Lecture 1 A Revolution in time

Classical Physics Timeline

Modern Physics Timeline

At the age of 16, Einstein puzzled over what it would be like to travel at the speed of light. In particular, what would one see in a mirror that is also moving at the speed of light? Einstein’s Mirror

In 1905, at the age of 26, Einstein published his theory of Special Relativity...

“I was sitting in a chair in the patent office at Bern when all of a sudden a thought occurred to me: If a person falls freely he will not feel his own weight... I was startled. This simple thought made a deep impression on me. It impelled me towards a theory of gravitation.” The “Happiest Thought” of Einstein’s Life (1907)

Almost a decade later, in 1916, Einstein completed his theory of General Relativity...

And both theories completely revolutionised our concept of time...

“What, then, is time? I know well enough what it is, provided that nobody asks me; but if I am asked what it is and try to explain, I am baffled.” St. Augustine (5 th Century A.D.) Easier question: How do we measure time?

The ‘second’ is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom Definition of a Second

Accurate to seconds! Modern Atomic Clock (NIST-7)

In a galaxy far, far away...

Fact 1

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Conclusion:  Time is not absolute, as common sense and everyday experience might suggest  It depends on the observer  In other words, it is relative “When the Special Theory of Relativity began to germinate in me, I was visited by all sorts of nervous conflicts... I used to go away for weeks in a state of confusion.”

Evidence for ‘Time Dilation’  Atomic clock travelling around the world in a plane  Muons reaching Earth from a cosmic ray shower  Muons travelling round in a storage ring  Global Positioning System (GPS)

Hafele and Keating’s Classic Experiment Hafele and Keating taking the atomic clocks around the world to test Einstein’s relativity theory

Muons  Muons are a type of subatomic particle  Same family as electron, but 200 times more massive  They are unstable, with a lifetime of about 2 x seconds

Cosmic Ray Shower

Cosmic Ray Shower (cont’d)  Muons are created in a cosmic ray shower, and travel near the speed of light down to the ground  They should only travel about 600 m before dis- integrating in the upper atmosphere  But from experiments, they can travel 4.8 km to ground level — because of time dilation!

Muon Storage Ring  Can make muons ‘last longer’ by keeping them circulating round a ring at very high speeds  If there were no time dilation, muons can only orbit the ring around 15 times before decaying  Instead, muons are observed to orbit hundred of times before decaying A muon storage ring at CERN

The Global Positioning System (GPS)  24 satellites in orbit  4 of them visible from any point on Earth  Accurate to less than a few metres Hand-held GPS Receiver

GPS Accuracy Depends on Relativity  A clock error of seconds would result in a position error of around 30 m!  Motion of satellite would slow clock by but the reduced gravity would speed it up slightly  Thus effects of special and general relativity need to be accounted for