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Comment by everdev | Hacker News Reader

> this proposal implied something strange: that the multiverse is infinite, with endless, uncountable parallel universes existing alongside our own

My biggest skepticism of the multiverse or even an infinite universe is the amount of energy required. Here on Earth energy is finite and cannot be recreated without using more energy in the process.

For an infinite universe or multiverse with infinite mass, you'd need infinite energy, right?

Infinity seems more like a concept than a physical property. It's easy to imagine an infinite number line, but harder to think about an infinite number of intelligent beings (which I suppose would be possible in an infinite universe).

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> For an infinite universe or multiverse with infinite mass, you'd need infinite energy, right?

Given that the current best guess for the geometry of the universe is (conformally) flat, this universe already contains infinite energy. From what I recall, the normal definitions of total energy of a spacetime are not possible to formulate for non-asymptotically flat spacetimes. So it is a bit nonsensical to talk about the total mass/energy.

The easiest skeptical take on multiverses is simply that there are no measurables defined by multiverse theories yet. I usually lump them in with anthropic theories, which a lot of GR theorists seem to love.

Talking about asymptotics already assumes that the universe is infinite. Any smooth, finite spacetime will have finite energy. The interesting thing about asymptotically flat spacetimes is that they can be infinite in volume but have finite energy.

But our universe is believed to be isotropic, and it has positive energy density everywhere, so it seems that should imply infinite total energy.

Why would this lead to infinite energy? Positive density summed over a finite volume does not equal infinity.

The "anthropic theories" aren't really something you can be skeptical about in the normal sense of skepticism, unless I'm misunderstanding your point.

Well, indeed I disregard anthropic theories largely on the grounds of non-cognitivism, which I suppose is not an ordinary form of skepticism. I lump them both together in my own mind not because of how I rationalize ignoring them, but instead because I think the popularity of both of them is driven more out of spiritualism than science.

I personally find the idea of finiteness much harder to accept.

Infinity makes sense to me. It just means there’s no limit to stuff. Why should reality have a limit?

A finite reality is just plain weird because it implies there’s a single actual number describing how much stuff there is. The universe contains X godzillion kilograms, or is Y billion light years across. Where did those numbers come from? What determined them? Why isn’t there more stuff, or less?

This reminds me of the kind of magical thinking that people like Aristotle used. Why should the universe be easy or hard for you to accept? Why is it important that these numbers "come from somewhere" or be determined by something?

Why doesn't the same thing apply to e.g. energy density? Fine structure constant? Gravitational constant? Why isn't the gravitational constant higher or lower?

Where did I say the universe should be easy for me to accept? You seem to have inferred a “therefore that’s the way things really are” ending to my comment that I did not write nor intend to imply.

I mean, it makes some sense if there was a big bang X years ago, that everything ends up withing X lightyears of each other.

I find it easy enough to think of there being a beginning, since we know that stars age, and use up energy, so they can't have been running forever.

Since we think the big bang was 13.7 billion years ago, we can put Y at about 2*13.7 billion light years across as a first guess. Though there's something funkey with the spacetime stretching or whatever. So maybe more.

Stars cannot run forever, they explode eventually, and other stars can form from the debris: https://www.quora.com/What-is-a-third-generation-star?utm_me...

For the Universe having certain age - I don't understand it, b/c whatever is 13 billion years for one, might be 1 sec for another, due to relativistic time dilation. I think of this number as a kind of metaphor :)

But eventually a region runs out of fusable (fuseable?) matter (Hydrogen / free protons).

https://en.wikipedia.org/wiki/Elliptical_galaxy

Lots more. https://en.wikipedia.org/wiki/Observable_universe#Size

Of particular note to your post:

https://en.wikipedia.org/wiki/Observable_universe#Misconcept... (#1)

Yeah, but in my defense, 46 billion is the same order of magnitude as 13 billion. And a lot easier to understand why it'd be around that magnitude without the stretching of spacetime.

