Why I am reading my Electricity And Magnetism Text Book Again
As ever when I start one thing it leads to another. We Were taking a short break in North Norfolk and our host had built a personal philosophy based on his layman understanding of General Relativity and Quantum Physics.
This inspired me to look again at some of my physics which I studied at London University in the late 1960s. I remember that Special Relativity was mathematically easy but General Relativity was deemed to be much harder. According to the Times Reporter who covered Einstein’s lecture in London ‘there are only three people in the world who understood it’. I was told this by my lecturer in 1967 and assumed it to be true. However according to Simon Singh in his book The Big Bang Theory this was written by The Times Sports Reporter who was standing in for their Science Reporter who was on holiday. Oh, and he did not even go to the lecture but chose to spend the time in a London pub where he wrote his report.
Quantum Mechanics was taught in my final year as an undergraduate but had little appeal for me. I was and still am only capable of learning topics when I can follow a logical argument and in Physics this usually meant understanding the mathametical basis. I am no good at learning “parrot fashion” and found that when in exams I would always falter at the point I relied on just memory and not logic. For these reasons I quickly decided that in my endeavour for my Hon.s degree I would be better off skipping the Quantum Mechanis lecture and concentrating on other topics. The tactic was succesful in achieving my short term goal but unfortunate in the long term as Quantum Mechanics features heavily in present day science. Free of any need to pass exams and approaching 70 it is well past time I looked into this.
Special Theory of Relativity Postulates
Before delving into the details I thought I would read Why E=MC2? by Brian Cox & Jeff Foreshaw
which was a Christmas present yet to be read. I assumed it would be good overview of modern thinking but teach would me ittle new. I was wrong.
Einstein’s Special Relativity deals with Frames of Reference in constant motion whereas his General Theory includes Frames of Reference that can be accelerating. Einstein’s two postulates for Special Realtivity are :
- There is no preffered frame of reference.
- The velocity of light is the same in all frames of reference and independent of the velocity of its source.
There is no preffered frame of reference.
The first postulate is no great deal, already discussed way back by Galileo. Basically movement is relative to something which appears stationary. You drop a pen in a plane it lands at your feet but to an observer on the ground if he could have seen it it would have travelled in an arc. An observer on Mars whould see a more complicated path, and so on.
The velocity of light is the same in all frames of reference.
The second postulate for me was more surprising. Fire a missile capable of travelling a 100 mph from a plane which is travelling at 500 mph means that the missile will travel at 600 mph. If the missile is aimed at an approaching plane travelling towards you at 500 mph then the missile is travelling towards the target at 1100 mph.
Not so with light. Using a similar example as before, but this time the planes travel at half the speed of light c/2 and the weapon uses light. To the attacker the light travels away at c and not 3c/2. To the target the light also approaches as c and not 2c (c/2 + c +c/2).
This is hard to accept at first but from these simple postulates you can derive all the equations concerning the changes in time, length and mass with increased velocity and also E=mc2.
I never understood how Einstein made this apparent leap concerning the independence of the speed of light but this was where Brian Cox and John Foreshaw book helps.
Maxwell’s equations describe how electric and magnetic fields are generated and altered by each other and by charges and currents. They were the culmination of the Electricity and Magnetism lectures.
I recall they express mathematically electro-magnateic waves that cover the whole spectrum from gamma rays, x-rays, ultra-violet, visible light, infra-red, micro-waves, short band, medium band etc. These various wavelengths all share the same speed, c and Cox and Foreshaw state that these equations show that c is a physical constant and independent of its frame of reference. Was this why Einstein specified his second postulate?
Anyway I enjoy this type of physics so I thought I would lok again at Electricity and Magnetism and being free of exams enjoy the exploration of the maths that accompany it. The text book I am reading is by W. J. Duffin published in 1965 (1st Edition), contemporary with my undergraduate days.
At the end of the 19th Century shortly after the publication of Maxwell’s Equations the scientific world could not accept that light waves could travel througout the universe without a media to support it. They named this media ether and also hoped it would prove to be a preferred frame of reference.
Michelson-Morley performed an experiment to measure the earths movement through the ether in an elaborate version of the age old problem of swimming up and down stream of a river compared to swimming across and back. The stream was the earth’s orbit at opposite sides of it’s orbit and the swimmer was light. A brief description of this can be found at Michelson-Morley Experiment
The experiment revealed no effect at all and so the existance of ether was dismissed, and with it the hope of finding a preferred frame of reference.