What is space?

Not what you think

Ask a group of physicists and philosophers to define “space”, and you will most likely get bogged down in a long discussion in which such deep but meaningless combinations of words as “the very fabric of space-time is a physical manifestation of the concepts of quantum entropy woven together into a universal the nature of the location. " If you think about it, maybe you should not start deep discussions between physicists and philosophers.

Is space just the endless void underlying everything? Or is it a void between objects? What if space is neither one nor the other, but even a physical entity that can splash like a bath of water?

It turns out that the nature of space is one of the greatest and strangest mysteries of the universe. So get ready for ... lengthy reasoning.


- I do space exploration

Space is a physical entity

Like many deep questions, the question of the nature of space at first seems simple. But if you challenge your intuition and rethink the question, you will find that it is hard to find a clear answer.

Most people imagine space as a void in which everything happens - like a large empty warehouse, or a theatrical stage on which the events of the universe unfold. In this sense, space is the absence of things. This is a void waiting to be filled, such as “I left a place for dessert” or “I found a great parking place.”


Exhibit A: Space

If you follow this idea, then space can exist on its own, without matter filling it. For example, if you imagine that there is a finite amount of matter in the Universe, you can imagine that you have flown so far that you have reached a point beyond which there is no matter at all, and all the matter is left behind. A clean and empty space would stretch before you, and beyond it, space can extend to infinity. From this point of view, space is an infinite void.


- Space must be very lonely

Could such a thing exist?

Such an idea of ​​space seems reasonable and seems to correspond to our experience. But one of the lessons of history is that every time we think that something is obviously true (the Earth is flat, eating a lot of cookies sold by scout girls is useful), we must be skeptical and re-examine this concept. Moreover, we must consider completely different explanations for the same experience. Maybe we did not think about any theories. Maybe there are those associated with this theory where our experience with the universe is just one of the weird examples. Sometimes the most difficult is the definition of our assumptions, especially when they seem natural and simple.

In our case, there are other rationally looking ideas of what space can be. What if space cannot exist without matter - what if it is nothing but the relationship between the parts of matter? From this point of view, there is no pure empty space, since the idea of ​​a space extending beyond the last particle of matter does not make sense. For example, you cannot measure the distance between two particles if you do not have particles. The concept of space will end when there are no particles defining it. And what will appear after that? Not an empty space.


Exhibit B: space

This is a strange and counterintuitive view of space, especially considering that we have never encountered the concept of non-space. But strangeness has never been an obstacle to physics, so do not reject this possibility.

What space do we have?

Which of the above concepts of space is correct? Does space look like an endless void waiting to be filled? Or does it exist only in the context of matter?

It turns out that science is quite confident that space is not described by any of them. Space is clearly not a void, and obviously not a simple interconnection of particles of matter. We know this because we have observed the behavior of space that does not fit into any of these ideas. We watched how space bends, ripples and expands.

At this point, your brain should say: "Whaaaaaaaa? .."

If you were not distracted from reading, you should have been surprised by phrases such as “curvature of space” and “expansion of space”. What can they mean? What is the point of them? If space is an idea, it cannot be curved or expanded, as it cannot be chopped into cubes and fried with coriander (except in California, where anything can be done with coriander). If space is our ruler measuring the location of things, how can we measure the curvature or expansion of space?

Great questions! The reason the idea of ​​curving space is confusing is because most of us grow up with the idea of ​​space as an invisible backdrop, against which everything happens. Perhaps you imagine the space as a theatrical stage, mentioned earlier, with hard wooden planks as the floor and with hard walls on the sides. And, perhaps, you imagine that nothing in the Universe is capable of bending this scene, since this abstract construction is not part of the Universe, but is what contains the Universe.


- In my opinion, direct

Unfortunately, here your idea is wrong. To understand the meaning of the general theory of relativity and reflect on modern theories of space, you need to part with the idea of ​​space as an abstract scene and accept its physical essence. You need to imagine that space has properties and behavior, and that it responds to the matter of the universe. It can be pinched, squeezed, and even filled with coriander.

