Infinite amount of matter in the universe?

[phinds]

OK, so the idea is that we could never develop good enough technology to see past the cosmic horizon. However, if the universe is actually finite and wrapped around the 4th dimension, would there be some way of proving that (given that we haven't been able to thus far and it may be more difficult to do so in the future due to the continued expansion of space)?

There is absolutely zero evidence of such a thing.

 

[ChrisisC]

Yes.

Edit: Merged another thread into this one.

But if there is a infinite amount of matter, wouldn't that require new energy to be constantly made forever, violating the law of energy conservation?

 

[phinds]

But if there is a infinite amount of matter, wouldn't that require new energy to be constantly made forever, violating the law of energy conservation?

(1) How do you figure that? I don't get that conclusion at all
(2) Irrespective of whether or not the universe is infinite, "dark energy" is being created constantly
(3) The law of conservation of energy does not apply on cosmological scales.

 

[mfb]

OK, so the idea is that we could never develop good enough technology to see past the cosmic horizon. However, if the universe is actually finite and wrapped around the 4th dimension, would there be some way of proving that (given that we haven't been able to thus far and it may be more difficult to do so in the future due to the continued expansion of space)?

Yes, and in fact scientists have looked for evidence of that (repeating structures in the cosmic microwave background). No indication of that has been found.

But if there is a infinite amount of matter, wouldn't that require new energy to be constantly made forever, violating the law of energy conservation?

No. There is no need to "make new energy" (this is identical for a finite and an infinite universe), and global conservation of energy does not hold in general relativity anyway.

 

[ChrisisC]

(1) How do you figure that? I don't get that conclusion at all
(2) Irrespective of whether or not the universe is infinite, "dark energy" is being created constantly
(3) The law of conservation of energy does not apply on cosmological scales.

If the law of energy conservation does not apply on a cosmological scale... where does the new energy come from?

 

[lifeonmercury]

Yes, and in fact scientists have looked for evidence of that (repeating structures in the cosmic microwave background). No indication of that has been found.

Not sure if I follow. If scientistics look toward
the cosmic horizon in one direction and detect radiation or star constellations that they also detect in the opposite direction, this would be evidence of a finite universe wrapped around a fourth dimension?

 

[mfb]

If the law of energy conservation does not apply on a cosmological scale... where does the new energy come from?

What new energy?
You cannot define "the total energy in the universe now" in a meaningful way, because you cannot even define a global "now" in a meaningful way. In GR there is no reason to expect that energy is conserved globally, energy does not have to "come from something" - that is the point of non-conservation. If energy would have to come from something, it would be conserved.

Not sure if I follow. If scientistics look toward
the cosmic horizon in one direction and detect radiation or star constellations that they also detect in the opposite direction, this would be evidence of a finite universe wrapped around a fourth dimension?

Forget the "fourth dimension", that concept doesn't work. It would be evidence that the universe is finite, yes, and that you get back to the starting point if you fly in one direction long enough.

 

[PeterDonis]

the law of energy conservation does not apply on a cosmological scale

It's more than that. In GR, the only law of energy conservation is local--it's that the covariant divergence of the stress-energy tensor is zero. Physically that means that stress-energy can't be created or destroyed in any infinitesimal volume of spacetime. Stress-energy includes other things besides energy, or more precisely energy density--it includes momentum flux, pressure, and shear stresses. For a simple discussion it's OK to think of it as "energy", but it's important to understand that it's a density--a local thing--not a "total energy". There is no invariant way to "add up" local contributions of stress-energy at different points to get a "total energy"--not just on cosmological scales, but strictly speaking, on any scale where spacetime curvature is large enough to be significant.

 

[stoomart]

What's the problem with infinite Universe? Do you also have difficulty with infinite real line? Infinite number of natural numbers?

Writing ∞ is one thing, actually constructing an infinite number of natural numbers or an infinite real line is non-sense.

EDIT: I should point out that philosophically I believe the quantum scale is infinitely small and the cosmological scale is infinitely great, but this is a science forum.

 

[Chronos]

Philosophy is irrelevant. Only observational evidence has any scientific validity. Apparently, that approach is a challenge, given the lack of credible citations supporting your conclusions.

 

[andrew s 1905]

What's the problem with infinite Universe? Do you also have difficulty with infinite real line? Infinite number of natural numbers?

It seems to me physicists are comfortable with some infinities but not others. Some, as in QED, are happily renormalised away while others infinities, such as the density as a singularity at the center of a black hole is approached, are seen as a sign that GR breaks down.

Regards Andrew

 

[stoomart]

Philosophy is irrelevant. Only observational evidence has any scientific validity. Apparently, that approach is a challenge, given the lack of credible citations supporting your conclusions.

My philosophy is not dependent on observational evidence, but is supported by it.

 

[phinds]

My philosophy is not dependent on observational evidence, but is supported by it.

