- The focus of tonight's lecture
is the magnetic universe. Magnetic fields pervade space. All of outer space is
threaded by magnetic fields. And if you thought that magnetism was maybe a rather eclectic, perhaps even irrelevant part of physics, then you'll find out in
this evening's lecture that that is simply not the case. Magnetism is critically important, not just for the preservation
of life on Earth, but as the explanation for
some of the exotic phenomena that we see in outer space. Things behave very differently in the presence of a magnetic field, differently from what
we might first expect. So let's begin with a
familiar manifestation of how an everyday object can interact with a magnetic field. The compass, of course,
which we now understand responds to Earth's magnetic field, was critically important
for developing navigation, for being able to navigate ships to a particular newfound
land and back again. But magnetism should not
be regarded as man-made, some kind of unusual
manufactured artifact. Far from it, magnetism is
absolutely naturally occurring. Indeed, Thales of Miletus
discovered in the sixth century before Christ that certain minerals, this one particularly known as lodestone, had highly magnetic properties. And if it came together with something that was ferromagnetic, for example, a spanner made out of iron, which possibly wasn't around
in the sixth century BCE, there would be an interaction. Indeed, the lodestone would
somehow keep an invisible grip on whatever ferromagnetic
object was to hand, and it wouldn't fall to
the ground under gravity, as it would do in the absence of any magnetic field interaction. So lodestone, this mineral that's in the rocky-type substance
in that human hand there, is very much naturally occurring, but bar magnets are, of course, something we are perhaps a bit more familiar with. I'm going to show a photograph now of what happens if you take a bar magnet and you put a sheet of white card on top, and then you sprinkle
some iron filings on top. Turns out that repeatably,
every single time, you get this beautiful pattern, if, that is, you keep the bar magnet still and safely underneath the white card, which has all the iron
filings on top of it. Underneath the sheet of
white card is a bar magnet, with a north pole, which
is over on the left, and a south pole, which
is over on the right. And it's this, underneath
the sheet of white card, that gives rise to this
beautiful structure, this beautiful distribution
of the iron filings. Sometimes, you need to tap the white card a little bit to get the iron filings to respond to their magnetic environment. But you can see, there's
something of a north-south divide in the distribution of the iron filings. They're in very specific locations and they seem to follow very
specific shapes of lines. So let's look further at this bar magnet. What happens, for example,
if we chop it in half. If you do that, and you do actually need something a little bit more robust than just a pair of kitchen scissors, you end up with two bar magnets, and the north-south orientation
is the same for each and the same as the original bar magnet. So how do these half bar magnets
interact with one another? Let's imagine that we
keep the original set with the south and the north
in the middle together, but we reverse in a second experiment, we reverse the right hand magnet, so that south is closer
to the other south pole, and we do a similar kind of experiment where we have the north poles together. What happens next? Well, the like poles will
experience a force of attraction and the similar poles
will repel one another regardless of whether you are talking about the south-south example
or the north-north example. They will move, the bar magnets will shift in response to those
forces of either attraction or of repulsion. This is completely repeatable, happens every single
time with a bar magnet. So what about the magnetic
field lines that emanate from a bar magnet manifested
by the distribution of the iron filings on the white card? Well, the very idea of
magnetic field lines is a conceptualization of what's going on with the magnetic field. Strictly speaking, there's no such thing as lines of magnetic field at all, but it is helpful to talk in
terms of that picture language, because it reminds us that
there is a directionality to the distribution of
magnetic field lines, which will make a difference to the way nearby matter interacts with it, and that will become
really quite important later on in the talk. But before it does, I want to talk to you about how a charged particle,
such as an electron, will interact in the presence
of magnetic field lines. And that interaction
will be very different according to the relative direction with which the electron is moving with respect to a given
magnetic field line and the orientation of
that magnetic field line. Let's take a couple of extreme examples. So, whereas with gravity,
we understand that, our intuition is very well developed. We have an apple, we let go of it, it drops to the ground. Completely repeatable, happens every time. What about the direction with
which a charged particle, such as an electron,
will move with respect to magnetic field lines. Now another big conceptualization here, the electron is represented
as that pink circle. And if that, if the electronic appears moving from left to
right across the screen, and it's exactly parallel
to the local direction of the magnetic field lines, well, it just moves across, left to right, if and only if it is parallel to the local direction of
the magnetic field lines. An electron moving parallel
to magnetic field lines will continue to move parallel. It won't respond at all to the presence of the magnetic field, assuming
