Einstein’s Laws

First Law

• Newton’s first law says that with no net force there is no acceleration.– Objects at rest

– Objects at constant velocity

• If there is no observed acceleration on an object with no net force, the observer is in an inertial reference frame.– Newton’s laws of mechanics apply equally

– No absolute motion

Inertial Frame

• An observer on the table sees two ball fall.– First straight down

– Second in a curve

• An moving with the second ball sees the reverse.– Second straight down

– First in a backwards parabola

• Both frames are inertial.– Motion consistent with

Newton

Accelerated Frame

• A rotating observer throws a ball across a merry-go-round.– Ball veers to the side

– No external force

• This is a non-inertial frame.– Observed motion

inconsistent with Newton’s laws

– Fictitious forces

Galilean Relativity

• Galileo described the conversion between inertial frames in 1638.– Galilean relativity

– Basis for Newton

• The Galilean transformation describes velocities as sums.

vtxx

y

x

S y’

x’

S’

Event Px

x’

v

yy

zz

tt

vuu xx

yy uu

zz uu

Moving Light

• Light as a wave should have a medium for transmission.– Velocity like a plane in the

wind

– Speed c without motion

• The earth’s velocity and the light velocity must add to get the result from the earth.

observed source light

medium

22 vcu

Michelson-Morley

• An interferometer can split and recombine light. – Interference shifts for

different paths

• In 1887 an experiment sowed the direction didn’t matter.

• The speed c must be a universal constant.mirror 2

mirror 1semi-silvered mirror

Einstein’s Principles

• In 1905 Einstein described two basic principles.

I. The laws of physics are the same in all inertial frames.

II. The speed of light in a vacuum is the same for all inertial frames, from all sources.

Equivalence Principle

• The 1905 principles became known as special relativity.

• In 1915 Einstein added another principle to form general relativity.

III.Gravitational mass is the same as inertial mass.

descsite.nl