eli5 If matter cannot be destroyed or created, how does space constantly expand?

Question

Maybe I’m misinformed about this. But I was always told matter cannot be destroyed or created. But I have been told space is constantly expanding. Is this expansion of space not made up of “matter”? Where does it come from?

In: 282

Answer 1

You are right and wrong about matter not being able to be created or destroyed.

An older model of physics/chemistry was the *conservation of mass*, the idea that no matter what chemical reactions you cause, stuff you throw around, or *whatever you do*, the combined *mass* of everything involved would stay the same. You can re-arrange all the atoms like legos, but the quantity will stay the same.

While this model is still useful for a lot of everyday chemical reactions, it’s also not true thanks to nuclear physics: Mass can be converted to *energy* and back. That’s what Einstein’s famous E=MC^(2) equation actually means: The conversion ratio between Energy and Mass (in the proper units) is the speed of light squared.

Side note: Conservation of mass is also not *quite true* even at the chemical level, but the difference in mass between two hydrogen atoms and an Oxygen atom unbonded, and bonded into a single molecule of water is *very small*, small enough that it went undetected by science until precise enough tools of measurement were developed. The mass lost is converted into the energy that keeps those molecules connected with molecular bonds.

The modern version of this model is instead the *conservation of* ***energy***, which acknowledges mass and matter to be a kind of energy given physical form, and states that while Energy can be converted to mass and vice-versa, the overall sum of *energy* in the system never decreases, energy simply changes forms.

I don’t have good commentary on the space part, other than that “space” is not defined by the mater that inhabits it: “Space” can be just that- literal, empty void. I leave that subject to other smart people who have more to offer than a good memory of lessons from a good highschool science teacher, but I hope I at least gave you useful perspective on the conservation of energy, and how that works.