By this time, your brain should already sound alarms related to the meaninglessness of what is happening: “What the # @ # $?!?!” This is clear. But hold on, for the real craziness is yet to come. By the end of the story, your alarms will run out. But we need to carefully disclose these concepts in order to understand ideas and appreciate the truly strange and basic puzzles of space that remain unsolved.

Cosmic jelly - and you swim in it

How can space be a physical object that can ripple or bend, and what does this mean?

This means that instead of being something like an empty room, the space is more like a huge piece of thick jelly. Usually objects can move in jelly without problems, as we can move in a room filled with air without noticing air particles. But under certain conditions, this jelly can be bent, changing the process of passing objects through it. It can also shrink and go in waves, changing the shape of things inside it.


Exhibit B: space

This jelly (let's call it “spatial jelly”) will not be an ideal analogy to the nature of space (jelly exists in space, but we don’t know if there is space inside anything else), but it will help you imagine that the space in which you are Now, not necessarily fixed and abstract. You are in a certain environment, and this environment can stretch or tremble or distort so that you may not feel it.

Perhaps a wave of space has just passed through you. Or you are now being pulled in a strange direction, but you don’t know. Until recently, we did not even notice that jelly generally behaves somehow, except that it simply is in its place and does not go anywhere, which is why we confused it with emptiness.

So what can this spatial jelly do? It turns out a lot of strange things.

Firstly, space can expand. Let's think carefully about what expansion means. It means that objects move away from each other without moving with respect to jelly. In our analogy, imagine that you are in jelly, and suddenly the jelly begins to grow and expand. If you are sitting opposite another person, that person is now further away from you, despite the fact that both of you have not moved relatively jelly.


- I think we are moving away from each other

How do we know that jelly has expanded? Will the ruler with which we measure jelly also expand? The space between the atoms of the line would expand and stretch them to the sides. If the line was made of very soft toffee, it would expand. But if your ruler is solid, all its atoms hold tight to each other (using electromagnetic forces), and the ruler remains the same length, so you notice the appearance of additional space.

And we know about the expansion of space, because we see its expansion - this was how dark energy was discovered. We know that in the early Universe, space expanded at shocking speeds, and that similar expansion is occurring today.



We also know that space can bend. Our jelly can be squeezed and deformed, like a toffee. We know this from Einstein's general theory of relativity, where it is described that gravity is the curvature of space. When an object has mass, it causes the surrounding space to distort and change shape.

When space changes shape, objects no longer move through it as you might imagine. Instead of moving in a straight line, a baseball passing through a pile of twisted jelly will twist along with it. If the jelly is severely curved by something heavy, such as a bowling ball, a baseball can even begin to move around it - just like the moon moves around the earth, or the earth around the sun.

This we can observe with the naked eye! For example, light distorts its path, passing near massive objects, for example, our Sun or giant lumps of dark matter. If gravity were just a force acting between objects with mass, it would not be able to attract photons without mass. The only way to explain the curvature of the path of light is the curvature of space itself.


Einstein's Tricky Throw

Finally, we know that space can go in waves. This is not so strange, considering that we know that space can stretch and bend. Interestingly, curvature and extension can spread through the jelly - and this is called the gravitational wave. If you make a sudden curvature of space, it will spread outward like a sound wave or like a wave in a liquid. Such behavior can only occur if space has a certain physical nature, and it is not a simple abstract concept of pure emptiness.

We know that these waves are real because GTR predicts them, and that we actually registered them. Somewhere in the Universe, two black holes rotated wildly in each other's arms, and thereby caused enormous perturbations of space, spreading outward. Using very sensitive equipment, we found these waves here on Earth.

These waves can be represented as waves of stretching and contracting space. With the passage of such a wave, space contracts in one direction and expands in the other.


