Why do like forces repel each other, and unlike attract

krd

Put to electrons near each other and they will repel. And the same for like poles of a magnet.

Why is that?

By what mechanism do they repel or attract?

What is the fundamental reason behind them doing so?

Maximus Alexander

These are called fundamental interactions because they cannot be broken down any further, at least currently.

In short, like forces repel one another because they have to in order for the universe to exist.

krd

LeighH wrote: »

In short, like forces repel one another because they have to in order for the universe to exist.

That's a bit like saying if holy god and the virgin mary didn't exist the whole universe couldn't exist either.

Come on. There must be a better answer than that. If I take two magnets and face the norths to the norths, they'll repel. A south to a north and they'll snap together. Play with magnets, and you can feel the forces passing invisibly through the air.

Two electrons will repel. Electricity flows.

I'm not saying there has to be a simple answer. But "they do, because they do" isn't an answer.

Why does the repulsion and attraction happen?

Maximus Alexander

krd wrote: »

That's a bit like saying if holy god and the virgin mary didn't exist the whole universe couldn't exist either.

Come on. There must be a better answer than that. If I take two magnets and face the norths to the norths, they'll repel. A south to a north and they'll snap together. Play with magnets, and you can feel the forces passing invisibly through the air.

Two electrons will repel. Electricity flows.

I'm not saying there has to be a simple answer. But "they do, because they do" isn't an answer.

I don't agree with this at all. The fundamental forces can be observed and measured. Nothing has to be taken on faith.

There has to be some intrinsic behaviour upon which matter and the universe are built, otherwise you end up with infinite regression. The fundamental forces just happen to be this intrinsic behaviour.

There may be places beyond the observable universe where the forces behave differently, but we could never have come to exist there. In order for us to observe the universe we had to be in a place where they behave the way they do. This is the basis for Anthropic Principle. See also the Many-Worlds interpretation of Quantum Mechanics for an example of how people have tried to deal with this.

Why does the repulsion and attraction happen?

I'll do my best here. Let's take two electrons as an example. It is not possible for two electrons to occupy the same space at the same time. If you imagine that you could push two of them together, as they get closer you will need to use more and more energy to keep them moving toward one another. To actually put them in same place would require infinite energy.

Once you release them, all of that potential energy is released and they fly apart. In just the same way if I take an apple to the top of a building it costs energy to pull it away from the Earth. When I drop it off the roof that stored potential energy flings it toward the ground again.

Morbert

krd wrote: »

Put to electrons near each other and they will repel. And the same for like poles of a magnet.

Why is that?

By what mechanism do they repel or attract?

What is the fundamental reason behind them doing so?

They repel and attract through exchange particles. "Exchange particles" are transient disturbances in fields.

For example, an electron is an excitation of the electron field. This excitation is also accompanied by transient excitations in the photon field (i.e. The electromagnetic field). If we consider two electrons, their transient excitations in the electromagnetic field will exchange momentum between the electrons. This is the electromagnetic force. Similarly, gluons act as the exchange particles for the colour force. Intermediate vector bosons act as the exchange particles for the weak nuclear force. Gravity is a little more complicated, as the gravitational field is space-time itself.

These exchange particles are sometimes called "virtual particles". A virtual particle is very much real, but it does not satisfy relations that would classify it as a particle. It is an "almost particle".

krd

LeighH wrote: »

I don't agree with this at all. The fundamental forces can be observed and measured. Nothing has to be taken on faith.

There has to be some intrinsic behaviour upon which matter and the universe are built, otherwise you end up with infinite regression. The fundamental forces just happen to be this intrinsic behaviour.

I was familiar with that idea. I've read few things by different people, where you just have to change some of the rules slightly and our existence becomes impossible.

It still does leave the question, what are the rules and why are the rules.

I'll do my best here. Let's take two electrons as an example. It is not possible for two electrons to occupy the same space at the same time.

Pauli's exclusion principle? I'm trying to chip away at these ideas by reading. I've heard the ideas before but never really went into that much depth. I'm slowly getting there though.

Under the exclusion principle, two electrons cannot be in the same space. As they would repel each other. I know there are huge gaps in my knowledge - but as electrons are waves, of they same kind, what's to stop them from simply joining up and becoming a larger resultant wave - with more energy/greater amplitude.

Another peculiar thing I've read recently - I don't know if this is correct - the centre of the electron orbiting a hydrogen atom (or should I say the electron orbiting the proton - or should I try to forget the Bohr model) , is in the same space as the centre of the proton.

If you imagine that you could push two of them together, as they get closer you will need to use more and more energy to keep them moving toward one another. To actually put them in same place would require infinite energy.

Infinite energy or something happens to the wave function of both electrons before the energy required to force them together reaches infinity. If it was possible for infinities to be reached that would have other problems for existence. I understand the mathematical model of the electron wave, as the centre, the amplitudes reach off into infinity. Whereas in the lab, the central amplitude is shown not to be infinite.

Does this infinite energy also occur on an attractive basis between an electron and a proton.

Once you release them, all of that potential energy is released and they fly apart. In just the same way if I take an apple to the top of a building it costs energy to pull it away from the Earth. When I drop it off the roof that stored potential energy flings it toward the ground again.