they're exactly parallel. However, it's a little bit different if the electron is moving
at a different angle with respect to the magnetic field lines. Let's imagine that the electron is moving exactly perpendicular to
the magnetic field lines when it appears in the
middle of our experiment. If that happens, the
electron will just trace circles around the magnetic field line, just going round and round and round. Now, if you have some intermediate angle between perfectly parallel
and perpendicular, which gives you the circular motion and the straight parallel
motion irrespectively, then you trace out a helical path, much like the gray line
is illustrated here. You will get that every single
time, that helical path, for an electron moving
at an arbitrary angle with respect to magnetic field lines. When this happens, something
remarkable takes place, and that remarkable thing
is that the electron, or whatever charged particle
we are talking about, will emit light. And that happens because
when a charge particle is forced to go into some
kind of circular orbit, whether we're talking
about a perfect circle or whether we're talking
about a helical path, it will be undergoing acceleration. And it turns out because
of quantum mechanics that you can't have that
kind of acceleration without simultaneously emitting a photon. In a way, the direction of the photon also knows exactly about the direction of the electron and of
the magnetic field lines. This works out in a very detailed way. It's important to realize
that what I've just said has been very well
established over the past, well over a century now, thanks to the work of
the Scottish physicist, James Clerk Maxwell. He demonstrated that light is comprised of a wave in electric field
and a wave in magnetic field. That is what comprises a photon. It travels at the speed of light, and the speed of light
is a very special number in the universe. It's defined simply by
the magnetic properties of the universe and the
electrical properties of the universe. And I went into this aspect
in a little more detail in my very first Gresham lecture, which was entitled, "Faster Than Light?" So if you are not too familiar with this, I refer you to that lecture. But back to today's story. If you have interesting
magnetic field structures, and by interesting, I
don't just mean parallel, but if you have interesting
magnetic field structures, then you get remarkable patterns in the distribution of light if you've got charged
particles rocking up, rattling around, following helical paths around your interesting
magnetic field structure. Can we see this in the night sky? Well, we absolutely can. And here is such an
example of the aurorae, which we see if you go
to northern latitudes, in which case they're often
called the Northern Lights, or if you go closer to the south pole, in which case they're called
the Southern Lights, of course. You see absolutely remarkable
and beautiful structures if it's dark enough so
that light from the sun doesn't swamp out these
beautiful structures. Whether it's green or whether it's pink depends very much on the
nature of the charged particles that suddenly rock up on
Earth's magnetic field. Whether you've got helium,
or whether you've got oxygen, or whether you've got something
a little bit more unusual, that determines the color,
but the distribution, the shapes that you see in space, depend on the clouds of
charged particles that rock up and the magnetic field structure
of planet Earth itself. Now, this is a cartoon representation of the kind of magnetic
field that we believe to emanate from within planet Earth. And you can see close to
the north magnetic pole and the south magnetic pole, you get a bunching of
these magnetic field lines, and that's why you see
the aurorae associated with closer, you see it
closer to the north pole or closer to the south pole. It's because you've got a
much greater concentration of magnetic field lines, a much stronger magnetic field strength, and so that's why it's much easier to see the aurorae in or
nearer to those polar regions. Besides being really,
very stunningly beautiful, planet Earth's magnetic
field is significant and important for the
survival of life itself. Planet Earth's magnetic
field doesn't just give us the decorative aurorae in the night sky, our planet's magnetic field protects us, and it stops planet Earth from
being an even more dangerous place on which to try and live. Because the very fact that
those charged particles, which come from usually the sun, the sun belches and erupts and in something that
we call the solar wind, blasts charged particles in the direction of all the planets in the Solar System, those charged particles from
the sun that arrive on Earth start following helical paths around the magnetic field lines of Earth, we see the beautiful or
patterns in the night sky, but in the meantime,
those charged particles are trapped in their helical
orbits, their helical paths, and so they don't actually
usually end up penetrating our human bodies and causing mutations and doing damage when that happens. It is a wonderful thing for
the survival of the human race that planet Earth has a magnetosphere. Earth's magnetic fields
that form the magnetosphere keep us safe from high
doses of radiation poisoning that would otherwise
come from the solar wind that's ejected from our nearest star. In contrast, planet Mars
has no magnetic PPE. PPE, of course, standing for
planetary protective equipment. (audience laughs) Here's an image of planet Mars that I took from my back garden in
the, two Octobers ago to try and capture its rotation, which you can just about see there. Now, my back garden in
