Einstein and Relativity

The Special Theory of Relativity (1905) underpins much of modern science and technology, from our view of the Big Bang to the harnessing of nuclear power for industry. A short description of the theory and the supporting evidence will be presented, and the impact of relativity on modern science will be described.

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Einstein and 1905

March 1905 - On the nature of light

Cornerstone of Quantum Theory

May 1905 - Brownian Motion

Verification of the existence of molecules

June 1905 - Special Theory of Relativity

Physics of bodies moving at high speed

1915 - General Theory of Relativity

Physics of accelerating bodies

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Special Theory of Relativity

- One of the world’s most successful scientific theories

- Dramatic impact on 20th century science and technology

- Changed our view of time and space

What is Special Relativity ?

“Modification of traditional physics required to describe bodies moving at tremendously high speed”

Special Relativity - bodies in uniform (non-accelerated) relative motion

General Relativity - bodies in non-uniform (accelerated) relative motion

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Galileo and Relativity

Galileo : Object released from a height on the deck of a ship experiences the same drop irrespective of motion of ship

→

Principle of Relativity

“..laws of motion indifferent as to whether a system is at rest or moving at constant velocity”

‘no absolute frame of reference for the laws of motion’

Note : motion of ship cannot be detected inside the ship

In tune with Copernican view of earth

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Relativity and the Ether

Maxwell - electricity and magnetism = EM

speed of EM wave = speed of light c

→

light = EM wave

→

c = speed of light wave relative to ether ?

(wave must travel through medium)

Absolute frame of reference for EM ?

- Experiments undertaken to measure earth’s motion relative to ether

o null result

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Einstein and Relativity

- EM theory in apparent conflict with Galilean relativity

- unsuccessful attempts to detect earth’s motion relative to the ether

- induction of an electric current in a loop of wire by a magnet depends only on the relative motion of magnet and loop

Relativity applies to EM ?

No absolute frame of reference for EM?

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The Special Theory of Relativity

Einstein 1905 : Galileo’s relativity must be replaced by new relativity principle that includes EM

1.“…the laws of all physics are the same for observers in uniform relative motion”

“the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest”

From EM, Einstein added a second postulate

2. “…the velocity of light in empty space has a constant value, independent of the motion of source or observer...”

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Derivation

Postulates could not be predicted by traditional mechanics

→

traditional ideas about time and distance must be revised!

→

New equations for distance and time assuming constancy of speed of light

Lorentz Transformation

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Predictions

No absolute values for distance, mass or time!

1. Length of body will shrink* when travelling at high speed

L = L0

* relative to a stationary observer

2. Mass of body will increase when travelling at high speed

m =

Note c = ULTIMATE SPEED LIMIT

3. Time interval will increase for body travelling at high speed

t =

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Implications

1. Time is relative

- two observers in relative motion could measure elapsed time differently and not agree that two events were simultaneous.

2. Mass is a form of energy

E = mc2

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Experimental Evidence

Mass increase

Mass of high-energy particles (β- rays of radioactivity)

Ultimate Velocity

Speed of high-energy particles

Speed of light emitted by high-energy particles

Time Dilation

Lifetimes of high-energy particles (muon detection at sea level)

Mass-energy

Radioactivity

Nuclear fission, fusion

Particle anti-particle creation

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High-energy particles conform to the speed limit set by the speed of light

Relativistic mass increase with increasing velocity

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General Relativity (1915)

- motion of bodies with accelerated motion.

- gravity = geometric curvature of space-time.

Predicitions

Light bending in gravitational field

Clocks run slow in gravitational field

Evolution of the Universe

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