Speaking of what I understood, spacetime is three dimensions of space and one of time. Now, if we look at general relativity, spacetime is generally reckoned as a 'fabric'. So my question is, whether spacetime is real or is just a mathematical construct to understand various things. In addition to this, if it's a mathematical thing, then what does its distortion/bending mean?
TL;DR This is a complicated question and anyone who tells you a definitive answer one way or another is either a philosopher or is trying to sell you something. I justify arguments either way below, and conclude with the AdS/CFT correspondence, in which two theories on two vastly different spacetime manifolds are in fact equivalent physically.
First, let’s clear things up:
Now, if we look at general relativity, spacetime is generally reckoned as a 'fabric'.
No. This is simply how easy-to-digest PBS documentaries and popular science books explain the idea of spacetime. In reality, it is a (pseudo-Riemannian) manifold, meaning that it locally (for small enough observers) looks like regular flat spacetime that we are used to dealing with in special relativity. The main difference here, is that for larger observers, the geometry may start to look a bit foreign/strange when compared to the “flat” case (for instance, one might find a triangle whose angles don’t add up to 180 degrees). These are just geometrical properties of the world in which the observer lives, and it happens that the strange geometrical measurements happen to coincide with areas of concentrated mass/energy. This effect of wonky geometry, coupled with the fact that observers naturally follow the path of least spacetime “distance” (proper time) account for what we’re used to calling gravity.
In addition to this, if it's a mathematical thing, then what does its distortion/bending mean?
Again, this is a PBS documentary image that anyone wishing to actually understand physics needs to abandon. Spacetime doesn’t “warp,” “bend,” “stretch,” “distort,” or any other words popular science books care to tell you. What these terms are really referring to is the geometrical properties of different parts of spacetime being different than in the special spacetime of special relativity. In particular, they refer to the geometrical notion of curvature, which is simply a value measurable by local observers which is zero for flat spacetime and nonzero for others, but has nothing to do with stretching, pulling, distorting, or what-have-you.
Finally, let’s get to the meat of the question:
Is spacetime real, or is it a mathematical construct?
Short answer: Yes to both.
Spacetime is, from a mathematical viewpoint, a manifold, which is a set of points equipped with a certain structure (being locally flat). Physically, each point corresponds to an event (a place for something to happen, a time when it happens), and local flatness simply means that small enough observers can find a reference frame in which they would locally feel like they are in flat spacetime (this is Einstein’s equivalence principle).
Mathematically, spacetime has a little more structure. It has a metric tensor, which is the fundamental geometric variable in relativity, and physically corresponds to being able to measure distances between nearby “events” and angles between nearby “lines.” These both certainly seem physical.
As you can see, each mathematical property of spacetime manifests itself to the observer as a physically measurable property of the world. In this sense, spacetime is very physical. However, one could argue the other way as well.
I really like the way that Terry Gannon puts it in his book “Moonshine Beyond the Monster.”
...we access space-time only indirectly, via the functions (‘quantum fields’) living on it. (Gannon, 117)
And this is the sense in which spacetime is just a mathematical tool. We never interact with “spacetime.” What we interact with are the functions whose domains are the abstract manifold we call spacetime when describing them (gravitational fields, electromagnetic fields, etc.), and to make any measurement about spacetime occurs only indirectly through the measurements of these fields. Even something as simple as measuring distance requires a ruler, which can only be read through electromagnetic interaction (light).
The truth is, this is a completely and utterly complicated question which we may never know the answer to. Instead, I leave you with this:
Holographic theories (AdS/CFT and variations thereupon) suggest that a gravitational system (spacetime + curvature) and a certain non-gravitational quantum theory of fields (spacetime + fields + no curvature) in one dimension fewer have exactly the same physics. That is, no measurement could meaningfully tell you which system you’re in, because, physically, they are the same system.
