Pulsars as Physical Laboratories
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00:00
RSD-10 PioneerPulsarApertureStarMeasurementRRS DiscoverySunlightAngeregter ZustandModel buildingLaserNeutron starSpare partNeutron starYearPhotonic crystalTypesettingLaserIntensity (physics)Orbital periodFunkgerätPresspassungSpaceflightMorningRadiation therapyFACTS (newspaper)Electric beaconRotationSeeschiffRadio telescopePulsarWind waveBeam emittancePlasma (physics)GeokoronaPlasma (physics)MaterialEmissionsvermögen
04:01
ApertureWavelengthParticleRestkernNyquist stability criterionPresspassungHypothetisches TeilchenOrbitKilogramMaterialQuantumDensityOrder and disorder (physics)MeasurementNeutron starSeparation processLimiterMassNeutron starPulsarAtomismElektronenkonfigurationGround (electricity)Atmospheric pressureSpare partArray data structureBackpackTypesettingConduction bandGrey
06:24
Boeing 777ApertureRSD-10 PioneerJuli FernándezVery low frequencyNeutron starAtmospheric pressureParticleReaction (physics)FACTS (newspaper)Conduction bandNeutrinoDensityRadioactive decayMinuteAngeregter ZustandBeta particleMaterialSizingRRS DiscoveryParticle physicsArray data structureMassRestkernSeparation processOrder and disorder (physics)Air compressorDirect currentAvro Canada CF-105 ArrowKilogramEffects unitGrey
08:47
PulsarRSD-10 PioneerIonSunlightStarSkySizingNuclear fuelNeutron starNeutron starGamma rayFormation flyingBlack holeElektronenkonfigurationReaction (physics)NeutrinoTypesettingGeokoronaMaterialPulsarSupernovaPlanetary nebulaFuelFACTS (newspaper)RRS DiscoveryAstronomerCondensed matter physicsYearWhite dwarfOrbital periodSunriseCartridge (firearms)Bending (metalworking)FunkgerätHot workingDaylightMine flailRutschungPhase (matter)K-EinfangRelative articulationMassOpticsKilogramLightAngeregter ZustandBallpoint penNegativer WiderstandPower (physics)Crystal structureVakuumphysikAtmospheric pressureCentre Party (Germany)Air compressorGreyDefecationCondenser (heat transfer)Radioactive decayGround (electricity)
13:57
SawWirePulsarGearRestkernRoll formingIonMaterialAngeregter ZustandSkyParticleUniverseRutschungSolidSpantStarPionQuantumNeutron starNyquist stability criterionQuarkTesla (unit)Magnetic coreGreyLiquidEddy currentFACTS (newspaper)K-EinfangWill-o'-the-wispHypothetisches TeilchenDensityPulsarScale (map)AtomismSunlightNeutron starEffects unitMine flailAnalog signalCrystal structureBallpoint penStandard cellPlanetary nebulaAlcohol proofTemperatureSuperconductivityAtomAir compressorConductivity (electrolytic)PhotographyMagnetarCurrent densitySewing needleGasTelephoneRelative datingOrbital periodVoltaic pile
17:56
ApertureRSD-10 PioneerPulsarLaserStream bedQuantumVortexTemperatureLiquidCartridge (firearms)Field-effect transistorOrbital periodVideoPair productionCasting defectArray data structureStarParticleMagnetarCrystal structureScale (map)Neutron starQuantization (physics)Ring strainRelative datingMicroscopeNeutron starHose couplingViscosityDifferential rotationVacuum tubeSpare partPulsarZirkulatorVolumetric flow rateMaterialEffects unitSpin (physics)Angeregter ZustandSolidTypesettingRoll formingFACTS (newspaper)Radiation therapyHypothetisches TeilchenSource (album)Angle of attackWater vaporRemotely operated underwater vehicleEmissionsvermögenPaintYearGentlemanComputer animation
21:54
WednesdayRSD-10 PioneerNeutron starHot workingCrystal structureZeitkonstantePulsarPIN diodeQuantumVideoCasting defectProzessleittechnikSolidVortexRelaxation (physics)Spin (physics)ElectromigrationAngeregter ZustandPhysicistLiquidYearHose couplingModel buildingEffects unitDayShip breakingAtomhülleDensityClockSpare partBird vocalizationFunkgerätWoodWater vaporNeutron starWind wave
24:17
Receiver (radio)MagnetosphereColor chargeArbeitszylinderFACTS (newspaper)MagnetStarDipolTesla (unit)Spin (physics)Neutron starLaserSpannungsmessung <Elektrizität>Neutron starGround (electricity)Spare partVoltageSunriseWindPlanetForceMaterialLaserCylinder (geometry)Ballpoint penRutschungCosmic distance ladderSpeed of lightParticleVideoElectrical conductorCartridge (firearms)Wind waveMagnetarFunkgerätClockOrbitSolar windShort circuitModel building
