This might sound weird and without any practical relevancy, but I’ve wondered about this for some time: Is there a precise definition of when two things touch? My guess is that two things touch when they get so close that forces of electromagnetic repulsion between their molecules and atoms prevent them from getting closer together, but then again those forces work continously, theoretically at infinite distance (although getting weaker proportionately to the distance squared, IIRC), and surely they can get even closer after a slight first touch.
I know there’s not too much of importance in it, but I hope I could make clear what I was pondering about in my idle hours.
Physically, some subatomic space will continue to exist between the two objects.
I like to think of it as the closest two objects can come to one another.
But won’t there still be some place where there isn’t any subatomic space, which is shown by the fact that there’s still friction? I move to answer that it is “the distance at which there is a place where the 2 objects do not have any space between them.”
Space exists between the molecules within a heterogenous object. How could you eliminate the space between two seperate objects that share no molecular bonds?
I would define it as the point when the objects begin to exert a normal forces on one another.
The normal force is the parctical effect of the electromagnetic forces in the OP. It’s what keeps objects from passing through one another. For example, consider a book sitting on a table. The book has weight, i.e. it feels a force of gravity. Without the table to support it, it would fall, accelerating because of the gravitational force exerted upon it by the Earth. But put the book on a table, and the table exerts a normal force that is equal and opposite to the gravitational force on the book, so that the net force is zero, and the book does not accelerate.
If you want to know when a book touches a table, hang the book from a spring scale, then slowly lower the book toward the table. As soon as the force registered by the spring scale begins to decrease, the book is touching the table.
That’s a very big-scale definition, so maybe it’s not what you’re looking for.
I think that’s about the best definition you’re going to get. On the quantum level, nothing “touches” anything else, because what we think of as particles are actually just fields centered about a focus that we sometimes express mathematically as a single point (or very small volume). In the macro world of everyday life, particles can theoretically be anywhere in space, but their interactions are limited to the region about their locus; an electron isn’t going to spontaneously satisfy an unfilled electron shell in an atom across the room even though out could, briefly, be in the position to do so. So the extertion of normal (and lateral “friction” forces that occur by various methods) is as exact a definition as one can reasonably draft.
Now, as to a discussion on when such interaction between particles becomes an issue of sexual harassment, it all depends upon how charming the quarks are…
But at a large distance, an electron in an object “sees” the other object as electrically neutral; the forces from the positively charged protons in the nuclei and the negatively charged electrons cancel out.
It’s only when the electrons can cozy up real close that the electron will feel repulsive force from the electrons that’s significantly greater than the attactive force from the atom’s nucleus.
I can’t resist shamelessly ripping off someone’s sig. (Chronos?)
“Is that a lepton in your pocket or are you just happy to see me?”
I don’t remember whose sig that was; perhaps CalMeacham? But the joke originated with DrMatrix, in the classic Expanding Universe thread.
And strictly speaking, normal forces, being electromagnetic, never quite go to zero, even when the objects are far apart. But there is generally some well-defined characteristic distance at which the force suddenly becomes much greater, so we can reasonably define objects at that distance to be touching.
