M7 Topic 4 Special relativity

Question 1

what is the classical physics understanding of light (3)

Answer 1

• waves require a medium (aether)
• light where considered as continuous waves due to their wave behavior (such as reflection refraction, diffraction and interference)
• Maxwell predicted the speed of light 3x10⁸ms⁻¹

Question 2

What are the assumptions in Galilean Relativity (based on newtonian assuptions) (2)

Answer 2

• The laws of motion are the same in all inertial frames of reference ( cannot do any experiment to determine if you are moving)
• Relative velocity is calculated by adding vectors

Question 3

What were the issues with Galilean Relativity (2)

Answer 3

• Implied that the speed of light was not constant and could be increased
• Light was recorded to be constantly 3x10⁸ by Hertz

Question 4

What is the aether (2)

Answer 4

• The medium of propagation for light, based of the wave model of light
• Seen as the absolute frame of reference

Question 5

what was the justification for the aether model

Answer 5

Classical Physics, used the wave model of light due to light’s wave nature/behaviour (diffraction, refraction, reflection, interference)

Question 6

what are the features of the aether and their justifications

Answer 6

1. Feature: Filled the universe
   Explanation: light was everywhere
2. • Feature: Solid
   • Explanation: Propagate through solid due to light’s high speed)
3. • Feature: Transparent
   • Explanation: Light energy was not diminished as it
     passed through the aether
4. • Feature: Thin (very low density) and high elasticity
   • Explanation: Allow’s planets to move through it
     unimpeded(no friction)

Question 7

What is the significance of the aether (2)

Answer 7

• The Earth would experience an aether wind as it moves at 30kms⁻¹ through space
• Motion experiments on Earth would be relative to the aether( so it must be taken into account)

Question 8

What was the aim of the Michelson Morley experiment

Answer 8

Prove the existence of the aether, by identifying changes in the velocity of light , depending on its relative movement through space.

Question 9

how did the Michelson Morley experiment work (2)

Answer 9

• A laser was shot at a half transparent mirror where two light beams will form
• An interferometer was used to detect the difference in phases (interference pattern) between the two light beams that would have resulted in different travel times by rotating the apparatus.

Question 10

What were the results of Michelson Morley experiment

Answer 10

Michelson and Morley found NO change in the interference pattern at the interferometer as the apparatus was rotated through 90°

Question 11

What did the results in the Michelson Morley experiment mean (3)

Answer 11

• The light beams arrived at the same time irrespective of their motion relative to the aether
• results opposed the aether model of light transmission
• provided evidence for Einstein’s theory of special relativity, and its theory of light transmission

Question 12

what is a frame of reference

Answer 12

• The point of view or a perspective from where an observation can be made.

Question 13

What are inertial frames of reference

Answer 13

when the observer is not accelerating

Question 14

What are Einstein’s postulates of special relativity (2)

Answer 14

1. All inertial frames of reference are equivalent (The laws of physics are the same in all inertial frames of reference).
2. The speed of light (in a vacuum) is an absolute constant for all observers in an inertial frame of reference.

Question 15

What was Einstein’s thought experiment to explain special relativity

Answer 15

• He questioned, if you were in a train that moved at the speed of light, would you be able to see your reflection in a mirror

Question 16

what was the answer to Einstein’s thought experiment to explain special relativity (2)

Answer 16

Yes you can see your reflection as:

• Galilean relativity states that in a inertial frame of reference you cannot do an experiment, that can determine if you are moving or not.
• Light is moving at constant, therefore observers outside of the frame of reference would see the train contract (length contraction) and time move slower in the train( time dilation)

Question 17

What was Einstein’s explanation of Simultaneity within special relativity

Answer 17

• Simultaneity was relative, therefore no reference frame is correct (each perspective will see something different)

Question 18

What was Einstein’s example thought experiment of relativity

Answer 18

A train is moving close to the speed of light and there are two observers one outside the train and on inside the train, lightning strikes on both the front and the back of the train, what does each observer see?

Question 19

what was the result to Einstein’s thought experiment to explain relative simultaneity (2)

Answer 19

• The observer outside the train sees the lightning strike both the front and the back simultaneously, as the lightning is in the same frame of reference
• The observer inside the train sees the lightning at the front strike first and the back second as the train is moving towards light emitted from the front and moving away from the lighting at the back

Question 20

What was Einstein’s example thought experiment of time dilation

Answer 20

The light clock: a train moves close to the speed of light, a light source shines a light which is reflected off a mirror which reflects onto another mirror directly below it, and there are two observers one in the train and one outside, what does each observer see

Question 21

What was the result to einstein’s thought experiment of time dilation (2)

Answer 21

• Observer in the train is in the same frame of reference as the light clock , and sees the proper time ( the shortest possible amount of time t₀)
• Observer outside the train sees the clock ticking more slowly than the other observer as the approaching mirror is moving away from where the light was previously reflected causing the light to travel a longer distance

Question 22

What is the time dialation equation

Answer 22

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