26:40
IonRRS DiscoveryPulsarApertureRSD-10 PioneerMagnetosphereMorningGravity waveOrbitClockPulsarAlcohol proofAvalancheProzessleittechnikAccelerationPrecessionModel buildingEmissionsvermögenMinuteStarMaterialNeutron starPair productionVoltageGamma rayDipolYearRoll formingToolAtomic clockHourVideoPlanetPresspassungRRS DiscoveryPeriheldrehungPlasma (physics)MassSynchrotron radiationWindFord MercuryEffects unitRemotely operated underwater vehicleColor chargeParticlePower (physics)Short circuitMeasurementElektromotorische KraftArc lampOverhead cablePlasma (physics)LaserSpeed of lightFunkgerätPositronCaesiumRedshiftTurningWind waveSocial network analysisWellenenergieGround station
31:50
PulsarScatteringAtomic clockPulsarMint-made errorsPhysicistStandard cellClockTypesettingYearSeasonFACTS (newspaper)
33:02
IonPulsarHomogeneous isotropic turbulenceClockPulsarNeutron starAtomic clockYearSkyWatch
33:50
RSD-10 PioneerVideoMaterialCommercial vehicleElectric power distributionNanotechnologyWill-o'-the-wispComputer animation
Transcript: English(auto-generated)
00:22
Ladies and gentlemen, one of the exciting things, as I hope you're becoming aware, as we've heard from lectures today, one of the exciting things about studying astrophysics is that you need to consider physics under conditions which are very extreme,
00:40
conditions that you simply cannot achieve in a terrestrial laboratory. And this, of course, makes the physics exciting, and when my research led to the discovery of pulsars around 25 years ago, I had little idea of the wide range of really wonderful physics
01:01
that would be needed to understand them. In my short talk now, I would like to describe as simply as I can some of the main interesting ideas. Of course, astrophysics still has many problems. The basic phenomenon of a pulsar is that you receive with a radio telescope a regular succession of pulses.
01:22
Here is a rather extreme example. This is a pulsar that was discovered around 10 years ago and has the remarkable property that the periodic time between one pulse and the next is around 1.5 milliseconds. It is, in fact, an extremely regular pulsar and it's possible to measure that periodic time rather accurately.
01:43
You've seen this morning some very accurate physical measurements and I think this one ranks alongside those others. One can measure the periodicity to about one part in 10 to the 14 with an uncertainty here of about plus or minus 3 at the end. Now, it's not quite right in the introduction you were told
02:03
that we decided when the discovery was published that we were dealing with rotating neutron stars. I have to correct that slightly. We considered seriously the possibility of neutron stars. That seemed to be the most likely solution to this problem but we hadn't tumbled to the idea that they would be rotating.
02:22
I at that time thought that they would be vibrating like the crystals that Professor Ramsey spoke about. However, that turned out to be wrong. Now, in understanding neutron stars and pulsars you need a wide range of physics and I've illustrated some of it here. The model we have to produce pulses of this kind
02:40
is that you need a star, a highly compressed star where you can fit as much material as the Sun contains into a sphere of radius around 10 km. There will be a powerful magnetic field, as I shall explain and if the star has a magnetic axis which is oblique to the rotation axis
03:01
then you can have a beam of coherent radiation and the pulses simply represent the beam rotating past the observer and it is really exactly like a terrestrial beacon or a lighthouse for navigating ships. The physics that you need covers a wide range. I have sketched a few titles here.
