Physicists may have found a fifth force. Uh, that sounds exciting. And since it sounds so exciting, you often see it in the headlines, so often you probably wonder how many of these Fifth Forces there are. And what is a fifth force anyway? Could It Really Exist? If it exists, is it good for everything? That’s what we’ll talk about today.
Before we can talk about the fifth force, we need to briefly talk about the first four forces. According to our current state of knowledge, all matter in the universe consists of 25 particles. Physicists collect them in the “Standard Model”, which is something like the periodic table for subatomic particles. These 25 particles are held together by four forces. This is 1) gravity, apples falling and all that, 2) the electromagnetic force, which is a combination of electric and magnetic force that really belong together, 3) the strong nuclear force that holds atomic nuclei together against the electromagnetic force, and 4) the weak nuclear power responsible for nuclear decay.
All other forces that we know, for example the van der Waals force that holds atoms together in molecules, frictional forces, muscle forces, these are all emergent forces. That they are emergent means that they are derived from these four fundamental forces. And that these forces are fundamental means that they do not arise – they cannot be derived from anything else. Or at least we don’t currently know anything simpler from which to derive them.
Now if you say that gravity is a force in the wrong society, someone might point out that Einstein taught us that gravity is not a force. Yeah, the guy again. According to Einstein, gravitation is the effect of a curved space-time. Looks like a force, but it isn’t. That is the reason why physicists, if they want to be very precise, do not speak of four fundamental forces, but of four fundamental interactions. But in reality I hear them talking about gravitational force all the time, so I would say if you want to call gravitation a force please go ahead, we all know what you mean.
As you can see from this, what physicists call a force is not very precisely defined. For example, the three forces besides gravity – the electromagnetic and the strong and the weak nuclear force – are similar in that we know that they are mediated by exchange particles. That is, if there is a force between two particles, such as a positively charged proton and a negatively charged electron, then you can understand that force as the exchange of another particle between them. In the case of electromagnetism, this exchange particle is the photon, the light quantum. We also have exchange particles for strong and weak nuclear forces. For the strong nuclear force these are called “gluons” because they “stick together” quarks, and for the weak nuclear force these are called Z and W bosons.
Gravity is the weird again. We believe that it has an exchange particle – this particle is called a “graviton” – but we do not know whether this particle actually exists, it has never been measured. And on the other hand, we have an exchange particle to which we do not assign any force, and that is the Higgs boson. The Higgs boson is the particle that gives the other particles mass. It does this by interacting with these particles, and it acts pretty much like a power carrier. In fact, some physicists call the Higgs exchange a force. But most of them don’t.
The reason for this is that the exchange particles of electromagnetism, the strong and weak nuclear force, and even gravity, hypothetically all result from symmetry requirements. Not the Higgs boson. That might not be a particularly good reason not to call it a powerhouse, but that’s the common terminology. Four fundamental forces including gravity which is not a force but not the Higgs exchange which is a force. Yes it is confusing.
So what about that fifth force? The fifth force is a hypothetical, new, fundamental force for which we do not yet have any evidence. If we found it, it would be the biggest physics news in 100 years. That’s why it makes headlines a lot. There is not a particular fifth force, but there are a large number of “fifth” forces that physicists invented and are now looking for.
We know that it is difficult to observe when a fifth force exists because otherwise we would have noticed it. This means that this force is either only noticeable at very great distances – this is how you would see it in cosmology or astrophysics – or it is noticeable at very short distances and is hidden somewhere in particle physics.
For example, the anomaly in muon g-2 could be a sign of a new force carrier, i.e. a fifth force. Or maybe not. There is also an alleged anomaly in some nuclear junctions that could be mediated by a new particle called X17 that would carry a fifth force. Or maybe not. Neither of these anomalies is very compelling evidence, the most likely explanation in both cases is difficult nuclear physics.
The most plausible case for a fifth force, in my opinion, comes from the observations we normally attribute to dark matter. Astrophysicists introduce dark matter because they see a force acting on normal matter. The most widespread hypothesis for this observation right now is that this force is just gravity, an old force if you will, but that there is a new kind of matter instead. That doesn’t fit all of the observations so well, so instead it could be that it’s not just gravity, but actually a new force, and that would be a fifth force. Dark energy is also sometimes assigned to a fifth force. But that’s not really necessary to explain observations, at least not for now.
If we could find evidence of such a new force, could we do anything with it? Almost certainly not, at least not anytime soon. The reason is, if such forces exist, their effects must be very, very small, otherwise we would have noticed them sooner. So you definitely can’t do it for yogic flying or to pin your enemies on the wall. But who knows, if we find a new strength, one day we may find something to do with it. Definitely worth looking for.
So if you read the headlines about a fifth force, it just means that there is an anomalous observation that can be explained by a new fundamental interaction, mostly a new particle. It’s a catchy phrase, but it’s actually pretty vague and not very informative.