# Mildly interesting: is the number of objects in the universe at any given time finite? [closed]

My guess is yes. Here's my reasoning.

There are a finite number of atoms in the universe (the current estimate is about 10^80). In other words, the set of all atoms in the (observable) universe is a finite set.

Now define an object to be any (nonempty) subset of this set of atoms. For example, a book is a subset of the set of all atoms, and so a book is an object. This definition also includes many many things we would normally not consider as objects, such as the set of exactly two atoms: one atom on Earth and one on Jupiter. But this is okay; I take this very general definition of an "object" in order to achieve an upper bound on the number of objects.

What, given our definition of an object, is the number of objects in the universe? The number of subsets of the set of all atoms is the cardinality of the power set of the set of all atoms.

This is 2^(10^80), which is a ridiculously large number (10^80 is difficult to imagine but you can imagine writing down 80 zeroes following a one ... with 2^(10^80) there are around 10^(80) digits).

While there are a lot of objects that I've counted that shouldn't probably be counted, we've calculated an upper bound. So, in a specific sense, the universe is finite.

This is quite the grandiose claim, and it probably isn't that unsurprising or significant. I guess my questions are simply: Is this reasoning correct, and if so is there a name for this strange definition of an object? Are there any papers I could read on this kind of thing?

• the concept of infinite is pretty confusing to me ;-) if the universe is finite then the number of things in it will always be finite. But thats matter of definition: from 0 to 1 there are a infinite amount of numbers
– yamm
Commented Dec 18, 2014 at 10:03
• If an object changes over time (ie, the atoms it consists of change), is it a different object at each time interval? If not, and if you believe time is continuous, there can be an infinite number of objects.
– user935
Commented Dec 19, 2014 at 17:35
• This question appears to be off-topic because it is about physics. Commented Dec 19, 2014 at 18:10
• You have performed a correct calculation : assuming that the premise : "There are a finite number of atoms in the universe" is true, the conclusion follows by mathematics alone. Commented Dec 21, 2014 at 15:13
• The idea of virtual particles does not suggest there are a stable finite number of basic elements in the universe. The fact we call them 'atoms' aside, the things that make up molecules are not the smallest possible objects. Schrodinger's equation is expressed in continuous terms. That suggests that matter manifests energy in a way that may or may not be continuously divisible.
– user9166
Commented Dec 31, 2014 at 2:29

Your position is phrased as a definition, so the assumption that it is true is tautological. If you assume that the word "object" can be defined by a set of atoms, and you assume there is a finite number of atoms (which is science's best guess, but not certain), then by definition there are a finite number of objects.

The question that does remain is "is this definition of 'object' useful." Is it useful enough to supercede all other definitions of "object" in the English language such that when you say "object" there is no ambiguity on the part of the listener.

That is a linguistics debate for you, which does not have an answer one way or another. However, I would suggest there is a linguistic argument to say "No, your reasoning is not correct: your definition declares that an electron is not an object." You are free to use your definition, but you must accept that there are things called "electrons" that others may think of as objects, but you define them to be "not objects."

This process can continue deeper and deeper if you so choose. It will get interesting if you reach the QM level, where the fundamental object is a waveform. At that point, the definition of "object" muddies greatly because we are pretty sure the waveforms start to blur together in some cases.

If I may suggest an interesting alternative definition...

The strange world of QM can be interesting if you define an object with respect to the way we think, rather than physics. Consider that nature really doesn't care if a table is one object, or five (a top and four legs). The way you define your objects matters when you start trying to predict what the table will do in certain situations (how will it break apart if I put too much weight on it?). This predictive value seems to be closely tied to our concept of what is an "object" and what is not; I consider this important.

With this definition, I can go through a a series of historical experiments that lead up to QM. You will see that, as we go along, the meaning of "object" shifts.

Physics purists: I'm certain I'm getting some terminology wrong. I am sorry in advance.

Consider a lightbeam. It is a meaningful enough thing that we might dare to call it an object (or maybe not... but I'm going to call it an object). If you have a lightbeam pouring into a room, the room is lit. If there is no lightbeam, the room is dark.

