If all of the particles that made up the Empire State Building had the EMPTY space removed from them there would be an object about the size of a grain of rice left. With so much empty space in every object it might be possible to squeeze in another object or two. How the hell it could be done I have no idea though. — Sir2u

If you are talking about the atoms from one "object" fitting between the atoms in another, you are not actually talking about two actual physical things being in exactly the same place at the same time. Here no two elementary particles are actually in exactly the same place at the same time. It is a situation like my earlier illustration. And if you were to do that, say with two buildings, you wouldn't have anything resembling buildings at the end of it, given all the inter-atomic forces that would be at work.

You might be interested to read this:

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

I looked at the link. As far as I can tell with a cursory skim, it doesn't talk about two identical particles being in the same place at the same time while remaining two separate particles. — petrichor

It says that bosons can share the same quantum state (which includes position). Fermions can't share the same quantum state but they can still share the same position if some other property differs (such as spin state).

Note that there are interpretive issues about what position entails given Heisenberg uncertainty. But the general point is that two particles can be indistinguishable in principle and it is this feature that leads to quantum interference effects. For a nice example of this with two particles, see the Hong-Ou-Mandel effect (where the two particles enter the beam splitter at the same time and produce interference effects).

What does it mean, precisely, for two things to be in the same quantum state?

Also, when it comes to interference effects, aren't we just adding waves, like in the example of water ripples I gave earlier? And isn't the wave in this case a probability wave?

Well think of a camera taking a timelapse photo of a flower. The colors are changing but the object exists spatially in the same place? I'm on my phone so sorry for the crappy posts. — Wallows

Now think of that timelapse as a GIF and you'll see it's layered and not spatially the same.

If two objects occupy the same space, they meld - and become qualities of one object.

Fundamental particles can occupy the same space at the same time. See identical particles.

I, at least, consider particles to be physical objects. — Andrew M

You are just making my point. You choose to include elementary particles into things that you call "objects." I don't think it's a conventional use of words, but whatever - the point is that your choice of whether or not to call something an "object" has no metaphysical implications.

Elementary particles are tricky if you want to talk about them being in the same place at the same time. The best you can do is talk about their quantum states and their superpositions. But then you might as well talk about superpositions of classical fields - here at least being in the same place at the same time is well-defined. If you want to call classical fields objects, then of course you will find that such objects can be in the same place at the same time, but again, this is just word manipulation, nothing more.

Here's the conventional usage:

1. A material thing that can be seen and touched.

1.1 Philosophy A thing external to the thinking mind or subject. — Andrew M

That's a different meaning of "object." Sumerian grammar can be an object of a study in this sense.

There is no such physical thing as a temperature field. — petrichor

That's kind of a silly thing to say, on the one hand. A field is "a physical quantity... that has a value for each point in space-time." And temperature is, of course, a physical quantity. One can talk about temperature fields, and electron fields, and all sorts of other physical fields, and they all exist in the same place (all place) at the same time (all time). But they are not physical objects, you would object! Well, yeah, when we talk about physical objects, we usually talk about things like chairs and stuff. So don't call things that are not object-like objects, and you'll get the conclusion that objects cannot be in the same place at the same time. Or do call them objects, and you'll get a different conclusion. Whoop-de-doo.

Hi everyone,

I have been thinking about this for a while now, and I do not understand why people claim that two objects cannot occupy the same space at the same time. — elucid

Have you seen anything to the contrary? Everyone would be surprised, even shocked, if you have.

One piece of evidence that may or may not convince you is solid objects are incompressible. In terms of particles consider an object A composed of 2 particle x and y. If two particles could occupy the same space then we should be able to compress A to the size of an x/y. This isn't possible. I haven't seen it being done.

You may say gases are compressible but that's explained in terms of squeezing out the space between the particles

An interesting fact is liquids are compressible too. Strange because there's more inter-particle space in liquids in gases.


Liquid behavior is interesting because despite the existence of bigger spaces between particles they aren't compressible. It may be that particles, and by extension any object, can occupy the same space but some kind of force, electric repulsion for example, keeps them from doing so.

Therefore, while objects, except gases, at a human scale can't occupy the same space, it may be that that's possible but prevented by inter-particle repulsive force of some kind.

