Empty space as the soil of the universe: Part 1
One of the most peculiar and elusive concepts in physics is that of the vacuum.
When I was in high school, my physics teacher told me that the 'vacuum' meant empty space. It was simply space with nothing occupying it. By that stage in our education we had learned that air is made of molecules. The teacher had a sealed glass jar and a pump that would pump most of the air out of it. The teacher asked us: if the pump were strong enough to remove every single molecule of air from the jar, what would be left inside? The answer, he said, is empty space, which is called vacuum.
Pondering this after class, I encountered a paradox. Take two objects, say two chairs or (using your imagination) two atoms, and declare that 'there is nothing between them'. That means there cannot even be space between them. But then that means they must be touching. So, empty space is not the same as nothing! What we call "vacuum" is not the same as "nothing at all". But if there are no atoms in a vacuum, and the vacuum itself is not nothing, then what kind of 'thing' is it?
You might expect that if a child, wondering these things, had gone on to study physics at university, then to write a PhD on physics and become a professional career physicist for years, that eventually they would arrive at a good answer. Well, I did all of those things, and the best answer I can come up with is not really an answer — at least not the kind of answer that counts as a rigorous scientific answer — but only a metaphor.
The vacuum is like soil. It is the soil of the universe.
When we think of soil, if we do so at all, we tend to think of it either as something horrible and disgusting that is made from rotting and decaying plants and animals, or else we think of it as something wonderful, fertile, dark and mysterious that has an almost magical power to rejuvenate old life and produce new life. The paradox of soil is the way that it embodies and synthesizes death and decay with life and growth. The poet Walt Whitman wrote:
A child said What is the grass? fetching it to me with full hands; How could I answer the child? I do not know what it is any more than he. I guess it must be the flag of my disposition, out of hopeful green stuff woven. Or I guess it is the handkerchief of the Lord, A scented gift and remembrancer designedly dropt, Bearing the owner’s name someway in the corners, that we may see and remark, and say Whose? Or I guess the grass is itself a child, the produced babe of the vegetation. Or I guess it is a uniform hieroglyphic, And it means, Sprouting alike in broad zones and narrow zones, Growing among black folks as among white, Kanuck, Tuckahoe, Congressman, Cuff, I give them the same, I receive them the same. And now it seems to me the beautiful uncut hair of graves. Tenderly will I use you curling grass, It may be you transpire from the breasts of young men, It may be if I had known them I would have loved them, It may be you are from old people, or from offspring taken soon out of their mothers’ laps, And here you are the mothers’ laps. This grass is very dark to be from the white heads of old mothers, Darker than the colorless beards of old men, Dark to come from under the faint red roofs of mouths. O I perceive after all so many uttering tongues, And I perceive they do not come from the roofs of mouths for nothing. I wish I could translate the hints about the dead young men and women, And the hints about old men and mothers, and the offspring taken soon out of their laps. What do you think has become of the young and old men? And what do you think has become of the women and children? They are alive and well somewhere, The smallest sprout shows there is really no death, And if ever there was it led forward life, and does not wait at the end to arrest it, And ceas’d the moment life appear’d. All goes onward and outward, nothing collapses, And to die is different from what any one supposed, and luckier.
When I was taught that the vacuum is empty space, it seemed to me that it must be a barren, like a desert. A desert is a place that is dry, and therefore it is also empty of life. Antarctica is the Earth's biggest desert, which by being so cold is also dry, because all water is frozen and cannot sustain life in that form. Bigger still is the desert of interstellar space, an environment that is nearly a true vacuum. To enter this vast emptiness means to have the life sucked from your dessicated body in seconds.
The idea of empty space, as familiar as it is today, was not always so natural or obvious. The argument that I was able to articulate even as a schoolboy, that empty space must be different from 'nothing at all', was the basis for more sophisticated arguments by philosophers since antiquity, who insisted that there could be no such thing as a true void. Aristotle was one of the first strident proponents of this idea, which was perpetuated for centuries afterwards by the medieval scholastics. After the Renaissance, various prominent thinkers including Descartes, Galileo, and Mach would give their own personal twist to the idea. For Descartes, the concept of 'spatial extension' was synonymous with matter — there could be no notion of space except as embodied by some material substance. Furthermore, an influence of one material body on another could only be achieved through a chain of intermediate material influences, like a row of dominoes toppling one another. All natural forces had to be reducible to a chain of physical impacts of material bodies placed side-by-side with no space in between. The notion that a force could be transmitted over truly empty space was absurd.
For these thinkers, therefore, space could not be empty at all — it had to be full of some kind of matter. Even if this matter was only very thinly distributed and invisible, it had to be there in order to explain how apparently empty space could have extension, and to explain how long-distance forces could be transmitted across it.
Newton was the one who really upset this orthodoxy with his universal theory of gravitation. According to Newton, not only did space exist absolutely and independently of matter, but the gravitational force could act instantaneously between two bodies across empty space. This was an absolute shock to thinkers at the time, and created much controversy. Newton was accused of introducing 'occult forces' into physics, bringing science backwards to the middle ages. Even Newton himself displayed some discomfort with the idea in his private correspondence.
Newton's vision of empty space was not equivalent to 'nothingness', for space had a definite existence. In Newton's paradigm space was a kind of container, or stage, in which matter existed and moved. Just like a stage in a theater, it exists mainly for the actors to deliver their performance, but once the curtains close and the actors depart, the stage continues to exist apart from them. With Newton, space was deprived of all its material being and became just pure, barren extension. A true desert... until Einstein came on the scene.
(to be continued...)