I got into a long and protracted argument with someone on an online forum recently over a seemingly minor point and got accused of not knowing basic physics. It would be funny if the guy weren’t so rude and dismissive about it. At any rate, part of the argument came out of the following thought experiment I concocted (actually the version I present is a much better version than the one that sparked the argument). In it, I devise a situation (albeit highly contrived) in which it is impossible to tell whether a certain object is attracted to a certain type of box due to gravitation or electrostatics. This highly contrived story takes place in a universe in which we have massless, charged particles (something we don’t presently see in nature) and assumes that the given materials are all that you have in your possession (actually, in theory, you wouldn’t even need massless, charged particles as long as it was plausible that you could have a small black hole inside the box).
You are in possession of a neutral object of some mass like, for instance, a piece of Scotch tape or a balloon or something of that nature. You are also in possession of special type of box that happens to be both massless and neutral. In the box is either a strong gravitational field source (a neutral, massive object) or a strong electromagnetic field source (a massless, charged object). You do not know which. You begin to bring the object (i.e. the balloon or tape or whatever) closer to the box. At some point it becomes attracted to the box. It is your job to determine whether the attraction is due to gravity or electrostatics.
If it is an electrostatic attraction that occurs, this is due to polarization which means the sign of the charge producing it doesn’t matter (this is a simple experiment/demo I do with Scotch tape in one of my classes). Since you can’t see inside the box you have no idea how much charge or mass is present (so, for example, the box might even contain a small black hole) which means you can’t determine what the source is based on the rate at which the object was attracted to the box. Therefore, you have absolutely no way to tell whether it was attracted due to gravity or electromagnetism!
Are there holes in the argument? Remember that this is all you have access to (i.e. you can’t go running out to get an electrically charged object to see if it is repelled).