Oxford is a distance of about 60 million
kilometers from planet Mars. But, of course, you get a
much closer view of a planet if you are much nearer to your subject. And so, last year, in fact,
NASA's Perseverance rover touch down on the surface of planet Mars in order to explore it
rather more thoroughly. This was a very exciting advance in terms of the engineering prowess that got it there successfully. It's very exciting in terms of being able to explore another celestial body. It was crucially important, it is crucially important
for astrobiology research, searching for signs of
ancient microbial life. It's exciting because we can learn about the planet's
geology and past climate, but it's really important to realize, however it is to, however exciting it is to think of flying over to Mars, rocking up there and
establishing a human colony, it's important not to be naive about how dangerous that would be. Mars is hostile for human
life because we would get the full radiative poisoning
force of the solar wind. There's a lack of any, of evidence of any strong magnetic field on Mars, and it would be really dangerous
to think of living there. The magnetic field of
planet Earth is one reason why this planet belongs to what
we call the habitable zone, but more of that in the final lecture of this series later in the year. So, the magnetic fields
that give the aurorae are the same magnetic
fields that are our PPE. There are no aurorae, no
comparable aurorae on Mars. Well, that's a little
bit about magnetic fields belonging to planet, but
now let's look at the origin of the solar wind, the sun itself. So back in August of 2017, I was watching the total solar eclipse from the vantage point of Idaho in the United States of America. And, of course, the
main action was the fact that the sun was increasingly being eaten by the moon traversing in front of it. But what you may just be able to make out, there's not quite enough
contrast perhaps to see this, is that within that green circle, there are black blotchy regions. Now this was not dust on
the sensor of my camera, I can assure you, but it absolutely was dark
blotches on the sun itself. Now in 2017, the sun
wasn't particularly active, and so that's why only
a few little blotches, little sunspots, were
detectable at this time. However, back in 2003, my
colleague Steven Lee in Australia was observing the sun at a time when it was much more active than in 2017. And I'm sure you can
see very, very clearly indeed the dark, very contrasty sunspots that were observable in the sun at this time of high solar activity. I'm just going to zoom
in, in the green box so that perhaps you can see
a little bit more clearly. Right at the center of each blotch you see an extra dark region, and that's because something quite special is going on with the magnetic fields. Really, really strong
bunches of magnetic fields are coming out of these sun spots, we'll see that in a moment, and where you have magnetic field lines threading through the plasma of the sun, of the star itself, that
inhibits the motion, the convective motion, within the sun, and so you can't redistribute the thermal energy within the sun. And so that plasma counterintuitively actually becomes a bit colder, cooler, it's not that cold, and
is seen as being darker. Here are some more sunspots that was seen in the sun in 2011. The sun wasn't quite so active then, but indeed this image, also
by my colleague, Steve Lee, was taken from light
that only permits light in a very, very narrow filter, a filter corresponding to where hydrogen predominantly emits light, and you can perhaps
see if I zoom in again, there's a tendril coming
out of that sun spot. Well, what's going on, and how do magnetic fields
connect up with sunspots? In fact, if you zoom in, and to do this, we aren't talking about a
telescope in a back garden, we're talking about a
satellite that's purpose-built to closely inspect the surface of the sun, you can see streaks and indeed
ropes of emission coming out. So these are images of sunspots
from NASA's TRACE satellite, and you can see big loops
of magnetic field lines coming out. As I've said, the presence
of a concentration of magnetic field hinders
convection within the sun, and so that inhibits,
convection normally brings hot plasma from the interior
of the sun to the surface, but the magnetic fields
put the brakes on that, they really put a stop to it, and so you do get cooler plasma in the vicinity of these sun spots, if you're looking at optical light, as I showed you in those earlier images. If we take a more wide
angle view now of the sun, again, from the TRACE satellite, you can see that the sun is a
place of tremendous activity. So the TRACE satellite is NASA's Heliophysics
and Solar Observatory. It was precisely designed to investigate the link between the solar plasma itself and the amazing coronal loops that tell us about the
magnetic field structure. Now, the key thing to realize
is that these coronal loops store huge amounts of magnetic energy. When they touch, you get a short circuit, and this gives you
something of a big bang, not the original big bang,
but still pretty serious bang. These coronal loops will
short-circuit one another, and they will set off
explosions and solar flares, solar flares being what
launches the solar wind, which can be in a very impulsive way. Sometimes, these events
are called solar storms. And this happened at a
particularly vulnerable time in December, 2006. It was a particularly vulnerable time between, because these two astronauts, Robert Curbeam and Christer Fuglesang, were doing a space walk at the time. They were outside of the ISS, the International Space Station. Moments earlier, from Mission Control, there was warning that big
