An illustration of heavily curved spacetime, outside the event horizon of a black hole. As you get closer and closer to the mass’s location, space becomes more severely curved, eventually leading to a location from within which even light cannot escape: the event horizon. At large distances, the spatial curvature is indistinguishable for equal mass black holes, neutron stars, white dwarfs, or any other comparably massed object. Credit: JohnsonMartin/Pixabay
When most of us think about the Universe, we think about the material objects that are out there across the great cosmic distances. Matter collapses under its own gravity to form cosmic structures like galaxies, while gas clouds contract to form stars and planets. Stars then emit light by burning their fuel through nuclear fusion, and then that light travels throughout the Universe, illuminating anything it comes into contact with.
But there’s more to the Universe than the objects within it. There’s also the fabric of spacetime, which has its own set of rules that it plays by: General Relativity. The fabric of spacetime is curved by the presence of matter and energy, and curved spacetime itself tells matter and energy how to move through it.
But what, exactly, is the physical nature of spacetime? Is it a real, physical thing, like atoms are, or is it merely a calculational tool that we use to give the right answers for the motion and behavior of the matter within the Universe?
It’s an excellent question and a tough one to wrap your head around. Moreover, before Einstein came along, our conception of the Universe was very different from the one we have today. Let’s go way back to the Universe before we even had the concept of spacetime, and then come forward to where we are today.
But what, exactly, is the physical nature of spacetime? Is it a real, physical thing, like atoms are, or is it merely a calculational tool that we use to give the right answers for the motion and behavior of the matter within the Universe?
It’s an excellent question and a tough one to wrap your head around. Moreover, before Einstein came along, our conception of the Universe was very different from the one we have today. Let’s go way back to the Universe before we even had the concept of spacetime, and then come forward to where we are today.
The journey from macroscopic scales down to subatomic ones spans many orders of magnitude, but going down in small steps can make each new one more accessible from the previous one. Humans are made of organs, cells, organelles, molecules, atoms, then electrons and nuclei, then protons and neutrons, and then quarks and gluons inside of them. This is the limit to how far we’ve ever probed nature.Credit: Magdalena Kowalska/CERN/ISOLDE team
At a fundamental level, we had long supposed that if you took everything that was in the Universe and cut it up into smaller and smaller constituents, you’d eventually reach something that was indivisible. Quite literally, that’s what the word “atom” means: from the Greek ἄτομος: not able to be cut.
The first record we have of this idea goes back some 2400 years to Democritus of Abdera, but it’s plausible that it may go back even farther. These “uncuttable” entities do exist; each one is known as a quantum particle. Despite the fact that we took the name “atom” for the elements of the periodic table, it’s actually subatomic particles like quarks, gluons, and electrons (as well as particles that aren’t found in atoms at all) that are truly indivisible. READ MORE...