03:21
We need to consider the compressed matter the physics of the matter which is inside the neutron star that involves you with superfluids. In understanding the radiation you need to come to terms with relativistic plasma physics around the star. Also, radiation theory to account for the intensity of the radio waves
03:42
you require something like laser type emission which is very unusual in astrophysics. The radio emitter has to be a highly organized system as I shall explain. Finally, having come to terms with the physics of a neutron star you can then begin to use, as Professor Ramsey already mentioned
04:01
use the high stability of pulsar timing to do physics experiments and you can make useful experiments as has already been done and I shall mention in general relativity you can, I believe, show very clearly
04:20
that gravitational waves must exist and finally the measurements have a bearing on cosmology. I am sure there will be many other features which will be discovered as time goes on. Well, in order to understand the basic physics of a neutron star let me just remind you of some elementary physics
04:42
and let us consider in the simplest possible fashion what would happen to ordinary material as you subject it to steadily increasing pressure. So we start with normal matter here on the top. Normal solids, as we understand them in a physics lab here on Earth
05:00
would be a close-packed arrangement of atoms like this with the mass content mainly in the nucleus composed of neutrons and protons and the separation of those nuclei is set by the electron orbits. You can squash material in a terrestrial lab until the orbits are just about overlapping and then you have a solid and a typical density, of course
05:22
about one gram per cubic centimeter. Now, the state of matter here is set by the most elementary quantum consideration the particle wavelength which, as you all know, is Planck's constant divided by the momentum of the particle. The particle in a box has been discussed several times already
05:41
by previous lecturers. So if you squeeze matter down let's suppose we can squeeze it without limit and use quantum mechanics to work out what the configuration of that matter will be. Well, if you compress the material until it's reached a density of around ten to the sixth grams per cubic centimeter
06:02
that's to say about one ton of material in a cubic centimeter then, because you're fitting the electrons into a smaller and smaller volume their quantum wavelength has to reduce correspondingly and the electron energy simply rises. It becomes great enough to free the electrons
06:20
from a particular nucleus so you don't anymore get standard atoms you get an array of nuclei with electrons moving randomly as conduction electrons do in a metal but in a degenerate state of this sort all the electrons behave like conduction electrons and move randomly through the material. That is degenerate matter
06:41
and we can find it in astrophysics. Go on squeezing the matter down and what happens then? Well, there's an interesting reaction takes place. Under normal conditions in a terrestrial physics lab if you create neutrons then they decay in about ten minutes or so into a proton, an electron
07:01
and an antineutrino that's a very well known and well understood reaction but under high pressure under very high compression you see the electron in order to fit into the allowed space has to have an extremely high energy the Fermi energy has to become
07:20
several tens of NeVs if you squeeze matter sufficiently and this means the electron begins to acquire relativistic energy the relativistic mass has to be considered in this simple particle relationship here and under high enough pressure the relativistic mass will dominate which means that the normal beta decay
07:43
of a neutron goes in the reverse direction because of the high energy of the electron here when you sum the energies on this side in fact this arrow goes in the reverse direction so if you have a box containing protons, electrons under high enough compression
08:01
you will end up with a box of nearly all neutrons that can be predicted and in fact was predicted soon after the discovery of the neutron particle so that one has matter almost entirely in the state of neutrons well, I'll be saying a little more about it but its simplest properties could perhaps be best understood
08:24
if I had a sample of neutron matter to show you I can't quite do that it would be rather uncomfortable if I did supposing this little lump of material which I have in my hand here was a lump of neutron matter about that physical size
08:41
that in fact is a lump of sugar but the density of the material would be as I've shown here something like 10 to the 14 grams per cubic centimeter that's to say something in the region of 10 to 100 million tons in a piece this size here of course I would be wrapped around it
09:02
rather quickly by gravitational retraction if that was actually a piece of material and it would not simply sit on the table like this it would rapidly pass through the Earth oscillate for some time and come to rest at the center of the Earth so that when we talk about solid state and the condensed matter physics
09:22
condensed matter as we understand it in a physics lab is almost a pure vacuum to the kind of condensed matter you have to consider inside a neutron star so that is the basic physics of compressed material and this has a big relation to what you would expect
09:41
stars to do as they run short of nuclear fuel could I have the slide please the compressions I've been talking about can be obtained by gravitational forces inside stars and these states of matter are highly relevant to the death throes of a normal star like the Sun our Sun let's suppose is up here
10:03
when it runs short of nuclear fuel ultimately it will be compressed down to what we call a white dwarf star a lump of degenerate matter with a density of around 1 ton per cubic centimeter it would have a size then about the size of our planet Earth this is well known and white dwarf stars of course
10:21
are very well understood and we can see that physics going on in the sky a slightly heavier star when it runs short of nuclear fuel and gravity begins to compress it as ultimately it must will give rise to the formation of neutrons and in that case the stellar mass ultimately can be contained in a sphere
10:42
