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Originally Posted by Sam5
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Originally Posted by Taibak
Why should gravity stop just because you've taken something apart? Put another way, general relativity says that it should be possible for two neutrons to orbit a common center of mass in well-defined, completely predictable orbits. Quantum mechanics says that would violate uncertainty. Something has to give.
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Thanks for the info!
I don’t think I would say that “gravity stops” when things are taken apart. I’m thinking about something like a large electro-magnet that can overcome the pull of gravity by means of the pull of the magnetic field, and the magnet can hold a steel bar up in the air. Switch off the magnet, and the steel bar falls. With the magnet on, we are not doing away with the gravity, we are just overpowering it with a strong magnetic field.
It’s like a piece of cellophane. I can’t throw it away. It sticks to my hand and doesn’t fall because of an electric field at my hand. At close range to my hand, the electric field is stronger than the gravity field at that same place. |
That's what I'm getting at, actually. The electromagnetic force is stronger than gravitation, but only over shorter distances. Nobody has any understanding as to why this is true.
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Originally Posted by Sam5
The “completely predictable orbits” you mention, would that be in the total absence of the other fields? Could the presence of the other fields, on the small scale, break up the “predictability” of the orbits of the Neutrons? Can we totally isolate the two Neutrons from all other fields? In large-scale space in our solar system, we don’t have other fields that are strong on a large scale, and so the gravity field dominates the large-scale motion of the planets. But wouldn’t the other fields be stronger at short distances on the small scale?
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In theory, yes to all of this. Neutrons are electrically neutral, so they don't interact with electric fields. They have no magnetic moment, so they don't interact with magnetic fields. As such, even if either type of field was present, they wouldn't make any difference whatsoever. The two neutrons aren't confined to the nucleus of an atom so the weak magnetic force. All we have are two neutrons in otherwise empty space - which isn't at all unlikely.
*But* the neutrons have mass and therefore they produce gravitational fields that tug on each other. It's not a strong pull by any means, but to the neutrons it's not negligible. Both general relativity and Newtonian mechanics saw that they should be able to rotate around a common center of mass - just like two stars of equal masses would. Calculating those orbits with those theories should be a simple matter of applying Kepler's laws and cruching the numbers.
The thing is, neutrons are small enough that you have to apply quantum mechanics. The two neutrons have wave-like properties that, because the neutrons are so small, can't be ignored. In fact, the longer the neutrons go without colliding with something (and the tiny amount of gravity we're talking here isn't going to be enough to do this) the more wave like they become. Neither general relativity nor classical mechanics has any way of dealing with this.
And then there's the Heisenberg Uncertainty Principle. At the risk of over simplifying it, the more precisely you meausre the neutron's velocity the less precisely you know the orbit, and vice versa. Well, to figure out the orbit you need to know the velocity of the neutrons. The catch is, when you measure the celocities
you force the neutrons to collide with something that will change its position and therefore change its orbit. That change, according to quantum mechanics, is unpredictable whereas general relativity says that the change in orbit should be calculable. On the other hand, you could measure the distance between the neutrons and plug that into Kepler's laws. The inverse happens - you've changed the
velocities and that will also change the orbit. Again, that change is unpredictable under quantum mechanics but deterministic under relativity and classical mechanics. Taking this a step further, it's not just a matter of the neutrons being so small that bouncing photons off of them to measure their position or velocity sends the neutrons flying. According to quantum mechanics, the neutrons
don't actually have a specific, finite position or velocity until the photon hits it forcing it to take a proverbial stand. The longer they go without hitting a photon (or anything else, for that matter), the less specific their position becomes and the more *wave-like* they become. Neither relativity nor classical mechanics offer any explanation as to how that works, let alone how you can get two waves orbiting each other.