# How do we know it's not some kind of attraction?

1. Jun 15, 2012

### scijeebus

I can't remember what the experiment is called, but it was to test for virtual anti-pair particles, and two plates where put together so close that at certain energies of virtual pairs should be excluded, thus creating a difference of pressure as to push the plates closer together. But when you have things that close together, how do we know it's virtual particles and not anything to do with the electro-magnetic force in any way shape or form or perhaps even weak gravitational attraction?

2. Jun 15, 2012

### Staff: Mentor

You are talking about the Casimir effect, I think?

We know the masses of the plates, so we can calculate the expected gravitational force between them (very damn small). And likewise we can measure and control the electric charges on the two plates, so we can calculate the expected electromagnetic forces between them (much larger, but still very small). And then we measure the actual force between them, and we find that it's much larger than either and consistent with the predicted Casimir effect.

So we put all of this together, and we conclude that EITHER:
1) We've observed the Casimir effect; OR
2) We've observed some unknown effect. It might be that gravity or electro-magnetic forces behave differently at very small separations so our "expected" forces are wrong, or there might be some new and hitherto unknown force at work here.

We can't ever completely exclude #2, but because we haven't seen any evidence of this hypothetical unknown effect in other experiments, and because what we have is consistent with the Casimir explanation, it's natural to go with #1 unless and until there's evidence to the contrary.

One caveat: the stuff above makes it all sound a lot easier than it really is. The experiments that demonstrate the Casimir effect are hard to do and easy to screw up. Getting to where we're really sure of our results can be a lot of work and take a long time.

3. Jul 4, 2012

### scijeebus

Ok, the proximity you would have to bring plates close enough together to exclude something as small a virtual particle is way more than enough for the EM force AND gravity to bring the plates close enough together. We're talking about nano-meters here, less than that even, how wouldn't that be other forces? The EM force that holds electrons to a nucleus is a lot stronger than just 1 nano-meter, even for a single proton.

Last edited: Jul 4, 2012
4. Jul 4, 2012

### Staff: Mentor

Yes, we agree that gravitational and/or electromagnetic forces can draw two objects that are very close even closer.
But the point is that we can calculate the strengths of these forces, and we can measure the actual force between the plates. If the measured force is much greater than the calculated forces... Then we know that something other than the gravitational and electromagnetic forces as we understand them today are producing the measured force.

It's worth noting that the behavior of electromagnetic forces as the nanometer scale is pretty well understood. The people who design and build computer chips would definitely have noticed, big time, if gravity or electromagnetism behaved differently at those scales.

You're going to have to fix that last sentence before i'll be able to understand it... I don't understand how a force can be stronger than a nano-meter, which is a distance.

5. Jul 4, 2012

### scijeebus

No, I don't think they would notice necessarily, it's a small and virtually immeasurable realm of the strangest properties of the universe. If someone as smart as Newton can be wrong about something as simple as gravity's speed, I'd hate to think how wrong people could be making all these assumptions of the existence of such outlandish things.

I don't really need to fix it that much, maybe I need to add the word "range", but otherwise there's plenty of instances where scientists measure a force as so small they consider it 0. True, Pluto's gravity does reach Earth, but it's so small scientists consider it 0 on Earth. Even with particle probability, eventually a particle's probability becomes so small scientists just treat the range as 0. But, this is does not happen with the EM force of a subatomic particle for at least a few nano-meters.