h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Seems obvious to some, but it vexes me? What is it about the fine structure of atoms of iron that make them act this way? Why would anything create a magnetic field sans electric field when the rest of the universe is brimming with electromagnetic radiation (where the two travel together)? How come the electric field isn't trapped inside that iron stuff too? What was it about magnetisim that made it choose iron as a home? At the micro sub atomic level, what the hell is going on in iron that doesn't happen in aluminium, for example? How come you can walk up to any piece of iron and get it to stick to your magnet? You didn't have to prime it, it ate no fuel, no energy seemed to go in. What's going on in there?

Oh boy, you do ask difficult questions. I did a PhD involving magnetism, and I am not 100% sure I can explain it convincingly.....
It is to do with unmatched spins in d-shell electrons. Then the explanation gets technical, and I need a text book. I'll get back to you on this one. Sometime......

And another thing....why are electric and magnetic fields perpendicular to one another in electromagnetic radiation? Do they have to be? What happens if they aren't? What if they were in the same plane of polarisation? Is there a way to lead or lag the spatial phase of the fields to force them onto or near to the same plane? Would doing this cause invisibility? Was the Philadelphia Experiment an attempt to try this? You know what they say...the simplest fool can ask questions that go beyond the knowledge of the wisest man!

The maths says that self-propagating EM fields have H and E perpendicular - look up the Poynting vector (and this is spelled Poynting). However, you can of course have H and E fields at any angles, but only the perpendicular components propogate (I think this is correct, but I did do this bit 17 years ago!). For how the phases of the two fields interact, I think you need to know Special Relativity, specifically the Lorenz transformation equations/tensors for relativistic electric fields. Maxwell's equations are also necessary. As far as I can remember the answer is that the phases are fixed, but if one could vary the phase this would be wonderful.

Invisibility - no idea; please remind me what the Philadelphia Experiment was. And the best of luck!

This is a very simplified answer, based on a physics degree from a number of years ago. Jan*'s answer is more accurate; I can understand mine.

It sounds like you're already aware of magnetic induction, i.e. a changing electric field produces a magnetic field. Well, when an electron orbits an atom (this is the sort of simplification that really annoys some pedantic scientists, so let's just keep this between ourselves, ok?), that's a little electric current going round in a circle, like an induction loop, and it creates a litle magnetic field.

Now, most electrons don't actually go round in simple ecliptical orbits like planets; they go in more complicated spherical patterns, so they don't produce a constant magnetic field. Your iron atom, though, has one electron that does exactly that, and so it produces a magnetic field. Why iron and not aluminium? Well, that's got to do with unmatched spins in d-shell electrons, and you'd better ask Jan* about that.

A lump of iron is magnetic when all those little magnetic fields point in the same direction. You don't get one big electric field because the magnetic field is produced by the change in the electric current, not the electric field. So the electric fields of the electrons are cancelled out by those of the protons, the same as in any other material.

I hope that makes sense.

The Philadelphia Experiment was one of those secret experiments that the US apparently conducted to make ships invisible to RADAR. It used "flux capacitors" to make battel ships actually disappear, not just at RADAR frequencies. There is a lot of myth surrounding the very existence of the experiment and the US denies it ever happened, but there was a film of the same name made, suggesting what had taken place, then wandering off in a fanciful direction.

Could be rubbish.

It's the clearest explanation I've ever seen! Thanks!

By the way, does anyone know why electric fields induce magnetic fields and vice versa? I know they do in terms of Maxwell's equations, but why? It's a really strange thing to happen in space, don't you think?

Also, has anyone figured out how gravity acts on other bodies at a distance? I don't mean in terms of inverse square laws, I mean what fields are carrying the gravitational forces or particles? (is that a meaningful question?)

OK, I know this one (but its realy hard to explain without pictures). I'm basing this on a cartoon I saw about one of Einstien's "Thought Experiments" on the subject. OK take for instance, a man in an elevator. if the the elevator cable snaps, both the man and the elevator fall at the same rate (they just do, its the way gravity works). Since they are traveling at the same speed, in relation to each other, the man will float, weightless in the elevator. (this is true, and has actually happened to people who've been in plummeting elevators) Now, once the elevator hits the bottom, the man will again feel the effects of gravity (and most likely break a leg in the process) Now take another man and stick him in a rocket. He floats weightless in the rocket as well. Now if the rocket takes off, the man will not move untill the floor (or crash couch, or whatever) in the rocket hits him, and he will continue to be "pushed" along by the rocket. This will feel like gravity to the man however as he will have a constant push on him from behind (underneath). This push will affect every oject in the rocket, at the same rate (Just like Gravity). Einstien then surmised that since the two felt thes same, and reacted the same, then they must be the same.

Now this next part is where I'm a little fuzzy.

so, Einstein somehow realized that space "fabric" wasn't flat, it was curved. And being curved, things (matter) was actually moving along this curve in space. He also surmised that objects sitting on this space fabric would also create a smaller "curve" around them (have four people hold a bedsheet so that it is flat, then put a bowling ball in the middle, this will create a "dent" or curve surounding the ball. Thats a simple visualization of the effect) So as objects passed each other, they would be drawn "into" the resulting mini curve, and start falling towards each other. The movement of "falling" towards each other is what creates gravity. We are mearly being drawn down the curve in space created by the earths mass (called earths "gravity well") only the Earth is getting in the way. since we are so small in comparison, our gravity well has no real effect against the Earth.

