home
 
 

 
Astrophysics wants its physics back! Part 1: Geomagnetism
© Charles Chandler
 
[intro (60 seconds) showing images of great scientists and philosophers (with the names and what they did), People, with background music, suddenly stopping with Einstein]
 
First I'd like to explain the title, "Astrophysics wants its physics back!" Over the years, astrophysics has become more and more dominated by abstract conceptual and mathematical models. It's common these days to hear scientists say, "There are some aspects of the phenomena that are not perfectly understood, so here's the conceptual model that we're using to frame our thoughts on the matter." Then typically, they'll lay out a concept that is intuitive enough, but on closer inspection, we see that the explanation defies the relevant principles of physics. For example, tidal forces are explained as a consequence of gravity, and yet gravity should act on the entire body all at once, not selectively tugging at the near side. This will be examined in greater detail later, for suffice it to say for now that to scientists, tidal forces are mathematical abstractions. The forces correspond directly to the position of the Sun and Moon, but the laws of gravity do not predict such selective forces, and the formulas that are actually used were derived heuristically. In other words, scientists made them up, because they couldn't get the gravity formulas to work.
 
Well, OK, they said that it was just a conceptual model, so it doesn't have to be made of physical stuff. But scientists have been at this for so long now, that these temporary constructs have become accepted by the broader scientific community. And when that happens, such models become convenient foundations for other inquiries. And then, not surprisingly, more models are developed, and they're conceptual too, because once you're broken away from physical constraints, it's always easier just to invent new concepts whenever you encounter new problems. But then, as time goes on, the whole thing just keeps getting more and more abstract.
 
Now, from the scientists' perspective, the good news is that it's tough to find fault with research that never claimed to be real in the first place. OK, it's just a conceptual model — how could that be wrong? But from the general public's perspective, the bad news is that the chance any sort of practical application coming out of such research goes down as the work gets further and further from reality.
 
So the objective here is to reintroduce the concept of physics into the study of astrophysics. There aren't any abstract entities or mechanisms or processes anywhere in this — it's all just known physical forces. Interestingly, it actually seems as if it's been a while since anybody has tried this. There are many problems in modern astrophysics that are intractable within the accepted frameworks, but which appear to have simple solutions. All we have to do is toss the abstractions and start over with proven physics. The cool thing is that we now have a fairly complete toolkit for doing this. We can calculate interactions among inertial, gravitational, electromagnetic, and nuclear forces to a high degree of precision, and all of that is now built into the atomic theory of matter, which has been verified every time somebody tried. All we have to do is apply this proven science to the outstanding issues, and maybe we'll get physical solutions, and we'll have no need for abstractions. Here we have to remember that many of the concepts in astrophysics date back to a time when not all of these pieces were in place, and it's understandable why scientists were forced into the realm of abstractions. They didn't have the tools to solve the problems, so they had to gloss over them. But that was then — this is now. We now have a complete toolkit, and it's time that we try addressing the problems head on.
 

The best place to start is with something close to home, which would mean planetary science, and more specifically, let's make a physical study of the Earth. We should start at the largest scale, so that what we learn will provide the context for studying finer problems. And the largest structure in geophysics is the Earth's magnetic field. This is something that continues to baffle scientists.
 
 
Figure 1. Earth's magnetic field, courtesy J. Marvin Herndon.
 
 
It has been known since the that a lodestone suspended by a string will always turn and point in the same direction. By the 1100s, Chinese mariners were using lodestone compasses to navigate in open waters. It was later found that when molten iron solidifies, it takes on the polarity of the Earth's magnetic field. This begged the question of where the Earth itself got its field.
 
In 1820, Ørsted discovered that a compass needle can be deflected by an electric current in a nearby wire. In 1876, Helmholtz & Rowland demonstrated that a rotating electrostatic charge could produce a magnetic field, and in 1879, Perry & Ayrton were the first to suggest that the Earth's magnetic field was generated by its rotation. But there is a problem with dynamo theories: neutrally charged matter doesn't generate a net magnetic field. A charged particle in motion generates a magnetic field, but oppositely charged particles generate opposite magnetic fields that cancel each other out. So just an equal quantities of opposite charges form an electrically neutral group, in motion they are magnetically neutral. So to be a dynamo, the Earth would have to possess a net charge. But sustaining a net charge requires resistance, and this is something that the Earth doesn't have. Below the water table, the Earth is an excellent conductor, and net charges equalize nearly at the speed of light in an excellent conductor. Thus the Earth can only be net neutral, and therefore shouldn't be capable of producing a net magnetic field by the dynamo effect. So scientists went back to thinking that the magnetic field was simply frozen in when the Earth cooled, leaving the Earth as a permanent bar magnet. This idea persisted until the early 1960s, when exploration of the sea floor near mid-ocean ridges revealed polarity reversals, called magnetic striping.
 
