EINSTEIN'S SPECIAL RELATIVITY 1905                    back to ether      back to relative motion

Einstein had been thinking about time and relative motion since he was a boy. He posed the thought experiment based on a tram moving at the speed of light away from a clock showing noon. What would the clock show to the people on the tram - hence how would time intervals be seen in the clock's frame  according to people on the tram?

He had also been thinking about the unusual feature of Maxwell's Equations that the speed of light seems to be the same no matter how it is looked at.

What he did was radical - assume the speed of light IS the same, and see what happens!

At this time he was a patent officer going through physics and engineering patent applications to see if they were viable or infringed existing patents. He had enough time on his hands to do physics. He had just finished his equivalent to our PhD and it was published in this year.

Born in 1879, Einstein was but 2 when the Michelson-Morley experiments were first carried out. They were to affect his thinking though he was not to mention the results in any of his work. He thought long and hard about Lorentz's idea of physical interaction of the ether with physical material leading to physical contractions.

ASSUMPTIONS in Special relativity

1. ALL laws of physics apply equally in all inertial reference frames - NO absolute frame - This is an extension of Newton's assumption now covering electromag as well as Newton's Laws.

2. The velocity of light is independent of all motion - it is the same value in a vacuum to all observers regardless of what they are doing - it is an invariant.

RESULTS

• Newton's universal time - "time is the same for everybody, everywhere," is now incorrect.

It is replaced with relative time intervals - time differs from observer to observer depending on what they are doing. We talk of PROPER time and RELATIVISTIC time.

• Maxwell's Laws come into line with the Conservation Laws of momentum and energy and operate equally in all frames of reference with no preferred frame - the absolute frame has gone, the ether has gone. Magnetism becomes a relative motional effect of the Electrostatic Force Laws.

• Newton's Laws are now "low speed" - say less than 1/10th speed of light - approximations of a wider mechanics
• Perceptions of length and mass now become relative.
• The relative motion formula   v_BrelA = v_BrelG - v_ArelG    is now incorrect.

This is pretty clear if two passing spaceships are to see the same speed for light.

Consider the diagram below - we have three frames

 

1. Earth, where we are, is "stationary" - the ground G
2. Alien A coming from the left at 0.9 x speed of light ( 0.9c) rel to Earth G
3. Alien B coming from the right at 0.9 x speed of light ( 0.9c) rel to Earth G

The formula gives      Vel Alien B rel to A = 1.8c >c ,

but if a laser beam shines from B towards A, it must

• leave B at the speed of light according to everyone
• and pass A also at the speed of light according to everyone!
• So the light must leave B and pass A before the spaceship B passes A according to everyone.

Vel Alien B rel to A < speed of light ! The formula is stuffed.

Relative Time/Proper Time