Sometimes the dumbest questions can be the toughest ones to answer. Take this one, for example: “What does space look like if you take everything out of it?”. On the face of it, that would seem to be a trivial question. A philosophical, or even a metaphysical one perhaps. But really, it’s a tough old nut when you look into it deeply enough.
Lets start with the entire universe. We’ll push a magic button that makes everything in it disappear. All of the galaxies, the stars, the interstellar dust and gas … poof! All gone. Even you as an observer can no longer exist, since you would render the universe non-empty. So the first thing to consider would be this. If the universe is infinite, but there is nothing in it, how could we even know that? How can we measure or assess the size of an absolutely empty space? There is nothing for any measurement tool to measure. It raises the troubling question of whether space can actually exist at all in the absence of anything to occupy it. I mean, how would we actually know one way or the other?
We know quite a few things about “essentially-empty” space, since there is quite a lot of it out there. Our entire observable universe is in practice a totally empty void, populated with the odd observable object that we can use as reference points. That observation process is a key tool here, because the light that travels in our direction from a remote galaxy has to cross vast distances that could amount to as much as several billion light years without encountering anything to block its path. And the number of stars and galaxies out there which send their light our way is actually uncountably large. So even though the vastness of space is as close to totally empty as makes no difference, it is still absolutely teeming with so much stuff that it really isn’t a good enough stand-in for our notion of “absolutely empty space”.
But nonetheless, it is a fair observation to say that the light which has travelled across vast expanses of interstellar and even intergalactic space to get to us had to cross large tracts of totally empty space en route. Let’s think about that … about light travelling across vast tracts of empty space. How does that work, exactly? How does light travel across nothing? We might be tempted to dismiss this as a trivial question if it were not for one key thing. In 1887, the famous Michelson-Morley experiment discovered that the speed of light appears to be independent of the relative motions of the observer and the source. This stumped the scientists of the time, which led to all sorts of new theories to attempt to explain the result, but none of them proved adequate until Albert Einstein’s 1905 Special Theory of Relativity, and his more complete 1915 General Theory of Relativity.
So now we know that light travels through the vacuum of empty space at a very particular speed, and we know that this speed is invariant regardless of the relative speed of the observer. But if there aren’t any observers, let alone any reference points against which to establish the relative speed of an observer, what are we to make of the propagation of light? Indeed, what, if anything, would be its “speed”? It harks back quite remarkably to the old philosophical chestnut of whether a tree exists if there is nobody around to observe it:
There was a young man who said: “God,
must think it exceedingly odd
that our sycamore tree
ceases to be
when there’s no-one about in the quad.”
In other words, how is it possible for light to propagate at all in a totally empty universe … and can light therefore exist in such a universe? After all, waves must ultimately be observable disturbances in something. One approach postulates that light waves are related to disturbances in the fabric of space-time. So if space itself was well and truly empty, then it could no longer be defined, and it would follow that it would also be impossible to speak of time. [The prominent physicist-mathematician Roger Penrose has a lot to say about this in his Conformal Cyclic Cosmology theorem.] Given that speed is ultimately a relationship between space and time, this has a measure of appeal to it. But it remains more of a mathematical argument than a theory of physics, and doesn’t ultimately take us anywhere (… unless Penrose is ultimately proved correct).
There is another philosophical question that relates to the concept of totally empty space. If there was nothing whatsoever in space, we could no longer talk about the relative sizes of things. The most colossal volume of space – say, for example, the entirety of the current observable universe – would be no different from the tiniest conceivable volume, such as the space occupied by the most fundamental of fundamental particles, a quark. With nothing in it to give it scale, there would be no way whatsoever to distinguish the infinite from the infinitesimal. This is a big problem for physicists, because there are troubling issues (i.e. quantum effects) that come into play in all aspects of physics as dimensionality reduces, just as other troubling issues (i.e. the expansion of the universe) come into play when we consider dimensionalities that head toward the infinite. Just as nature abhors a vacuum, physics abhors the infinite.
So it is that the consensus among physicists is now coalescing around various notions that empty space is not in fact empty at all. At a quantum level, empty space comprises discrete chunks of ‘space-time’, formulated according to the various theories under consideration. These are sometimes described as “virtual particles of space-time” which are perpetually popping into and out of existence, and are often referred to by various catchy terms such as “cosmic foam”. A popular terminology these days is the “Higgs Field”, with its related detectable particle the “Higgs Boson” which has recently been detected by the boffins at CERN. If these theories are correct, then space itself comprises a kind of roiling foam of virtual space-time particles. It also places lower limits, not only on the physical size of anything, but also on time itself, which then becomes a “temporal foam” comprising quanta of “discrete time” which ping in and out of existence. Something you could perhaps mention next time you’re late for a meeting.
These theories actually have remarkable depth to them, and delve into concepts that would seriously trouble the layman, if not all but a tiny minority of the experts themselves. And of course, they open themselves to all sorts of yet more complex layers of questions, such as “what is a quantum foam of the Higgs Field actually made of”, and similar tasty tidbits. Whole fields of physics are now concerning themselves with this stuff.
Meanwhile, back in our own particular corner of civilization, we prefer to argue over whether power cords sound different … J