🏷️ ป้ายกํากับ: physics

When someone says “spacetime is doomed,” the phrase sounds apocalyptic. It sounds as if space and time are about to be thrown into the cosmic furnace, along with our kitchens, calendars, bicycles, coffee cups, and Tuesday afternoons.

But in modern physics and philosophy of mind, the claim is usually subtler than that.

“Spacetime is doomed” does not mean that the world around us is fake in the ordinary sense. It does not mean that distances are useless, clocks are meaningless, or the room you are sitting in is imaginary. Spacetime works astonishingly well as a description of the world at the scales where we normally live. Einstein’s general relativity treats gravity not as a force pulling through space, but as the curvature of spacetime itself. That idea has been tested again and again, from the orbit of Mercury to gravitational waves.

So why would anyone say spacetime is doomed?

The short answer is: spacetime may not be fundamental.

That is different from saying it is unreal.

Real But Not Fundamental

A thing can be real without being the deepest layer of reality.

Temperature is real. You can feel heat. You can burn your hand. You can freeze water. But temperature is not fundamental in the same way that individual molecular motion is. At a deeper level, temperature describes the average behavior of many particles moving around.

A wave is real. It can knock over a boat. But a wave is also a pattern in water, not a separate substance floating above it.

We have seen that three spatial dimensions are not just the stage on which physics happens.

They help determine what kinds of physics can happen at all. Change the number of large spatial dimensions, and familiar structures begin to strain. Gravity and electric forces behave differently. Stable orbits become harder to arrange. Atoms and chemistry may lose the reliable bound states they need. Even topology changes: knots, entanglements, and certain kinds of persistent structure belong very naturally to three dimensions.

So the question becomes sharper.

Does this explain why space is three-dimensional?

Or does it only explain why beings like us could only find themselves in a universe where something like three-dimensional space was already in place?

That is a different kind of question. It is not mainly about calculating orbits or atoms anymore. It is about what counts as an explanation.

Two different kinds of answer

There are at least two ways an explanation of three-dimensional space might work.

One would be a physical necessity explanation.

That kind of explanation would say: the deeper laws of reality require exactly three large spatial dimensions. Space is 3D because, at a more fundamental level, it could not have been otherwise. Some hidden principle, symmetry, consistency condition, or deeper theory would force this structure into place.

In schematic form:

deeper law → three large spatial dimensions

A common objection to the idea that three-dimensional space is special goes something like this:

If modern physics talks about 10 or 11 dimensions, why make such a big deal out of three?

It is a fair question. On the surface, it sounds like a contradiction. One page says that three spatial dimensions are unusually friendly to stable orbits, atoms, chemistry, and durable complexity. Another part of physics seems to say that reality may have many more dimensions than that.

The way out is not to choose one claim and throw away the other. The way out is to notice that the word dimension is doing more than one job.

The claim that three dimensions are special is a claim about large spatial dimensions: the extended directions in which ordinary objects can move freely. Left and right. Forward and back. Up and down.

The extra dimensions in Kaluza-Klein theory, string theory, M-theory, and related frameworks are usually not imagined as additional large directions like those. They are hidden, compactified, constrained, or otherwise inaccessible at ordinary scales.

That difference matters.

The dimensions we live through

Everyday life appears to unfold in what physicists often call (3+1)-dimensional spacetime:

• three spatial dimensions
• one time dimension

The “3” is not just a label on a coordinate system. It affects how physical influence spreads through space. Gravity, electric fields, orbital motion, and atomic structure all depend on the dimensional setting in which they occur.

A knot feels ordinary because we meet it in ordinary places: shoelaces, cords, thread, rope, hair, fishing line. It feels like a nuisance rather than a deep fact about space.

But a knot is not just “something tangled.”

A real knot is a kind of geometric trap. It is a structure that cannot be undone by ordinary motion. You can pull, twist, stretch, and slide it around, but unless you cut the strand or let it pass through itself, the knot remains.

That raises a quiet question:

Why is a knot possible at all?

The answer depends on something so familiar that it is easy to overlook: we live in three spatial dimensions.

A knot is not just a drawing

In mathematics, a knot is usually thought of as a closed loop sitting in space.

