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Comment by kazinator | Hacker News Reader

The question "would space still exist if matter disappeared" is posed with an ontological bias which influences the answer.

A better question is: does a universe in which there is nothing (and never has been) still contain space? (Though a better question, I wrecked it by introducing "never", which presupposes that the universe has time---even though it contains nothing, and therefore no events take place.)

The question creates a bias because when we imagine matter being removed from the universe, we firstly imagine the removal as an event unfolding in time. Secondly, we continue to imagine the locations where pieces of matter used to be, and those locations continue to be separated by the abstract space which we continue to imagine.

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> does a universe in which there is nothing (and never has been) still contain space?

Within the framework of our best current theories, the answer is yes, in the sense that flat Minkowski spacetime, containing no matter or energy anywhere, is a valid solution of the Einstein Field Equation of general relativity.

Physicists have been thinking about this for a long time, and I seem to recall having seen it pointed out before that the question of a space-time that never had matter in it could be different than one that had it but somehow had it removed. Which then implies the possibility that the space-time has a state in it, in which case, where did it come from and where is it being stored?

Given our current state of understanding, there really isn't a need to insist that one question is wrong and some other question is better, seeing as we are currently equally incapable of answering any of them.

Time is simply a measure of change, so if there is nothing to change how can there be time?

This isn't entirely accurate.

Think of the exact same system performing the same intra-system permutations (changes) but in two different reference frames (one relativistic and the other not). The changes are identical but the time required by each will be different due to their different reference frames. If time was simply a measure of change than you would expect the time taken by these two systems to be identical. Time then takes on a quality (or rather perhaps has a dependency) that exists beyond simply a "measure of change".

I've struggled several times to formulate this question properly. I'm not convinced by your formulation. I'll give it an other try.

Does space have an existence per se, that is regardless of the matter it contains, or is it just a mathematical framework for the interactions between particles?

This requires a definition of space. For your question to be logical, space should be taken as the thing that is among and around matter. Then on it may be researched physically. Buf if space is taken to mean the thing that exists among and around perceivable matter, then your question becomes obscure, as this means that space is matter, but just not perceivable. And therefore it exists, also without relation.

We know what space is, at least operationally. Or rather space-time. Space is what can be measured with rigid rods. Time is what clocks measure. Einstein painstakingly defined those concepts this way.

Plank's question is more metaphysical : beyond what it means experimentally, does space have a -physical- existence per se, even in a completely empty universe? In other words, is an empty universe different than no universe at all? If not, one way to look at space it is to consider it as a set of abstract rules regarding the possible interactions between particles, those rules being very close to what we call geometry.

Nothing is an abstract concept that we use for various things, when we say nothing is just a way of saying the absence of something we know, our own existence (Descartes) is proof that absolute nothingness is not possible.

How about this question: Is there a space with no matter?

Is there matter with no space in it?

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