The strange behavior that space is capable of: expanding, bending, worrying, walking with a wheel

Well, the disordered jelly this is the case? Are you sure?

No matter how crazy the idea sounds that space is an object, and not just emptiness, this is what the Universe tells us through our perception. Our experimental observations clearly show that the distance between objects in space is not measured on an invisible abstract backdrop, but depends on the properties of the spatial jelly in which we all live, eat cookies and cut cilantro.

But if the idea of ​​space as a dynamic object with physical properties can explain such phenomena as the expansion and curvature of space, it as a result leads us to new questions.

For example, you may be tempted to say that what we called space now needs to be called physical jelly (“physel”), but this jelly should be in something that we can now call space again. It would be very tricky, but as far as we know (and we know a little), jelly does not have to be in something else. When the jelly is distorted and distorted, this distortion changes the interaction of its parts - this is not a distortion of the jelly in relation to some larger room that it fills.

But just because our spatial jelly does not have to be inside something else does not mean that it is not inside something else. Perhaps what we call space is within the larger “superspace.” It is possible that this superspace is like an infinite void - but we do not know this.

Is it possible that in some parts of the universe there is no space? In other words, if space is jelly, is it possible for the absence of jelly, non-jelly, to exist? The meaning of these concepts is not entirely clear, since all our physical laws imply the existence of space - so which laws could work outside of space? We do not know.


“So what's jelly on the outside?”
- To co- jelly -niyu unknown.

A new concept of space as an object appeared recently, and we are at the very beginning of understanding what space is. In a sense, we are so far connected by our intuitive concepts. These concepts worked well when the early men and women hunted and collected prehistoric cilantro, but we need to break free from the shackles of these concepts and understand that space is completely different from how we imagined it.

Directly about curved space

If your brain is not yet sore from all these jelly concepts of space distortion, here's another mystery: the space is flat or curved (and if curved, which way?) The



questions are crazy, but they can be asked by accepting the idea of ​​compliant space . If space can bend around objects with mass, can it have a common curvature? This is how to ask if our jelly is flat: do you know that it can be deformed by clicking on some point, but does it sag on the whole? Or is it perfectly straight? You can ask the same questions about space.



The answers to these questions will seriously affect our understanding of the universe. For example, if the space is flat, it means that traveling in one direction, you simply will continue to move, possibly endlessly.

But if the space is curved, then other interesting things can happen. If the curvature of the space is positive, then, traveling in one direction, you will make a loop and return to the same place on the other side! This is useful information if, for example, you do not like it when you are sneaked up behind.


The longest draw in the Universe It is

quite difficult to explain the idea of ​​curved space, because our brain is not very well adapted to visualize such concepts. And why would he need it? Most of our everyday experience (avoiding predators or finding car keys) deals with a three-dimensional world that looks motionless (although if we were attacked by aliens capable of changing the curvature of space, I hope that we could deal with this as well).

What would the presence of curvature mean for space? One way to visualize is to pretend that we live in a two-dimensional world, caught on a piece of paper. This means that we can only move in two directions. If this piece of paper on which we live lies flat, we can say that our space is flat.



But if for some reason it is curved, we say that space is curved.



And the paper can be curved in two ways. It can be curved in one direction (positive curvature), or in different directions, like a saddle or Pringles chips (this will be negative curvature, or a violation of the diet).

And that’s great: if we find that the space is flat everywhere, it will mean that a sheet of paper (space) can go on forever. But if we find that space has positive curvature, then there is only one shape in which positive curvature is retained everywhere: a sphere. Or, technically speaking, a spheroid (for example, potato). This is one way in which our universe can lock itself in. We can all live in the three-dimensional equivalent of potatoes, which means that regardless of the chosen direction of movement, in the end you will come back.



In this case, it turns out that we have an answer to a similar question - the space, apparently, is “fairly flat”, that is, flat with an error of 0.4%. Scientists through two very different methods have calculated that the curvature of space (at least what we see) is practically zero.