Some one else will have to explain orbits though... I can picture it in my mind, but I'm having trouble forming the picture into words.

Could be rubbish, but it was officially denied, so it's probably true. Nice idea, but the chances of us finding out are minimal and not worth the time involved. I am happy that the CIA tried to assassinate Castro with an exploding cigar - that was officially denied too.
Was 'the Philadelphia Experiment' a prequel to 'Back to the Future'?

Magnetic fields are caused by electrons moving in space. This is where the Lorenz transformation comes in - it unifies Maxwell's 4 equations into one. It's a very elegant bit of physics, but a bit hard to explain in a forum. Think of electrons moving along a wire (i.e. a current) this produces a magnetic field. The effect is very weak, but is measureable. µ = 4Pi x 10^-7 H/m is not a lot. As to the fundamental reason why a moving electric charge generates a magnetic field - it does. Ask Prof. Hawking - he might know. I'm afraid I just accept it. Sorry.

The rubber sheet analogy is very useful, as gravity is just potential energy. Going back a bit, when Newton was hit on the head by an apple falling under gravity, he deduced that the moon was also falling towards earth under gravity, but was going so fast round the earth that it missed hitting us, and so was in orbit.
If you've ever seen one of those potential well models in a science museum, you will know what I mean. The depth of the well is 1/r where r is the radius from the centre, just like a gravitational field. If you send a ball round the edge of the well it will spiral into the well and disappear, like a satellite returning to earth. If you hit it hard enough, it will orbit a few times until friction reduces the speed and it falls in. There is very little friction in space so the moon, for example, keeps orbiting for a long time, but it will eventually spiral in (DON'T PANIC this takes a veeeery long time...). If you just let a ball go with no tangential velocity, it is like an apple - it hits the earth immediately.
So space is curved in the sense that the potential energy of any object in space is affected by surrounding masses, so it's behaviour is affected by those masses.
As to why mass should have this effect, the current thoery is that it is mediated by particles called gravitons, but noone has detected any yet,as far as I know. If they do, this would enable the unification of all the forces (gravitation, electro-weak and strong) as the other two are due to exchange of (virtual) particles. The problem is that gravity acts over vast distances, which the other two don't, and is far weaker. These two features may well be linked, but I am not an astrophysicist, so don't know the up-to-the-minute state of play. It's a fascinating subject though.

Hmmm. I worry that the moment you 'just accept it' is the moment you stop being a scientist. That's probably a bit of a generalisation, as when you're doing science for a living there isn't time to read everyone else's results, but from my experience as a physics undergraduate, you get some things proved to you and you just get told some things. Now, five years on, I have difficulty remembering which is which. For instance, like everyone here I can understand magnetism from the point of view of Maxwell’s laws, but have no idea of the physical derivation of them. Of course, at the time I was more interested in passing exams than understanding things about how the universe works, which is a pity because understanding things about how the universe works was the reason I chose to study physics in the first place.

But rambling aside, I know someone who’s registered as a researcher and hasn’t written anything yet, but is doing a PhD in fundamental fields or something, so I’ll ask him if he can shed any light on the subject.

As for gravitons acting over vast distances, it’s not a problem if they have no mass – in that case the attractive force tends to zero as distance tends to infinity, which happens to be observed in nature. Electromagnetic attraction works like that, as the particles are massless virtual photons. I think (& I’m not sure) that the main obstacle to theories involving gravitons is, as you said, that nobody’s seen one.

Theres a lot of stuff here - the topics being discussed would fill a respectable physics course. Here's some thoughts on various stuff, starting with

1: Magnetism from relativity:
You can see how to get magnetism from relativity in some situations - the easiest one I can remember is the force between two parallel wires conducting current in opposite directions. There is a magnetic force between the wires which you can calculate a la Maxwell.
Or , you can take the following view. Suppose I sit on an electron in one of the wires, and I am travelling say from left to right. There is a stream of electrons in the other wire going from right to left. Suppose they are all going past in an orderly fashion, a fixed distance apart. Then becuase they are moving, there will be a Lorentz contraction in the distance I see between them. This is a special relativistic effect which basically says that length isn't constant, but depends on how fast the object being measured is moving with respect to the measurer. Sounds odd I know, but it happens. Anyway, because of the Lorentz contraction, from the point of view of the electrons in the first wire, the ones in the other wire look closer together. So from my point of view, sitting on an electron is the first wire, there is a difference between the charge on the two wires, and so there is therefore an *electrostatic* force between the wires.
Note that from my point of view, sat on an electron, that electron is at rest, and so is NOT experiencing a magnetic force.
You can calculate that the new electric force is the same as what is usally perceived to be the magnetic force. So, from the point of view of relativity, electric and magnetic force is definitely the same thing - you can get from one to the other just by changing point of view - the physical size of the force will of course always be the same.

