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From Aristotle to Hiroshima

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?


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


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? Canwe 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.