I recently learned what it really means when people say space is constantly expanding. At first I thought it just meant more matter is getting created on the outer areas of the universe or something. But it’s moreso space in the spacial sense is expanding between everything, like a balloon being inflated. This opened up a realm of stuff I hadn’t thought about, with my brain struggling to comprehend how there is finite ‘space’ in that sense. Like how does existence itself have a limit to size? For distance as a concept to exist, the space between atoms has to be finite, and doesn’t break down infinitely. But my brain can’t comprehend this, similar to how it can’t comprehend there being nothing before the big bang. It obviously can’t be infinite because there’d be no existsnce as we know it, but how can it be finite and exist at all?
I guess the question is, how is there a limit to the space between atoms?
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> At first I thought it just meant more matter is getting created on the outer areas of the universe or something
The expansion of space has nothing to do with creating matter or energy. Matter and energy cannot be created or destroyed so however much the Universe had when it was created is how much it will have forever. It may change forms, but the total amount of matter and energy will remain constant due to conservation laws. This is an oversimplification, but the expansion of space is the empty space between matter and energy getting larger. However, it’s a function of distance so it doesn’t effect atoms because the other forces are stronger at those scales.
> But it’s moreso space in the spacial sense is expanding between everything, like a balloon being inflated.
The problem is you’re looking at the expansion of space in isolation. You have to consider all the forces acting on an object, how strong they are, and how their strength changes with distance. This gets you the net force and will tell you if an object should stay clumped together or get ripped apart. The latter only occurs on very large distance scales because the expansion of space increases very slowly and requires millions of lights years to create a noticeable cumulative effect. Below that scale (galaxy clusters and below) the other forces win out and hold everything together. Here’s an easy dividing line:
Galaxy cluster and below: The fundamental forces win and anything bound by them will remain bound forever. Gravity holds everything together on these scales. Below the limit of self-gravitation, the electromagnetic force holds things together (roughly between human-sized scales and individual atoms. While the electromagnetic force holds individual atoms together, the protons and neutrons in every atomic nucleus is held together by the strong force. All these forces are strong enough to counter the expansion of space at these scales.
Clusters of galaxy clusters and above: The expansion of space is stronger than the fundamental forces so these objects will recede at an accelerating rate until one day they leave the observable Universe as seen from Earth. Notice how this scale involves many millions of billions of light years. It takes large distances for the expansion of space to create a noticeable, cumulative effect because it grows very slowly with distance.
> how is there a limit to the space between atoms?
The expansion of space isn’t a factor at atomic distance scales. Atoms have a preferred distance because of the strong force and electromagnetic forces. The EM force causes atoms to bind together. If they get too close, the strong force will repel them away. The strong force is only attractive at the scale of an atomic nucleus. Beyond that it actually pushes other nuclei away to prevent multiple nuclei from merging together. Since there’s an attractive and repulsive force, there’s a sweet spot where the forces cancel out and that creates the preferred distance for atom. Above this distance and the EM force will attract two atoms so they get closer. Below this distance the strong force will push them away.
Atoms can get closer if there’s enough mass. This is of course due to gravity which is attractive and only acts towards the center of mass. However, gravity is many orders of magnitude weaker than the other forces, so you need A LOT of mass to over come them. That’s why stars can only form when massive amounts of gas come together and initiate stellar fusion. There’s a minimum mass required for gravity’s crushing force to overcome the repulsive effect of the strong and EM forces (protons repel protons and electrons repel electrons due to having the same charge.)
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