of only some tens of kilometers in radius and the final collapse under gravity is rather violent you would expect a massive star of this type to form a neutron star near the middle and the energy and the neutrino flux from that reaction, that inverse beta decay reaction
11:01
will blow the remainder of the star which hasn't yet reached the neutron configuration into pieces and will blow it off into space to form a supernova heavier stars still are not necessarily stable even as a neutron state of matter they will become black holes well that in a nutshell is the importance of
11:21
this new type of matter, compressed matter with regard to stellar evolution so the old age of stars in fact when they run short of fuel becomes really an exciting period and it's not right to think of stellar old age as being a dull phase Could I have the next slide please? Well you've just been told about the Crab Nebula
11:42
here is a picture of it and these ideas about neutron stars as the explanation of pulsars were confirmed most beautifully by the discovery of a pulsar in the Crab Nebula I haven't time to tell you about all this work but this nebula is the remains of a supernova
12:01
which was seen by Oriental astronomers in the year 1054 with just visible observations the explosion gave rise to what looked like a stellar object which could be seen even by daylight so it was a rather dramatic event in the sky but we now know that the you see these two little stars right at the very middle here
12:21
it's the bottom right hand star there which is the remains of the star which actually erupted to form that nebula well this picture was made long before pulsars were discovered but after the pulsar discovery within about a year of it it was discovered that this bottom star here was in fact flashing light
12:40
the light from that star flashes regularly about 30 times each second so that there you have what must be a neutron star in the place a neutron star would be expected and it fits the whole theoretical concept very well indeed there is much more supporting evidence here which I would like to explain but time prevents it
13:02
so there is one of the few examples of a neutron star you can see there are a handful like that much weaker, emitting much weaker light than this Crab Nebula and they're very hard to see but they can just be detected with optical telescopes of high power normally neutron stars emit radio but in this case they emit from radio
13:22
right through to high gamma ray energies well now what would a neutron star be like? it's wrong to think of a neutron star just as a ball of neutrons it involves a lot of interesting physics there's a huge pressure gradient as you go from the outside of a neutron star into the middle and this causes the structure to vary
13:41
in an interesting fashion as you go from the outside of the star to the centre so as you come downwards the neutron star, well this one is 15 km it may be somewhere between 10 and 15 km depending on the total mass of the neutron star here you have degenerate matter which would probably be the most stable nucleus that would exist
14:02
which is ion 56, Fe56 so you meet first a very rigid crust where the nuclei Fe56 form a regular cubic lattice like that and it's degenerate because the electrons already have high enough energies that they're not bound to a particular nucleus
14:21
and they move randomly through the material as you come down there's an increase in compression and somewhere down here you get inverse beta decay you get the protons and electrons effectively being crushed together to form the more stable state neutrons and that begins to happen here as you come further down
14:40
you get more and more neutrons with just a trace of electrons and protons now remarkably enough solid state physics predicts that neutrons which are Fermi particles will actually form Cooper pairs and that this state of neutron matter although it's so enormously dense will in fact be a liquid
15:01
it will be a quantum liquid, a superfluid the neutral material analogue of a superconductor this will occur for any temperature much below 10 to the 10 degrees Kelvin so we simply have the standard results of low temperature solid state or rather superfluid state physics and that is what would be predicted to happen
15:23
so as you come down in fact much of the material of the star is in the form of a quantum liquid, a superfluid the trace of electrons and protons that still reside there the protons would be a superconductor and the electrons probably normal closer to the middle of the star
15:41
we don't actually know enough yet about the behaviour of fundamental particles to know what goes on here there might be a quite exotic core near the middle where you have stable quark material or perhaps stable pion material we simply don't know so that in general is what simple physics would predict for the structure of a neutron star
16:01
as you go from the surface to the middle and remember that it's going to be threaded by a very powerful magnetic field all stars have magnetic fields and if you compress a star like the sun down into a ball about 10 km across then the residual magnetic field will still be there
16:21
one of the things you learn in astrophysics very quickly is that the scale of the physical objects and the conductivity of the material is such that you can't get rid of magnetic flux it takes longer than the history of the universe to destroy magnetic flux the eddy currents looking at it from simple physics simply flow forever so you can't get rid of magnetic flux
16:40
and you would predict that the magnetic field would be something like 10 to the 8 teslas 10 to the 12 gas, an enormously strong field this does modify atomic structure in an interesting way you know, the surface here it turns atoms into more needle-like things and I'd like to talk about that but unfortunately there's no time
17:00
well now, we do have observations which relate to this you can do physics which relates to this structure and begin to check it out could I have the next slide please? oh, while it's there I'll just show you the actual flashing from that pulsar in the Crab Nebula I showed you the two stars in the middle of the nebula it's the bottom right hand one which is the pulsar
17:22
by clever photography and using stroboscopic techniques with your telescope you can make the pulsar quite visible on a photograph and effectively if you take frames about 16 milliseconds separated in time you can sometimes see the pulsar turned on and sometimes see it turned off here when the neutron star is not pointing towards you could I have the next slide please?