This is, of course, an approximation of reality. It is convenient to think of the lightbeam as a thing because we can use that to predict whether rooms are light or dark. This approximation is totally valid, up until the point where we start to strain it.

The double-slit experiment is a famous experiment. If you shine light on a slit, and observe the light on a surface behind the slit, you see a diffusion pattern: a bright patch in the middle which fades to darkness on either side. If you shine light on two slits, side by side, you see two diffusion patterns. However, if you get your hands on some monochromatic light (only one color/wavelength), strange behaviors occur. "Fringing" patterns appear, alternating bars of light and dark, much unexpected from our simple lightbeam model.

So we update the model, and have light travel as waves. We can do the math and show that the waves cancel out at the surface to generate these fringing patterns. We have had to modify our lightbeam definition a little: now each wavelength is broken out and has "phase" and "amplitude," but this is of minor concern. It's not that much more complicated, and it explains all we have seen.

Now consider a strange effect: the photoelectric effect. We observe that some materials, metals mostly, will emit electrons when lit up by light (which, of course, is how solar cells work). However, experiments show that it has some curious behaviors. If you use too low of a frequency of light, it doesn't emit an electron at all, no matter how powerful the light source is. Use a higher frequency of light, and it starts knocking electrons off, even with faint light sources. Curious.

The only way this makes sense is if light was quantized into little objects which knocked the electrons off like colliding billiard balls. We have to shift our definition of object... now there are billions of small objects, "photons," skittering about. High energy photons can kick an electron off of our metalic surface with ease. Low energy photons just lack the oomph. En masse, they sum to form wavelight behaviors, explaining the double-slit behavior. We've got a definition for an object that explains everything we've seen!

This is where it gets odd. Let's go back to our double-slit, now that we know light is quantized into photons. We can construct a light source that emits just one photon at a time. We can record where the photon hits the surface behind the slits, then send another photon. If we repeat this process and overlay all of the impact locations, we expect to see two bands, one behind each slit, because the photon has to go through one slit or the other.

Wrong.

Classical physics hangs its head in shame, because that is NOT what we see. We see fringing, just like before. Somehow the photon is traveling through both slits at the same time, and interfering with itself!

Quantum Mechanics can explain this behavior. QM states that photons are not particles, per se, but wave packets. They are waves that are localized in space, just like a staccato note played by a flute is localized in time. Within the note, there are vibrations, but they taper off as you approach the edges. If you model your "single-photon light source" like this, usually you will find its behavior to be approximately identical to the classical model. However, the double-slit setup amplifies the characteristics of this wave packet which classical modeling assumes "don't matter much." It shows that you should expect a fringe pattern, which you see.

Freaky, isn't it.

Note that, at each step, the "object" is whatever makes sense to help you predict behavior

In 99.99% of your life, you will not have to think about light as anything more than a beam.

A "lightbeam" is a useful object for you to work with. In a handful of odd cases, you will need to think of a light beam as something more than just one object. Perhaps you put a prism in the way of the light beam.

In 99.99999% of your life, you will not have to think about light as anything more than a wave.

However, perhaps you are interested in solar energy. You will have to think of photons as the fundamental "object" that light is made of.

In 99.99999 999% of your life, you will not have to think about light as anything more than photons.

However, if you play with the deep dark corners of physics, where new discoveries are found you wont have such luxuries. You will have to deal with light in the truest form we know: waveforms

In 0.00000 0001% of your life, you will have to think about light as a quantum waveform. If you deal with experiments such as the delayed-eraser-double-slit-experiment, you will defecate building materials when you see the results if you try to think in terms of classical mechanics instead of quantum waveforms. That being said, in all other cases, simpler objects are fine. You don't always need to use the most exacting model.

In 99.99999 99999 99% of your life, waveforms will be enough.

Are you a theoretical physicist? How cool would it be to find that the QM definition of light is insufficient because you ran an experiment that got different results than QM predicted.

Are you ready to redefine object once again?

