Gravity

Question 1

Name the 4 fundamental forces of nature, rank them by relative strength, and identify what role each one plays in the universe

Answer 1

1. Strong nuclear force
   - strength = 1
   - holds atomic nuclei together
2. Electromagnetism
   - strength = 1/137
   - most of the forced you experience in your daily life are manifestations of this force
3. Weak nuclear force
   - strength = 10^-5
   - involved in radioactive decay
4. Gravity
   - strength = 10^-39
   - makes masses attract one another

Question 2

Describe how forces are related to accelerations

Answer 2

Newton’s second law:
‣ The change in speed (acceleration), a, is equal to the applied force, F, divided by the mass of the object, m
‣ a = F/m
‣ F = ma

Question 3

Distinguish between Newtonian gravity and Einstein’ s general relativity (2 examples of Einstein’ s GR)

Answer 3

• Newtonian gravity: the force between two objects is proportional t. It’s mass and distance between them
• F = G * (m1*m2)/d^2
• double the mass = double the attraction
• double the distance = divide attraction by 2◦ Einstein replaced Newtonian concept of gravity with GENERAL RELATIVITY
  ‣ Gravity is a consequence of spacetime curvature
  ‣ objects with mass curves spacetime
  ‣ THEREFORE ACCELERATION IS PRODUCED BY SPACETIME CURVEATURE
  ◦ Example: the precession of the perihelion of mercury
  ‣ Planets do not orbit in fixed orientations, the axis of orbit precesses over time
  ‣ So mercury with is closest to sun - orbit precesses more because since it is closer to a large mass (the sun) it is more affected by the curvature of spacetime
  ◦ Example: gravitational lensing: massive objects bend spacetime around them, forcing light to follow its curved contours

Question 4

• Explain the Equivalence Principle and describe what it tells us about the nature of gravity (1 eg)

Answer 4

◦ The laws of physics including special relativity would work exactly the same in an accelerating FoR as they would in a gravitational field
‣ Eg on earth vs an elevator in space accelerating 9.8m/s^2 - a dropped ball will fall to the floor at the same rate

Question 5

• Describe how the presence of a mass deforms spacetime

Answer 5

◦ SPACETIME: is a four dimensional space: length, width, depth, time
◦ In general relativity, we think of objects bending space and time like a ball on a rubber sheet,
‣ It deforms the fabric
‣ the more mass, the more it bends spacetime

Objects must follow curved trajectories through that curved spacetime

Question 6

• Describe gravity in terms of spacetime curvature

Answer 6

◦ Earth is not orbiting because of a force at its core, it is following the straightest possible path through spacetime
◦ The earth orbits the sun because the sun is curving spacetime
◦ The rubber sheet analogy