turning points - special relativity Flashcards
what is a frame of reference?
a set of coordinates to record the position of time and events
what is an inertial frame of reference?
a non-accelerating frame of reference (stationary or moving at a constant speed)
what is absolute motion?
note - this theory has now been rejected
movement with respect to a fixed reference frame. if an object is moving at a particular speed according to 1 observer, all other observers will observe the same thing
what was the “ether” meant to be?
a theoretical universal substance believed to be the medium for transmission of electromagnetic waves.
in the Michelson-Morley interferometer experiment, what did the semi-silvered block do to the light beam?
split the light beam from the source into 2 beams at the back surface of the semi silvered block (beam was partially reflected and partially refracted)
in the Michelson-Morley interferometer experiment what did they use the plane glass block for (compensatir?)
to ensure both beams travel through the same thickness of glass, otherwise the 2 wave trains would not overlap.
ensures they have the same optical path length.
in the Michelson-Morley interferometer experiment, what did the observer see through the viewing telescope?
a bright fringe where the 2 beams arrive in phase.
a dark fringe where they arrive 180 degrees out of phase.
interference pattern with concentric rings.
in the Michelson-Morley interferometer experiment, what did they do to the apparatus after their initial observations?
rotated it 90 degrees.
in the Michelson-Morley interferometer experiment, what did they predict would happen when the apparatus was turned 90 degrees? what was the sensitivity of their apparatus?
they predicted the fringes would shift by ~ 0.4 fringe widths. their apparatus was sensitive enough to detects a 0.05 fringe width shift.
explain why, in the Michelson-Morley interferometer experiment, they expected their prediction
initially, the equipment was aligned so that the beam in the direction of the plane glass block travelled parallel to the direction of the motion of the earth through the theoretical ether and the reflected part of the beam travelled perpendicular to the earths motion
turning the apparatus 90 degrees in a horizontal plane would swap the beams directions relative to the direction of the motion of the earth.
if the ether was real, they predicted the rotation would cause the difference in trave times on the 2 beams to reverse which would cause a shift in the position of the interference fringes
what observations were made in the Michelson-Morley interferometer experiment, and what did they conclude?
the detected no shift in the fringes - a null result
this disproved the ether theory
this meant Galileo’s laws of dynamics and Newton’s laws of motion didn’t work for light.
speed of light is invariant in free space.
what are the 2 postulates of special relativity?
the laws of physics are the same in all inertial frames of reference
the speed of light in a vacuum is invariant
explain the speed of light postulate
all observers measure the speed of light in a vacuum as the same speed ‘c’ regardless of the speed of the light source or speed of the observer.
what is the “proper time”
the time interval between 2 events measured by an observer at rest relative to the events
how does the time measured by an observer in another inertial frame compare to the proper time?
proper time is always smaller
how does the detection of muons at the Earths surface act as proof for time dialation?
muons are created several kilometres above the Earths surface, but from Newtonian laws of motion and the half life of muons, the predicted number of muons reaching the Earths surface if considerably less than the observed number that reach the surface.
this is a result of time dilation.
what is the proper length?
the length of an object measured by an observer at rest relative to the object.
a rod moving in the same direction as its length appears shorter than when it is stationary.
a rocket is travelling from Earth to a distant star, as a speed close to the speed of light. who (from an observer on the rocket and an observer on Earth) measures the proper length of the rocket, and who measures the proper length of the total journey?
assume the Earth and star are at rest relative to each other
observer on the rocket measures the proper length of the rocket, as they are stationary relative to the rocket
observer on Earth measures the proper length of the journey, as they are at rest relative to the Earth and the distant star.
the proper length is always ________ than the length measured by an observer who is moving relative to the object
longer
which is larger - the relativistic mass of a moving object or the rest mass of the object?
the relativistic mass is larger
mass increases with speed
what is the equation for total energy?
E = m c^2
where m = relativistic mass
what is the equation for rest energy?
E = m0 c^2
where m0 = rest mass
what is the equation for kinetic energy
KE = total energy - rest energy
which energy equation cannot be used when dealing with relativistic masses/speeds?
E = 0.5mc^2