Welcome back to the world of the sane. Objects are made of atoms again. Your sanity is intact. But you will never be the same, because you have seen the rabbit hole, and realized you may never truly comprehend how far it goes.

At each step of that journey, there was a thing that was worthy of being called an "object." Between each step, that definition was shaken until it crumbled. However, for most of your life, you are free to define an "object" to be something simple. In fact, it turns out that modeling the QM waveforms emitted by an incandescent lightbulb is virtually intractable. There's just too much math! If you use the best definition for an object made of light, you'll never get off the drafting paper. By choosing a less precise definition, you can make a useful product, like lightbulbs that we use every day!

These "less precise" defintions of "object" are useful because they help us understand the universe around us!

• That makes sense. I didn't consider electrons or other subatomic particles. I don't know much about QM but are the waveforms you mentioned fundamental building blocks in the way atoms were once thought to be? And if so, could we reach the same conclusion of a finite universe? Commented Dec 18, 2014 at 2:03
• The tricky part about waves is that it is hard to define where one begins and where one ends, so it becomes hard to "slice" the universe up into "atomic-wave-pieces" without missing a detail that matters. If you take that process to one extreme, you end up with uncountably infinite slices (objects). If you take the process to another extreme, there is one Universal Waveform, so there is a finite number of objects (specifically one, and its a universe). Commented Dec 18, 2014 at 2:10
• Okay. So it seems like questions like "is the universe finite?" aren't very useful, because they are ambiguous ... ? Commented Dec 18, 2014 at 2:13
• If you want to go into crazy math-land, the QM "wave functions" are infinite-dimensional function spaces called Hilbert-spaces. They're fun to try to make sense of. However, if you go down that path, remember that QM is just the best model we have so far... not necessarily the true nature of the universe. Commented Dec 18, 2014 at 2:13
• I would agree with the statement that they are ambiguous. It is meaningful to ask if the "number of atoms in the universe is finite," or if "the spatial extents of the universe are finite," and I think they are useful questions. The only reason I would say "is the universe finite" is not useful is just because its so easy to get two listeners to interpret that question two different ways, and arrive at totally different and irreconcilable outcomes without either listener actually being wrong. Commented Dec 18, 2014 at 2:15

At any given resolution, the number of atoms in the universe are finite. However, that resolution is disputed. For example Feynman path integrals already take an infinite number of virtual particles to make sense.

In fact, this was also the position of the ancient atomists, when they said that there were an infinite number of atoms in the the universe.

• Thanks for the answer regardless. I don't know anything about these Feynman path integrals but do they include time as a variable in any way? My post was talking about the number of objects at any fixed time. Commented Dec 18, 2014 at 23:44
• @Mukerjee: It depends on how you look at it; there definition is obviously different from todays, given how much more we know; but in essence it is correct and is still discernible in our latest definition: consider an atom is that which can no longer be divided - the greek definition; and the contemporary definition of a particle as an 'iredducible representation of the Poincare group of space-time': there is a long (and zig-zaggy) line that connects non-divisibility and iredducibity. Commented Dec 20, 2014 at 16:47

It depends on how you're defining object. A book is an object. But so are the pages of a book and so are chapters (collections of pages) of the book. There are a potentially infinite number of ways to split a finite number of atoms into objects.

• Actually, no. The point I was trying to make was that even if you leniently consider every subset of the set of atoms to be an object in its own right, then there are still a finite number of objects. This is because the power set of a finite set is itself finite. Commented Dec 18, 2014 at 23:42
• Is a fraction of an object an object into itself? So for a given book, you have 1/2 the book, 1/4 the book, 1/8 the book, etc. You can argue that this hits a limit at the point where the fraction has come down to the point of a single atom but you've not demonstrated that a single set is identical to every set that contains the same members. That is to say that the set of books by left handed radical feminists with red hair may have the same members as the set of books by Czech baristas who are into Morrisey. Yet they are different sets because their members don't define the sets. Commented Dec 19, 2014 at 20:34
• Elements of sets completely describe sets. Sets are equal if they are subsets of each other. Commented Dec 20, 2014 at 2:16