Special Relativity How does light behave in moving reference frames?

Some lingering questions remain… Maxwell’s equations describe and predict electromagnetic phenomena amazingly well. but… 1)What is the medium for electromagnetic waves? 2)In what frame of reference is the speed of light 3.00 x 10 8 m/s?

Luminiferous ether? Hypothesized as medium for light Hypothesized as frame of reference for light Does a microwave work on a fast-moving spaceship? Does a motor work? If there is an ether, it’s odd. Does not noticeable interact with matter Is it dragged by Earth or is there an ethereal wind through which the Earth (and everything else) travels?

Michelson-Morley Interferometer In 1880’s, Albert Michelson and Edward Morley designed an experiment to test the speed of the ethereal wind. Swimmers’ analogy Stationary relative to ether? waves should constructively interfere. Moving relative to ether? waves should destructively interfere

Null results Michelson and Morley discovered no evidence of the Earth’s motion through ether.

Consequence of no ether 1.No ‘absolute’ frame of reference 2.If no ‘absolute’ frame, laws of physics (including electricity and magnetism) work in all reference frames 3.If laws of physics work in all reference frames, speed of light is the same in all frames of reference Speed of light is the same regardless of reference frames?

Postulates of special relativity 1 st postulate: All laws of nature are the same in all uniformly moving frames of reference Sometimes called “inertial reference frames” 2 nd postulate: The speed of light in empty space is the same, regardless of the motion of the source or the motion of the observer

Frames of reference (with a ball) The thrower throws the ball at, say, 10 m/s in each situation. The receiver perceives the ball to travel at 1)10 m/s (truck at rest) 2)>10 m/s (moving towards) 3)<10 m/s (moving away) BLuI118nhzc

Frames of reference & light 1)What happens if you travel at the speed of light and turn on a flashlight? Does that light travel at twice the speed as the ‘slow’ light? 2)Imagine an electric current traveling down a wire. It induces a magnetic field. Would you detect a magnetic field if you traveled the same speed as the electric current?

Frames of reference (with a flashlight) The person on the truck shines a flashlight at the person standing on the ground. The person on the ground perceives the light to travel at 1)3 x 10 8 m/s (truck at rest) 2)3 x 10 8 (moving towards) 3)3 x 10 8 (moving away)

Explaining null results Hendrik Lorentz proposed a mathematical trick as a solution Trick = hypothesis with no physical explanation Suppose that objects contract (i.e., shrink) in the direction of their motion

What if length contraction not a trick? Einstein’s insight: if speed of light is the same in all reference frames, 1.Clocks aren’t 2.Rulers aren’t Read the original paper here: s/einstein/specrel/www/ s/einstein/specrel/www/

Time and distance in stationary frames Person on high-speed rocket ship measures time with a light clock, a device in which light bounces back and forth between two mirrors

Time and distance in moving frames Person NOT on the ship observes the following: Moment 1Moment 3Moment 2

Time and distance in moving frames Person NOT on the ship observes the following: Moment 1Moment 3Moment 2

Time and distance in moving frames By Pythagorean Theorem

Time dilation If physics works regardless of reference frame, then observers note that time slows downs for events in moving reference frames. NOT “seems to slow”, but really and actually and counterintuitively slows At everyday speeds (v << c), v/c ~0, so time dilation is barely noticeable. At relativistic speeds, (v some appreciable fraction of c), time dilation is significant.

Length contraction Length contracts in direction of travel.

Measurement between frames Measurements of time and length between one frame of reference and another will agree only on the speed of light.

Other implications

Using relativity to explain electromagnets Imagine an electric current traveling down a wire. It induces a magnetic field. Would you detect a magnetic field if you traveled the same speed as the electric current?Would you detect a magnetic field if you traveled the same speed as the electric current

Think about… Joe Burpy is 30 years old. He has a daughter who is 6 years old. If Joe leaves on a Greyhound space bus which takes a 5-year (space bus time) round trip at 0.99 c. a)How old will Joe be when he returns? b)How old will his daughter be?

More… Minute Physics: Special relativity: Energy & mass equivalence: O9M O9M How far away is tomorrow: Adding velocity: Breaking speed of light: Why you can’t go speed of light: Nw Nw New York University – Physics Dept: elativity.html# elativity.html#