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A Level Physics - Turning Points > Special Relativity > Flashcards

Flashcards in Special Relativity Deck (13):

What is absolute motion?

Many physicists thought that electromagnetic waves were vibrations in an invisible substance they called the ether.

Light was thought to travel at a fixed speed relative to the ether.

Detection of the ether was thought to be possible as a result of comparing the time taken by light to travel the same distance in different directions.


Describe the Michelson-Morley experiment.

They used an interferometer to test the ether hypothesis.

The light beam is split into 2 beams at the back surface of the semi-silvered glass block.
The beams reflect at separate mirrors.
When the reflected beams meet back at the semi-silvered block, they form an interference pattern.

(See notes for diagram)


Why was a compensator block in the Michelson-Morley experiment?

The compensator block makes sure both beams travel through the same thickness of glass so they will overlap.


What was the purpose of the Michelson-Morley experiment?

They were trying to prove the ether hypothesis. If it was correct, there would be a noticeable shift in the interference fringes when the apparatus was shifted through 90 degrees.
It would change the travel times of the beams.


What was the significance of the Michelson-Morley experiment?

The experiment produced a null result. They were unable to detect the predicted fringe shift.

They concluded that the ether doesn't exist. Absolute rest disproved.
The speed of light has the same value for all observers.


What is relative motion?

To detect the relative motion of an object, you need to observe the object's position against your frame of reference (markers that are fixed relative to your own position).


Describe Einstein's theory of special relativity.

Physical laws have the same form in all inertial frames of reference (frames of reference that move at constant velocity).

The speed of light in free space is invariant (always the same, independent of motion of light and motion of any observer).


What is time dilation?

The time between events is stretched out according to a moving observer.


What is length contraction?

A rod moving in the same direction as its length appears shorter than when it is stationary.


What is relativistic mass?

Mass of an object increases with speed.


What is the cosmic speed limit?

It is the speed of light in free space.

No material object can ever reach this speed as mass would become infinite and can never travel faster than light.


What is the special relativistic relationship between mass and energy?

Increasing the speed of an object increases its kinetic energy.


Describe Bertozzi's experiment to prove special relativity.

Used linear particle accelerators to accelerate pulses of electrons from rest through a measured p.d. between 0.5MeV and 15MeV.

Measured time taken for the electrons to reach a target electrode at a fixed distance so speed could be calculated.

Checked the kinetic energy of the electrons by measuring the metal target's temperature rise caused by the gain of kinetic energy from the electrons.

Plotted a graph of KE against v/c, closely matched that predicted by Einstein's formula.