solar flare had erupted on the sun over an area
of about 10 planet Earths. It was a major solar flare. And so they were told to
race back into the ISS with moments to spare, because, of course, they were in a much weaker
region of Earth's magnetic field. They were much more
vulnerable to the radiation coming from the sun. If they hadn't got back
into the ISS in time, they would've either had become very sick with radiation poisoning,
if not actually dead. So much like for those
of us living in the UK, last Friday, we were told to get indoors and stay indoors because of Storm Eunice, back in December, 2006, they were told to get indoors, where indoors for them was the ISS. A bit like getting inside
because of Storm Eunice, only, I imagine, rather scarier. Robert Curbeam, the astronaut on the left, they're now both retired as astronauts, but Robert Curbeam now works
in the safety office for NASA. (audience laughs) I'm sure he has a particularly unusual perspective on safety. So, we've talked a little bit
about how charged particles respond in the presence
of magnetic fields. But I want you to know that if you have charged particles which are moving, that movement gives rise
to a magnetic field, and the details of the
cause and effect processes between moving current, which
is moving charged particles, and consequent magnetic field lines, or indeed, varying magnetic field lines, giving rise to particular
behavior of charged particles, is all described in a set of equations known as Maxwell's equations, the same James Clerk
Maxwell that I mentioned to you earlier in the context of light. I won't show you those equations tonight because not everyone likes equations, but for people who do like equations, these are particularly beautiful
ones, Maxwell's equations. You might like to look them up later. But it's important to realize
that electromagnetism, encapsulating the behavior
of charged particles, and the consequent
behavior in magnetic fields and vice versa is a thing. It's not just a thing here on Earth, it's a thing in the plasma of the sun, it's a thing in plasma
all across the universe. Now, if you take magnetic field lines and then you just add motion
of the bulk of the plasma, forming the star, then that can twist up the magnetic field lines and alter the magnetic field structure you thought you had to begin with. So even if you start
with magnetic field lines that just go from the south
pole to the north pole, if you've got the
equatorial belly of the sun rotating a bit faster
than plasma is rotating closer to the poles of the sun, you are going to be rotating
the magnetic fields, which are frozen into that plasma, and you are going to end up with plasma that goes round in loops
parallel to the equator, and that's going to give
you very different behavior. Depending on the details
of the starting point and the turbulence of the plasma
on the surface of the sun, you will end up by creating loops, which, being highly
energized, want to expand. And when they start to expand, they might expand into one another, giving those short circuits I mentioned, or they may just break, either way, launching the solar wind, endangering the lives of astronauts, but not really endangering
the lives of us here on Earth, thanks to the magnetic field that we have here on planet Earth. So, magnetic fields are significant in terms of life on planet Earth, and they are significant in
terms of the behavior of stars. I want to just very briefly
take you through a timeline of magnetic revolutions. I think you can make the case for saying that magnetism and our
understanding of magnetism have had considerable
geopolitical influence in human history. And it really began in, although the discovery of
magnetic minerals like lodestone took place back in the sixth century BCE, it was really the 10th or 11th century that the magnetic compass was invented. This took place in China. And the significance, of
course, of a magnetic compass is that you could navigate. The minute people could navigate, then it meant it was possible
to establish colonies because you could
navigate across the ocean, you could navigate back again, and so on. Around the time that
Gresham College was formed, William Gilbert was doing lots
of experiments with lodestone and investigating the different phenomena that you could get from that. And he wrote a bestseller,
seriously, called De Magnete. And this truly was a bestseller, and it influenced a lot of the thinking of physicists at the time. A century and a half later,
the horseshoe magnet, a very strong, permanent
magnet, was invented. Couple of centuries ago, Oersted discovered that magnetic fields emanated from current-carrying wire. In other words, wires that
had charged particles, current moving along them. Just the following year, Michael Faraday discovered
electromagnetic induction, the idea that you could reverse the consequences of changing current in presence of a magnetic
field, or vice versa. That led to the first electric motor, and we all know where that led
to, cars all over the roads. A few years later, the first
electromagnet was constructed, so this is not something
that was a permanent magnet, like a horseshoe magnet or a bar magnet, this was something that was
controllable via electricity, the movement of charged particles. It was in 1864 that James Clerk Maxwell, the Scottish physicist
I mentioned earlier, formulated his famous equations. Then later came transformers and AC power, alternating current power, the kind that flows
through our homes today. But it was only just over
