The Never-Ending Days of Being Dead by Marcus Chown

Author:Marcus Chown [Marcus Chown]
Language: eng
Format: epub
ISBN: 9780571246199
Publisher: Faber & Faber
Published: 2007-09-14T16:00:00+00:00

General Relativity

There are clearly situations, however, when the laws of physics do depend on your viewpoint. Say you are in a plane which is changing its speed, or ‘accelerating’. Drop a ball in the aisle and it will not fall vertically to the ground as it does in a plane flying at constant speed. The greater the acceleration, as indicated by how hard you are pushed back in your seat, the farther from the vertical the ball will fall. Obviously, the laws of physics do not appear the same from the viewpoint of two people undergoing different accelerations.

Nevertheless, the desire to write down the laws of physics in a way that does not depend on viewpoint is a powerful one. How, after all, could physicists hold up their heads if their ‘universal’ laws were different in different situations – for instance, for people undergoing different accelerations?

For a decade after his discovery of the special theory of relativity, Einstein struggled to find a way to write down the laws of motion in a way that is the same for everyone, no matter what their acceleration. In 1915, at the height of the First World War, he succeeded. He could do it on one condition. The condition was that a force with a very particular behaviour be added to region of space which is undergoing acceleration. The force is gravity!

Gravity, according to Einstein, is therefore not a real force at all. It is a force we invent merely so that the laws of physics still make sense to people accelerating at different rates. In fact, gravity is like other ‘fictitious’ forces such as ‘centrifugal force’. If you are in car rounding a sharp bend, you will find yourself flung outwards. So that the laws of physics – for instance, the laws that describe how a ball dropped in the car falls to the floor – still make sense, it is necessary to invent an outward force called ‘centrifugal force’. No such force exists, however, as is obvious to anyone observing you from outside the car. They can see that you are not flung outwards towards the car door. Rather, it is the car door that comes towards you because the car is following a curved path and you are continuing to move in a straight line under your ‘inertia’.

The Earth orbiting the Sun is like you in the car going round the bend. It is not being constrained to circle the Sun by the ‘force’ of gravity. It is simply flying through space in a ‘straight’ line because of its inertia. Of course, it does not look like the Earth is moving in a straight line. But that is just because we are using the wrong definition of a straight line!

It was Einstein’s genius to realise that space-time is actually warped by the presence of a mass like the Sun. As mentioned before, we cannot see this warpage because, as lowly three-dimensional creatures, we cannot directly experience something four-dimensional. But, just as

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