Special Relativity Flashcards
Why was it assumed that light travelled through a medium
At the time, all other known waves such as sound and water travelled through a medium, so it was assumed that light did as well
What was the medium for light waves called
Luminiferous aether/ aether
What was the aim of the Michelson-Morley experiment
To use the difference in speeds of light to determine the absolute motion of the Earth relative to the aether and therefore prove the existence of the aether
What was the interferometer
A device consisting of two arms of identical lengths at right angles to each other, with mirrors at each end and a beam splitter at their intersection
What is a beam splitter
A semi-silvered mirror which allows some light to pass through and reflects some light
Why is a pane of glass used in the Michelson-Morley experiment
To ensure both beams of light travelled the same distance of glass and air
To make time a controlled variable
To obtain equal optical path lengths
What happened to the light when it reached the beam splitter
Some light was reflected and some was transmitted, forming two beams that were initially coherent
What does the observer see through the telescope in the Michelson-Morley experiment when the distances are equal
An interference pattern formed of concentric circles
Outline how the Michelson-Morley interferometer apparatus was used and state the result and its significance
Rotate the apparatus through 90 degrees.
Observe the fringes at the same time.
Observed fringes did not change or shift.
This means speed of light in free space is invariant and there is no aether
The interference fringe pattern did not shift when the apparatus was rotated by 90 degrees. Explain the significance of this null observation
Speed of light is invariant.
Rotation by 90 degrees realigns beams relative to direction of Earth’s motion.
No shift means no change in optical path difference between the 2 beams.
Time taken by light to travel to each mirror unchanged by rotation.
Distance to mirrors unchanged by rotation.
No shift means speed of light is unaffected
Einstein postulated that the speed of light in free space is invariant. Explain what is meant by this postulate
Speed of light does not depend on the motion of the light source or that of the observer
What conclusions were made from the null result of the Michelson-Morley experiment
The aether does not exist therefore light can travel without a medium
Speed of light is unchanged by the Earth’s motion - it is invariant
Explain why Michelson and Morley predicted the fringes in the interference pattern would shift when the interferometer was rotated 90 degrees
They predicted the speed of light depended on the motion of the Earth, relative to the aether.
Therefore, the time difference would change between the two beams when they were rotated.
So there would be a change in phase difference, shifting the fringes
PHASE DIFFERENCE, NOT PATH DIFFERENCE AS LENGTHS REMAIN THE SAME
What does invariance of light mean
When in a vacuum, the speed of light is always ‘c’ for every observer
What is an inertial frame of reference
A reference frame that is non-accelerating.
All inertial reference frames are moving at constant velocity with respect to each other
What are Einstein’s two postulates of relativity
The laws of physics are the same in all inertial frames of reference
The speed of light in free space is invariant/ same in all inertial frames of reference
What is t0 in time dilation equation
Proper time, measured in the same frame of reference as the event. This means they are stationary relative to the event.
E.g. Measuring the time taken to travel from Mars to Earth when you are on the spaceship would be T0 but if you were on Earth measuring the time, it would be T
What is t in the time dilation equation
Dilated time, measured by an observer in a different frame of reference to the event
What is l0 in length contraction equation
Proper length, the length measured by an observer at rest to the event
L<L0
What is l in length contraction equation
Contracted length, the length measured by an observer moving relative to the event
e.g. the length measured by a person running past a car
What is the evidence for time dilation and length contraction
Muon decay
Observer on Earth viewed muon’s half-life as longer (time dilation)
Muons’s reference frame views the distance needed to travel as shorter (length contraction)
What is mass-energy equivalence
Mass can be converted into energy
Energy can be converted into mass
E = mc^2
Examples of where mass is converted into energy or energy to mass
Fusion of hydrogen into helium in the centre of the Sun
Fission of Uranium in nuclear power plants
Nuclear weapons
High-energy particle collisions in particle accelerators
What is mo
An objects proper mass, measured by an observer at rest relative to the object
What happens to the mass of an object as its speed near the speed of light
Its mass gets larger and larger and vert close to the speed of light, the object’s mass tends towards an infinite mass
Rest energy of an object in motion relative to an observer =
E0 = mo x C^2
Kinetic energy of an object in motion with total energy E which is greater than its rest mass energy E0
Ek = mc^2 - moc^2
What is Bertozzi’s experiment direct evidence for
The variation of kinetic energy with speed
What did Bertozzi do
He accelerated electrons to speeds close to the speed of light and measured their KE.
KE of electrons in Bertozzi’s experiment
eV
How did Bertozzi calculate the KE of electrons
He measured the temperature change of the target and used E = mc x delta theta.
This energy change is equal to the total KE lost by incoming electrons