a century and a quarter ago that the electron was discovered. It must have been astonishing
to have been uncovering all these electromagnetic properties without really having any idea
that electrons were a thing. It must have been remarkable. It was only a century and a decade ago that the solar magnetic
field was discovered, and that came via spectroscopy, the fact that if you
try and do spectroscopy on certain emission lines arising from certain
elements in the solar corona, the lines would split in a way that they simply didn't if
you had no magnetic field, so that's how recent our understanding of the solar magnetic field is. Quantum mechanics was what explained the phenomenon of magnetism
from ferromagnets, magnetism in iron spanners. That too was really very recent. Magnetic resonance, which
is the crucial ingredient in MRI machines that help diagnose problems in our bodies in hospitals, again, not quite a century old. And then as recently as the 1960s, magnetic recording was made possible. The recording of data from
scientific measurement, from satellite data on
tapes became possible, and, of course, that propelled
the information revolution. Spin electronics is
only three decades old. Without spin electronics, hard
discs wouldn't be a thing. Laptops wouldn't be doing
the projection of slides. So, magnetism has had
an astonishing history. And if you are interested
in how our understanding, humanity's understanding of magnetism, has shaped the geopolitical world, then I refer you to the
Oxford University Press's Very Short Introduction to Magnetism, which happens to be written by my husband. (audience laughs) End of plug. Let's go back to stars,
a bit like our sun, but stars that have even
stronger magnetic fields. So if you take a star that's
more massive than our sun, but got lots of magnetic
fields threading it because of the motion of its plasma, sometimes with an internal dynamo that whips up and enhances and amplifies the magnetic fields coming from it, and then imagine that that star has used up all its nuclear fuel, so it can't keep the star
undergoing nuclear fusion, collapse soon follows in a process called a supernova explosion. A big shell is blown off, but the bit in the middle collapses to become a compact object. And depending on the mass
of the original star, it can either become a
neutron star or a black hole, something I've addressed in
lectures in my first year. Let's just consider that
the original star had a mass between say 10 and 25
times the mass of our sun, and then it undergoes,
collapse to a compact object because of a supernova explosion, the resulting neutron
star will be something like 20 kilometers across. Big by the standards
of the lecture theater that we're in this evening, but bijou in comparison with the radius of the original star. We're talking about a collapse
from the original star down to the compact
object, the neutron star, by factor of tens of thousands, at least. If you start compressing
and collapsing a star into just 20 kilometers across
into the ultra dense matter that comprises a compact
object, like a neutron star, the magnetic field strengths
become absolutely huge. You're squashing them all together. Magnetic flux is a conserved quantity. Energy doesn't, magnetic
energy doesn't just disappear because the plasma squashes
down into a neutron star. And so you can end up
with extremely strong magnetic field strengths
for such neutron stars. The magnetic field strengths can exceed the magnetic field strength in our sun, which gives rise to all
those amazing coronal loops and gives a scary time to
astronauts, by a factor of 10^15. That's a one and 15 zeros. Hugely strong magnetic fields. When you have such strong magnetic fields threading a neutron star, you
have something that we call, it's obviously a magnetized star, when you compress it, you end up with something
called a magnetar. The word is, of course, a
contraction of magnetized star, but it's important to realize we are talking about
extremely magnetized star. So how strong are these magnetic fields? In units of Tesla, which I'll calibrate for you
in a couple of slides' time, the magnetic field strengths
are about 10^12 Tesla, a million, million Tesla. Hold that number for a moment and we'll calibrate it in a moment. It's huge, by the way, but I'll demonstrate just
how huge in just a moment. But I want you to just
think about the fact, you've got this huge magnetic field and it's frozen into this ultra dense compact form of matter
known as a neutron star, what happens when you get seismic activity within that neutron star? Well, it's pretty dramatic. The resulting blast that can
happen when you abruptly shear, or attempt to shear, such
magnetic field strengths dumps a huge amount of energy
into the surrounding space in a mere 10th of one second. And the amount of energy
that seismic activity in a highly magnetized neutron star, the amount of energy that's dumped is, in that one 10th of a
second, is as much as the energy as the sun emits in 150,000 years. Kind of energies we're
talking about are just huge, easily trivially measurable on Earth in all sorts of different wave bands, principally x-rays and gamma rays, the most energetic wave bands. We see huge flares in those
wave bands from seismic activity in highly magnetized neutron
stars in these magnetars. There's another type of
magnetized neutron star, which is known as a pulsar, and these are still
very highly magnetized, not quite such high magnetic
fields as in the magnetars. Their magnetic field strengths are a bit more like 10^10 or 10^11 Tesla. Famously, pulsars were discovered
by Jocelyn Bell Burnell. This is her pictured in 1967, the year that she made the discovery. Jocelyn confounded