17:42
now quite early on in the history of pulsars this was back in 1969 one of the fairly rapid pulsars detectable only in the southern sky mostly showed a remarkable effect what is being plotted here is the periodic time of the pulses as a function of date and you see here's the period
18:01
steadily increasing corresponding to a systematic slowing down of the rotation neutron stars can rotate at high speeds but they must slow down because they're losing energy and the kinetic energy they're born with is all the energy they have so if they're losing energy in the forms of radiation or other types of energy particle emissions, they must slow down
18:23
we expect that to be happening and here, here, here you see it however, there was a discontinuous change around about late February when the period suddenly decreased in other words it looked as though the neutron star was suddenly spinning a bit faster but what does that mean?
18:41
everybody knows in physics that the angular momentum of an isolated body and you can't have a body more isolated than an astrophysical neutron star has to maintain angular momentum but it suddenly goes a bit faster well the answer is, it has a complex structure if you have angular momentum distributed through the star and there may be differential rotation
19:02
then you can couple angular momentum from one part to another and reproduce effects of this sort and that is what we believe is going on in this case so how do we actually understand this increase of periodicity? increase of spin rate well, some elementary facts
19:21
about quantum liquids which are very well understood from studies of materials like liquid helium close to absolute zero temperature if you have a quantum liquid it has zero viscosity you simply can't stir it when you have a cup of tea or coffee you simply put a spoon in and swivel it around and give the material angular momentum
19:42
that's not possible with a quantum liquid with a quantum liquid a quantum liquid can't contain sources of angular momentum put a little more mathematically this means that the fluid flow the velocity of the flow, v curl v must vanish which means, looking at it rather crudely you can't stir a quantum liquid
20:02
well, what happens in the neutron star? of course it has high angular momentum before it's enduring the collapse and as it becomes a super fluid what then happens? because it is beginning before it is rotating before it becomes a super fluid well, the physics of that is that you get the creation of vortex lines
20:21
you get little vortices of microscopic scale containing normal fluid which has angular momentum which is quantized you have quantized little quantized tubes of angular momentum and if you just have an isolated tube the flow around it would have to be the flow velocity, the circulation the velocity would have to be proportional to one over r
20:43
and such a system is allowed because the curl of that vector field vanishes so if you originally had a spinning liquid and it becomes a super fluid you get an array of vortices like this set up within the liquid such phenomena have been observed of course in terrestrial physics laboratories
21:01
magnetic flux is also contained in magnetic flux tubes something like this well in a neutron star you have the super fluid interior and the rigid crust now these vortex lines can pin just in the same way as solid state physics they can pin to nuclear sites
21:21
so as the star gradually slows down you must have the array becoming less and less dense you need less and less vortices and that you could reproduce by having the vortices move steadily outwards if these vortices move gradually outwards as the star spins down then that is what you need
21:41
to reproduce macroscopically what is going on in a massive rotating star gradually slowing however, as it slows if the vortex lines are pinned rigidly to the surface here then of course you set up a strain these vortex lines begin to bend they want to move outwards
22:00
but they are held back by the pinning and it could happen, for example that a batch of vortex lines wanting to migrate steadily outwards actually breaks this surface here or it could come uncoupled in other ways what I've drawn is a rather approximate model of course these vortex lines actually thread up
22:22
into the lattice here and it's a more complicated situation than I've sketched but you can have a sudden discontinuous migration of vortex lines outwards and of course when they reach the outside shell they impart their angular momentum to that and can actually spin it up a little bit
22:41
they can speed it up so it's processes of this kind which actually tell us that we are dealing with a complicated rotating object and if you look in detail and as much work has been done by solid state physicists particularly in the United States on this problem you can account and understand
23:01
most of the observational phenomena it's rather difficult to explain this in detail but when you watch carefully the timing of pulsar then you get these periodic you get these changes of pulse rate suddenly and then they tend to creep back they relax back to a state
