I Is anything in the physical world infinite?

[LightningInAJar]

TL;DR Summary

Is anything in the physical universe unlimited or just in math?

Is anything in the physical universe unlimited or infinite or does Infinity only exist to make some math work? Is Infinity a real thing outside of math?

 

[Godot_]

The distance you can travel on the surface of a sphere, Moebius band, ...

 

[anuttarasammyak]

It is still unknown whether the size of the universe is finite or infinite. It could be infinite.

 

[Drakkith]

It depends on what you mean by 'anything in the physical universe'. The number of partitions something continuous can be divided into, such the space between two points or part of the electromagnetic spectrum, is infinite. As is how far you can travel on something like a sphere, as Godot said above. But if you're talking about something like the number of particles then the answer is that we don't know. If the universe is infinite in size then the number of particles is likely infinite as well, but we don't know how big the universe is.

 

[phinds]

If the universe is infinite in size then the number of particles is likely infinite as well,

I think if the universe is infinite in size then the number of particles must be infinite (I'm assuming isotropic and homogeneous, on large scales, over the whole of it)

 

[anorlunda]

Is anything in the physical universe unlimited or infinite or does Infinity only exist to make some math work?

That's an excellent question. You already have some excellent answers.

 

[Dale]

Summary:: Is anything in the physical universe unlimited or just in math?

Is anything in the physical universe unlimited or infinite or does Infinity only exist to make some math work? Is Infinity a real thing outside of math?

Well, if you go with “anything” then that would also include infinitesimals since the inverse of an infinitesimal is infinite. And our best descriptions of nature involve infinitesimals, so there is a lot of evidence supporting them and not anything strongly opposing them. So I would keep an open mind on the topic

 

[sophiecentaur]

The distance you can travel on the surface of a sphere, Moebius band, ...

I remember, when I was a lad, this (the surface of a sphere plus others) was brought up as an 'infinity' example. However, it's finite but 'boundless'.
That's just a mathematical idea and doesn't prove anything ' physical' about the Universe.
Interestingly, the heroic and nerdy Mathematicians sorted out a lot about what infinity can mean, hundreds of years ago. But that's all based on Axioms and some of the results don't necessarily 'make sense' to our intuition.

 

[sophiecentaur]

if the universe is infinite in size then the number of particles must be infinite

That may or may not follow, depending on how you define a particle. When really big numbers of 'particles' are involved, the formula for the sum may or may not be convergent. i.e. is 'the number of particles' really an integer?

 

[Godot_]

I remember, when I was a lad, this (the surface of a sphere plus others) was brought up as an 'infinity' example. However, it's finite but 'boundless'.
That's just a mathematical idea and doesn't prove anything ' physical' about the Universe.
Interestingly, the heroic and nerdy Mathematicians sorted out a lot about what infinity can mean, hundreds of years ago. But that's all based on Axioms and some of the results don't necessarily 'make sense' to our intuition.

The surface per se is boundless, but finite - it has a finite area, that can be determined. The distance you can travel, however, is infinite.

Easy way to look at it would be e.g.:
You can "count" the area by painting it with exactly one layer of paint. You start somewhere... ...and eventually you run out of unpainted places. => Finite.

The number of steps you can take in any direction - that is infinite: After each step, you add one to the count, yet you still can take another, uncounted step... ...ad infinitum. Each step is a new one - even though you've been there before...

/nitpicking

[Edited for better understanding]

 

[jbriggs444]

If the universe or any particular thing were infinite, how could we distinguish that from finite-but-larger-than-we-can-measure?

If position or time or any other quantity were infinitely divisible, how could we distinguish that from discrete-but-smaller-than-we-can-slice?

If it turns out that these things are truly finite or discrete, we might eventually figure that out. If they are truly infinite or infinitesimal, we may never know.

 

[TeethWhitener]

I remember, when I was a lad, this (the surface of a sphere plus others) was brought up as an 'infinity' example. However, it's finite but 'boundless'.