expectations and searched her data with exquisite thoroughness. She didn't know a priori. She was looking for a new phenomenon. She subsequently refers
to the signal that she saw at the time, which opened
up the entire field of neutron stars, as scruff. That's what it looked like,
a bit of scruff, in the data. Well, she made the discovery, and she's now a professorial fellow at Mansfield College in Oxford. But her tenacity at that young age led to a discovery that revolutionized a lot of relativistic astrophysics. But I promised you I'd try and calibrate for you the different
magnetic field strengths that we've been talking about. So the most extreme
magnetic field strengths we know about in the universe at present, we might discover something
new in the coming years, but at present, the most strong
and extreme magnetic fields we know about are in the magnetars. Their magnetic field strengths are something like 10^12,
a million, million, Tesla. For those of you who are thinking, come on, tell me what a Tesla is, let me try and calibrate it for you. If anyone here has ever
had an MRI image done of a particular limb or part of their body to investigate a problem, the MRI scanner that you
will have been put inside for that diagnostic imaging will either have been a 1.5
Tesla superconducting magnet or three Tesla one. Now for the purposes of today's lecture, three is equal to one, so the magnetic field in
a superconducting magnet that you have in an MRI
scanner in a hospital, that's about a Tesla, so a million, million times that is what you have in a magnetar. Slightly less than that, 10^9 or 10^10 or maybe 10^11 Tesla, is what you have in a
neutron star or a pulsar, depending on what their progeny, what the original magnetic
field was in the star that ultimately gave rise
to that neutron star. At the opposite end of the scale, we have the magnetic field strengths that are going on in the human
heart and in the human brain. Now, while MRI scanners
are perfectly safe, it would not be safe to go
anywhere near a magnetar. A magnetic field strength of 10^12 Tesla near our human brains and our hearts would stuff them up entirely, because that magnetic field
strength would be so strong that the atoms and molecules
that comprise our hearts and our brains and the rest of our bodies would be so distorted, we'd be stuffed. People often fixate on
the spaghettification that is postulated to
happen in the vicinity of a black hole, but trust me, what would happen to
the atoms and molecules in our human bodies in
the vicinity of a magnetar or a pulsar, you just
don't want to go there. Please trust me on that. Well, let's go back to the presence of, the behavior of charged particles in the presence of magnetic fields, and let's go back to outer space. So I'm going to talk now about something called synchrotron radiation. This picks up on the idea that if you have a charged particle moving at an angle with respect
to a magnetic field line, pictured by the black line
going across the slide there, it will, as I said earlier,
follow a helical path. And as it does so, it will give rise, it will emit photons, it
will give rise to light. What's special about synchrotron radiation is that if that charged
particle has got so much energy, it's moving relativistically, and by relativistically,
I mean, moving at a speed that's comparable with the speed of light, you get very remarkable radiation indeed. It's completely different if the electron is just ambling around at normal speeds, but if the electron is whizzing around at speeds comparable with
the speed of light itself, you get extremely energetic radiation, even if the magnetic field strength is still relatively low,
and by relatively low, I mean, only 10 or a
hundred thousand times higher than the magnetic field in our human hearts and
in our human brains. If you do have sufficiently energetic, sufficiently rapidly
moving charged particles, like electrons, then that gives rise to these very beautiful radio
structures that will we see, or at least that radio telescopes see. We're not sensitive to these structures with our human eyes at all, but telescopes that are
sensitive to radio wavelengths absolutely can detect these with ease. This is a beautiful radio image of the radio galaxy Centaurus A, imaged by my friends, Ilana Feain, Tim Cornwell and Ron Ekers with the Australia Telescope Compact Array shown in the foreground there. That's an optical image
of those telescopes. And these balloons of charged particles whizzing around in helical paths along the different magnetic field lines that pervade these balloons
that had their origin in plasma ejected from nearby, the black hole that's at the
very center of Centaurus A, then it is marvelous to
behold these structures. It's just astonishing to think
of how they can balloon up to the size that they are. The extent of this is comparable with about 1 million light years. In other words, if
you've got a ray of light traveling from one end to the other, even traveling at the very rapid speed, that is, the speed of light, it would take you 1 million years. These structures are huge, but such are the energies
that we are talking about, even in the presence of very, relatively, very low magnetic fields, still quite a bit stronger
than the magnetic fields in our hearts and our brains, you see these amazing balloon structures. This is Fornax A, shown us
an image of radio wavelengths that are about 20 centimeters long. This was imaged with a Very
Large Array radio telescope in New Mexico, in the USA, only about a third of
the size of Centaurus A that I showed you in the previous image. It's actually rather
good fun to try an image different radio phenomena in space at very low radio frequencies,
