Question

Quantum gravity, if developed, would be an improvement on both general relativity and quantum mechanics, but more mathematically difficult. Under what circumstances would it be necessary to use quantum gravity? Similarly, under what circumstances could general relativity be used? When could special relativity, quantum mechanics, or classical physics be used?

a) Quantum gravity for large masses; general relativity for small masses
b) Quantum gravity for small masses; general relativity for large masses
c) General relativity for high speeds; special relativity for low speeds
d) Quantum mechanics for large masses; classical physics for small masses

Answer 1

Quantum gravity would be necessary for phenomena involving both quantum effects and strong gravitational fields, such as in black holes or the early universe.

General relativity is used for massive objects and strong gravitational fields, special relativity for high speeds, quantum mechanics for the very small, and classical physics for everyday phenomena.

Step-by-step explanation:

The circumstances under which quantum gravity would be necessary are at the intersection of quantum mechanics and general relativity, particularly with phenomena involving extremely small scales and large masses, such as black holes or the very early universe.

General relativity is used to describe the universe at large scales and strong gravitational fields, such as the motion of planets or the bending of light by stars. Special relativity is applicable when dealing with high velocities close to the speed of light, but without significant gravity effects, whereas classical physics can be used for everyday phenomena involving larger objects and lower speeds.

Quantum mechanics is required for the description of phenomena at atomic and subatomic scales, where particle-like and wave-like properties of matter come into play.

Each of these theories has its own domain of applicability, informed by the scales of energy, mass, velocity, and gravitation at which different physical effects dominate. Therefore, the correct answer to the circumstances requiring the use of quantum gravity or general relativity, as described in the parameters of the student's question, would be:

• Quantum gravity for scenarios involving both high gravitational effects and quantum effects, typically at the Planck scale.
• General relativity for large masses or strong gravitational fields.
• Special relativity for objects moving at speeds close to the speed of light with negligible gravitational influences.
• Quantum mechanics for processes involving the very small, at or smaller than atomic scales.
• Classical physics for daily scale phenomena where quantum and relativistic effects are not significant.