23:21
near the original spin rate not quite but maybe near it this is a relaxation phenomenon it has a time constant time constants vary from days to years according to the actual structure of the neutron star and these time constants can be measured by physical observation
23:40
they come out about right without a super fluid in the center of a neutron star you would not reproduce the right relaxation effects coupling would be instantaneous if you had anything but a quantum liquid so that you have very good evidence here that this actual structure of a neutron star exists and you can use it to study how super fluids
24:01
at a density of say 100 million tons per cubic centimeter would actually behave so there's a great deal of useful physics you can do here of course not all pulsars do this and I'll have more to say about the really steady rotating pulsars later on because you can use them as clocks now that part then is
24:22
fairly nicely confirmed by observation but why is it that neutron stars actually emit radio waves we don't really understand this as well as we should I think we're near a solution now but we don't understand it terribly well but we have to produce a beam of coherent radiation given a rotating
24:42
ball of neutrons with a powerful magnetic field well some elementary physics helps here you know that if you take a normal cylindrical bar magnet in the physics lab and spin it then you'll get a voltage between the center of the magnet and the outside if this is a slipping conductor here then with simple lab experiment
25:02
you can generate a millivolt or something of the kind do the same thing with a neutron star you've got a magnetic field of 10 to the 8 Tesla you can spin it up to speeds of up to several hundred revolutions per second and you get an enormous potential difference between the pole
25:21
and the equator can be as large as 10 to the 18 volts maybe even slightly more depending on the neutron star so what does that do that potential difference well if you had a symmetrical case like this you would get charges forming on the surface of the star and that gives rise to a magnetosphere charges are literally flung into space
25:41
by these forces could I have the next slide please you in fact will get around the star a charge separated magnetosphere of this kind extending many perhaps up to several thousand kilometers according to the spin rate from the neutron star you're going to get a charge magnetosphere
26:01
in which some charges are positive and some charges are negative and they will be trapped in the magnetic field except near the magnetic poles where particles can escape to infinity along field lines which never return to the star this happens because you must pass through at a certain distance something called the velocity of light cylinder
26:22
where if the magnetosphere continued to co-rotate like the magnetosphere of the earth or the magnetosphere of planets you would be trying to force material at speeds greater than c and that is not allowed so you must get some sort of a wind and particles breaking off into space just like that well within that general scheme
26:42
you can begin to understand why pulsars should radiate if you take a rather more complicated model where you tilt the magnetic field sideways you've got that magnetosphere around the star then mostly the magnetosphere actually short circuits the huge electromotive force
27:00
that I described but close to the magnetic poles this won't happen because material is escaping along open field lines and it doesn't as it were short circuit all that potential difference well in such a region you have a powerful acceleration and charges finding themselves in that region will be accelerated in the presence of the magnetic field
27:20
they will emit gamma rays the gamma rays will form electron-positron pairs the electron-positron pairs will be further accelerated they will emit gamma rays and this process is a cascade, a kind of avalanche and you can get copious electron-positron pair production above the poles which forms an electron-positron plasma
27:41
well it will of course be a relativistic plasma and one has to consider relativistic mass of particles and so on, it's not easy to work this out but a wind in that plasma this plasma is probably flowing outwards at somewhere near the speed of light plasmas are notoriously unstable phenomena as we all know from terrestrial physics
28:01
charge bunching can take place and these charge bunches can emit coherent radiation by the process of curvature radiation they're guided along the magnetic field and they cause radiation because of their acceleration well there are many details you can fit on to a model of this kind I've only dealt with that very very crudely
28:22
but that in principle is how we get radio emission from a pulsar one of the more interesting pulsars which was mentioned by Professor Ramsey this morning is rather famous now it was discovered around 1974-75 by Professor Taylor and his colleagues now at Princeton
28:40
