance, so-called Cooper pairs of electrons form; these pairs flow in precise harmony and with zero resistance through the metal, which is thus said to have achieved a superconductive condition. Similarly, helium-4 atoms (composed of 2 electrons, 2 protons, and 2 neutrons) can collectively display boson-like activity when cooled to a superfluid state. A swirl in a cup of superfluid helium will, amazingly, never dissipate. The observation that even-numbered groups of fermions can behave like bosons raises the corollary question of whether groups of bosons can ever exhibit fermionic characteristics. Some scientists argue for the existence of skyrmions (after the theorist Tony Skyrme who first described the behavior of these hypothetical fermion-like groups of bosons) in superconductors and other condensed-matter environments, where twists in the structure of the medium might permit skyrmions to form.

The example of “a crowd filling seats in a stadium” is intended to

• expand upon one consequence of the Pauli exclusion principle
• illustrate a behavior of certain fermions
• describe how electrons circle the nuclei of atoms in concentric, evenly-spaced orbits

Correct Answer - I and II- (click and drag your mouse to see the answer)

The author’s primary purpose in writing this passage is to

(A) explain the mechanism by which fermions can become bosons
(B) describe the two classes of subatomic particles
(C) provide examples of the different forms of matter
(D) explain the concept of particle “spin”
(E) argue that most matter is composed of one type of particle

Correct Answer - B - (click and drag your mouse to see the answer)

Which of the following is NOT mentioned as a characteristic of bosons?

(A) They can be composed of groups of fermions.
(B) They are measured in integer spin.
(C) They are the constituents of ordinary matter.
(D) They tend to bunch together in the same fundamental state.
(E) They lead to phenomena such as superconductors and superfluids.

Correct Answer - C - (click and drag your mouse to see the answer)

Which of the following can be properly inferred from the passage?

(A) An atom composed of two protons and a neutron would be considered a boson.
(B) Skyrmions have been discovered in superconductors and other condensed matter environments.
(C) Two electrons in an atom cannot circle the same nucleus at exactly the same distance.
(D) A current through a superconducting wire will never dissipate.
(E) Fermions cannot behave as bosons unless they are cooled to a temperature near absolute zero

Correct Answer - D - (click and drag your mouse to see the answer)

According to the passage, which of the following describes a difference between fermions and bosons?

(A) Fermions cannot inhabit the same fundamental state, whereas bosons bunch together in the samestate.
(B) Fermions contain many more types of particles than bosons.
(C) Fermions exist in groups, but bosons do not.
(D) Fermions have integral spin values, whereas Bosons have half-integer spin.
(E) Fermions do not obey the Pauli principle, whereas bosons do.

Correct Answer - A - (click and drag your mouse to see the answer)

Based on the information in the passage about the Pauli exclusion principle, to which one of the following situations would this principle be most relevant?

(A) Fermi Energy: The maximum energy that electrons in a solid will contain in order to avoid having identical energy levels.
(B) Particle Accelerators: Devices that will accelerate charged particles to very high speeds through the application of an external magnetic field.
(C) Quantum Entanglement: When particles interact physically and then become separated but still have interdependent properties.
(D) Double Slit Experiment: An experiment that revealed the particle and wave duality of photons.
(E) The Higgs Field: The field produced by the conjectured Higg’s particle that would explain why matter has mass.

Correct Answer - A - (click and drag your mouse to see the answer)

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