Chapter 12 - special_relativity

Question 1

What is Einstein’s first postulate of special relativity?

Answer 1

The speed of light in a vacuum is an absolute constant, independent of the motion of the source or observer.

Question 2

What is Einstein’s second postulate of special relativity?

Answer 2

All inertial frames of reference are equivalent; the laws of physics are the same for all observers moving at constant velocity.

No experimental device that can tell you travelling at a constant spped or standing still

Question 3

What experimental evidence supports the constancy of the speed of light?

Answer 3

The Michelson–Morley experiment, which failed to detect any difference in light’s speed due to Earth’s motion.

All known as a null reesult.

Question 4

What is time dilation?

Answer 4

The phenomenon where time appears to pass more slowly in a moving frame relative to a stationary observer.

the ticks of clock take longer

Question 5

Give an example of experimental evidence for time dilation.

Answer 5

Observation of cosmic-ray muons reaching Earth’s surface despite their short half-life.

without time dilation they would not be able to reach the each surface due to short half life (but we still detect them)

Question 6

What experiment confirmed time dilation using atomic clocks?

Answer 6

The Hafele–Keating experiment (1971), flying atomic clocks around Earth.

Question 7

What is length contraction?

Answer 7

The shortening of an object along its direction of motion relative to an observer, as its velocity approaches the speed of light.

note: length contraction only in direction of motion

Question 8

Where is evidence for relativistic effects also observed?

Answer 8

Particle accelerators and cosmological observations.

LHC-

Question 9

What is the formula for relativistic momentum?

Answer 9

p = γmv, where γ = 1/√(1-v^2/c^2).

Question 10

What consequence does relativistic momentum have for velocity?

Answer 10

It imposes a limit — no particle with mass can reach or exceed the speed of light.

Question 11

What is Einstein’s mass-energy equivalence relationship?

Answer 11

E = mc^2

Question 12

Give three examples of processes where mass is converted into energy.

Answer 12

1. Nuclear fusion in the Sun.
2. Particle–antiparticle annihilation (e.g., electron–positron).
3. Combustion of conventional fuel.

Question 13

Why is E = mc^2 significant?

Answer 13

It shows mass and energy are interchangeable, revolutionising physics and underpinning nuclear energy.

Question 14

What thought experiment did Einstein use with a moving train and lightning strikes?

Answer 14

Observers on the train and on the platform disagree on whether lightning strikes at both ends of the train happened simultaneously, showing that simultaneity is relative.

Question 15

What was Einstein’s ‘chasing a beam of light’ thought experiment?

Answer 15

He imagined running alongside a light beam. If classical physics were true, he would see stationary light waves, but this contradicted Maxwell’s equations — leading him to propose light’s speed is constant.

Question 16

How did Einstein’s light clock thought experiment demonstrate time dilation?

Answer 16

A light beam bouncing between mirrors takes longer for a moving observer compared to a stationary one, showing that moving clocks run slower.

Question 17

What experiment attempted to detect the aether and supported Special Relativity instead?

Answer 17

The Michelson–Morley experiment, which showed no difference in light speed regardless of Earth’s motion.

Question 18

How do cosmic-ray muons provide evidence for time dilation?

Answer 18

Muons created high in the atmosphere should decay before reaching Earth, but many are detected because their lifetime is extended by relativistic time dilation.

Question 19

What experiment confirmed time dilation using aircraft?

Answer 19

The Hafele–Keating experiment (1971), where atomic clocks flown around the world recorded time differences consistent with relativity.

Question 20

How do particle accelerators provide evidence for Special Relativity?

Answer 20

Particles gain huge momentum without exceeding the speed of light, and their lifetimes are extended at high velocities.