# How can a mug and a torus be equivalent if the mug is chiral?

I don't know much about topology, but the other day I was thinking about the (classic?) coffee cup - donut topological equivalence. I realised that a cylinder with one open end and a handle on the side (a mug) is a chiral object in 3D space, whereas a torus/donut is achiral ("meso", as indicated by the internal planes of symmetry present in a torus).

EDIT: I just realized the mug may not in fact be chiral, due to an internal plane of symmetry (vertical, through the handle). But I will leave the post as some of the questions are still relevant, and I'm still interested in the arbitrary example of how topology handles a chiral object being transformed into an achiral object.

As an outsider, my impression is that topology is roughly "the qualitative mathematics of shapes"; is this accurate at all? I would expect such a field would be quite concerned with phenomena like chirality, the lack of superposability of two mirror-images of a 3D object. From another perspective, chirality is the reason we have a "right hand rule" convention, or the very reason we can distinguish between a right- and left-handed coordinate system. To me chirality seems to be an emergent property, an asymmetry that arises when the object/space is sufficiently complex.

So when a chiral object and achiral object are claimed to be topologically equivalent, is there some unsung caveat about chirality? Perhaps topologists truly do not care about chirality, as it does not stop their infinitely malleable material from transforming between the two shapes. If so, then my question is what is the significance of chirality in topology?

And if I may, these questions built off this train of thought:

If a 3D object that is chiral in 3D space is viewed in a four-dimensional space, would it be considered achiral ("meso")? (Analogous to a 2D object being considered achiral in 3D space, if I am not mistaken.)

Is there any usefulness to defining chirality of 2D objects in 2D space? Is there any generalized notion of chirality for an $n$-dimensional space, some "$n$-chirality"?

• Add a lump to the handle to make the mug chiral, it will remain topologically equivalent to the torus. Apr 30, 2017 at 6:22

Other answers have said that topology cannot distinguish chirality, but this is not quite true. Topologically a left and right glove are identical, because the left-right reflection is continuous in both directions. The difference between the left and right gloves isn't intrinsic to the gloves themselves, but is in the way the gloves are embedded in three-dimensional space.

Consider this analogy: The symbol b can be turned into d, but to do it you have to lift the b out of the plane and flip it over. If you leave b and d embedded in the plane you can never turn one into the other (although you can turn b into q and d into p) so the difference is not in the shapes themselves but in the way they inhabit the plane. Left and right gloves are distinguishable if they remain in three-dimensional space. But in larger spaces, they're identical. It's easy to turn a left glove into a right glove if you can lift it into four-dimensional space and flip it over.

The branch of topology called knot theory concerns the way simple shapes like circles can be embedded into three dimensions. Topologically a knotted loop of string and an unknotted one are the same—but again, their embeddings into three-dimensional space are different. To deal with this topologically requires some extra effort. One embeds the knotted loop in space and then considers whether there is a continuous deformation of the entire space, including the loop, that transforms the knotted loop into the unknotted one. Viewed in this way one can say that a trefoil knot (shown below) is chiral, because although each is topologically equivalent to a plain circle, the left-handed trefoil knot can never be smoothly transformed into the right-handed one, in the sense of the previous paragraph, without removing it from three-dimensional space. This seems to be what you are looking for.

But what if we consider the coffee mug and torus example? The coffee mug is not chiral in this sense. It has a handle on the left side. Could we deform space to move the handle to the right side instead? Of course we could: just turn the cup around, now the handle is on the other side. No fourth dimension is required. (There are also many other ways to get the handle to the other side.)

I'm still interested in the arbitrary example of how topology handles a chiral object being transformed into an achiral object.

This never happens. Let $$C$$ be some chiral object and let $$\bar C$$ be the mirror image of the object. Since $$C$$ is chiral, $$C$$ cannot be transformed into $$\bar C$$ without removing it from the ambient space.

But you say that $$C$$ can be transformed into some achiral object $$A$$. But clearly $$\bar C$$ could be transformed into $$A$$ by the same method (except mirrored.) But then we can turn $$C$$ into $$A$$ using your method, and then $$A$$ into $$\bar C$$ by using the mirrored method, but in reverse. So there is a way to turn $$C$$ into $$\bar C$$ after all, and thus $$C$$ was never chiral in the first place.

