I am a third-year high schooler and I remember Michio Kaku once said (in some documentary which I can't sadly find now) That only pushing forces exist. Pulling a door is actually your fingers pushing it from the other side. Gravity is the curvature of space-time so it is pushing it towards earth. The charges probably push by their electric fields. My question is what is the pushing force in magnets, Thanks. [Sorry for bad english]
Answer
Not all forces push, some can pull, like gravity or the electrostatic force between oppositely charged particles. What Michio Kaku probably meant is that only contact forces exist: in other words, objects can only affect each other if the are exactly at the same point - they can't exert a force on something far away. Gravity and electromagnetism might seem to be counterexamples, but you can think of objects interacting via gravity or E&M as exchanging particles called gravitons or photons, respectively, that can't travel faster than the speed of light and "transmit" the force between the two objects. So when magnets attract, it's because of the exchange of photons traveling between them, and the one magnet is not directly influenced by the other magnet, just by the photons "coming out" of it.
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