So how are you guys? After ‘TIME’, it’s time for ‘SPACE’. I hope you are taking care of your personal space nicely. But, the ‘space’ we are going to discuss is different. This ‘Space’ is what physicists call “Space”. You might think this time this simple question about space is really simple because we know it is the emptiness. Well, is it really that simple?
Let’s find out!
According to Wikipedia:
“Space is the boundless three-dimensional extent in which objects and events have relative position and direction.” 
Space is one of those fundamental entities (like time and mass), which are with us since the beginning. It is so much integrated in our understanding of the universe, that we expect to know everything about it. But like many (previously discussed) deep questions, the question of “What space is?” sounds like simple one at first. But if we re-examine the question, by going against our intuition, we discover that a clear answer is hard to find.
Most people imagine that space is just the emptiness where things exist and perform their action. Just like an empty stage on which the events of the universe play out. Well, space has been an interesting topic to be discussed and debated for philosophers since ancient time.
This debate related to nature, essence and mode of existence of space dates back to the age of Plato, Socrates and Aristotle. The evidence of the study about space is found in the Timaeus of Plato. Socrates also gave his reflections on what the Greeks called khôra (i.e. space) and Aristotle in the Book IV, Delta of his Physics gave the definition of ‘topos’ (i.e. place). 
Philosophers kept on suggesting new ideas and theories about space. German philosopher-mathematician, Gottfried Leibniz argued that
“space is in fact a relationship between objects defined by their distance and direction from one another.” 
His ideas suggested that since space is just the relation between individual matter and their possible locations and therefore it cannot be continuous but must be discreet. He also believed that we can’t have pure ’empty space’ because the idea of any space beyond the last piece of matter doesn’t make any sense. We can’t measure the distance between two particles if we don’t have any particles. The concept of space would end at the very moment when there are no more particles to define it. Philosophy of Leibniz throws light on the fact that if matter is finite in the universe, then space must also be finite.
This philosophy is really weird and counterintuitive, especially when it is given that we never experienced the concept of non-space. (But in Physics, sometimes weird things are true, so keep an open mind.)
Well, his ideas also shocked Isaac Newton. Newton had just an opposite view regarding ‘space’. According to him, space was absolute and that it could exist permanently and independently without the presence of matter. It is a void that can exist by itself, waiting to be filled.  For example, if we imagine a finite amount of matter in the universe, then we could imagine travelling so far that we shall reach to a certain point beyond which there is no more stuff. At that point we will be facing a pure empty space, which actually extends out to infinity.
This picture of space looks reasonable and confirms with our experience of the universe. But history has taught us that, sometimes when we believe something to be very true and logical (like “Earth is flat”) we should be skeptical about that and take a step back and re-examine the thing carefully.
So, which of these ideas about space is correct? Does the space exist only in the context of matter? Or is it like an infinite void waiting to be filled?
Well, the answer is, it is neither of the two: Space is definitely not an empty void and it is definitely not just our relationship between objects. We know this because we have observed space do things which fit neither of these ideas. We have observed space bend, expand and even ripple. This is the part where your brain goes “Whaaaat…..?”
Yeah this is really weird and confusing, but this is what our physicists have actually discovered.
Does Space Exhibit Physical Properties?
If you are regularly reading my articles with good attention, you might 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 bend or expand. If space is our ruler for measuring the location of objects, then how can we measure the bending or expanding of space itself?
Good question. The reason behind this idea of space bending being so confusing is that, most of us grow with a picture of space as an invisible backdrop in which things happen.
Most of us imagine space to be like that theatre stage, we mentioned before, with hard wooden planks as the floor and rigid walls on all sides. And you imagine nothing in the universe can bend that stage because this abstract frame is not the part of the universe but something that contains the universe.
This is where our imagination about space goes wrong. To understand the modern concept of space and for the general relativity to make sense, we must give up the idea that space is an abstract stage and accept that it is actually a physical thing. We have to imagine that space has properties and behaviour and it reacts to the matter in the universe. We can pinch it, squeeze it and even fill it with anything (which we do actually).
After reading all this, you may have been thinking “What nonsense this article is talking about?” And you may have decided to stop. But if you are firm enough to continue, be prepared to bear with it, because the real craziness is yet to come.
You will start understanding the thing by the time we are done. But we need to unpack these concepts carefully, so that you grasp the ideas effectively and appreciate the truly strange and basic mysteries about space that remain unanswered.
How can space show physical properties like rippling and bending? What does that actually mean?
