Forces Do not Exist

with 6 Comments


Two electrons playing particle physics tennis with a photon.

Forces do not exist

Most people first hear about forces in high school, with Newton. But the reality is that forces do not exist. Instead, what we have is something similar to particle physics tennis: two particles exchange another one and get either closer and closer or further and further apart.

Imagine you’re in outer space with a friend. You take a tennis ball and you throw it at your friend: now, you are thrown backwards because of recoil. In more physical terms, this is an example of momentum conservation: the momentum the tennis ball carries has to be equal to your momentum going the other way. When your friend catches the ball, they will also be pushed, in this case in the direction of the ball. If they now return the ball to you, the process will happen again and you and your friend will be moving further and further apart from each other. It’s as if there was a force pushing you apart: however, there is no force. You are just passing a tennis ball around.

With particles, something very similar happens. An electron will give off a photon, a type of boson, which will be caught by another electron. The momentum of that photon will propel both particles in opposite directions, causing what looks as a repulsive force.

The force between opposite charges

The situation for opposite charges is a bit more complex. My explanation may strike as unorthodox to some particle physicists, but it is the only intuitive one I could find. The trick is to consider opposite charges as if they were moving back in time.

Back when Quantum Mechanics was starting, Paul Dirac made a startling discovery: the electron had to have an evil twin with positive charge, which he called the positron. Later on, Richard Feynman suggested you could view these positrons as electrons travelling back in time: a positron will do exactly the opposite of an electron, which means you can’t tell between a video of a positron and a video of an electron being played backwards.

When dealing with the effects of photons, this becomes important. Imagine an electron gives off a photon, which then hits the positron. An electron would be pushed forward, so the positron will do the exact opposite: it will move towards the electron! Then the positron will also give off photons, but those photons will also do the opposite of what the electron’s photons would do: this means that, instead of pushing the electron away, they will pull it closer together! This means that oppositely charged particles will attract, whereas same-charge particles will repel. We can explain all of this using only particles: forces do not exist.

Virtual and real particles

The story above is understandable and quite close to reality, but far from a complete description. For example, if electrons are giving off photons in all directions, there shouldn’t feel pushed in any direction more than any other. The trick here is that only the photon that ends up being absorbed by a nearby particle “counts”: the other ones have energies that are too small to have an effect. In fact, all of those photons, including the one that gets absorbed, are undetectable and live only in our calculations: we call them virtual photons. Only the photons we get to detect are real photons.

Virtual photons live within the uncertainty principle. They are allowed to exist for a brief time, as long as their energy does not exceed a certain threshold. The less energy they have, the longer they can survive. This is why forces that are transmitted by massive particles, like the weak nuclear force, have such a short range: the virtual particles are not allowed to exist for long enough to get far!

However you consider this, one thing is still clear: forces do not exist. They are a side-effect of particle exchange, but have no existence of their own.

6 Responses

  1. […] left- and right-handed particles? It turns out there are many good reasons. For starters, some forces will only interact with right-handed particles, a bit like teachers in the 1950s. This creates a […]

  2. Ezo
    | Reply

    “The trick here is that only the photon that ends up being absorbed by a nearby particle “counts”: the other ones have energies that are too small to have an effect.”

    But how does that happen? Why particles hitting other objects have higher amount of energy? How does ‘sender’ “know” which virtual photons are gonna hit other particles?

    • Bossy Boson
      | Reply

      Hi Ezo,
      Nice to see you in my other blog. The reason only the particles that hit other objects count is the uncertainty principle. I don’t know how familiar you are with this (probably quite a bit, judging the comments on my other blog) but I’ll give you a summary just in case.
      The idea is that, when I look at a particle, there is a limit to the precision with which I can know its state. The uncertainty principle tells me that, the better I know a particle’s position, the worse I know its momentum and vice-versa. This is a direct consequence of the fact that particles behave like waves.
      However, the uncertainty principle works for other physical quantities. In particular, it works for time an energy: if I measure a particle for a long time, its uncertainty in the energy will be small; if I measure it for a short time, its uncertainty in the energy will be big.
      Suppose now we have an electron that’s giving off virtual photons. These photons are allowed to have some energy thanks to the uncertainty principle. However, the longer they live, the smaller this energy can be because of the uncertainty principle. If a photon is emitted and travels very far, this means it’s been around for a long time, so the uncertainty in its energy has to be very small. Since the “average” value of the energy is zero to begin with, the maximum energy this photon can have is the value of the uncertainty in the energy. Therefore, the longer a photon travels, the less energy it can have. Photons that travel very far (an “infinite” distance) will have zero energy. Which photons travel an infinite distance? Those that don’t get absorbed.
      However, those that do get absorbed live less time, which means that they are allowed to have more energy. Therefore, only the photons that get absorbed “count” as they are the only ones that have any energy whatsoever.
      Did that make sense? I’ll be happy to explain myself better if this wasn’t clear enough.

      • Ezo
        | Reply

        Hi 🙂

        Yeah, that makes sense. So, these emited photons all exist because of uncertainty principle. As their lifetime increases, uncertainty decreases – since they ‘really’ didn’t have much(or at all) energy in the first place(it only appeared that they had due to uncertainty), their energy decreases. That’s why ‘forces’ grow weaker as distance between objects increases. Is that (approximately) right?

        Btw, do you plan to start writing again on your second blog, sometime in the future?

        • Bossy Boson
          | Reply

          Exactly. However, the fact that the strength of the interaction decays with distance is also due to the simple fact that the density of photons decreases, since they get spread out over larger and larger areas.
          I’ve been meaning to start writing my “old” blog again, but it’s one of these things where every time I decide to do so I then have an exhausting week and cannot muster the energy. Also, this one takes up quite a bit of my time: I’m a terrible artist, so I’m basically learning from scratch. In fact, the particle drawings were done by my wife! Maybe when this one starts to run a bit more smoothly and I have a decent amount of content I’ll start publishing in my other one more often.

  3. […] were initially straight to curve, giving rise to the familiar orbits around the Sun, for example. There is no force: particles just move in a “straight” line, it’s just that this straight line is […]

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