That's kind of a silly thing to say, on the one hand. A field is "a physical quantity... that has a value for each point in space-time." And temperature is, of course, a physical quantity. One can talk about temperature fields, and electron fields, and all sorts of other physical fields, and they all exist in the same place (all place) at the same time (all time). But they are not physical objects, you would object! Well, yeah, when we talk about physical objects, we usually talk about things like chairs and stuff. So don't call things that are not object-like objects, and you'll get the conclusion that objects cannot be in the same place at the same time. Or do call them objects, and you'll get a different conclusion. Whoop-de-doo — SophistiCat

In this discussion, I think the intuitive image that most of us have of what is actually being disputed is whether two pieces of actual physical matter can actually overlap while remaining distinct. As everyone who has taken high-school physics or chemistry knows, a temperature field is an abstraction that represents such things as the average kinetic energy in the particles of a gas at a given point in space. For our purposes though, we are talking about the actual stuff, the particles themselves, not a smeared-out representation of their average kinetic energy.

Maybe some others aren't even thinking about the same problem as I am, in which case we are talking past one another. In my view, if we allow our high-level abstractions to be considered physical objects for the purposes of asking whether two physical objects can overlap, it is trivial to say that two such objects can indeed overlap. Sure, a storm can be in the same place as the sky. A dog can also be in the same place at the same time as a collection of hairs, blood vessels, kidneys, lymph nodes, and so on. Trivial. Carve up the world however you like and name what's inside the boundaries you arbitrarily define whatever you want and then show that some of them overlap. Can urban blight overlap with a sunny day? Sure, why not? Can a wealth-concentration field overlap with a happiness-concentration field? Sure. We can even have another field that represents some relation between wealth and happiness, a happiness-over-wealth field. It might be interesting to see if it varies from region to region.

Personally, I find it interesting to ask about the nature of fields and particles and whatnot. What are we really talking about? Are they real physical things? Or are they abstractions we use to represent things, like temperature fields? Air traffic controllers use little strips of paper to represent airplanes. And their system works well for managing the traffic. But it would be a mistake to take the map for the territory. When we think of a field as a real physical thing, are we taking the map for the territory? I don't know! I would like to know!

And yes, as you seem to have noticed, I don't consider such objects as chairs to be real beyond the way our minds carve up the world. The matter that composes them is real. But there is no true boundary between a chair and a pillow sitting on it that says each is a truly distinct thing. It is like the strips of paper representing airplanes. It would be too much for the air traffic controllers to think about everything involved. So they take a shortcut and used a much-simplified model. That's what all high-level scientific theories do.

But for the purposes of inquiries such as the present one, we need to get as low-level as possible. What is the actual stuff? Is it multiple? When we talk about two fields coexisting in the same place as two separate objects, are we confused? Is it really just two aspects of one more fundamental thing? What about two particles in the same place? What are we talking about, really?

But, in a world where higher order dimensions influence lower dimensions in a manner of totality, — Wallows

In other words, if we make up some incoherent gobbledygook . . .

What does it mean, precisely, for two things to be in the same quantum state? — petrichor

A quantum state contains all the information about a quantum system. For two photons to be in the same state means there is no information, in principle, that distinguishes them. Which leaves us with cardinality (i.e., 2 photons) but not individual photon identities.

For why this is significant, consider the Hong–Ou–Mandel experiment again. Suppose the photon entering the beam splitter at the top is named A and the photon entering at the bottom is named B. In state 2, photon A ends up at the bottom and photon B ends ups at the top. Whereas in state 3 photon A remains at the top and photon B remains at the bottom. However according to QM, there is no information that would physically distinguish those two states - they are physically equivalent. So their respective amplitudes add which manifests as destructive interference, as observed. Whereas classical physics (in which those states are physically different) is unable to predict what is observed.

Also, when it comes to interference effects, aren't we just adding waves, like in the example of water ripples I gave earlier? And isn't the wave in this case a probability wave? — petrichor

Yes. But it's worth noting we're not observing waves, we're observing particles at specific positions or with specific spins (or whatever we choose to measure).

You are just making my point. You choose to include elementary particles into things that you call "objects." — SophistiCat

Yes, but it's motivated by the ordinary definition, not an arbitrary choice. I think the relevant characteristic of objects is that they are concrete things that can be observed or have their properties measured. That's compatible with "a material thing that can be seen and touched".

The question of whether objects can occupy the same space at the same time or not seems to me to be more associated with classical physics rather than something necessarily associated with the term.

The problem with this discussion is that no one, including, I am sure, the OP @elucid, quite knows what question is being asked, and what kind of answer is expected. Much of philosophy is about asking good questions and being alert and skeptical about assumptions, especially your own.

"why people claim that two objects cannot occupy the same space at the same time"

What is meant by "objects"? Are they bulky solid things that we can see and touch, as per one common meaning of the word "object"? Or anything to which one can refer, as per the grammatical sense of the word? Or something else?

And what does "cannot" mean in this sentence? Cannot in fact (in our world, to the best of our knowledge)? Or in principle - what principle? If we relax the grip on reality and allow other possible worlds, then we have to have a good grip on the trans-world "objecthood", lest the word loses its meaning.