at very long wavelengths. And I've done this quite a bit. It's really good fun, using the Very Large
Array radio telescope. Now, the images that I'm showing you here are contour images, so where
you see them bunched together, that tells you've got
a very dramatic change in how bright the radio distribution is. And if you just, for a moment, compare these two contour images, the top one is taken at a
wavelength of four meters, so pretty much half the span
of this lecture theater here, half the width, whereas the image below is a wavelength of only about one meter. What you may notice and
just be able to discern is spurs in the contour levels in the top low frequency,
long wavelength image, for which there is no evidence in the slightly higher frequency, slightly shorter wavelength image. It's really good fun
to image these things. This is per CSA, by the way, another famous local radio galaxy. It's really good fun to observe these, unless you are in a state
of high solar activity. If you are near solar
maximum and you try observing at these very long
wavelengths and then the sun starts emitting a big
impulsive solar flare, that really stuffs up imaging at these long radio wavelengths, so I have struggled
with that a little bit. It's another reason, beautiful
though the aurorae are, they're dangerous for
astronauts, as I said, and they're inconvenient when you're doing long wavelength radio astronomy. But despite that, with such images, if you combine them with
images at x-ray wavelengths, then you start to see beautiful
interactions going on. So on this right image
here in the color scale, corresponding to where
those spurs are in emission, you can see they're feeding holes, evacuated regions in the
underlying x-ray emission shown in the yellow color scale. So these spurs previously
inflated bubbles, which have now cooled, and so the combination of investigating the behavior of magnetized plasma with radio imaging at
different wavelengths, particularly long
wavelength radio imaging, together with x-ray imaging tells us a lot about the evolving nature
of this magnetized plasma. Well, radio imaging,
particularly long wavelength, low frequency radio imaging, is really the flavor of
the month in astronomy. And one of the new
so-called next-generation radio telescopes is one located
in the Karoo, pictured here, in the Northern part of South
Africa, known as MeerKAT. MeerKAT consists of 64 antennas, each one 13 and a half meters in diameter, so you would struggle to fit
one in this lecture theater. You certainly wouldn't
get it through the door. So this next-generation radio
telescope inaugurated in 2018 is starting to yield some fantastic data. And what I'm going to show you now is a new image of the Galactic Center that was publicly
released only a month ago. It was published in the
Astrophysical Journal, which is one of the premiere journals where new scientific data, new scientific results are published. It was also published
in the New York Times, which is where I've taken this image from. You probably can't read
the small subtitle there, but the subtext, the
subtitle is out there. So this is an image of just
the very central few degrees of the galactic plane in the Milky Way. And that yellow bit in the
middle is the brightest part, that's the location of
Sagittarius A* star, which is the location of a black hole whose mass is 4 million
times the mass of our sun. This particular image is
about three and a half degrees on the sky, by two and a half degrees. And it was made by using
a lot of telescope time with MeerKAT. MeerKAT successively observed
these, this region of sky scanning across in a mosaic using over 200 hours of telescope time. It yielded something like
70 terabytes of data. This image is only about 800 megabytes, so that's quite a compression. It took 40 days to process the data on two supercomputers in Cape Town. And this is a truly remarkable image. I'm going to zoom in on different
features within the image, but I just want to introduce
you to two of the team that were involved in creating
this beautiful structure. One of them is my friend
and colleague in Oxford, Ian Heywood, and another
is Isabella Rammala, who's at Rhodes University studying for her PhD on this beautiful data. What a PhD topic to be studying,
the center of our galaxy. Well, if I now zoom in
on that central region, then you can see really, very clearly these distinct linear features, which seem to just go across
some of the brighter features, that Sgr star down there
looking very bright in its own little bubble. Also seen in this image are
spherical, or at least circular, in projection presumed
to be spherical bubbles in radio emission. Again, the whole sort of streaky thing is seen with that thing on the left, seemingly blasting away at
some high speed or other. Well, these circular,
presumably spherical, regions are believed to be bubbles,
blast waves, shockwaves, generated by supernova explosions. When a supernova explodes, you get a massive overpressured
blast wave of plasma, so overpressured, it blasts into space, and with time, you see that blast wave get bigger and bigger and bigger. Over the course of about
five years, half a decade, you see these bubbles get
bigger and bigger and bigger. It's, by the time you get
to the right hand side, the supernova remnant is
now about one light month in extent, meaning it would take light a month to travel from one
side across to the other side. Some of the bubbles seen in that new data are absolutely astonishingly circular, as though they've
expanded into very uniform and pristine conditions,
which is astonishing given the violent activity that's implied by some of the images I've