I think the question is where do they come from.
I guess that originally, there were phenomenological, and have since been seen to have all sorts of nice mathematical structure (some people get very exited about the fact that they can be simpified so much; you can write them in terms of nice differential geometric quantities very easily).
From another point of view, we now know more about how electricity and magnetism work at the really fundamental level, not in terms of electric and magnetic fields, but in terms of exchanges of particles. This is QED - quantum electrodynamics. This is what all those squiggly Feynman diagrams are about. It is a staggeringly successful theory - it has been tested to some ridiculous precision. It's very good at telling you about really tiny stuff (what happens when an electron hits a photon) but for practical purposes, Maxwell's equations are still more usefull. You can, however, derive Maxwells equations from QED - although it's not easy. This was of course one of the necessary checks that QED was correct - it needed to agree with stuff we knew was right. So although it came after Maxwell, you could say that Maxwells equations are dependent on QED. This really just leads to the question - "so where does QED come from?" but I guess that you can always ask questions of this kind

Yes - we reckon gravitons- particles that carry the gravitational force should exist.
Why? Because.
I guess because the other forces (electro-weak and strong nuclear) are described in terms of particles.

So why isn't it done?
Because quantum gravity is a right mess. There are hundreds of people trying to make a quantum theory of gravity, but no-one's doing very well. It's a mess because, as jan* said, matter changes the "shape" of space. For the other theories (electro-weak (QED) and strong) you assume space is fixed, and then have your force carrying particles (photons and gluons) zipping about and popping into and out of existence. In quantum gravity, every time space changes, it has an effect on how gravitons are produced, and each graviton has a back reaction on the space. So everything depends on everything else in a horribly complicated way, providing needed employment for theoretical physicists who would otherwise have to be let loose on the world instead of being sensibly shut up in universities. Given that the effects of such a theory will only have a tiny tiny effect on anything, its questionable if there really is any other reason for the research

I'll shut up now. Does anyone think any of the above ramblings should go in an article somewhere? I guess I should do it, if no-one else has.

"There is very little friction in space so the moon, for example, keeps orbiting for a long time, but it will eventually spiral in (DON'T PANIC this takes a veeeery long time...)"

Now, I'm not as up on all the theories as you (hence the reliance on cartoons to educate me), but the above statement doesn't seem to take into acount that we (the earth) are also falling towards the moon, since it also has a gravity well (this accounts for the eliptical orbits of celestial bodies as well). As we both "fall past" each other we end up putting the other body slightly further into each others well, thereby increasing potential energy, speeding us up enough to shoot up out of the well slightly, there by insuring that the two bodies will actually remain in orbit longer...

Wouldn't the other possibility be that this increase in potential energy would over come the friction, and eventually the moon would build up enough momentum to break free of the Earth. (I'm thinking back to that old "Space 1999" show)

That remark was not entirely serious. Lagrange points and things probably come into it too, but then I never did astrophysics,as I may have mentioned, I preferred looking into electron microscopes. However, the relative masses of the earth and the moon would seem to indicate that a) the moon crashes into the earth, b) the earth crashes into the moon, or c) the huge tidal strains induced will break one or the other up. Whichever way you look at it, we're doomed, but I am not losing sleep over it.
Have a nice millenium, or two

The electo-weak force is not described by QED.

QED describes all interations involving electrons (and their anti-particle the positron) and photons. The Weak Interation is responsible for some forms of radioactive decay, e.g. the decay of a neutron into a proton, electron and electron neutrino (the real action is at the quark level, but I won't go into that now).

In general, particle physicists talk about four fundamental forces (listed from weakest to strongest):
Gravity.....very weak, but significant because of the amount of matter in the universe.
Weak Interaction.....responsible for some radioactive decay.
Electromagnetic Interation.....responsible for chemistry, etc. Described by QED
Strong Force.....holds nuclei together.

All of these forces are mediated by particles (or thought to be - gravity hasn't yielded to this form yet, as already discussed).

In our current universe, these forces are quite seperate. However, there is a unified theory of Electromagnetism and the Weak Interaction, called the Electro-Weak Theory, which shows that they are two sides of the same coin and, in an environment containing enough energy (e.g. in the early universe) it is not possible to distinguish between them. There are also various proposed theories which combine the Strong and Electro-Weak Interactions, and the hope is to be able to add Gravity in as well.

I'm thinking of writting a guide entry on this stuff. Or maybe several, covering different aspects.

I am enormously glad that someone is writing a Guide entry on this, as we seem to be trying to teach a physics degree, at the very least, using words alone - this is very hard. This is very interesting stuff, but it is easier to learn from a book, and as my ageing neurons will testify very difficult to explain without maths, diagrams and a large blackboard. I wish you all the luck in the world.

An ex-Physicist.
(like an ex-Parrot, only slightly less funny)

If any of you respectable physicists can write an itelligible article on the above topics, I promise to invite you next time I have one of 'those' sort of parties. (Can't make any promises about the hostess's undergarments, though.)