eli5: How does gravity bend space-time when space is…nothingness? How do you bend nothing?


I understand time has more of a construct to it making it easy to manipulate and bend but I don’t understand how the same can be true for space. Doesn’t the fact that space bends means it something fundamentally rather than nothing emptiness?

In: 7

Space is (almost completely) empty, in the sense that there is almost zero **matter**.

HOWEVER, space-time is most akin to a scientific **field**, similar to the electro-magnetic field.

This field (like the electro-magnetic field) extends everywhere, and essentially “just exists”, no matter what is around it (matter or not).

It is the presence of matter (or energy) that bends the field, “creating” (the effects of) gravity.

Objects and particles near or at your location are bent as they move toward or away from you by gravity. This bending can only be measured from your location, and this is true from the object’s point of view too.

We don’t know if “spacetime” is fundamentally a real thing. In GR it’s basically a coordinate system. That doesn’t mean it’s real.

Einstein himself cautioned against interpreting the geometric math of GR as something that is fundamental.

If it turns out that spacetime is just a tool of math, your question is moot.

I think you’ve started to answer your own question: what we think of as “nothing” is actually something that’s turned out to be pretty complicated.

Our observations lead us to conclude that reality behaves as though we can assign a degree of “bend” to every point in space relative to us, even if a lot of space happens to have zero bend to it.

Maybe think of space being like sheet music: even when there’s nothing being played in a song, there’s still notation for how many beats of rest are in the song at that point. The silence isn’t empty *nothingness,* exactly, because each silence has its own specific qualities like duration.

There is frankly one answer to this:

Nobody knows.

I mean, we know “empty space” isn’t empty, there’s plenty of quantum stuff going on. That part of the question is easy.

But as far as GR goes, it brilliantly describes the curving of spacetime.

DESCRIBES. Not explains, though. If you ask GR “why does mass bend spacetime”, it is silent. It tells what the mathematical relationship it obeys, but that’s it. Same goes for “what IS spacetime”. In GR, it’s just a 4D mathematical grid that distorts based on the mathematical relationships with mass an energy. It is amazingly accurate in describing and predicting observations, but doesn’t give us any insights into WHY it works. It just does.

Well, actually, we know “why” it works. Because we plug in the correct numbers which we’ve physically measured.

Because it’s just mathematics, it also describes “nonphysical” things, like naked singularities. Objects that are mathematically possible, but our universe seems to find ways to avoid at all costs. In fact, you can use the maths of GR to make all sorts of wonderful and weird spacetime curvatures, because it doesn’t have restrictions. If you plug in negative mass or energy you’re free to go faster than light. Do you want to CHANGE the speed of light? Go ahead, GR doesn’t predict it, we measure it and plug it in.

Unfortunately, currently there is no testable theory to plug that hole (there are plenty of untestable hypothetical models, obviously).