What are these ways? One of them is the measurement of triangles. Curvature has an interesting property - triangles in curved space do not obey the same rules as triangles in flat space. Let us return to our analogy with a sheet of paper. A triangle drawn on a flat sheet will be different from a triangle drawn on a curved surface.



Scientists conducted an experiment equivalent to measuring triangles in three-dimensional space, having studied the spatial relationship of its different points captured in the image of the early Universe. They found that the triangles they measured correspond to flat space.

Another way that tells us that space is flat is to study what leads to the curvature of space: energy in the universe. According to general relativity, in the Universe there is a certain amount of energy (more precisely, energy density) that causes space to bend in one of the directions. It turns out that the value of the energy density that we can measure in our Universe just corresponds to the fact that space does not bend at all (with an error of 0.4%).

Some of you may be disappointed that we don’t live in a cool three-dimensional potato tuber, bending itself, if you fly in the same direction. Of course, who did not dream of cutting circles around the universe on a rocket motorcycle in the style of Yvel Knivevel ? But instead of being upset about the boring plane of the universe, you might be interested. Why? Because, as far as we know, the flat essence of our Universe is a gigantic coincidence of cosmic proportions.


- In my opinion, this is a bad idea, regardless of the curvature of space
- Well, bliiiin ...

Think about it. All the mass and energy of the Universe gives a curvature to space (remember that mass and energy distort space), and if we had at least a little more mass and energy than now, space would curve in one direction. And if we had them a little less, then the space would have curved in the other direction. But we, apparently, have just so much energy to make the space perfectly flat. The exact amount of matter necessary for the absence of curvature is five hydrogen atoms per cubic meter of space. If we had 6 or 4 hydrogen atoms, our entire Universe would be completely different (more curved and interesting, but different).

Further - stranger. Since the curvature of space affects the movement of matter, and matter affects the curvature of space, there are feedback effects. If in the early days of the Universe, matter would be at least a little more or not enough, and we would not come to this critical density of matter, necessary for space to be flat, then as a result, everything would move away from the flat state even further. In order for space to be flat now, it needs to be extremely flat in the early Universe, or something else to hold it in a flat state.

This is one of the greatest mysteries of space. We not only do not know what space is, we also do not know why it is what it is.

Space shape

The curvature of space is not the only topic on which we have questions related to the nature of space. After you accept that space is not an infinite void, but perhaps an infinite physical entity with properties, you may have many strange questions about this. For example - what is the size and shape of the space?

The size and shape of the space tells us how much the existing space is and how it relates to itself. You may decide that since the space is flat and does not have the shape of a potato or a saddle (or a potato in the saddle), the idea of ​​the size and shape of the space does not make sense. Indeed, if the space is flat, it means that it continues indefinitely, right? Not necessary!


This clearly does not fit the shape of the space.

Space can be flat and infinite. Or it can be flat and have an edge. Or, stranger, it can be flat and still lock onto itself.

How can space have an edge? In principle, there are no reasons prohibiting a space from having an edge, even if it is flat. For example, a disk is a flat two-dimensional surface with a smooth continuous edge. Perhaps three-dimensional space also has a border due to strange geometric properties at its edge.

Even more interesting is the possibility that space can be flat and still lock onto itself. It will be similar to one of those video games such as Asteroids or Pac-Man, where you, going beyond the edge of the screen, simply appear on the other side. Space may in some way unknown to us connect with ourselves. For example, wormholes are theoretically predicted by GR. At the wormhole, two different distant points of space can be connected to each other. What if the edges of space are connected together in the same way? We do not know this.

Quantum space

Finally, one may ask whether the space consists of tiny discrete pieces, like pixels on the body of the screen, or is it infinitely smooth, such that there are infinitely many positions between the two points that you can find yourself in?