If the zero-energy hypothesis[0] is correct, wouldn't the energy cost of infinite universes still be zero?

[0] https://en.wikipedia.org/wiki/Zero-energy_universe

The idea that the multiverse can continue to expand at the incredible rate that would be required based on the idea as stated always made it seem implausible to me. Furthermore, it bothers me that the idea discards the notion of probability inherent in quantum mechanics, since everything that can happen, does.

A more sane model for me is to imagine the state of the universe as a quantized distribution over a set of set of states with finite measure. This is sort of like the multiverse in that the wavefunction doesn't "collapse" but instead reality "branches." Unlike the multiverse, branches are constantly being created and dying off, since the information content of the universe is conserved. This also preserves the probability derived from the wavefunction - states are kept or discard based on their probability.

"Some have posited that there are an infinite number of molecules outside of the one we call home. They vary in size and shape, according to multimolecule theory. In some of them it is said the hydrogen galaxy is in the outer section, some the exact center, and some have no hydrogen galaxy at all!"

"My biggest skepticism of this theory is that it has been proven here in our molecule we cannot create or destroy atoms. Insofar as we know no atom has ever been added or removed from our molecule in the millions of plancks we have observed the known molecule for and none ever will."

"And for there to be an infinite number of molecules beyond the confines of our own - each with their own strange and exotic configuration expanding and changing off into eternity - you would need an infinite amount of atoms and electron fields to properly bind the vast regions of atoms. And as it's been well proven, atoms in this molecule are finite and unchanging."

What if we only get precisely the amount of energy we require? Each universe won't need infinite energy.

Energy is just a concept in our particular universe. The idea behind the multiverse hypothesis is that everything is just "math all the way down". Other universes do not all have the pieces of math which underlie "conservation of energy", and so on. The equations of each universe determine whether or not it has any phenomenon resembling our energy.

Multiverse theories aside, as far as we know our current universe could, or should (depending on who you ask), be infinite. There is no evidence that its topology is closed (from estimates of curvature), so infinitely large is indeed the default assumption.

Is the theory that space-time is infinitely large, or also that there is an infinite amount of matter/energy within it?

The theory is that space-time is infinitely large, and there is an infinite amount of energy within it (afaik).

Actually the theory (General Relativity)is that a closed universe should have measurable positive curvature, unless it has some weird topology that isn't "naturally" expected (although everything consistent is possible of course). We measure a curvature approximately 0, so that would imply the universe is in fact infinite. Since it is also expected to be isotropic (have approx. the same distribution properties everywhere and in any direction), then it also would have infinite energy. There's no proof of this infiniteness of course, it is only the most natural expectation (most natural of course is always controversial since some physicists probably don't like infinite universes). What we observe, and what can affect us in any way, is of course the 'Observable Universe', and up to its edge it appears boringly uniform (except for a minor anomaly[1]), arranging in a web of huge filaments[2] that extend across its entirety.

[1] https://en.wikipedia.org/wiki/Axis_of_evil_(cosmology)

[2] https://en.wikipedia.org/wiki/Galaxy_filament

>> For an infinite universe or multiverse with infinite mass, you'd need infinite energy, right?

Except that under some theories the net energy in our universe is actually zero. Antimatter+matter = nothing. So the concept of unlimited energy can be acceptable under the rubric that energy in one area is mirrored by anti-energy in another.

> Antimatter+matter = nothing.

Matter and antimatter both are forms in which energy (and not “anti-energy”) is bound up; matter + antimatter = energy, as is observable in particle / anti-particle interactions.

> Except that under some theories the net energy in our universe is actually zero. Antimatter+matter = nothing. So the concept of unlimited energy can be acceptable under the rubric that energy in one area is mirrored by anti-energy in another.

No, anti-matter still has positive energy. It's the gravitational potential energy that balances out all of the other energy to leave a total of zero - every particle's energy is cancelled by its negative potential energy of its gravitational field.

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