From Aristotle to Hiroshima

From Aristotle to Hiroshima
From Aristotle to Hiroshima

Space, Time and Einstein

J. B. Kennedy

PART I – Einstein’s revolution

CHAPTER 1: From Aristotle to Hiroshima

Cup your hands together and peer down between your palms.

What is between them?

One answer is “air”. But we think of air as composed of separate molecules, like isolated islands. What lies between the molecules?

Nothing?

The distances between the molecules differ. Could there be more “nothing” between some, and less “nothing” between others? Could nothing really exist?

The empty space does seem to be nothing. It is tasteless, colourless and weightless. It does not move, and the gentlest breeze can pass through it without resistance.

This is our first question. What is between your cupped palms? Is it space, a vacuum, a place? Is it there at all? Is it something or nothing?

Now pause silently for a moment until you can feel the blood pulsing through your hands. Time is flowing. Your brain is sensitive to the physical passage of time and as each second or so passes it rouses itself and decides to stimulate your heartbeat, sending blood coursing down through your palms.

Does time flow invisibly through the space between your palms, as blood flows through your fingers or as a river flows past it banks? Can you feel time flowing there? Is that the right metaphor?

Does time flow more slowly and more quickly, or at a steady rate?

If steady, then steady compared to what? Does it flow at a speed of one hour per . . . hour?

If no body moves through a space does time still flow there? Can time proceed without change? This is our second question. What is
the flow of time? Is it happening there in the empty space between your palms, or in the space your brain occupies? Is time the same as physical change, or is it the cause of change?

These questions about space and time seem idle at first. It is not clear even how to begin, how to get a grip on them. But we have learned otherwise.

Consider one time and place. On 6 August 1945, early on a bright sunny morning in the city of Hiroshima, tea was being made in offices, children were being bundled off to school and a lonely, propeller-driven plane buzzed unnoticed through the sky above.

When the atom bomb fell, the furious, boiling ball of fire killed some one hundred thousand human beings at once. The city centre disappeared, rivers and criss-crossing canals were vaporized and buildings were blown apart for miles. Pedestrians walking across a distant bridge were suddenly sooty silhouettes on scarred concrete.

Many more who at first survived the initial blast soon died horribly as their flesh peeled from their bones, and their organs were eaten away by the radiation.

The atom bombs dropped on Hiroshima and Nagasaki, like those still poised and ready in missile silos around the world today, stand as emblems of the power – of the depth and the danger – of our new ideas about space and time. The basic theory of the bombs is given by Albert Einstein’s famous equation that says that ordinary matter can be converted into tremendous explosions of pure force and energy.

The following chapters will trace Einstein’s surprisingly simple theories, showing how new ideas about time led to new ideas about energy, and give instructions for constructing an atomic bomb. But here we should pause to contemplate the power of ideas, the possibility that seemingly idle questions may have far-reaching consequences.

Modern answers to the two questions above mix great tragedy and great beauty, and are known as the “philosophy of space and time”.

This subject has played a central role in European philosophy since the time of the ancient Greeks. It is sometimes traditional to divide philosophy – the “love of wisdom” – into three branches according to the three leading questions:

• What is there? What exists? What is reality composed of? Does it include atoms, space, ghosts, souls, Beauty, God?

• What can we know? Which sorts of knowledge are reliable? Can we trust our senses? Who should we believe? What is truth?

• What should we do? What is good or evil? Is our aim successful survival or saving our souls? Should we tell lies? Should we be guided by reason or emotion, or both?

For each question, the corresponding branch of philosophy is:

• Metaphysics – the study of reality

• Epistemology – the study of knowledge

• Ethics – the study of good and evil, of values The philosophy of space and time is part of metaphysics. Some people mistakenly think that the word “metaphysics” means “after or beyond physical science”, but the word is really an historical accident.

Historians explain that Aristotle (384–322BCE) wrote many books, which were kept in a chest after his death in 322BCE. A later editor bound them together into volumes and gave each volume a title. One dealt with “Physics”, and was so entitled. The next dealt with more basic questions but had no title. It came to be called “the book that came after the one entitled Physics”, and this name, “After-the- Physics” or “Metaphysics” (“meta” being Greek for “after”), has stuck through the ages. Aristotle would have probably preferred to call it “First Philosophy”, simply because it dealt with the most basic and general questions that could be asked. It was thus a deeper continuation of physics, not a separate subject.

This is important because the philosophy of space and time deals with many ideas that are part of modern physical science: it is not “after” or “beyond” physics. Here, there is no dividing line between philosophy and science.

In fact, the division between philosophy and science may have been a temporary aberration. A little history will help explain this. What we call “science” in the modern sense grew from a small movement in the 1600s led by a few philosophers, aristocrats and mechanics. At that time the new vogue in studies of nature was simply known as “philosophy”.

Only some two centuries later, when the trend had caught on and attracted many investigators, was a need felt for some new name for the discipline. “Science” slowly came to have the sense of a study of nature that emphasized experiment and mathematics.

The word “scientist” was not coined until 1863.

These new terms signalled a novel and peculiar split between philosophy and the emergent “science”; suddenly there were two disciplines and two communities of thinkers, where before there had been one loose community of philosophers. Crudely put, the philosophers withdrew from experimenting and observing the world while scientists tried to restrict themselves to measurement, calculation and deduction. Philosophers thought in their armchairs: scientists looked through their telescopes and microscopes. The split widened so much in the twentieth century that some people complained that Europe had “two cultures”: the humanities were separate and isolated from the sciences.

There are now healthy signs that this split is healing, and the philosophy of space and time is one area where philosophy and science are converging and overlapping again. After all, both are studying the same world. One reason for this convergence is an extraordinary and unexpected crisis in our understanding of space and time.

Physicists had been optimistic that Einstein’s theories were both correct and fundamental. Now there is a widespread sense that, although his theories make many correct predictions, they are somehow wrong and mistaken. Just as Einstein overthrew earlier physics, we may now be on the verge of a new revolution. The new problems are so surprising and so deep that ambitious philosophers have invaded physics and thoughtful physicists have begun raising broad and searching metaphysical questions again. The quantum theory of matter, the new theory of gravitation (“quantum gravity”), astronomy and attempts at unified theories of physics are all throwing up challenges to our understanding of space and time. These are deep enough to be called philosophical.

It is an exciting moment to study the philosophy of space and time.

We possess deep and beautiful theories that seem right and illuminat- ing, and make many verifiable predictions. We also know now that they are not fundamentally correct, but we do not understand why.

We do not understand how to proceed.

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