just been showing you. These linear streaks that are seen here, the authors have cataloged into a whole great, big collection
of these linear streaks. Each of them is quite a
few light years in extent, and their magnetic properties
are truly astonishing. The kind of, let me skip to this slide, the kind of magnetic field
strengths that we are talking about for these streaked linear regions is about 1% of the magnetic field strength in an MRI scanner, in a
superconducting magnet, that you have in a hospital MRI scanner. So by astronomical standards,
leaving aside magnetas, these are quite significant
magnetic field strengths. What's their origin, dunno. It's completely unknown at present why you should have these one-dimensional magnetized ropes in space, seemingly perpendicular to
the plane of our galaxy, but for sure, this is
one of many discoveries that will come from the
next-generation space, next-generation radio
telescopes, such as MeerKAT. There's absolutely no
doubt that investigating the magnetic universe is a very rich and exciting aspect of the
universe for us to study. Thank you very much, indeed. (audience clapping) - So the first question is, is the Earth's magnetic
field growing or waning? And if it is one or the
other, do we know why? If it's static, do we know why? (audience laughs) Tell us. (laughs) - So, one of the one of the reasons why we think we have a magnetic field arising from the center of planet Earth is because of the nature of Earth's core. It's molten iron, amongst
other things or other minerals, but molten iron, meaning liquid iron, means you have charged metal,
liquid metal, moving around. And as I indicated earlier, that gives you a consequent
magnetic field structure, somewhat resemblant of
the magnetic field lines that emanate from a bar magnet. Now, is Earth's magnetic field changing, is the question, or is it static. To take the second question
first, no, it's not static. It's changing. It's always changed. Excitingly, or maybe, I mean, worryingly, Earth's magnetic field has flipped. South was previously north,
north was previously south, and it's going to do that again. In fact, it's overdue for doing that. Now, exactly why that happens
isn't fully understood, but it's some kind of very
grandiose phase transition to do with the internal
nature of Earth's core. When it has happens, it is
going to be quite disruptive. - [Man] Hi. Thank you for the lecture,
super interesting. You know the right hand rule, so I can find everything explaining what the right hand rule is, but no matter how much I search, can I find anything explaining why, what gives the directionality 'cuz, and maybe Maxwell's equations explain it, and I need to go and look at them, but it feels to me like it speaks of some underlying
asymmetry, which is baffling. Yeah, so, do you know why? - Well, the first thing to say is, I mean, I absolutely do commend Maxwell's equations to you. However, Maxwell's
equations are a description of the electromagnetic
phenomena in the universe. They describe what happens in exquisite and beautiful detail. But they don't really us why. It's important to realize
they are a description of the physical universe
as we discover it, as we measure it, as we see it playing out in front of us, whether that's on planet
Earth or in space, but the fact is, magnetic
field feel this way, if you're talking about electrons,
then things go that way, if you're talking about
positively charged particles, then it's reversed. So there's a lot of consistency
about it, but you're right, it's not isotropic in any way. Maxwell's equations will give
you a beautiful description, but they're not really telling you why, but they are telling you about the nature of the physical universe. - [Man] Does anybody know? - I think why isn't really
the question to consider. I mean, seriously, we, as physicists, we are measuring and we're saying how. We are trying to explain all
sorts of different phenomena in the light of physical laws
that we think we understand. Why is a bit of a different question. How is more the domain of physicists. - [Woman] Hi, thank you for your talk. I was just wondering if, oh, sorry, if we now have any ways of predicting solar activity and solar flares so that we can potentially
schedule spacewalks so that they're not at the same time. - That's the most interesting thought, and I feel sure the
astronauts of the world are very grateful for your concern. (audience laughs) I mean, loosely, generally, you can say, scheduling spacewalks for
when we're at solar maximum, as we were in 2002, 2003, for example, when all those sunspots appeared
on the surface of the sun, you are at greater risk then. When you're at solar minimum, you are at much less risk, but nonetheless, the
appearance of sunspots and the ejection of solar flares
is a little bit stochastic, and it does depend on the turbulent and the convective processes
going on internally to the sun, so until we can probe
internally to the sun, and I can't see that
happening anytime soon, sadly, I don't think we can make any detailed space weather forecasting of the kind that you have in mind. - The online audience
is also a little worried about the astronauts, so
there's one last question, if I may, does the moon
have a magnetic field? In other words, were the lunar astronauts subject to the full
strength of the solar wind during the various Apollo missions? - I think they were. I mean, I think the moon
has some kind of low-level residual magnetic field, but it's nothing compared
with Earth's magnetic field. - Okay. Well, thank you very much, and I hope you'll all join me in thanking the professor for a wonderful lecture. (audience clapping)
