WHAT IS THE SHAPE OF SPACE? We’re used to thinking of space as the emptiness
in which things happen, like an empty warehouse ready to be filled, or a theater stage on
which the events of the Universe play out. But General Relativity predicts that space
is not just emptiness, it’s a physical, dynamic thing, and that prediction has been
borne out by many, many experiments. Space can bend because of matter and energy,
curving the paths of objects that move inside of it. It can ripple with gravitational waves And
it can expand, creating more and more space between two objects. All of these phenomena can be described by
one idea: curvature of space (or spacetime). In flat regions of spacetime (like, if there’s
no energy or matter nearby), objects traveling along parallel paths stay along parallel paths. In positively curved regions of spacetime
(like near planets or black holes), parallel paths converge, and in negatively curved regions
of spacetime parallel paths (or even paths pointed at each other!) diverge. But what about space as a whole ? If space
is positively curved everywhere, then there’s only one shape space can be: a giant hyper-potato. If you went in one direction for long enough,
eventually you’d end up in the same place you started. If space is flat everywhere, its shape could
be simple: just extend out straight to infinity. Or it could loop around in a periodic way,
like in some video games: And if space is negatively curved everywhere,
sports would be impossible So which is it? There are basically two ways to measure the
large-scale curvature of the Universe. One is to measure the angles inside of triangles. If the space is flat, then the angles will
add up to 180 degrees. But if the space is curved, those angles will
add up to more or less than 180 degrees depending on the type of curvature. Cosmologists have done the equivalent of measuring
our Universe’s triangles by looking at a picture of the early Universe, and studying
the spatial relationship between different points on that picture. The second way to measure curvature is to
measure the thing that causes space to curve in the first place: the density of energy
and matter throughout the Universe. Which cosmologists have also measured. It turns out that in both cases, measurements
show the Universe to be… pretty much flat (within 0.4% margin of error). But before you get disappointed that we don’t
live in a cool cosmic hyper-potato, let me tell you one big problem The fact that we live in a flat Universe appears
to be a GIGANTIC, COSMIC-LEVEL COINCIDENCE. If the Universe had just a little bit more
mass and energy, space would have curved one way. And if it had just a little bit less mass
and energy, space would have curved the other way. But we seem to have just the right amount
to make space perfectly flat as far as we can tell. This perfect amount is the equivalent of five
hydrogen atoms per cubic meter of space, on average. If instead there were six hydrogen atoms per
cubic meter of space on average, or four, the entire Universe would have been a lot
more curved or a lot less . And we so far have no idea why our universe
has the density that it does. When it comes to the curvature of the universe,
our knowledge falls flat.
I'm more interested in the margin of error than the "probably flat" answer
"What is reality? - 'What we have called matter is energy (light), whose vibration has been so lowered as to be perceptible to the senses. There is no matter.'"
Albert Einstein