Is there some substance or material out there that penetrates us and all other objects and binds the universe together? Or perhaps is there just a vast emptiness in between those 100 billion galaxies plus assorted other objects?
This is a question that has been asked for centuries, dating back to when the physicist Isaac Newton first promoted the idea that there is nothing out there between objects in space.
Now then, an opponent of this model can ask how Earth knows how to orbit the Sun so reliably for the past 4.6 billion years if there is nothing in space. Yes, Earth
Earth feels the gravitational pull of the Sun, but then how does the Sun communicate to Earth how much of a gravitational pull it should “feel,” and anyway how does a big rock like Earth “feel” anything?
Einstein proposed a very different view of space from Newton. He said we should instead imagine space as a material which is not visible to us but nonetheless is there. This so-called “space time” can be approximated in the classroom as a flexible rubber sheet, such that if you and your friends pull a sheet taut and place say a lead ball in the middle of this sheet, it will make a depression.
Now if a friend rolls another ball on the sheet, the course of its rolling will be to careen towards the other ball. In this scheme one can usefully conclude that the Earth knows about the Sun, and therefore how much of a gravitational pull to “feel,” because Earth travels in a depression along an “indentation” that the Sun makes in the surrounding space time. In reality this pure inward pull is offset by the planet’s own velocity in a way that is not fully conveyed by the rubber sheet analogy.
The point is that by using the rubber sheet as an analogy, one can explain how a massive object can communicate how much gravitational pull a smaller object should have.
All that may be well and good, but as far even Einsteins’ rubber sheet model for space existed only in Einstein’s imagination. It is by the discovery of gravitational waves that the rubber sheet model is upheld that the universe may indeed be entwined by a “rubbery” space time material.
