Ask a group of physicists and philosophers to define “space” and you will likely be stuck in a long discussion that involves deep-sounding but meaningless word combinations such as “the very fabric of space-time itself is a physical manifestation of quantum entropy concepts woven together by the universal nature of location.” On second thought, maybe you should avoid starting deep conversations between philosophers and physicists.
Is space just an infinite emptiness that underlies everything? Or is it the emptiness between things? What if space is neither of these but is a physical thing that can slosh around, like a bathtub full of water?
It turns out that the nature of space itself is one of the biggest and strangest mysteries in the universe. So get ready, because things are about to get … spacey.
Space, It’s a Thing
Like many deep questions, the question of what space is sounds like a simple one at first. But if you challenge your intuition and reexamine the question, you discover that a clear answer is hard to find.
Most people imagine that space is just the emptiness in which things happen, like a big empty warehouse or a theater stage on which the events of the universe play out. In this view, space is literally the lack of stuff. It is a void that sits there waiting to be filled, as in “I saved space for dessert” or “I found a great parking space.”
If you follow this notion, then space is something that can exist by itself without any matter to fill it. For example, if you imagine that the universe has a finite amount of matter in it, you could imagine traveling so far that you reach a point beyond which there is no more stuff and all the matter in the universe is behind you.1 You would be facing pure empty space, and beyond that, space might extend out to infinity. In this view, space is the emptiness that stretches out forever.
Could Such a Thing Exist?
That picture of space is reasonable and seems to fit with our experience. But one lesson of history is that anytime we think something is obviously true (e.g., the Earth is flat, or eating a lot of Girl Scout cookies is good for you), we should be skeptical and take a step back to examine it carefully. More than that, we should consider radically different explanations that also describe the same experience. Maybe there are theories we haven’t thought of. Or maybe there are related theories where our experience of the universe is just one weird example. Sometimes the hard part is identifying our assumptions, especially when they seem natural and straightforward.
In this case, there are other reasonable-sounding ideas for what space could be. What if space can’t exist without matter—what if it’s nothing more than the relationship between matter? In this view, you can’t have pure “empty space” because the idea of any space at all beyond the last piece of matter doesn’t make any sense. For example, you can’t measure the distance between two particles if you don’t have any particles. The concept of “space” would end when there are no more matter particles left to define it. What would be beyond that? Not empty space.
That is a pretty weird and counterintuitive way of thinking about space, especially given that we have never experienced the concept of non-space. But weird never stood in the way of physics, so keep an open mind.
Which Space Is the Place?
Which of these ideas about space is correct? Is space like an infinite void waiting to be filled? Or does it only exist in the context of matter?
It turns out that we are fairly certain that space is neither of these things. Space is definitely not an empty void and it is definitely not just a relationship between matter. We know this because we have seen space do things that fit neither of those ideas. We have observed space bend and ripple and expand.
This is the part where your brain goes, “Whaaaaat … ?”
If you are paying attention, you should be a little confused when you read the phrases “bending of space” and “expanding of space.” What could that possibly mean? How does it make any sense? If space is an idea, then it can’t be bent or expanded any more than it can be chopped into cubes and sautéed with cilantro.2 If space is our ruler for measuring the location of stuff, how do you measure the bending or expanding of space?
Good questions! The reason this idea of space bending is so confusing is that most of us grow up with a mental picture of space as an invisible backdrop in which things happen. Maybe you imagine space to be like that theater stage we mentioned before, with hard wooden planks as a floor and rigid walls on all sides. And maybe you imagine that nothing in the universe could bend that stage because this abstract frame is not part of the universe but something that contains the universe.
Unfortunately, that is where your mental picture goes wrong. To make sense of general relativity and think about modern ideas of space, you have to give up the idea of space as an abstract stage and accept that it is a physical thing. You have to imagine that space has properties and behaviors, and that it reacts to the matter in the universe. You can pinch space, squeeze it, and, yes, even fill it with cilantro.3