Ten Days in Physics that Shook the World by Brian Clegg

Author:Brian Clegg [Clegg, Brian]
Language: eng
Format: epub
ISBN: 9781785787485
Publisher: Icon
Published: 2021-09-15T00:00:00+00:00

Thinking in a relative way

We tend to think of Einstein as an ageing man with a shock of white hair, recognised throughout the world as the epitome of scientific genius. This was anything but the Einstein who took FitzGerald and Lorentz’s ideas to the next level and disposed of the ether. In 1905, Einstein was 26, and was yet to achieve an academic post. He was working as a clerk (third class) in the Swiss Patent Office in Bern.

When Einstein’s paper on the special theory of relativity, ‘Zur Elektrodynamik bewegter Körper’ (On the Electrodynamics of Moving Bodies), was published on 26 September 1905, it came up with the same contraction result as Lorentz’s – but as a small part of a more radical shift in understanding of the nature of space and time. For Lorentz, the ether provided an unmovable spatial reference frame. Many years before, Isaac Newton had argued for ‘absolute’ time and space – a fixed background against which everything occurred, and which underpinned Lorentz’s concept of a fixed ether. But Einstein threw out this concept. In the special theory, there was no fixed thing against which everything could be measured. All positions and movement were relative. Measurements in space and time could be based on any viewpoint, apparently static or moving. Each viewpoint (or ‘reference frame’ in physicists’ terms) had equal validity.

All that Einstein needed to achieve this new way of looking at things was the combination of Maxwell’s realisation that light would always travel at the same speed in a particular medium and traditional Newtonian mechanics. This was a major breakthrough, although it was a theory that was in the air. It really only took the rejection of the ether to make it possible to move on from Lorentz–FitzGerald contraction to a more comprehensive description of the impact of light’s behaviour on relativity.

The special theory of relativity predicted a range of effects when objects are in so-called inertial frames – situations where the objects are not under the influence of acceleration. As well as contraction in the direction of motion, the theory predicted that objects would also increase in mass and have a slower passage of time (a concept known as time dilation). However, the nature of relativity doesn’t fit well with the English language. It’s tempting, for example, to say that someone in motion experiences a slower passage of time. But they don’t. From the viewpoint of the ‘moving’ person, they are not in motion. They are still, and the universe around them is moving in the opposite direction.

Because no frame of reference is privileged to especially denote being stationary, what we are saying is not that the moving person experiences length contraction, mass increase and time dilation. Rather, when observed to be moving by someone else, from the viewpoint of that observer the moving person undergoes length contraction, mass increase and time dilation. Note that this does not mean that the moving person only appears to undergo these changes. They really happen, from the point of view of the observer.

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