# How many forces do we know?

1. Dec 4, 2013

### Dash-IQ

Other than the 4 natural forces that are conservative, and friction(the none conservative one).
Are there anything else? In the whole universe that's all the forces we know?
Can there other forces to be discovered?

2. Dec 4, 2013

### Pythagorean

I believe every force can be explained in terms of fundamental forces or inertial reference frames. For instance, friction is a result of electromagnetic repulsion between matter (and also dissipation via entropy). In the general relativity framework, even gravity can be considered a reference frame force.

I'm fairly confident that classical forces we experience in every day life can be reduced to the electromagnetic force, repulsion (Newton's third through electromagnetic force) experienced during collisions... and gravity.

Last edited: Dec 4, 2013
3. Dec 5, 2013

### Khashishi

There are 4 fundamental forces: gravity, strong, weak, and electromagnetic. Electromagnetic and weak forces are unified in the electroweak theory, so maybe you can say there are 3 fundamental forces now. There is another force, the entropic force, which typically isn't considered a fundamental force, because it acts somewhat differently, through thermodynamics, and doesn't actually exist at a microscopic level. It is associated with the second law of thermodynamics, and it is as important as the other four.

All other forces can be derived from these 5. There's probably hundreds of non-fundamental forces, and their definitions are proliferated in various special topics, so there's no point in listing them. For example, the Van Der Waals force is just some manifestation electromagnetic force viewed at a molecular scale.

4. Dec 5, 2013

### dauto

The electroweak theory describes the electromagnetic force and the weak force in terms of two separate forces - the weak isospin and the weak hypercharge - so the count remains 4 fundamental forces. In GUT theories the weak hypercharge, the weak isospin and the strong force are truly unified under a single force so the count comes down to 2 - GUT interaction and gravity.

I don't see why entropy should be counted as a force.

5. Dec 5, 2013

### Pythagorean

Entropy results in statistical behavior that can be conceived as a force balance. For example, the Nernst potential can be viewed as a force balance between electrodynamics and the "force " of diffusion.

6. Dec 6, 2013

### dauto

That is a force, but it's not a new fundamental force. In your example, the nature of the force is also aerodynamical.

7. Dec 6, 2013

### Pythagorean

Whether it's a fundamental force, or a force at all, is a matter of metaphysics. Some people even argue that some of our fundamental forces are actually a result of the so-called entropic force. I don't really know (or care much) about the metaphysics.

What's unambiguous (and useful in modelling) is that it can be treated as a force phenomenologically.