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Minutes of the 373rd meeting held at 10.00 in Stokesley Town Hall on Tuesday 19th March 2019:

Speaker Peter Atkinson: ‘Is everything relative?’

Peter took a PhD in medical physics developing early methods of ultrasonic imaging. After working in Bristol and at Yale University, he joined ICI in the late 1970s, where he worked as a science/engineering manager. He gave an introduction to Einstein’s Special Theory of Relativity, showing that we live in a 4 dimensional world and time is not what we think it is!

He explained that at the end of the 19th century, many scientists thought that most things in physics were well understood, although there were one or two anomalies. In 1902, Einstein started to think about these. At the time only a handful of physicists understood his theories, but they would revolutionise not only the subject, but also our understanding of reality. Today everyone accepts his Special Theory.

Peter considered two questions: ‘Is time the same for everyone?’ and ‘What is space/time?’ He talked about relative motion, pointing out that if you are moving at the same speed as objects nearby, you would feel as though you are stationary. At the poles our speed is zero, but at the equator, it is 1,000mph. The earth is moving around the sun at 60,000mph, while ths solar system is moving around the Milky Way at 500,000 mph. Speed is always measured relative to something else. Einstein’s simple but vital insight was that all ‘frames of reference’ are equal and the laws of physics are the same in any and all of these frames. The Special condition he applied is that frames are not accelerating.

Peter said that the other important fact of which Einstein was aware was that the speed of light is constant. This concept is difficult to grasp because at everyday speeds, the relative speed of two objects is variable and can be easily calculated. Light however travels through space at a constant speed, regardless of the frame of reference. If a bus is travelling at half the speed of light and it shines a beam of light forwards, the beam will travel at the speed of light relative to the ground. What goes with this fact is that time is no longer constant in the way we are used to thinking of it. Time passes at different rates for people travelling relative to one another. If a man travels through space in a rocket close to the speed of light, when he returns to earth his colleagues will all have aged relative to him. The difference is acceleration – he had not remained in an inertial time frame.

This does not matter at all in real life. Newtonian physics works really well at ‘normal speeds’. However, on the International Space Station, a correction needs to be made. GPS is also affected and without making corrections, the system would become progressively less accurate.

Peter explained that Einstein had discovered that space and time are part of the same thing, a concept he called spacetime. Although this has been long accepted, it has features which are very unfamiliar. A clock which is moving relative to an observer will go slower that one which is not moving. Also a clock which is under the influence of a stronger gravitational field than an observer’s will also be observed to go more slowly. This is known as time dilation.

Peter demonstrated that the distance travelled by a beam of light from one side to the other of a very high speed train would appear to be slightly longer to a person on the platform than to a passenger on the train. What causes the distortion in time is that the speed of light being constant means that the time taken for the beam to travel between the two points must be different to the two observers. Because the distances appear to be different, but the speed is constant, the time must also be different.

For accelerating frames of reference, Einstein developed his General Theory of Relativity. This lets us begin to understand some of those phenomena which are still a mystery, such as black holes.