It means that instead of being like an empty room space is more like a huge blob of thick goo. Normally things can move around inside goo, just like we can move around inside a room full of air without noticing all the air particles. But under certain circumstances this space-goo can bend, changing the paths of objects moving through it.
However, this goo is not a perfect analogy for the true nature of space, but it at least gives an idea that the space you are sitting in right now is not necessarily fixed and abstract. Instead, it is like a concrete thing which can expand, bend and ripple in such a way that we cannot perceive it.
Maybe a ripple of space just passed through you. Maybe we are being stretched in an odd direction with space, and we don’t even know. Or maybe space is sitting silently and going nowhere but we can’t perceive that as well. This is why we normally confuse space with nothingness.
But, what can the space-goo do? It turns out that it can do a lot of weird things.
Space Can Expand
Let’s think carefully for a moment about what it means for space to expand. It means that things get farther apart from each other without actually moving through the space-goo. In our analogy, imagine that you are sitting in the goo and it suddenly starts growing. And if you are sitting in front of another person, you will find that the person is now farther away from you without either of you have moved relative to the goo.
How could we know if the goo has expanded? Wouldn’t a ruler we use to measure the distance also expand? Well, if use a soft taffy ruler then it would definitely expand. But if you use a rigid ruler with all its atoms tightly packed together (with electromagnetic forces), then the ruler would stay the same length, allowing you to measure the expansion of space.
And we know that space can expand because we have seen it expanding – this is how dark energy was discovered. We know that space expanded and stretched at unimaginable rate at the time of big bang and that a similar expansion is it still going on.
Space Can Bend
We also know that space can bend. Our space-goo can be squeezed and deformed. In Einstein’s general relativity, this is what gravity is – the bending of space. When something has mass, it causes space around it to distort and change shape.
When space changes shape, objects do not move through it in a path you might first imagine. Rather than moving in a straight line, a cricket ball passing through a blob of bent space-goo will curve along with it. And if the goo is severely distorted by some massive object then the cricket ball might even move in a loop around it — in the same way the moon revolves round the Earth, or the Earth moves around the Sun.
And this is something which we actually see with our naked eyes! Light, for example, bends its path when it passes near a massive object like our Sun or a black-hole. If gravity was just the force between objects with mass, then how the photons of light which are said to have no mass get attracted towards these massive objects? The only reason we can point out is that, the space is actually distorted by the massive objects and as a result the path of light which passes through that distorted space gets curved.
Space Can Ripple
Again, we also know that space can ripple. This is actually not very unlikely to imagine when we know that space can bend and expand. But what is more interesting is that, this stretching and bending of space can propagate through our space-goo : this is called gravitational waves. If something massive causes a sudden distortion of space, that distortion will radiate outward like a sound wave or a ripple inside a liquid. You can think of these ripples as waves of space stretching and compressing.
We know the ripple behaviour is real because:
- Einstein’s general relativity predicts it.
- We have actually sensed these ripples recently. Somewhere in the universe, two massive black-holes got locked in a spin around each other and thus creating huge distortion in the space, which got radiated outwards. Using very sensitive equipment, we detected those space ripples here on Earth. 
These kinds of behaviour could only happen if space has a certain physical nature to it and is not just an abstract concept or pure emptiness.
What’s beyond Space?
Now we can say that space is a thing and not just pure emptiness, this is what experience of the universe tells us. But while thinking space as a dynamic thing with physical properties which cause it to stretch and bend, it only leads to more questions.
You may be tempted to say that if space is like a goo which can stretch and bend, then it must be inside some other empty thing which we can call ‘space’ again. This is really a clever idea but as far as we know, space doesn’t need to be inside anything else. When it bends and curves, this is intrinsic bending that changes relationships between its parts, not the whole space-goo is bending relative to some larger room that it fills.
But it doesn’t mean that if space doesn’t need to be inside anything, then it is not sitting inside something else. Perhaps, what we call space is sitting inside some larger ‘metaspace’. And perhaps, this metaspace is like infinite emptiness, but we have no clue.
Is it possible to have parts of the universe without space? In other words, if space is a goo can there be places without goo? This is a tough for us to imagine because all of our physical laws assume the existence of space. So what physical laws will operate outside space we can’t tell.
The fact is that this new idea of space as a physical thing has come recently, and we are in the very beginning of understanding what space actually is. Sometimes we are still hobbled by our intuitive notions about space. These notions served as well in early times, but now to dive very deep into the mysteries of this universe, we need to break the shackles of these concepts. We must now realize that space is very much different from what we imagined.
