How can nothing expand? Isn’t space made of nothing? You can’t have 5 pounds of space.
In: Physics
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Our current theory that what space is is special relativity. General relativity connects space and matter. So no space isn’t the lack of stuff, stuff just may or may not be in space.
You solve the Einstein field equations for homogeneous matter distribution and you get expanding space. Similarly of how things aren’t stationary, their fundamental nature is to move and change in their motion is the interesting thing, the fundamental nature of space is to expand (or contrast).
So no, space isnt the “lack of matter” as you still have space when you have matter.
We have no clue, actually. We just found mathematical models that accurately describe part of what we can observe.
I think part of the confusion here is because of the terms. When we say space is expanding, it means space as in space-time, the fundamental idea of where things exist, not space as in specifically the area in between planets where there’s almost nothing. The second is included in the first, but it’s more general than that. If everything is observably moving away from everything else in every direction, then this must mean that there is more space as in “area to exist” in between them. Now practically this also means there is more space (the vacuum) but that’s because of space (the area of existence) expanding, not a fundamental part of specifically the vacuum areas.
You can’t have 5 pounds of lightning either.
Space is ***space***, it is not nothing. It is a thing. It is the medium in which things happen in existence.
Think about an Ocean. Water is the medium in which things happen in the ocean. That medium has properties, like mass, a volume and density. It has a boiling point, it will flow and fill the shape of the container it is in, those are properties of the medium, water.
The medium, space has properties as well but vastly different than anything you have encountered before. It has properties that allow it to expand and contract. Space can expand faster than the speed of light. Space can contract faster than the speed of light. You can have an infinite of amount of space in a single point. Mass bends space. Spinning mass twists space. Bends in space direct orbital path lines of light and other massive objects. These are properties of space.
Nothing expanding is actually really easy! If you start with nothing, and now have more nothing, you haven’t actually added anything. This works great with conservation laws.
In the case of space expanding; given two things, after a bit of time, they are now further away than they were before (specifically by a factor of about 10^(-19) per second). The distance between them has grown without either of them moving (for a given value of “moving”).
Space expanding means that things that are far away are constantly getting further apart from each other.
If there *was* stuff in space this would cause us problems, as that stuff would have be being created as the universe expands.
Except – as usual with physics – quantum mechanics and general relativity mess this up. It turns out there is stuff in space (for a given value of “stuff” and “is”), and universal expansion is accelerating. Which means there must be some kind of energy pouring into the universe driving this. Hence “dark energy” – one of the big questions in modern physics.
Imagine a blanket, with balls on it. The distance between the balls is empty space.
Now imagine the blanket is growing – getting larger. The balls start to get farther apart.
In our current models, and backed up by most observations, the universe is like the blanket, and everything is being pushed apart by the blanket expanding.
This gets hard to understand, because our day to day experience says it isn’t happening. My desk doesn’t get further away from my wall, cities don’t get further apart. So go back to the balls on the blanket. Individual objects have other forces keeping them together, that overcome the push from the growing blanket. So as the blanket grows, the balls don’t have enough force from the blanket to make individual balls expand, and they stay together as a ball in the same spot.
Now add gravity. Imagine the blanket is on a soft surface, and the balls push down on the surface. Other balls that are nearby may roll together, and overcome the speed of the blanket growing, so they seem to stay the same distance apart. That’s our solar system and Galaxy. There’s enough gravity to overcome the speed of the expanding universe, so it stays together.
But, if two objects are very far apart, the gravity keeping them together is less than the force of the blanket growing, and they move apart. That is what we observe when we look at galaxies that are far away.
And finally…. You’ll hear that the expansion is accelerating. We also observe this. Very slowly, so everything I’m about to to say is estimated to occur after the sun goes out and we’re all gone, billions of years in the future… But imagine if the blanket is growing, but going so fast, the balls start to roll really fast, and the blanket starts to push hard enough on the balls that they start to break apart from the force of the blanket expanding. That is what will eventually happen, as the speed of the universe expanding increases to the point that we could actually see our chair start to be further from the wall.
Obligatory not a physicist, so if anyone can correct what I understand, please make our thoughts experiment better!
Space is just the thing matter exists in. When you draw something, you need a place to draw on. The paper is that space. Its not “nothing” expanding, your paper is stretchy. We’re not quite 100% sure what is stretching that paper yet, but we know it’s happening because we can measure the increase in distance between drawings.
You can’t have 5 pounds of space because a pound is a unit of mass (and also weight, in this god forsaken system of units). Mass is a measure of how much matter there is, it’s like trying to measure how much velocity this lightbulb is shining with. It just doesn’t make sense, it’s the wrong unit. Space does, however, have *volume*. You *can* have 5 ft^3 of space. When something expands, it doesn’t change in mass, it changes in volume. It gets bigger, not necessarily heavier.
The lack of existence makes room for existence.
Until there is no more of nothing, that is.
“Isn’t space the lack of matter”
No. The vacuum has energy. Virtual particles pop in and out of being all the time. Because the vacuum has energy, it isn’t “nothing.” Philosophical “nothing” doesn’t exist.
Space can expand because there is a thing we only vaguely know that is anti-gravitational, we call it Dark Energy because it hasn’t been directly observed. It has, however been measured, it makes up more of the universe than all the “stuff” that can be observed. This is why it is forcing the universe to expand against all the gravity that “stuff” provides.
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