it's a binary pulsar in which you have two neutron stars orbiting each other with a period of about 7 hours just over 7 hours the orbital radius here is about the same as the solar diameter so it's a very compressed compact object and it's a wonderful tool laboratory tool for testing general relativity
29:01
one of the things you can observe very clearly because this pulsar is an accurate clock you can use it for timing and calculations of this orbit with very high precision is orbital precession one of the classical tests of general relativity was the precession of the orbit of the planet Mercury Mercury precesses
29:22
because of general relativistic effects by about 1 arc minute every 100 years a very small effect but detectable this pulsar orbit here is changing by about 4 degrees per year it's an enormously more powerful effect and the orbital precession is highly measurable and that checks general relativity
29:42
very very accurately doing the full relativistic calculations such a system ought to be radiating gravitational wave energy that is a prediction of general relativity but as you know gravity waves have not yet been detected by actual gravity wave detectors however if that system is losing energy
30:00
by gravity waves then the orbits the binary orbit ought to be shrinking because it's losing energy and therefore getting a little bit faster well professor Taylor has been measuring this orbit ever since the discovery of that pulsar in 1975 and here you see the timing measurements where you're looking
30:22
at the difference from the systematic behaviour such as would be accounted for by gravitational waves there's an orbital phase shift telling you the orbit is actually shrinking the binary orbit is getting more rapid now those points
30:40
are the observations and the curved line through it is the prediction using Einstein general relativity theory if the system is radiating gravity waves according to general relativity and the fit you see is absolutely superb if you're a physics professor looking after students who are learning physics in the lab and they fit points to a curve
31:02
and they come with results like this you're a little bit suspicious you think maybe they're too good they've been adjusting the points a little to come down onto the curve here but professor Taylor is a skilled observer he's not cheating, and this I think is really certain, I would regard
31:21
it as 99.99 percent confidence that we really have to be detecting gravity waves well, professor Ramsey this morning was talking about pulsars as accurate clocks and I would just like to show one more overhead here how good really are they if you squeeze them hard well, the first pulsar I showed you
31:42
on my viewgraph, the millisecond pulsar is an extremely accurate clock and you can start comparing it with atomic time, with the banks of cesium clocks that professor Ramsey was talking about and here you see over the years for which that pulsar has been available, the comparison between pulsar time, if you like and atomic clocks
32:01
and if you compare the pulsar with one set of atomic clocks maintained at the National Bureau of Standards in Boulder, you can see the scatter here, you can see the residual errors in comparing those two clocks and there's a certain scatter here, and you can see systematic variations and in fact they're not agreeing perfectly as you might expect, the scatter
32:22
is random, if you compare the pulsar with the world's best clock that's to say, you put together all the cesium clocks in the world and adjust them to give the best mean rate of time, as professor Ramsey explained, this is carried out in Paris that's the world's best clock and if you plot the millisecond pulsar against that I think any physicist would agree that the scatter of points here is more systematic
32:41
and more satisfactory than comparing, than the comparison with one set of atomic clocks so it looks as though the pulsar is at least as good as the best atomic clocks available when you put them all together well is it better? I wish we knew I think we will know in a few years time we need more millisecond pulsars if you have clocks, the only way to know
33:01
which clock is better than another one is to stick them all together and see which ones agree and which ones disagree, if we find the pulsars are giving better residuals a greater accuracy than the atomic clocks, then we know we're winning of course atomic clocks are getting better as you heard from professor Ramsey but the neutron star clock, the pulsar clock has a certain advantage
33:22
it will be up there for a million years it is not subject to funding problems and who knows maybe next year we shall be setting our watches by looking at the sky well I've outlined some of the some of the really interesting
33:41
exciting physics you have to consider when you're looking into the sky at what goes on in astrophysics there are many challenging problems waiting to be solved out there it's a field where we want the best scientists with new ideas for both observation and theory and I hope that some of you out there in the audience will in the future be helping in this wonderful quest
34:00
thank you very much
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