How far can you follow a compass needle?

As far as the north magnetic pole, where the needle starts spinning wildly?

Compass needles align with magnetic field lines, and on the precise spot of magnetic north, those field lines are vertical.

So just tilt your compass 90 degrees and you can continue your journey - either down to the molten iron dynamo surrounding the Earth’s core, or up.

But up to where?

The answer - to everywhere - and today, that’s where we’re going to go.

Imagine you can see gravitational fields.

It would mean seeing the literal threads of the fabric of spacetime.

Threads tugged lightly towards the Earth, tightly towards the Sun, or into inescapable knots towards black holes.

But there’s really only one gravitational field in the universe - manifest as the fabric of spacetime itself.

And it governs the formation of every major structure in the universe, from the smallest moon to the largest cluster of galaxies.

But for all its importance, gravity is a bit boring.

The universal gravitation field is just a grid with some dips in it.

Personally, I’d rather be able to see magnetic fields.

Magnetism is one half of electromagnetism, one of the four fundamental forces alongside gravity.

Magnetism shares a property with gravity that none of the other forces have - it manifests on enormous scales.

The nuclear forces are short range, and the electrostatic force never adds up to much because its positive and negative charges tend to cancel it out.

But not magnetism.

It’s generated when electrically charged particles move.

Even if the substance is electrically neutral you’ll still get a magnetic field as long as the charges are moving in opposite directions.

That means magnetic fields can add up - and magnetism adds up to having enormous influence on the development of structure in our universe.

Understanding magnetic fields is of fundamental importance to astrophysicists.

And so we should probably understand them too.

First up, let’s review how magnetism works.

Magnetic field lines form in concentric circles around moving charges.

If that charge moves in a loop, that results in a dipole field - sort of a torus around the loop with the field threading the loop and shooting off at the poles.

So those are magnetic field lines - but what do they do?

Well, a moving charged particle will feel a force perpendicular to both its direction of motion and to the field lines - and the net result of that is that charged particles tend to spiral around magnetic field lines.

And if that charge already has a circular motion - for example the electric current in an electromagnet, or the aligned electron spins in a ferromagnet, then that current will want to loop around the magnetic field.

But the circular current produces its own dipole field, and the net result is that dipole fields always try to align themselves with other dipole fields.

And that, by the way, is how compasses work.

Speaking of compasses - a minute ago we were standing at the north pole with a compass needle pointing straight up.

Where exactly does that field line go?

As with the gravitational field, in a sense there’s only one universal magnetic field.

So while most of the Earth’s dipole field loops back - but some of those field lines connect to this greater magnetic field of the solar system - and even of the galaxy.

So let’s hitch a ride on a field line and see how far it takes us.

Now I don’t want to spend too much time in the solar system - greater magnetic wonders lie beyond.

But while we’re here, it’s worth following one of Earth’s field lines that connects directly to the surface of the Sun.

This is a violent place, magnetically speaking.

Here the field is generated by electrical currents flowing in the searing plasma near the Sun’s surface.

The Earth’s solid inner core and mantle regulate the flow in its liquid outer core, resulting in a clean dipole field.

But the Sun is entirely fluid, and its rate of rotation speeds up towards the equator.

That means the dipole field gets twisted up over time.

Magnetic field lines cross each other, and enormous magnetic energy densities pile up.

We can see those tangled field lines in ultraviolet light as charged particles spiral along them, up and down from the Sun’s surface.

When the pressure gets too high, these field lines snap and then reconnect, and in the process spray that magnetic field out into the solar system - carrying high energy particles with them.

These coronal mass ejections join the solar wind.

Follow one of these magnetic blasts and you’ll spiral through the solar system on a giangantic magnetic tornado.

This is still the Sun’s magnetic field, which connects here and there to the piddling little fields of the planets.

About 4x the distance to Pluto, the Sun’s magnetic field connects to the field of the galaxy itself.

Or smashes into it, depending on how you look at it.

This is the heliopause, the boundary of the heliosphere, which defines the limit of the Sun’s influence in the galaxy.

Although it’s less of a sphere and more of a teardrop - dragged into that shape by the Sun’s orbital motion through the galaxy.

How do we know that?

Well, we have sent compasses that far - or rather magnetometers - on board the Voyager 1 and 2 spacecraft which crossed the boundary into interstellar space a few years ago.

But those didn’t reveal the shape of the heliosphere.

That was measured for the first time only last year.

NASA’s IBEX mission, which used a sort of solar wind sonar - it mapped how bursts of solar wind material were reflected back towards the Earth from the edge of the heliosphere.

Beyond the heliosphere, seeing magnetic fields gets trickier.

Fortunately astronomers are also tricky, and so have tricks.

It turns out that the Milky Way is full of natural compasses.

The interstellar medium - the space between the stars - is scattered with tiny specks of dust produced in past supernova explosions.

These specks tend to align with the local magnetic field of the Galaxy in exactly the same way as our iron filings align around a bar magnet.

When light passes through the dusty interstellar medium it gets scattered by these grains - it bounces off them.

But the pattern of alignment of these grains imprints a pattern on the scattered light.

The light gets polarized - which means the direction of its electric and magnetic fields pick up a preferred direction rather than being random.