Ancient scientists may not have imagined that air consists of tiny discrete molecules. The air seems continuous. It fills any volume and has interesting dynamic properties (wind and weather). But we know that everything that we like about the air (how it gently touches the cheek in the form of a cool summer breeze or how it prevents us from suffocating) is actually obtained due to the combined behavior of billions of individual molecules and is not a property of the molecules themselves.

The option with a smooth space seems to us more meaningful. After all, it seems to us that we move in space smoothly and continuously. We do not jump from pixel to pixel, twitching like a video game character moving across the screen.

Or jump?


- Run! This is a pie chart!

Given our current understanding of the Universe, it would be more surprising if space were infinitely smooth. After all, we know that everything else is quantized. Matter is quantized, energy is quantized, interactions are quantized, girls scout cookies are quantized. Moreover, quantum physics assumes the existence of a minimum meaningful length, 10 ^-35 m. So from the point of view of quantum mechanics it would be natural if space were quantized. But we do not know.

But the fact that we do not know this did not stop physicists from insane assumptions! If space is quantized, it means that when we move, we actually jump from one small place to another. From this point of view, space is a network of connected nodes, such as a metro station. Each node is a location, and the connections between them represent the relationship between the locations (which one is next to which). This is different from the idea that space is simply the relationship between the parts of matter, since these nodes can be empty and exist anyway.

Interestingly, these nodes do not have to be inside a large space or structure. They may just exist. In this case, what we call space will be just a set of relationships between nodes, and all the particles of the Universe will be just properties of this space, and not the elements that are in it. For example, they can be vibration modes of these nodes.



It is not as far-fetched as it seems. The modern theory of particles is based on quantum fields filling all space. A field means that there is a number, or value, associated with each point in space. From this point of view, particles are just the excited states of these fields. So we are not far from such a theory.

By the way, physicists adore such ideas when something that seems fundamental to us (for example, space) comes from something deeper. This gives them the feeling of peeking behind the curtain and discovering a deeper level of reality. Some even suspect that the interconnection of the nodes of space is formed by the entanglement of particles, but these are mathematical fantasies of a handful of theorists who have drunk coffee.

Riddles of space

If you read up to this point and either understood everything or turned off your anxiety of nonsense so that it would not distract you, then we can without hesitation explore the craziest theory of space (yeah, even crazier).

If space is a physical object, not a backdrop and a design, and has such dynamic properties as distortions and waves, and is possibly created from quantum pieces, then you should think: what else can space do?

Perhaps he, like air, has various states and phases. It is possible that under extreme conditions it can be structured in a very unexpected way or exhibit unexpected properties, just as air behaves differently, depending on whether it is in liquid, gaseous or solid form. Perhaps the space we know that we enjoy occupying is just one rare type of space, and there are other types of spaces in the Universe that are just waiting for us to understand how to create and work with them.



The most interesting tool that can be useful to us in search of an answer to this question is that space is distorted by mass and energy. In order to understand what space is and what it is capable of, it is best to carefully study its extreme conditions, where huge masses compress and stretch it: black holes. If we could study the surroundings of black holes, we could see a space cut and shredded so that our anxiety of nonsense would simply explode.

And what is most interesting, we are already very close to the ability to probe the extreme deformations of space. Because, if before we were deaf to the ripples of gravitational waves moving through the Universe, now we have the opportunity to listen to cosmic events, amazing and disturbing jellies of space. Perhaps in the near future we will more understand the exact nature of space and answer these deep questions that literally surround us from all sides.

So, do not fall into prostration, and leave space for answers in your brain.


“We still have room for one joke about space.”
- the disordered jelly ? No, that's enough for me.

Excerpt from the book “We Have No Idea: A Guide to the Unknown Universe” [Guide to the Unknown Universe].

The author of the drawings is George Cham, creator of the popular online comic books Piled Higher and Deeper, Ph.D. in robotics.

The author of the text is Daniel Whitson, professor of experimental physics at the University of California at Irvine, a member of the American Physical Society. He conducts research at the Large Hadron Collider.