[show image of magnetic striping]
 
This means that the Earth's magnetic field flips every couple hundred thousand years, and magma oozing up from the Earth's interior takes on the polarity of the field when it solidifies, creating a record of the inversions. Such inversions cannot possibly be coming from a bar magnet. Only solids can be permanent magnets, and in the Earth, only the crust is solid. And if the majority of the magnetized crust shifted 180°, the mid-ocean ridges would shift along with it, since they are defined by the relationships between the continents. And if everything shifts together, there won't be any striping, because there won't be any relative differences. In other words, imagine two mountains, with a river running between them. Now grab the mountains and move them around. The river between the mountains always has the same orientation relative to the mountains.
 
So magnetic striping can only mean that the majority of the crust stays where it is, and somehow the thing that is generating the field flips. And the only way that that can happen is if moving electric charges start moving in the opposite direction. So we're back to the dynamo idea. But scientists never solved the problem of how electric charges could get separated inside the Earth. So they settled on a vague notion of some sort of convection in the Earth's core that is driving electric currents, and that sometimes, this convection decides to change direction, thus inverting the magnetic field. But that still isn't correct. Hot, high pressure nickel & iron are excellent conductors. So charges flow effortlessly if there is an electric field, or stay put if there isn't. They won't flow just because the atoms are convecting. That would take resistance, for the convection to transport electric charges and generate a magnetic field as they go. Magnetic pressure would prefer that the charges stay put, even as the atoms slip past them due to convection. But that's as far as the scientists could get, and since then, no further progress was been made.
 
Then, in the 1970s, a series of break-throughs in quantum mechanics provided the keys for working the rest of the way through this. There is a charge separation mechanism that was previously unknown, and that's extreme pressure. Gases are easily compressed, but when we get down to the density of a liquid, the going gets tough, because liquids are incompressible. This is because further compression brings the electron shells of neighboring atoms into conflict with each other.
 
[show image of Bohr model]
 
Due to the Pauli Exclusion Principle, no two electrons can occupy the same quantum state at the same place at the same time. So the shell conflicts require the expulsion of one of the electrons, converting it into a free particle. The extra force that it takes to do this accounts for the incompressibility of liquids and solids. So where does that electron go? If all of the atoms in the vicinity are squashed together, there's no room anywhere for that electron, and it is forced out of that matter altogether.
 
The implication is that inside the Earth, the pressure is sufficient for this kind of electron expulsion, and where the pressure is greatest is inside the core. Therefore, the core is going to get positively charged by the loss of electrons, and those electrons are going to congregate at a higher elevation, where there is less pressure, and thus there is the spacing between atoms for extra electrons.
 
So, the core is positively charged, topped by a negative layer in the lower mantle. The whole thing is still net neutral. So how is that going to generate a magnetic field? The answer is that the strength of a magnetic field is a function of the number of charged particles, and the speed at which they are moving. And because the lower mantle has a longer circumference than the core, the particles travel further per rotation than the core, and thus they travel faster. Therefore, they generate a more powerful magnetic field. Both the positive core and the negative lower mantle generate fields. But the net field is whatever is left over after the one is subtracted from the other.
 
 
 
Right Hand Rule: When you wrap your right hand around a solenoid, with your fingers in the direction of the conventional current (i.e., the motion of the positive charges), your thumb points in the direction of the magnetic north pole.
 
Polarity: All magnets have two poles, where the lines of magnetic flux enter and emerge. By analogy with the Earth's magnetic field, these are called the magnet's "north" and "south" poles. The convention in early compasses was to call the end of the needle pointing to the Earth's North Magnetic Pole the "north pole" (or "north-seeking pole") and the other end the "south pole" (the names are often abbreviated to "N" and "S"). Because opposite poles attract, this definition means that the Earth's North Magnetic Pole is actually a magnetic south pole and the Earth's South Magnetic Pole is a magnetic north pole. The direction of magnetic field lines are defined to emerge from the magnet's north pole and enter the magnet's south pole.
 
From this we know that the Earth is a net negative dynamo.
 
 
 
But how could such a field flip its polarity? Gravity is constant, so the charge separation due to pressure is constant, meaning that the core must always be positively charged, and the lower mantle must always be negatively charged. There is only one other variable: the rate at which these charges are rotating. Remember that the strength of a magnetic field is a function of the amount of charge, and the speed at which it is moving. So there must be variable speeds of rotation. And this is precisely what we find inside the Earth — the core actually rotates faster than the lower mantle. Now, if the faster layer slows down, and the slower layer speeds up, the net field will invert in polarity.
 
 
 
Figure 2. Layers of charge inside the Earth. Red is negative; blue is positive.
 
 

↑ UP Powered by Quick Disclosure Lite®
© 2010~2017 SCS-INC.US
NEXT →