And what does "why" mean? Are we looking for a reductive explanation in terms of some underlying physics? That would be a relatively easy question to answer. A metaphysical principle? Then we'll have to tangle with objecthood and counterfactuals.

In this discussion, I think the intuitive image that most of us have of what is actually being disputed is whether two pieces of actual physical matter can actually overlap while remaining distinct. As everyone who has taken high-school physics or chemistry knows, a temperature field is an abstraction that represents such things as the average kinetic energy in the particles of a gas at a given point in space. For our purposes though, we are talking about the actual stuff, the particles themselves, not a smeared-out representation of their average kinetic energy. — petrichor

Everything is an abstraction, elementary particles included. Physicalist reductionism - the position that only (some) entities posited by fundamental physics really exist, everything else being mere abstractions and pragmatic simplifications - is a defensible view, but it must not be assumed unconsciously, as a matter of fact.

By "Objects", I mean physical objects. By "Cannot", I mean impossible. By "Why", I mean the reason behind that belief. I am guessing the reason behind that belief is seeing objects crash into each other or lightly bump into each other and instead of occupying the same space, they move away from each other, break or just prevent each other's movement.

By "Objects", I mean physical objects. By "Cannot", I mean impossible. By "Why", I mean the reason behind that belief. I am guessing the reason behind that belief is seeing objects crash into each other or lightly bump into each other and instead of occupying the same space, they move away from each other, break or just prevent each other's movement. — elucid

In that case, the somewhat flippant answer that I gave you in the beginning still fits. There are plenty of things in our ordinary experience that fit this description (and you are not asking "why" such things exist, you take their existence and their properties for granted - which is fine, one has to ground the discussion somewhere). We refer to such things as "objects" in the English language. If we learn about certain entities that do not fit this description, such as rays of light or bosons or spirits, we may accept their existence, but we won't refer to them as "objects" in the same sense in which call chairs "objects." This is just a matter of categorizing and naming things.

Chomsky points out that the meaning of "physical" has changed over time. With acceptance of electromagnetism, the old type of physicalism died.

So what we're calling a "common" meaning for "object" is actually from an outdated scientific model.

Happy NY all philosophy lovers, I am happy to have joined the group and I hope you don't mid me joining this discussion which I also have been writing about for some time.

What I wanted to add is that if we are going to ‘conceptually’ tackle the problem of same space/time co-occupation, then it is important we make the argument about any possible entity and make it type agnostic. For what we are concerned with here is the logical possibility of co-occupation by anything that can either be measured, or observed, or detected, or all. Because if the notion of co-occupation is 'possible' say in the case of photons, then it makes it a fact that the notion qua notion is a logical possibility given the entities in question have certain properties which photons would be an example of. So we need to talk in terms of whether it is possible in principle for any number of observable, measurable, or detectable entities to occupy the same space.

The other complication is should we position the question around ‘space/time’ at all? Or should we find a better way to ask the question? Because if we can’t then it either means our conceptual understanding of space is such that it is different from everything else, in which case we need to be able and define it, or it means there is a problem related to our question with the notion of space/time at the heart of the problem.

A related issue is the notion of ‘same’ used in the ‘same space/time location’. We have to be very clear about what this notion means. For example; to say two identical entities could have different histories just begs the question. In possible world W1 we can have two intrinsically identical particles, identical mass, same shape, and so on, where each have been traveling in a straight line for 1 year since they came into being. Our description of their history is the same it seems. Nothing in the description of one is different than the other. We may now wish to track them back to their source of origin and how they came into being. But the same can be said about those original particles where their history is not descriptively different and so on. But nevertheless we have two numerically different particles that are descriptively the same. I am not making the argument that such indiscernible identicals are at all possible, I am just saying resorting to ‘same history' doesn’t do the job. We will soon realise that we have to resort back to the notion of different/same space/time location' as some definitive point of difference, which brings me back to how we define 'same' and 'space/time'.

Now, to say that ‘if two objects occupied the same space wouldn’t they be just one’ doesn’t solve the problem for we haven’t demonstrated why that would be the case. Of course the assertion seems self-evident and intuitive but it nevertheless doesn’t represent at argument.
Now, I am not defending the position that such co-occupation is possible, I am merely saying that in my humble opinion the structure of the arguments need to be tightened.

Finally there is a difference between qualitative identity and numerical identity and the question is whether two objects that are qualitatively identical are necessarily numerically identical, or whether it is possible that they are in fact two numerically different objects. And how does the notorious 'space/time location' come into play.
This last point helps with confusions regarding 'everything is essentially made from the same thing' and so on. Any such comment needs to make clearly make the distinction between
qualitative vs numerical sameness and how they include or exclude one another .