Special relativity Flashcards
Describe the idea of absolute motion.
Scientists believed that everything moved relative to a fixed background (an ether) which permeates all space including a vacuum and that a Sun is at rest relative to the ether.
How was it believed that the Michelson-Morley experiment would provide evidence for absolute motion?
They hoped to compare the variation (difference) in the speed of light parallel and perpendicular to the motion of the Earth to measure the absolute speed of the Earth.
Why was a semi-silvered glass block used?
To split the light source into two beams, by reflecting some light and transmitting the rest.
Why were plane mirrors used?
To reflect the light back to the semi-silvered glass block (A.K.A partial reflector ) where the two beams will converge and form an interference pattern.
Why was a (normal) glass block used?
To make that that both beams of light traveled through the same amount of glass and air.
What results were expected from the Michelson-Morley experiment?
It was expected that rotating the interferometer would result in a shift in the interference pattern produced, due to a change in the angle that each beam was moving relative to the absolute motion of the Earth.
The beam moving parallel to the motion of the Earth should take longer than the beam travelling at right angles to Earth’s motion.
The Michelson-Morley experiment failed to detect absolute motion. Explain why this is significant and what conclusions can be drawn from it.
The “null result” showed that the speed of light is the same value for all observers (it is invariant) and that it is impossible to detect absolute motion as the ether doesn’t exist.
Describe the measures Michelson and Morley went through to make sure their results were as accurate as possible.
They repeated their experiment at different times of day and different points in the year. )However, they detected no shift in the interference pattern).
What is meant by a frame of reference?
A space or system of coordinates that we use to describe the position of an object.
What is meant by a inertial frame of reference?
A frame of reference in which Newton’s first law is obeyed. Therefore, the object must be stationary or moving at constant velocity.
What are the two postulate of special relativity?
Physical laws have the same form in all inertial frames.
The speed of light in free space is invariant.
Describe what is meant by time dilation.
An observer of two events that is moving at constant velocity ,v, in relation to the two events will measure a longer time interval ,t, between the two events than the time measure by an observer who is stationary relative to the events ,t0.
Explain how muon decay provides evidence for time dilation.
Muons travel from the upper atmosphere to the ground at speeds close to c. They have a short half-life and we expect the intensity of the muons to decrease to a certain amount as they travel between the two points in the atmosphere. However, the time taken in their reference frame is shorter than the time taken in our reference frame so the intensity of the muons drops less than we expect. This is evidence for time dilation.
Describe length contraction.
An object moving in the same direction as its length looks shorter to an external observer.