Then names for the different energies; potential, kinetic, etc. Always bothered me. You can say a can of petrol has X Joules of potential energy - or a stretched rubber band has X joules of energy. But that potential energy depends on what you do with the rubber band or petrol. Fuse the petrol in a nuclear reactor, you'll have far more energy than you'll get from a chemical reactor, like a combustion engine.

Potential energy in relation to gravity is another kind of odd thing. By taking an apple to the top of a building, you could equally say you've done work to charge the earth with potential energy. And when you drop the apple the earth is releasing this potential energy.

When you drop the apple it will smash to pieces. Lot's of different things happen with the energy. It will make a sound as hits the ground. It will release light even though it can't be see. Chemical bonds within the apple will be broken by something.

The higher the building, the more potential energy the apple has - but if you keep taking it higher and higher - say 30,000 km above the surface of the earth and drop it - it may not fall, it could be in a geostationary position. Where has the potential energy gone now - it was increasing and increasing, and now it seems to be gone. The apple doesn't feel any different.

krd

Morbert wrote: »

They repel and attract through exchange particles. "Exchange particles" are transient disturbances in fields.

These exchange particles are sometimes called "virtual particles". A virtual particle is very much real, but it does not satisfy relations that would classify it as a particle. It is an "almost particle".

Are the exchange particles/virtual particles a quantisation of the disturbance.

For example, an electron is an excitation of the electron field. This excitation is also accompanied by transient excitations in the photon field (i.e. The electromagnetic field). If we consider two electrons, their transient excitations in the electromagnetic field will exchange momentum between the electrons. This is the electromagnetic force. Similarly, gluons act as the exchange particles for the colour force. Intermediate vector bosons act as the exchange particles for the weak nuclear force. Gravity is a little more complicated, as the gravitational field is space-time itself.

Thanks for your explanation. It will take me longer to grasp all this stuff though.

Of many of the bits and pieces I don't understand. I don't really understand what the electron field is. In one of the physics books I've read recently, there's a quote from Schrodinger - where he said something like the universe is infinitely full of empty electron fields, waiting for electrons to occupy them.I don't know if that idea has been superseded, or if I have Schrodinger wrong. It kind of implies, that an accelerated free electron, is hoping from one hole to another as it travels through space.

Is the idea of an electron field something that shouldn't be confused with a wave medium?

Morbert

krd wrote: »

Pauli's exclusion principle? I'm trying to chip away at these ideas by reading. I've heard the ideas before but never really went into that much depth. I'm slowly getting there though.

It's important to make a distinction between the coulomb/electromagnetic force, which I discussed in my above post, and the Pauli exclusion principle. The Pauli exclusion principle is not due to a force, but rather a quantum-mechanical correlation between electrons. This is why it only applies to electrons with the same spin.

Under the exclusion principle, two electrons cannot be in the same space. As they would repel each other. I know there are huge gaps in my knowledge - but as electrons are waves, of they same kind, what's to stop them from simply joining up and becoming a larger resultant wave - with more energy/greater amplitude.

The larger resultant wave is defined over all possible electron configurations. For those configurations with electrons occupying the same states, the wavefunction reports a probability of 0. This would happen even if electrons were attracted to one another, as it arises from the form of the wavefunction, and not through force-carrying particles.

Another peculiar thing I've read recently - I don't know if this is correct - the centre of the electron orbiting a hydrogen atom (or should I say the electron orbiting the proton - or should I try to forget the Bohr model) , is in the same space as the centre of the proton.

The 0-angular momentum wave-function is non-zero at the configuration where the electron is at the same place as the proton. A situation that would be nonsensical in classical physics. Incidentally, it is this "non-zero" behaviour at strange configurations is the motivation behind the hypothesis that the universe emerged from "nothing".

[edited to append the following response]

Are the exchange particles/virtual particles a quantisation of the disturbance.

Yes. But they don't satisfy the wave equation or energy-momentum relations, and are short lived.

Thanks for your explanation. It will take me longer to grasp all this stuff though.

Of many of the bits and pieces I don't understand. I don't really understand what the electron field is. In one of the physics books I've read recently, there's a quote from Schrodinger - where he said something like the universe is infinitely full of empty electron fields, waiting for electrons to occupy them.I don't know if that idea has been superseded, or if I have Schrodinger wrong. It kind of implies, that an accelerated free electron, is hoping from one hole to another as it travels through space.

This is where the difference between quantum mechanics and quantum field theory is important. Quantum mechanics, as the name suggests, described the mechanics of particles. Quantum-field theory describes not only the mechanics of particles, but their existence as excitations of fields. The fields themselves are hard to describe in lay-terms. They are not like an all pervasive fog or aether, as they satisfy Lorentz invariance. Instead, they reflect the fact that quantum systems are acted on by field operators, defined over all space.

Is the idea of an electron field something that shouldn't be confused with a wave medium?

Yes. There are certainly similarities. A phonon, a "quasiparticle of sound" is an excitation in a sound medium. But a field is much more subtle, and directly relates to the mathematical notion of a "field".