This is basically the intuition behind the mathematical notion of compactness: it's a more precise answer to the question of whether a sphere's surface is finite or infinite.

 

[anorlunda]

That may or may not follow, depending on how you define a particle. When really big numbers of 'particles' are involved, the formula for the sum may or may not be convergent. i.e. is 'the number of particles' really an integer?

I can't make sense of that. If the universe is infinite, and homogenous and isotropic, then it is filled with stars and galaxies everywhere. One way to define the number of particles is the number of atoms. No formula is involved. No sums.

 

[PeroK]

I can't make sense of that. If the universe is infinite, and homogenous and isotropic, then it is filled with stars and galaxies everywhere. One way to define the number of particles is the number of atoms. No formula is involved. No sums.

There are two ways to express the idea of an infinite number of particles:

Given any number $N$, you can find at least $N$ particles in the universe.

The set of all particles in the universe is infinite - in the sense that it has a countably infinite cardinality.

That said, you can prove or disprove these properties for abstract mathematical objects but you can't prove them for physical objects in the same way. You can have a model that predicts an infinite number of particles, but it's not clear how you could ever confirm that the model is correct in that respect.

 

[anorlunda]

but it's not clear how you could ever confirm that the model is correct in that respect.

Yes indeed. AFAIK, our speculations that the universe is or is not finite can never be confirmed. But the cosmological principle is not a product of mathematics, it stems from observations; correct?

 

[jbriggs444]

Yes indeed. AFAIK, our speculations that the universe is or is not finite can never be confirmed. But the cosmological principle is not a product of mathematics, it stems from observations; correct?

Like all of science. Our observations so far are consistent with the hypothesis.

 

[PeroK]

Yes indeed. AFAIK, our speculations that the universe is or is not finite can never be confirmed. But the cosmological principle is not a product of mathematics, it stems from observations; correct?

Yes, the cosmological principle is a physical assumption underlying the current model. Observations show no clear evidence of finiteness and make the infinite model appealing. But, I'm not sure how infiniteness could ever be confirmed.

 

[PeroK]

PS we could confirm that the universe is finite, in the same way that we have confirmed that the Earth is finite, and not infinite and flat. It's confirming infiniteness that's problematic.

 

[LightningInAJar]

I received the walking around the Earth's equator example in a philosophy forum as well, but you of course pass the same molecules each pass and receive no additional information if not for the passage of time rearranging things slightly. As far as infinitesimal stuff I don't quite understand it. Can you really not cease from dividing the electromagnetic spectrum into smaller slices or space into smaller lengths? Or can we simply not prove otherwise because we are so much larger and measuring things larger than us affords us better tools?

 

[Dale]

Can you really not cease from dividing the electromagnetic spectrum into smaller slices or space into smaller lengths?

According to our best models, the ones with the most experimental verification, there is no limit. These models underly all of physics, from Newtonian physics to quantum field theory and general relativity.

Or can we simply not prove otherwise because we are so much larger and measuring things larger than us affords us better tools?

It is more than that. As far as I am aware there isn’t a known model of physics that both entirely avoids calculus and is consistent with current data.

 

[DaveC426913]

The OP has equated unlimited with infinite. That's tricky.

There seems to be limits to pretty much everything in the universe, preventing infinites. Here's what I mean:

The distance you can travel on the surface of a sphere, Moebius band, ...

I received the walking around the Earth's equator example

I would not call such examples infinite or even unlimited.

The OP is asking about the physical world.

You can not actually travel an infinite distance on a sphere in reality. There are real barriers that will ultimately prevent it.

While it can be said to be theoretically infinite (which - by definition - means ignoring certain physical, real-world constraints), that doesn't meet the OP's explicit condition of infinite, IMO. I guess "can't occur in the finite lifespan of the universe" is a boundary.

Likewise - while there are an infinite number of "real numbers" - this can only occur in the abstract world of math. The physical world (as far we we understand) has real constraints on infinite subdivision. I guess "the Planck length" is a boundary.

