Turning points: special relativity

Question 1

What did Einstein do in 1905?

Answer 1

-Published the first of two papers that would set out his theory of special relativity

Question 2

What is the inertia frame of reference?

Answer 2

• One which has Constant (or zero) velocity and experiences no acceleration
• Objects in an inertial Fram of reference remain moving in a straight line with constant velocity unless acted on by a force
• Any situation where motion is in a straight line with a constant velocity

Question 3

What is postulate 1?

Answer 3

• Physical laws have the same form in all inertial (non-accerating) frames of reference
• Laws of physics are same for all observers in all parts of the universe

Question 4

What is postulate 2?

Answer 4

• The speed of light in a vacuum, c is invariant which means light travels at the same speed regardless of the speed of the light source of the observer
• He believed there was no absolute motion and further no absolute time
• The only. absolute was the value of the speed of light in free space (a vacuum)

Question 5

What did Huygen’s wave theory assume?

Answer 5

• Light would need a medium to travel through

- Suggested ghat space filled with luminiferous æther

Question 6

What were the different theories of the aether?

Answer 6

• Experiments to measure properties of ether
  1. ‘aether wind’ as the Earth moved through the aether, as the movement through the aether would mean that light would take longer to travel through a known distance depending on its direction
  2. By measuring the speed of light in different directions, though that presence of aether and the motion of the Earth through it could detected
  3. Thought that change in time would be very small as Earth travels round the sun at 3x10^4 which is 0.01% speed of light

Question 7

What did the Michelson-Morely experiment do?

Answer 7

• Published 1887
• Interferometer, allows light from single source to undergo constructive and destructive interference to produce an interference fringe pattern
  1. White light source
  2. Semi-silvered mirror
  3. Compensating plate
  4. Two full mirrors

Question 8

What is the function of the semi-silvered mirror in the Michelson and Morely experiment?

Answer 8

Acts as beam splitter and allows both a reflected and transmitter beam of light

Question 9

What is the function of the compensating plate in the Michelson and Morely experiment?

Answer 9

A plane of glass block same thickness and semi-silvered mirror ensures that light beams in parallel and perpendicular arms of the interpreter travel the same distance in total

Question 10

What happens in the Michelson and Morely experiment?

Answer 10

1. The two beams from the beam splitter travel towards the mirrors in the parallel and perpendicular arms of the interferometer
2. They are then reflected back to the beam splitter and travel through it to the eyepiece of a viewing telescope
3. The two beams are initially from the same light beam, so they are coherent
4. This means that there will be an interference pattern seen because of the difference in path lengths of the two beams

Question 11

When will bright and dark firings be seen in the Michelson and Morely experiment?

Answer 11

• Bright finger seen when the beams are in phase with each other
• Dark drinks seen when the beams are 180 degrees out of phase with each other

Question 12

What would happen if the Earth is moving through the luminiferous aether?

Answer 12

The time it took for the light to travel to mirror 1 and back would be affected bu the aether and one particular pattern of interferences fringes would be obtained

Question 13

What would happen if the apparatus is turned through 90 degrees?

Answer 13

• There would be a difference in the time it takes for the light to travel between the mirrors because the aether would affect the light differently
• The interference pattern should change position

Question 14

What happened during the experiment compared to what was expected?

Answer 14

• The aether theory rep ducted that the change in orientation would make the fringes shift by about 0.4 of. fringe width and the sensitive was 0.05
• After many attempts unable to detect predicted fringe shift
• Obtained null result suggesting aether did not exist

Question 15

What is relative motion?

Answer 15

The movement of one object in comparison to another. Relative motion may appear different to different observers

Question 16

What is absolute motion?

Answer 16

Motion which is independent of where it is being observed from. We know that everything is in motion and absolute motion does not exist

Question 17

How was the Michelson and Morely experiment affected by absolute motion?

Answer 17

1. Should have been able to detect the absolute motion of Earth through space, due to the effect this motion would have of motion of light
2. Arm travelling parallel to direction of motion of Earth through space, taken slightly longer to travel to mirror and back rather than arm perpendicular to motion of Earth
3. But speed of light invariant of motion of observer
4. Showed no way to measure absolute motion of Earth

Question 18

What is proper time?

Answer 18

The time t0 measured by the observer in the same frame of reference as the event being timed

Question 19

What is time dilation?

Answer 19

The observation that time in a moving frame of reference runs more slowly than the time for the same event in a stationary frame of reference
-Moving clocks run slow

Question 20

What is rest mass?

Answer 20

The mass of an object measured in its rest frame. it is not moving relative to an observer

Question 21

What happens as an object gets faster?

Answer 21

• Its mass increases (as measured by an observer)

- It requires more energy to increase the speed of the object

Question 22

What is the rest energy?

Answer 22

The energy equivalent to the rest mass of an object

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