Does Space Have Any Overall Shape?
If you are not yet impressed by the mind-bending concepts of stretching and bending of space, here’s another mystery for you: Is space flat or curved? If it is curved, then which way does it curve?
These crazy questions are not far-fetched if we accept the notion that space is malleable. If space can bend around objects with mass, then it is possible for it to have an overall curvature? It is like asking if our space-goo is flat or it has any shape.
Answering these questions about space would also affect our understanding of the universe. For example, if space is flat, it means if you start travelling in one direction you’ll keep going possibly to infinity.
But, if space is curved, then other interesting things might happen. If space has an overall positive curvature, then going off in one direction might actually bring you to the point from where you started. This will really be a fun.
But explaining idea of curved space is not easy because our brains are not well equipped to visualize the concept. We live in a three-dimensional world that seems pretty much fixed and our everyday life is connected only with that. But if some advanced extraterrestrials with ability to manipulate the curvature of space invade us, then the scenario will be different. I hope, we too will figure that out soon.
It’s really difficult to imagine our three-dimensional space to have a curvature. But we can visualize it, if we imagine that we live in a two dimensional world (like being trapped on a sheet of paper). In that situation we don’t have any z-axis, means there will be no above and below.
And now, if the sheet of paper lies straight we say our space is flat. But for some reason if the sheet of paper is bent, then we say that the space is curved. And there are two ways in which paper can be bent. If it is curved in one way like the surface of a sphere, it is called positive curvature. And if it is curved the other way like the surface of a horse saddle, then it is called negative curvature.
Now here is the interesting part: if it turns out that space is flat everywhere, it means that the sheet of paper (space) could potentially go on forever. But if we find that space has an overall positive curvature, then only one shape we can imagine of: a ‘sphere‘ or to be more specific a ‘spheroid’ (i.e. a potato). If this happens to be true, then our universe will loop around itself. We could be living in a three dimensional equivalent of a potato, which means doesn’t matter in which direction we go, will end up coming back to the same spot.
So, which one is our space like? Is it flat? Or does it have a shape of a potato or a horse saddle?
Well, in this case we have an answer. Our space does appear to be pretty much ‘flat’ with a minimal error of 0.4%. Scientists through two different methods have calculated that, the curvature of space, at least the space we can observe, is very near to zero.
But what are these two ways to calculate the curvature of space? One of the ways is by measuring triangles. One interesting fact about curvature is that, triangles on curved surface don’t follow the same rules as on flat surface. [i.e. on flat surface, the sum of all three interior angles of a triangle = 180°, but on curved surface it is either greater than 180° or less than 180°]
Scientists have carried out the equivalent of measuring the triangles drawn in our three-dimensional universe by looking at the picture of early universe with the help of Cosmic Microwave Background Radiation (CMBR) and calculated the special relationships between different points on that picture. They found something equivalent to the sum of angles of triangle equals to 180°, which corresponds to a flat space. 
The other way through which we can tell about the curvature of space is by looking at the thing that causes it in the first place: the energy in the universe. According to general relativity, there is a specific density of energy in the universe that can cause space to bend in one direction or the other. Measurements from the Wilkinson Microwave Anisotropy Probe (WMAP) have shown that the observable universe has an energy density very close to the critical density (with a 0.4% of error) causing space to be pretty much flat. 
Some of you might be disappointed by the fact that we don’t leave in a cool three-dimensional cosmic potato. But you should be more intrigued than disappointed. Why? Because, as far as we know that this flat universe we are living in is basically a gigantic cosmic level coincidence.
We should think about the fact that all the mass and energy is what that give space its curvature and we have just the right amount of them to make the space flat. To be more specific the exact amount for the universe to be flat is about five hydrogen atoms per cubic meter of space. If we had six hydrogen atoms per cubic meter of space, then it would have curved in one way and if we had four hydrogen atoms per cubic meter then it would have curved the other way.
But again, for space to be pretty flat now means it had to be extremely flat in the early phase of the universe or there has to be something else keeping it flat. This is one of the greatest mysteries about space.
What is the size of the Space?
The curvature of space is not the only thing we have questions about when it comes to the nature of space. When we accept the fact that space is not an infinite void, it opens up many strange questions like, what is the size and shape of space?
If space would have been curved and had a shape of a potato, then it would have been finite in size. But we know that space is flat and it means that it must go on forever, right? Not necessarily!