 

[DaveC426913]

According to our best models [of EMR], the ones with the most experimental verification, there is no limit. These models underly all of physics, from Newtonian physics to quantum field theory and general relativity.

That is a theoretical limit.

A physically infinite EM emission would have an infinitely long wavelength. Not only would it never oscillate; it would never even start its first oscillation. I guess "an EM wave that is too long to oscillate" is a boundary.

An infinitely short wavelength would have other problems. I think, including - but not limited to - having being an infinite amount of energy. I guess "would have more energy than the universe" is a boundary.

 

[Godot_]

[...]
A physically infinite EM emission would have an infinitely long wavelength. [...]

While your arguments about the zero and infinite wavelengths hold some water, ...

...it completely ignores what, for simplicity's sake, I'd call the "inner" infinity: You can find an infinity of wavelengths between any two arbitrarily chosen wavelengths. Black body radiation and doppler shift take care of that...

To learn more, try looking at the Bolzano-Weierstrass-Theorem e.g. @wikipedia or try googling Bolzano Weierstrass Epsilontics (or epsilon-delta) ...

While it's way easier to use math to prove it, it still is observable in the physical world - down to the best precision your measuring instrument permits. And when you then build a more finely graded instrument, you can open the next can... ...ad infinitum.

 

[Godot_]

[...]
You can not actually travel an infinite distance on a sphere in reality. There are real barriers that will ultimately prevent it.
[...]

Depends on the sphere. Where's the insurmountable barrier on a billard ball?

The only barrier there would be the lifespan of the traveler... ..and, meh, he'll just pass the baton to the next traveler...

Also, if an insurmountable barrier exists, it's actually, by definition, not a sphere...

 

[Godot_]

[...] that doesn't meet the OP's explicit condition of infinite [...]

He didn't give any other than that they have to be there in the real, physical world. All else is your implication only. And that they're easier to prove or explain with math doesn't make them any less real. If you take the time and a sufficiently finely graded (or infinitely long) measuring tool, they can be observed.

[...]IMO. I guess "can't occur in the finite lifespan of the universe" is a boundary.
[...]

Current paradigm has it that the universe keeps expanding, and hence should be infinite in the time-forward-dimension. Yes, there has been some beginning - but there won't be an end. Like with natural numbers: There's a lower limit - 1 or 0, depending on whether it's N or N₀ we're talking about, but no upper limit, so it still is infinite.

Likewise - while there are an infinite number of "real numbers" - this can only occur in the abstract world of math. The physical world (as far we we understand) has real constraints on infinite subdivision. I guess "the Planck length" is a boundary.

Slanting the measuring scale yields another set of subdivisions, thanks to trigonometry. And don't tell me that that only exists in math...

Also, there definitely are infinite angles. You can always nudge your observing tool a bit to the right or left...

To give a physical example:
All those faraway galaxies in the recently published JWST image... ...their apparent width is in the milliarcsecond range. But they're composed of individual stars, some of which will be observed when they go supernova, eventually... ...which opens up the micro- and nano-arcsecond scale. Also, they're composed of atoms. Those atoms' apparent width would then be in the ... I won't bother to look up the appropriate prefix right now. You get the image, I suppose.

Are those atoms not real and physical, just because the distance is too great to observe them?

As I said above: Using math as a convenient tool to explain / extra- / interpolate stuff doesn't mean that that stuff isn't real or not physical.

 

[PeroK]

You can find an infinity of wavelengths between any two arbitrarily chosen wavelengths. Black body radiation and doppler shift take care of that...

You don't need to invoke anything more than the uncountably infinite set of points in a portion of space. But, that (like the EM spectrum) is a mathematical model of the physical universe. Clearly, we have mathematical models that assume space, time and the EM spectrum are infinitely divisible. But, you can never confirm that fully and conclude that there are, for example, an infinite number of possible wavelengths.

To learn more, try looking at the Bolzano-Weierstrass-Theorem e.g. @wikipedia or try googling Bolzano Weierstrass Epsilontics (or epsilon-delta) ...