So where did the extra dimension of spacetime come from, and where did all the curvature come from? If spacetime is real, then why can I describe two identical theories on two very different spacetime manifolds?
As a final thought:
Physics does not aim to find “truth”(so much is the subject of philosophy or metaphysics). Physics seeks only to find models of reality which are useful in predicting the outcomes of experiments or processes. Thus, physics can say nothing of the “reality” of spacetime, so long as there are two different theories on two different spacetimes which give the same result of every possible experiment.
Gravitational waves carry energy, momentum, and angular momentum from one point to another. In my book, this makes spacetime just as real as an electromagnetic field, which also transports these quantities. You feel electromagnetic energy whenever you walk outside on a sunny day. If gravity weren’t so weak, you would feel the energy in gravitational waves.
Most physicists consider these fields just as real as matter, which in modern physics is explained using yet more fields!
spacetime is three dimensions of space and one of time. ... my question is, whether spacetime is real or is just a mathematical construct
Here is different from there and now is different from then whether you do the math or not. A mathematical construct cannot be the difference between a near miss and a catastrophic collision.
Spacetime furnishes the stage upon which all the physics in the universe is acted out, in one way or another. For it to behave in the way we have observed it to, it must have certain properties, and our measurements of those properties are in close correspondence with calculations based on our mathematical models.
Based on this, spacetime is not just a mathematical construct invented to make calculations easier. The constructs are our mathematical models, which are intended to represent the functioning of the universe as it is.
Several problems with the question. What is real? What is time? What is space?
If real is reality to humans; space being the 3 dimensional world metastructure; time being the mapping of change to space giving a 4th dimension. If these are the case, I believe we can answer yes to your question, spacetime WOULD BE real. The problem is that the question itself has many hypothetical predications. Leading to the answer inheriting the curse (the curse of ifs?).
I don't think physics will ever gives us absolute answers that satisfy this direction of questioning as it has humans in the centre of the question making it subjective. Reality, whatever it is, is not the reality people are used to in their normal lives. In science, the reality is made of models that have a logical structure through maths, and human languages. Our outlook on the world limits our understanding of it leading to cases in which theories have to be made. Theories are great, but not absolute. This leads to science being incomplete (possible forever).
I would argue that theories that contradict one and another lead to the existance of many possible realities in science. Some are more propable than others given context, sizing of the system, and time of relevance. A theory of everything would unify these realities, and maybe give an answer to your question. I hope we find one.
In philosophy of physics, there are two main positions on the reality of spacetime: substantivalist and relationalist. The former sees spacetime as more of a real "substance", the latter sees spacetime as only emergent and not an underlying reality. Other answers have alluded to these positions, but not labelled them explicitly.
Some thinkers love Einstein's 1916 quote:
Nothing is physically real but the totality of space-time point coincidences.
Such "coincidences" refer to the point of crossing of two worldlines, for instance. I hear this quoted by those who seek to incorporate Mach's principle more fully into general relativity (or alternate theories). I have also seen it advocated in quantum gravity.
In the least, I hope this answer provides a few terms to look up for further information.
Spacetime provides a language or vocabulary that allows to us talk in a very precise way about the motion of an idealised point object. The motion is represented by a curve or "worldline" in spacetime, each point on the curve has four real co-ordinates, and the curve as a whole can be represented by parametric equations that tell us how these co-ordinates are related at different points on the curve. All of this language is imported from the mathematical field of geometry, and specifically the geometry of differential manifolds.
Using the same language we can define a notion of distance along a curve (a "metric") and with a given metric we can distinguish particular curves called "geodesics" which generalize the notion of a straight line. The interesting thing is that if we make the right choice of geodesic then the distance along a worldline correspond to the elapsed time that passes for an object following that worldline, and the worldline of an object in free fall (with no external forces acting on it apart from gravity) is a geodesic.