Space can be flat yet have an edge to it or even stranger it could be flat and still loop around itself.
How can space have an edge? Actually, there is no reason why it can’t have. It can still have a boundary even if it is flat. For example, a disc is flat two-dimensional surface with smooth continuous is edge. Perhaps, three dimensional space may also have a boundary at some point with some strange geometrical properties.
Even more astonishing is that it is possible for space to be flat with edge and still loop around itself. It would be like one of those video games like Pac-Man, where if you move beyond one edge of the screen you simply appear on the opposite edge.
Space might be able to connect with itself in some mysterious way that we are not completely aware of. For example, ‘wormholes’, which are predicted by general relativity, can connect two different points in space that are farther apart. What if the edges of the universe are also connected in a similar way? We don’t know.
Is Space Quantized?
Finally, you may also ask whether space is made up of tiny discrete bits of space, just like the pixels on a TV screen. Or is the space infinitely smooth, such that we can have infinite number of places between two points.
In ancient times, scientists or philosophers might not have imagined that air can be made up of tiny discrete molecules. After all, it appears to be smooth and continuous when it blows. It also can fill any volume and has interesting dynamical properties like wind and tornados. Yet we now know that these all are actually combined behaviour of billions of individual air molecules and not the fundamental properties of the individual air molecules.
The smooth continuous space concept may appear to make more sense to us. Because this is what our experience about space tells us. We easily glide through space and don’t jump from pixel to pixel in a jerky fashion like a video game character. Or do we?
But, according to our current understanding of the universe, it will be more surprising if space turns out to be infinitely smooth. This is because, everything else we know in the universe is quantized. Matter is quantized, energy is quantized, forces are quantized and even time is quantized. Moreover, quantum mechanics suggests that we cannot have any length smaller than 10-35 m (Planck Length). So from quantum mechanics perspective, it will make more sense if space was quantized, but again we have no idea.
But having no idea hasn’t stopped physicists from imagining crazy possibilities about quantum space. If space is quantized, then it means that when we move across space we are actually jumping from small little locations to other small little locations. In that view, space is a network of connected nodes, like the stations in a subway system. Each node represents a location in space and the connection between nodes represents the relationship between those locations (which one is next or before to which one). This idea is, however, different from the philosophy that space is just a relationship between matter, because these nodes of space can be empty and still exist.
In that scenario, what we call space can be just a network of those connected nodes and all the particle in the universe would just be the property of this space rather than elements in it. For example, they might be some vibrational mode of these nodes.
Yeah, this is possible. The current theory of particles is based on quantum fields that fill all the space. A field is just a number or value associated with every point in space. According to this theory, particles of matter are just the excited states of these fields.  So, we are not too far from the amazing theories already.
So, we see space, which is everywhere and inside which we are living, is very much different from what we had imagined. However, now we have started discovering some mind blowing properties of space, but still we are unable to understand what exactly is space and why it happens to be in the way it is.
Maybe we are missing something to which this space is connected. According to Einstein, because of relativity of motion, space and time can be mathematically combined into one object : space-time. It becomes easier to understand the physics of the universe and bending and stretching of space if we consider space and time in combined form as a single entity. As we know that mass, which influences space also affects time. So, it is possible that space and time are a part of some larger continuum. Also space-time plane gives a better visualization of the gravitational waves and also explains the effect of mass and energy in a better way.
Yet, there are many crazy things which we can think of if we take space like a physical thing with amazing physical properties. For example, if space is like air, it is possible for it to have different states and phases. Under extreme conditions, maybe it can arrange itself in very unexpected way and have weird unimaginable properties, just like air behaves when it is solid, liquid or gas. Maybe there are different types of space out there in the universe just waiting for us to discovered them.
Maybe we can actually understand the weird properties of space if we observe it in extreme conditions where it is distorted by mass and energy. We can find such conditions by looking carefully at places where cosmically huge masses are squeezing and straining it: black holes. If we explore near black-holes we might see space shredded and chopped making us capable to observe and examine it from inside.
Perhaps, we are pretty much near to unveil the mysteries of space. Now we have the ability to detect the ripples of gravitational waves, to which we were deaf before. In the near future maybe we will understand more about the exact nature of space and get the answers of those deep questions which are literally all around us.
So don’t go anywhere before sharing and liking this post. And yes, save some space in your brain for the answers.
[•] – Jorge Cham and Daniel Whiteson (2017) : We Have No Idea