That's explicitly mathematics.

While it's way easier to use math to prove it, it still is observable in the physical world - down to the best precision your measuring instrument permits. And when you then build a more finely graded instrument, you can open the next can... ...ad infinitum.

In mathematics the "ad infinitum" is possible by definition. In physics, you would need to confirm by experiment. And an infinite sequence of experiments is physically impossible.

Note that you can plan an infinite sequence of experiments, but you'll never finish them all. You will only ever have conducted a finite sequence of experiments, regardless of how much time you have.

Note also that even if the universe will exist for infinite time, physically that means that time never ends. But, not that at some point you have experienced an infinite amount of time.

 

[sophiecentaur]

I can't make sense of that. If the universe is infinite, and homogenous and isotropic, then it is filled with stars and galaxies everywhere. One way to define the number of particles is the number of atoms. No formula is involved. No sums.

I'm not surprised as I didn't get my words together correctly. I've re-thought what I was trying to say and the following may not be so dodgy. The thread is discussing matters like 'how many particles' but at the extremes of conditions our familiar particles are not the same. Theory suggests that particles are created in very low densities and that, under conditions of very high density, the lose their identity so counting them becomes meaningless and any model runs out.

That said, you can prove or disprove these properties for abstract mathematical objects but you can't prove them for physical objects in the same way.

Yes - the established maths doesn't deal with all situations so we can't rely on it to deal with the Infinity question or even give any meaning to it.

This could be interpreted as a personal theory so I will stop now.

 

[DaveC426913]

If you take the time and a sufficiently finely graded (or infinitely long) measuring tool, they can be observed.

Physically, it is limited by the Plank length.

Current paradigm has it that the universe keeps expanding, and hence should be infinite in the time-forward-dimension. Yes, there has been some beginning - but there won't be an end.

There will be a physical end to any sphere and any person or device attempting to traverse it. They will decompose.

Like with natural numbers: There's a lower limit - 1 or 0, depending on whether it's N or N₀ we're talking about, but no upper limit, so it still is infinite.

Just because there is no upper limit in theory does not mean that an upper limit can be reached physically.

Take the example of the unlimited frequency of an EMR emanation. At some point, it has more energy than the universe itself. And that's self-contradictory, since it's part of the universe.

Slanting the measuring scale yields another set of subdivisions, thanks to trigonometry. And don't tell me that that only exists in math...

It exists physically down to some physical limit. It is at least limited by the Plank length.

Also, there definitely are infinite angles. You can always nudge your observing tool a bit to the right or left...

No. Your observing tool is physical, and - whether atomic or photonic - has some lower limit of resolution.
Even a science fiction tool of fabulous precision will still be limited by the Planck length.

All those faraway galaxies in the recently published JWST image... ...their apparent width is in the milliarcsecond range. But they're composed of individual stars, some of which will be observed when they go supernova, eventually... ...which opens up the micro- and nano-arcsecond scale. Also, they're composed of atoms. Those atoms' apparent width would then be in the ... I won't bother to look up the appropriate prefix right now. You get the image, I suppose.

Are those atoms not real and physical, just because the distance is too great to observe them?

Really really small is not the same as infinitely small.The OP's question was not 'Are there physical things in the universe that are really, really big or really, really small?' If it had been, this thread would have ended after post 2: "Yes. Lots."

 

[Dale]

That is a theoretical limit.

A physically infinite EM emission would have an infinitely long wavelength. Not only would it never oscillate; it would never even start it first oscillate. I guess "an EM wave that is too long to oscillate" is a boundary.

An infinitely short wavelength would have other problems. I think, including - but not limited to - having being an infinite amount of energy. I guess "would have more energy than the universe" is a boundary.

You shouldn’t have added the [of EMR] to my quote. I was responding with a very general statement about all of modern physics, not limiting my comments to EM at all. That is why I said “These models underly all of physics”.

To clarify, I was talking about calculus. All of our scientific models are based on calculus, and all of calculus is based on infinitesimals. So all of our physical models presume that infinitesimals are physically meaningful. To my knowledge we don’t have any valid physical models that lack this feature.

