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Re: GAMSAT Section 1 Questions Unit 10

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GAMSAT Section 1 Questions Unit 10

Unit 10
Questions 1-5

Analyse and evaluate this theoretical explanation of String Theory

String Theory was proposed to try to reconcile quantum mechanics and particle theory. Relativistic quantum field theory has worked very well to describe the observed behaviours and properties of elementary particles. But the theory itself only works well when gravity is so weak that it can be neglected. Particle theory only works when we pretend gravity doesn’t exist. General relativity has yielded a wealth of insight into the Universe, the orbits of planets, the evolution of stars and galaxies, the Big Bang and recently observed black holes and gravitational lenses. However, the theory itself only works when we pretend that the Universe is purely classical and that quantum mechanics is not needed in our description of Nature.

 Originally, string theory was proposed as an explanation for the observed relationship between mass and spin for certain particles called hadrons, which include the proton and neutron. Things didn’t work out, though, and Quantum Chromodynamics eventually proved a better theory for hadrons. But particles in string theory arise as excitations of the string, and included in the excitations of a string in string theory is a particle with zero mass and two units of spin.  If there were a good quantum theory of gravity, then the particle that would carry the gravitational force would have zero mass and two units of spin. This has been known by theoretical physicists for a long time. This theorized particle is called the graviton.

One can add a graviton to quantum field theory by hand, but the calculations that are supposed to describe Nature become useless. This is because particle interactions occur at a single point of space time, at zero distance between the interacting particles. For gravitons, the mathematics behaves so badly at zero distance that the answers just don’t make sense. In string theory, the strings collide over a small but finite distance, and the answers do make sense. This doesn’t mean that string theory is not without its deficiencies. But the zero distance behaviour is such that we can combine quantum mechanics and gravity, and we can talk sensibly about a string excitation that carries the gravitational force.

Think of a guitar string that has been tuned by stretching the string under tension across the guitar. Depending on how the string is plucked and how much tension is in the string, different musical notes will be created by the string. These musical notes could be said to be excitation modes of that guitar string under tension. In a similar manner, in string theory, the elementary particles we observe in particle accelerators could be thought of as the “musical notes” or excitation modes of elementary strings. In string theory, as in guitar playing, the string must be stretched under tension in order to become excited. However, the strings in string theory are floating in space-time, they aren’t tied down to a guitar. Nonetheless, they have tension. The string tension in string theory is denoted by the quantity 1/(2 p a’), where a’ is pronounced “alpha prime” and is equal to the square of the string length scale.

If string theory is to be a theory of quantum gravity, then the average size of a string should be somewhere near the length scale of quantum gravity, called the Planck length, which is about 10-33 centimetres, or about a millionth of a billionth of a billionth of a billionth of a centimetre. Unfortunately, this means that strings are way too small to see by current or expected particle physics technology and so string theorists must devise more clever methods to test the theory than just looking for little strings in particle experiments.

The assessments must include whether or not the particle spectrum includes fermions. In order to include fermions in string theory, there must be a special kind of symmetry called supersymmetry, which means for every boson (particle that transmits a force) there is a corresponding fermion (particle that makes up matter). So supersymmetry relates the particles that transmit forces to the particles that make up matter, though never directly observable thus far.

1 The main idea or thesis of the passage is:
A. String theory – an augmentation of quantum mechanics
B. Particle physics and an exploration of the implications of string theory
C. A general overview of string theory in relation to theoretical physics
D. String theory – from the graviton to supersymmetry

2 According to passage information, which of the following describes the properties of a graviton?
I    have a corresponding fermion and boson
II   has zero mass and two units of spin
III  are mathematically verifiable

A. I
B. I & II
C. II only
D. II & III

 3 It can be inferred from passage information, that at the basic core of discussions of String theory is:
A. The hadron, made up of a proton and neutron
B. The properties and degree of gravity
C. Supersymmetry between boson and fermion
D. Classical thermodynamic notions of energy and volume  

4 Based on passage information, which of the following assertions, in relation to String Theory is false?  Which is the exception?
A. All types of strings are closed loops
B. String Theory attempts to philosophically “verify” gravitational spin
C. All types of strings roughly correspond to the size of Planck length
D. The string tension is equal to the square root of the string length

5 What is the major theoretical drawback of the notions proposed by String Theory?
A. The graviton lacks explanatory power for gravitational spin
B. Supersymmetry is too specific, therefore considered a hasty generalization
C. Strings cannot be empirically quantified, nor observed in experiment
D. Classical Physics and Quantum Mechanics really cannot be reconciled on a scientific basis

Answers – GAMSAT Section 1 Questions Unit 10

1 C is the Correct Answer.

Since both quantum mechanics and particle physics are considered to be theoretical physics (C) represents the best choice answer, because the passage focuses on how string theory provides a general overview and a connection between the two.  (A) & (B) only reflect one of the two, while (D) is too specific on aspects of String theory in general 

2 C is the Correct Answer. Only II or (C) describes the properties of the graviton, which can be affirmed by a close reading or re-scan.  String theory itself is unobservable, its main weakness, ruling out III, Fermions and bosons are associated with supersymmetry

3 B is the Correct Answer. At the very basis of discussion, gravity must be the basis for the discussion, all the other notions are related, to a certain extent, but not at the core, as asked in the question. 

4 A is the Correct Answer. All are true except (A) it does not state this in the passage, but:

String theories are classified according to whether or not the strings are required to be closed loops, 

5 C is the Correct Answer.

Clearly (C) is the best answer.  Strings are only theoretical.  They cannot be observed.

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