The Einstein field equations then show us how to choose the "right" metric - they are a set of non-linear differential equations that tell us how this metric changes as we move across the spacetime manifold. The physical input that "drives" these differential equations is the distribution of matter and radiation (i.e. the electromagnetic field) across the spacetime manifold.
So the spacetime manifold is a mathematical construct, but it is one that allows us to talk about how real physical objects move along their worldlines and interact with (i.e. influence the worldlines of) other objects - and to do this in a very precise, quantitative way.
I can interpret your question in two different ways.
- "Is spacetime real?" <=> "Do real triangles exist"?
This is a ~2.5k years old idea which you can read about in more depth in Platonic Realism or the primary sources linked from it. One example from that page is that while you can certainly take a pencil, draw a triangle on a piece of paper, call that a triangle, and convince yourself that triangles exist in the real world (because you just created one), there is something completely different also: An ideal triangle, or in other words, "a polygon with three edges and three vertices". And this is not real (if you subscribe to Platonic Realism). Simply speaking, nowhere in the universe you will ever find three edges connecting three vertices. Those objects are all just ideas in our brains, abstract concepts, fantasy, or whatever you want to call them.
In this sense, our mathematical construct labeled spacetime is not real. It is just that, a mathematical construct, which you could, if you wanted to, trace back to a handful of axioms. It just so happens that if you go and make some calculations (which, also, are of course not "real"; you clearly do not influence the universe in any way by pushing your numbers around), the mathematical construct of spacetime seems to somehow come out to match what we are measuring in the real world. That's the nature of physics - it tries to explain observations; it retrofits mathematical constructs to the real world. But never the other way around. Nobody in their right mind would suppose that the real world behaves like it does because of our mathematical construct of spacetime.
And it would not matter either way. Our physics do not define reality; we can be happy if we are able to speak about reality with our physics.
- "Is spacetime real?" <=> "Is there 'ether'?"
If you are asking whether there is some substance, or "stuff" which is spacetime in reality, then the answer is also no. Yes, people may call it a "fabric", but that is just a word. It is not like a "canvas" or "piece of cloth" where the real physical objects are kind of painted on. The problem might be that we need some didactic tools to bring basic concepts over to the audience; we are all familiar with pictures of curved spacetime, where a planet sits in a bowl-shaped indent in some semi-2D spacetime picture. This is complete nonsense, physically. Sure, you can put layer upon layer of abstraction on the actual maths, and end up with something like that; or you can just explain it like this to make your reader/viewer feel that they know something about how the universe works. But it is not that.
This is the same as asking "what is an electron really". Nobody knows. We only know what our physical formulae / maths tell us about that phenomenon we call "electron", but we do not (and arguably cannot ever) know what it actually is. The same goes for basically everything else. We can always just approximate, and everything is filtered through our senses - in this kind our "brain sense" handling the mathematics of spacetime. We're still in the cave, and there is no reason to believe that we will ever come out.
N.B., I find it fascinating that Plato figured all of that out 2.5 kilo-years ago, and that mankind manages to keep remembering that until this day. I'm pretty sure little of what we're figuring out today will be around in 2500 years from now...
For some years I have been puzzled by the concept of the “twin paradox”; consider the following. Imagine you are in a space ship travelling at 0.75 times the speed of light and its cabin is 4 units of distance wide and 3 units of length long. A photon traversing the width of the cabin takes 4 units of length divided by the speed of light for the transit. A fixed observer sees the photon traveling a distance equal to the square root of 4 units of length squared plus 0.75 times 4 units of length squared i.e. 5 units of length. Hence a time dilation of 5 divided by 4, 1.25 Now let us fire a photo from the forward corner of the cabin to the aft corner of on the opposite side. A distance of 5 units of length. The fixed observer sees the photon travel only 4 units of distance because at the time the photon arrives the aft bulkhead has move forward 3 units of distance. This suggests a time dilation of 4 dived by 5, 0.8 This cannot be correct, is there a fault in the theory! Please have one of your experts clarify for me.