@LightningInAJar made a very broad original claim, and then brought in the EMR spectrum, but I was still responding to the broader context.

 

[PeroK]

All of our scientific models are based on calculus, and all of calculus is based on infinitesimals. So all of our physical models presume that infinitesimals are physically meaningful.

Calculus doesn't depend on infinitesimals, which technically are part of "non-standard" analysis. Standard calculus is based on limits, which require only the properties of real numbers. One such property is the Archimedean property (which is not compatible with infinitesimals):

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

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

 

[Vanadium 50]

Physically, it is limited by the Plank length.

Which plank are you ralking about?

If you want to talk about the Planck length, nothing magical happens there. I wish people would stop saying otherwise. The Planck resistance is 30 ohms. Nothing magic happens there.

 

[Dale]

Clearly, we have mathematical models that assume space, time and the EM spectrum are infinitely divisible. But, you can never confirm that fully and conclude that there are, for example, an infinite number of possible wavelengths.

To me, the salient point is the converse. There are no models that I am aware of that both explain the data and don’t involve a continuum or infinitesimals. So the idea of a physical continuum or physical infinitesimals is definitely plausible and the idea of the converse is very questionable. It is not required by experimental data and it not known to be even theoretically consistent with observation.

 

[Dale]

Standard calculus is based on limits

That is indeed a valid point but since the word “infinitesimals” is commonly used when describing such limits I felt it was fine. Particularly given the OP’s use of the word “infinite” which is also a limit in standard analysis. In whatever sense the OP intends “infinite” I can use “infinitesimal” in the same sense and my points remain valid without distorting their meaning.

 

[PeroK]

To me, the salient point is the converse. There are no models that I am aware of that both explain the data and don’t involve a continuum or infinitesimals. So the idea of a physical continuum or physical infinitesimals is definitely plausible and the idea of the converse is very questionable. It is not required by experimental data and it not known to be even theoretically consistent with observation.

The Real number system is, however, mathematically tricky. There is a countable subset of computable numbers, which can be described with a finite amount of information. "Most" Real numbers, therefore, are literally impossible to describe. If I asked you to give me a Real number that is not computable, then you would simply be unable to describe it. A lottery based on Real numbers, for example, is not possible!

So, when we say something as seemingly innocuous as "Let $x \in \mathbb R$", we are already doing something that is purely mathematical and cannot be represented physically or represent experimental data.

 

[Dale]

The Real number system is, however, mathematically tricky. There is a countable subset of computable numbers, which can be described with a finite amount of information. "Most" Real numbers, therefore, are literally impossible to describe. If I asked you to give me a Real number that is not computable, then you would simply be unable to describe it. A lottery based on Real numbers, for example, is not possible!

So, when we say something as seemingly innocuous as "Let $x \in \mathbb R$", we are already doing something that is purely mathematical and cannot be represented physically or represent experimental data.

The mathematical trickiness is not particularly worrisome to physicists. The models work. And models without those tricky features don’t.

 

[PeroK]

That is indeed a valid point but since the word “infinitesimals” is commonly used when describing such limits ...

Not by mathematics students. A major breakthrough in 19th mathematics was to develop calculus on a rigorous basis without recourse to "infinities" or "infinitesimals".

"Infinity" appears as a term in standard real analysis, but only in terms of the properties of real numbers. For example, in the construction $n \rightarrow \infty$, there is in fact to appeal to a mathematical object called $\infty$.

 

[weirdoguy]

It is at least limited by the Plank length.

No it is not: https://www.physicsforums.com/insights/hand-wavy-discussion-planck-length/

 

[Dale]

For example, in the construction n→∞,

This is still a limit. I understand your point but I don’t think it is important for this thread specifically nor to physics generally.

A major breakthrough in 19th mathematics was to develop calculus on a rigorous basis without recourse to "infinities" or "infinitesimals".