By measuring this polarization we can map the direction of these tiny compass needles, and so map the magnetic field of the Milky Way.

This has now been done in incredible detail by the Planck mission, which mapped polarization of the cosmic microwave background - the ubiquitous radiation left over from the big bang.

The resulting map reveals a whirlwind of magnetism tangled through the galaxy.

KInda makes me wonder if van Gogh could see magnetic fields.

Actually there’s a more traditional way to map the magnetic fields of galaxies.

These fields drive the motion of lone electrons throughout the interstellar medium.

When radio waves interact with those electrons, their polarizations are also affected.

This is a bit different though.

The presence of these electrons tends to slow light down - just as light is slowed down in air or glass - but to a much smaller degree.

But that slowing depends on the polarization of the light.

In this case, it depends on the circular polarization.

If the electric and magnetic fields of a collection of photons all tend to point in the same direction, we say the light is linearly polarized.

But if those fields are not fixed but rather rotate in the same direction, we say the light is circularly polarized - and it can be left- or right-polarized, meaning clockwise or counterclockwise rotation.

The electrons in their magnetic fields tend to slow one circular polarization direction more than the other.

The net result of this is that the linear polarization - which is sort of the sum of the circular polarizations - gets rotated in an effect called Faraday rotation.

So by measuring the Faraday rotation of distant radio sources we can also map magnetic fields.

This has been done for the Milky Way using the light from pulsars.

We even have clear views of magnetic fields in many distant spiral galaxies.

We see that the field tends to be threaded along the spiral arms.

These are the densest regions of those galactic disks - places where magnetic fields have confined the charged particles of the interstellar plasma.

And that plasma in turn drags the magnetic fields in orbit around the galaxy.

OK, so galaxies have magnetic fields.

But where do those magnetic fields come from?

This is a surprisingly open question.

Large-scale magnetic fields can grow and reinforce themselves in very particular configurations called dynamos.

In the Earth’s dynamo, swirls of magma are induced by the coriolis force, and while these are initially turbulent, they arrange themselves into a series of self-supporting flows.

These amplify what starts out as a very weak and disordered field into the ordered and powerful field that surrounds the Earth.

We did a whole episode on this cool effect if you want to learn more.

The exact process for the Milky Way isn’t as well understood, but the ingredients for a dynamo are all there: we have differential rotation in the disk which can lead to coriolis-induced helical flows, and those flows can also be produced by supernova explosions.

Those supernovae may also give us the seeds of magnetic fields that can then be amplified by the galactic dynamo.

However it got there, the Milky Way has built itself a substantial magnetic field.

And that field helps build the Milky Way in return.

Magnetic fields generated by collapsing gas clouds help to slow the rotation of those clouds - expel angular momentum.

Without that, those clouds would never be able to collapse all the way into stars.

And magnetic fields also facilitate star formation after stars die.

Magnetic blasts accompany every supernova explosion, and these help to compress gas in the path of that explosion, triggering bursts of new star formation.

Those same supernovae are expected to blast their own guts entirely out of the galaxy, which should result in all those newly-formed elements being lost to us.

But the galactic magnetic field constrains that flow, funneling some of it into vast galactic fountains erupting from the poles.

That’s right - if you follow a magnetic field line too far, you may accidentally leave the galaxy.

But actually, these galactic fountains are incredibly important for building galaxies.

Their material tends to spill into the space around the galaxy before slowly raining back in, where it can be used for new star formation.

The other cool thing that galactic magnetic fields do is that they act as colossal particle accelerators.

If you thought the 27 km ring of the large hadron collider was big, try the 300,000 light year ring around the Milky Way.

Electrons and atomic nuclei can be accelerated in this magnetic field to high energies - into what we call cosmic rays.

These can also be accelerated in the magnetic shock-fronts of supernova explosions.

But the most energetic cosmic rays are accelerated by the strongest magnetic fields.

Those occur deep in galactic cores near the gigantic black hole that dwells there.

If that black hole is feeding and surrounded by a disk of gas, we have what is known as an active galactic nucleus - the most powerful of which are called quasars.

Intense magnetic fields live just above the event horizon of some of thses black hole, and thread the infalling disk.

Those fields grab particles of matter and accelerate them to incredible energies, flinging cosmic rays out into the universe.

We’ve even taken our first picture of the such a magnetic field - in the polarized light surrounding the M81 supermassive black hole observed by the event horizon telescope.

But there’s an even more spectacular result of these magnetic fields.

Thick flows of gas can be catapulted through the surrounding galaxy in powerful jets.

In some cases, these jets puncture the galaxy and plume out in radio lobes which can dwarf the entire galaxy that spawned them.

These jets carry magnetic fields out into the cosmos, and we see them through the radio light emitted by electrons that spiral slowly in these vast structures.

OK, we’ve ridden our magnetic field lines pretty far and into some strange places.

To summarize what we’ve learned: magnetic fields are ubiquitous, powerful, and extremely complicated.

In some ways they’re the curse of the astrophysicist because they’re so tricky to model - far more so than simple-old gravity.

But it turns out that without cosmic-scale magnetic fields we probably wouldn’t be here today.

As such, astrophysicists are spending more and more time learning how to map and to model magnetic fields, to better understand the mysteries of this magnetic space time.