You need to be careful about making such a claim on a science forum. If I understood your post correctly, your philosophy says that, to quote you directly, "the quantum scale is infinitely small and the cosmological scale is infinitely great". I challenge you to offer any evidence that these statements are true. Not LIKELY, but true. Science is a lot like horseshoes in some ways. "Likely" counts as zero if you are looking for "empirically verifiable with no exceptions" which is the standard for "true" in science, to the extent that anything is ever "true" in science.

 

[stoomart]

In post #13 I said these kinds of threads are too philosophical, and I'm surprised it has been allowed to continue this long. I recognize that I appear to be talking from both sides of my mouth when you look at posts #20 and #39, so let me explain why this is a fair conclusion.

My brain is simply a tool that is used by my mind (concerned with physical existence) and my soul (concerned with philosophy); most people supress one or the other to varying degrees, but I find life is more interesting and fulfilling when you give them equal freedom, because our brains are built to process both. This allows me to accept naturalism/creationism and a finite/infinite universe without conflict.

 

[PeterDonis]

It seems to me physicists are comfortable with some infinities but not others. Some, as in QED, are happily renormalised away while others infinities, such as the density as a singularity at the center of a black hole is approached, are seen as a sign that GR breaks down.

The difference in these two cases is that, in QED, the "infinities" that are "fixed" by renormalization are artifacts of a particular theoretical framework (perturbation theory), so renormalization is just a way of fixing the theoretical framework so that useful predictions can be extracted from it. The infinite terms that get renormalized away have no direct physical interpretation; and in fact, in non-perturbative models they don't appear at all. There is no need to believe that QED is telling us that there are any actual infinities in nature in this case.

The singularity at $r = 0$ in a black hole, OTOH, involves spacetime curvature invariants increasing without bound. Spacetime curvature invariants have a direct physical interpretation; there is no way to "renormalize them away" by changing your theoretical framework. So spacetime curvature invariants increasing without bound means GR is telling us that there are actual infinities in nature. Which means that, if you don't believe there can be any actual infinities in nature, you have to believe that GR breaks down deep enough inside a black hole.

 

[PeterDonis]

I'm surprised it has been allowed to continue this long.

There are genuine issues of physics being discussed in this thread (the post I just responded to raises one). But you are correct that philosophy is out of bounds.

My brain is simply a tool that is used by my mind (concerned with physical existence) and my soul (concerned with philosophy)

The latter part is out of bounds. Such beliefs can't be usefully discussed here because there is no way to test them against evidence. That doesn't mean you can't hold them; it just means they can't be usefully discussed in this forum. Please bear that in mind for future posts.

 

[Chris Miller]

Infinity, to me, isn't a number or an amount, but an algorithm (or function) that can always generate new set members. So the term "infinite mass" is kind of an oxymoron... unless the universe is an algorithm, I guess.

 

[PeterDonis]

Infinity, to me, isn't a number or an amount, but an algorithm (or function) that can always generate new set members.

This is not the standard definition. The proper way to interpret the statement "the universe contains infinite mass" using the standard definition of "infinite" is "given any real number, the amount of mass in the universe is larger than that number".

the term "infinite mass" is kind of an oxymoron

No, it isn't; it has a perfectly well-defined meaning, which I just gave above. The statement might not be true, but if it turns out not to be true it won't be because it's an oxymoron.

You can't change physics by picking new definitions of words.

 

[Ernest S Walton]

If personal opinions are to be kept out of this forum, then the opinion that physics and philosophy are incompatible should also be disallowed because that is all it is, an opinion. The idea that Plato, Kant, Descartes and other geniuses would have no place on this forum is mind-numbingly primitive.

 

[phinds]

If personal opinions are to be kept out of this forum, then the opinion that physics and philosophy are incompatible should also be disallowed because that is all it is, an opinion. The idea that Plato, Kant, Descartes and other geniuses would have no place on this forum is mind-numbingly primitive.

Why? This is not a forum about philosophy or art or music or any number of fields in which there are geniuses who have no relevance to hard science and so are not part of this forum.

 

[PeterDonis]

If personal opinions are to be kept out of this forum

Personal theories are out of bounds for discussion on PF. That is not the same as personal opinions. Please check the PF rules for further information.

The idea that Plato, Kant, Descartes and other geniuses would have no place on this forum is mind-numbingly primitive.

Nobody has claimed this. Any ideas they had that are relevant to the forum subject matter, i.e., physics, would be within bounds.

 

[Chris Miller]

Then you must of necessity believe that the universe cannot be infinite. You MIGHT be right but you can't prove it so that's just a personal opinion, not science.

Infinity can only exist conceptually, as a function or algorithm for termlessly generating new set members (or proving another member will always exist as with primes). So, unless the universe is a manifestation of consciousness (instead of, or in addition to, the other way around) it cannot be infinite. And if so, its infinitude can only exist as it is defined/discovered, just as we know the set of integers is infinite, but can only ever actually define/discover an infinitesimal part of it.

 

[PeterDonis]

Infinity can only exist conceptually, as a function or algorithm for termlessly generating new set members

This is not a fact, it's your personal opinion. Since it's not amenable to experimental test, there's no way to usefully discuss it here.

 

[PeterDonis]

The OP question has been sufficiently answered. Thread closed.