You may consider it a breakthrough. I consider it a step backwards that mathematical pedagogy has still not overcome.

 

[DaveC426913]

Which plank are you ralking about?

If you want to talk about the Planck length, nothing magical happens there. I wish people would stop saying otherwise. The Planck resistance is 30 ohms. Nothing magic happens there.

.

That was a typo, I swear. I know how to spell Planck.

And mea culpa re: hand waving Planck length. That was sloppy of me.

But my point remains, in the physical world there are limits to precision. At the very least, atoms and wavelengths will limit actual measurements.
It doesn't mean we won't refine our tools but that's not the same as saying things are infinitesimal or that measurements can be infinitely precise.

 

[jbriggs444]

in the physical world there are limits to precision

If for no other reason, because it would take infinitely long to write down the decimal expansion of an infinitely precise result.

 

[Klystron]

Remaining in the physical realm, the cosmic microwave background radiation (CMBR) can be offered as an example of an infinite unlimited, as far as our instruments detect, electromagnetic system.

[edit: corrected language to match the original post.]

 

[PeroK]

Remaining in the physical realm, the cosmic microwave background radiation (CMBR) can be offered as an example of an infinite, as far as our instruments detect, electromagnetic system.

Infinite in what way?

 

[Klystron]

Infinite in what way?

See correction. I meant to use unlimited as in the OP.

We detect the CMBR In every direction as far as we can see.

 

[DaveC426913]

In every direction. As far as we detect.

At the risk of being glib, how is that any more significant than simply saying everything within my personal field of view (be it sky, ground, or my own feet) is an unlimited EM system?

 

[Vanadium 50]

But my point remains

In my view, it is either wrong or at best confusing and unhelpful. One can measure things smaller than atoms - which, by the way, is a completely different claim than your original one about Planck units.

 

[Vanadium 50]

As a PS, Dave, you have well and truly hijacked this thread.

 

[DaveC426913]

In my view, it is either wrong or at best confusing and unhelpful. One can measure things smaller than atoms - which, by the way, is a completely different claim than your original one about Planck units.

I wasn't suggesting one can't measure things smaller than atoms. Sorry if it seemed that way.

It sort of turns on what one means by an "infinity in the real world". Everyone seems to keep coming up with theoretical ideas, none of which sound like they would survive an actual demonstration (especially the ones that require infinite time to execute).

As a PS, Dave, you have well and truly hijacked this thread.

I don't see how that is possible. It am directly, literally addressing the OP's question.

Now, if you mean I am dominating the thread with my views, that's something different from hijacking.

 

[jbriggs444]

I am dominating the thread

The PF way -- everyone has to make sure that their two cents worth is taken seriously.

 

[Klystron]

At the risk of being glib, how is that any more significant than simply saying everything within my personal field of view (be it sky, ground, or my own feet) is an unlimited EM system?

Sorry, you lost me. The CMBR, first detected IMS in 1963 using a modified radar receiver, has no significant relation to your examples. Within our technological limits we detect the CMBR in all directions as far as we can see.

If you mean to argue that all EMR emissions in free space propagate unlimitedly, then I tend to agree except that EMR eventually dissipates below threshold of detection. According to Weinberg, Wilson and Dicke, among others, the CMBR persists since shortly after the Big Bang.

I also considered offering 'cosmic rays' as an example but disqualified them since theoretically cosmic rays have preferred directions and identifiable sources such as stellar objects, supernovae, galactic cores, etc. While pervasive, cosmic rays have distinct limits compared to CMBR.

 

[DaveC426913]

While it's way easier to use math to prove it, it still is observable in the physical world - down to the best precision your measuring instrument permits. And when you then build a more finely graded instrument, you can open the next can... ...ad infinitum.

It is only ad infinitum in theory. This is explicitly about what exists.

Again, the OP is not asking about theoretical infinities or near infinities; that is a trivial question to answer.

He is asking if there is anything in the physical world that is infinite. "can" and "might" don't cut it here.
That is entire crux of this thread (interpreting the opening post).