A loose piece of rope with two free ends is not quite the ideal object, because its ends can often be threaded backward through the tangle. A closed loop has no such escape hatch. It is more like a rubber band, a loop of string with its ends joined, or a tiny circular railway that must keep its track continuous.

One compact way to say this is:

\[ \text{a knot is an embedding of } S^1 \text{ in three-dimensional space.} \]

Here \(S^1\) just means a circle: a one-dimensional closed loop. “Embedded” means the loop is placed into space without cutting through itself.

The important part is not the notation. The important part is this:

A knot is a closed loop arranged in space so that it cannot be untangled without cutting the loop or letting it pass through itself.

It is tempting to think of planets as the main test.

Suppose, somehow, a universe with a different number of spatial dimensions managed to have something like gravity, stars, planets, and long-lived orbit-like motion. That would already be a remarkable achievement. It would mean the large-scale structure problem had passed one of its hardest exams.

But chemistry has its own exam.

Atoms are not tiny solar systems in any simple sense. They are quantum systems. An electron does not merely need to be pulled toward a nucleus. It needs to form a stable bound state with it: a durable arrangement with a definite size, definite energy levels, and enough individuality for one kind of atom to differ meaningfully from another.

Attraction is not enough. A trap can be too weak. It can also be too steep.

That is where dimensionality returns, quietly holding the tuning fork.

Atoms Need More Than a Pull

In ordinary three-dimensional space, a negatively charged electron is attracted to a positively charged nucleus. That much is familiar.

But if attraction alone guaranteed atoms, the story would be much simpler than it is. The electron would just fall inward, and the atom would be a collapse, not a structure.

Quantum mechanics prevents that simple fall. Confining an electron to a smaller region makes its momentum more uncertain, which raises its kinetic energy. So the atom exists through a balance: electrical attraction pulls inward, while quantum localization resists being squeezed too tightly.

Why should adding or removing a spatial dimension do anything so dramatic to gravity?

At first glance, it seems like it should only change the picture. Space would have a different geometry, certainly, but why should that alter the basic possibility of a world with stars, planets, and enduring motion?

The hidden hinge is that gravity does not merely happen in space. It spreads through space. Change the dimensionality, and you change the way influence thins out with distance. That means you change the force law itself. And once the force law changes, the architecture of possible motion changes with it.

This is why dimensionality is not just a background feature. It reaches directly into the question of whether bound orbital structure can persist.

Geometry comes first

Imagine a source whose influence spreads outward equally in all directions. In ordinary three-dimensional space, that influence is distributed over the surface of a sphere. The area of that sphere grows like $r^2$, so the same total influence is diluted over a larger and larger surface as distance increases.

That is why the force falls as

$$ F(r) \propto \frac{1}{r^2}. $$

This is not a numerical accident. It is the three-dimensional expression of geometric spreading.

In $d$ spatial dimensions, the enclosing surface around a point source grows instead like $r^{d-1}$. So the corresponding central force typically scales as

$$ F(r) \propto \frac{1}{r^{d-1}}. $$

Why does the universe have three large spatial dimensions?

That question can sound decorative at first, almost philosophical in the loose sense. We are used to treating space as the given background, the silent room in which physics happens. But dimensionality is not neutral scenery. It changes how influences spread, how forces weaken, and what kinds of structures can remain intact through time.

So the real question is sharper than it first appears:

Would anything familiar survive if space had fewer dimensions, or more?

Not just geometry in the abstract. Not just whether equations could still be written down. Would there still be long-lived orbits? Would there still be atoms? Would there still be the kind of persistent, layered complexity that gives you stars, chemistry, planets, and stable environments?

The striking possibility is that three-dimensional space is not an arbitrary stage. It may be one of the conditions that makes a durable world possible.

Space changes the law, not just the picture

A first way to see this is to notice that forces do not merely exist in space. They spread through it.

Imagine a source sending influence outward equally in all directions. In ordinary three-dimensional space, that influence spreads over the surface of a sphere, whose area grows like \( r^2 \). As the same total influence is smeared over more and more area, the strength falls like

$$ F(r) \propto \frac{1}{r^2}. $$