If a 3D object that is chiral in 3D space is viewed in a four-dimensional space, would it be considered achiral ("meso")?

Yes, always. Because if $$C$$ and $$\bar C$$ are left- and right-handed versions of the same thing, then they are identical under a reflection symmetry, and in four-space this is simply a rotation, just as a reflection of the plane can be realized as a rotation of three-space.

But it may interest you to learn that there are higher-dimensional analogues of knotted circles: in four dimensions one can have knottings of a sphere which can be left- or right-handed, and which become identical when these chiral knots are lifted into five dimensions.

• Thank you for seeing through my poor approach and targeting my underlying uncertainty. The trefoil knot example was very enlightening to me. Your last two paragraphs were the very things I was hoping to learn, but didn't quite know how to ask for. Thank you again for your assistance! Apr 30, 2017 at 5:39
• I'm glad I could help.
– MJD
Apr 30, 2017 at 6:42
• It may also be interesting to note that if you lift the trefoil knot into 4D, you not only can turn it around to the opposite 3D-chirality, you also can unknot it into a plain circle, which isn't chiral even after re-embedding it in 3D. Apr 30, 2017 at 11:08
• Minor quibble to an otherwise very clear answer: as someone who regularly wears gloves inside-out, I have a simple mapping from left- to right-handed gloves in three dimensions. (not that I have a better suggestion)
– kyle
May 1, 2017 at 14:19
• Related quibble: I have used, and in fact when I read this answer I was thinking of, cotton gloves (searching for "inspection gloves" seems to give some images) that have no "front" and "back", and are therefore achiral in a simpler way. (Another minor quibble: the sentence “Since $C$ is chiral, $C$ cannot be transformed into $\bar C$...” is not strictly necessary (and was a bit confusing); IMO the argument flows better without it. This may just be one of those “proof by unnecessary contradiction” things.) May 2, 2017 at 0:06

Usually it is stated that topology is "geometry minus shape", which means, we are completely ignoring the exact shape of an object. Annother classical example would be a ball (sphere + interior) being equivalent to a (solid) cube. The sphere having infinitely many symmetries, while a cube only has a finite symmetry group. Topological equivalence only cares about preserving a kind of a closeness relation between the points of an object, i.e. homeomorphism (topological "isomorphism") will not cut or rip the objects apart.

From a topological point of view, no object is distinguishable from its mirror image. The map $f:(x_1,...,x_n)\mapsto (-x_1,x_2,...,x_n)$ is a homemorphism in the most geometrically meaningful topological spaces. In the end, a topologist can determine an object only up to a continuous transformation. He will not recognize any translation, rotation, stretching, mirroring or shearing.

• Thank you for helping me better understand the focus of topology! Apr 30, 2017 at 5:41
• "or shearing" - doesn't shearing mean "cut or rip the objects"? May 1, 2017 at 16:49
• @wizzwizz4 It means a specific kind of linear transformation, which is always continuous. May 1, 2017 at 17:30

The mug as pictured is not chiral. It has a mirror plane through the handle and the center of where the liquid goes in. In any event topology does not depend on point group symmetry and so does not depend on chirality.

• Thanks for pointing that out, I realized it right after I posted. But the particular example of the mug is not really the heart of my question. Apr 30, 2017 at 0:46
• Hence the second sentence in the answer. Apr 30, 2017 at 0:50
• All that this answer says is “topology does not depend on point group symmetry and so does not depend on chirality”, which I think does not sufficiently address the question. May 1, 2017 at 6:46

So when a chiral object and achiral object are claimed to be topologically equivalent, is there some unsung caveat about chirality?

No, it's just a notion of equivalence that doesn't care about the number of symmetries the object has. Similarly, when we say that two integers are equivalent modulo two, there isn't some unstated caveat about them possibly being different sizes: we've chosen a notion of equivalence that only cares about whether the numbers are odd or even, and doesn't care about their sizes.