Extreme Physics in the Sky
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Lindau Nobel Laureate Meetings104 / 340
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00:00
Centre Party (Germany)Effects unitDensityMaterialColor chargeStarFlashtubePlasma (physics)Model buildingRotationField strengthTire balanceCartridge (firearms)TypesettingFunkgerätElectronPositronParticleRadiationDisplay deviceMine flailGround (electricity)Tesla (unit)Stellar atmosphereWavelengthMagnetizationAngeregter ZustandOrbitSpare partAstronomerSizingForceBallpoint penCocktail party effectScale (map)Leakage (electronics)AmplitudeElektronenkonfigurationVakuumphysikNeutron starMeasurementGeokoronaSource (album)Planetary nebulaMonthTemperatureProzessleittechnikAudio frequencyYearForgingRutschungArbeitszylinderDirect currentIrregular galaxyEmissionsvermögenDayRoll formingIntensity (physics)Orbital periodCosmic distance ladderReceiver (radio)Rail transport operationsColorfulnessVideoMicroscopeSubwooferGentlemanField-effect transistorBird vocalizationAM-Herculis-SternAtomhüllePlatingGenerationDipolQuantumElectricityLinkage (mechanical)Spannungsmessung <Elektrizität>ScatteringTransfer functionPlasma (physics)LightBubble chamberAtomismSunlightRRS DiscoveryStellar windIonLadungstrennungUniverseHochfrequenzübertragungSynchrotron radiationSkyGamma rayPair productionRadio telescopeTelephoneOrder and disorder (physics)Alcohol proofSpacecraftTin canAstronautIPadGreyMassSpeed of lightSuperconductivityParticle physicsThermodynamic equilibriumCylinder headLiquidData conversionPIN diodeAtmospheric pressureFoot (unit)Voltaic pileSwitcherFusion powerDrehmasseShot peeningCondenser (heat transfer)Nuclear reactorNanotechnologyCentre Party (Germany)NightNeutron starBlack holeHypothetisches TeilchenStellaratorCondensed matter physicsEnergy levelExotic atomReaction (physics)Interference (wave propagation)Atomic nucleusRadio astronomyShock waveQuantum Hall effectWhite dwarfAir compressorNuclear fusionSupernovaGround stationKelvinAschenwolkeGravitational accelerationRestkernAstronomical objectSolidOceanic climateFormation flyingCrystal structurePsyche (psychology)FeatherStrangenessHourFahrgeschwindigkeitSchwache LokalisationWolkengattungDuty cycleDaylightProfil <Bauelement>Spread spectrumSeparation processCamera lensBending (metalworking)Band gapBlanking and piercingFuelWeightGalaxyAccelerationRotationsenergieMachineNoise (electronics)LimiterCell (biology)Quantum fluctuationTelescopic sightKette <Zugmittel>MechanicMetalWind waveGaussian beamBroadbandConcentratorSpantSynchrotronHot workingSteckverbinderAngle of attackRelative articulationWeekElectric generatorSeries and parallel circuitsScrew capMagnetosphereFACTS (newspaper)Enigma machineBill of materialsVoltageZirkulatorFood storageNear field communicationPulsarTerrestrial planetLastCoulomb's lawFunksenderMagnetic coreVortexHose couplingSeeschiffFuse (electrical)SensorQuadrupoleController (control theory)Gas balloonInertiaBohr, NielsStroboscopeElectric beaconBelt (mechanical)FlywheelSchubvektorsteuerungViolet (color)NeutronDampfbügeleisenFermionPionPlanck unitsMesonHyperonMinuteProgressive lensMeeting/Interview
Transcript: English(auto-generated)
00:12
Ladies and gentlemen it seems to me that there are two major frontiers, major barriers to
00:22
our understanding of physics at the present time. One of these occurs on the sub-microscopic scale and we can refer to this as the inner space of fundamental particle physics. This region we can study in the
00:45
laboratory using high-energy particle accelerators although the techniques are becoming ever more complex and costly. The other frontier occurs at the other
01:04
end of the scale of length. It occurs in outer space and concerns the large-scale physics such problems as the very origins of space and time themselves, the
01:22
life history of galaxies, the evolution of the universe. This region cannot be studied in the laboratory. We have to extend physics beyond the walls of terrestrial laboratories and we have simply to observe the interplay of
01:44
phenomena on a cosmic scale. The disadvantage of this is that we cannot of course arrange our experiments as in a laboratory. We cannot adjust the
02:01
conditions. This is the major disadvantage. On the other hand the advantage is that we have at our disposal a range of parameters far beyond anything which can be achieved here on Earth. So our problem is to
02:22
extend physics in this indirect fashion and to extrapolate what we know in the laboratory to the limits of space and time. Many phenomena occur which are quite different from anything we have previously understood.
02:43
The effects of curvature of space-time in general relativity predicted by Einstein are just detectable with the best modern techniques in our own laboratories. The effects of space curvature can become quite dominant
03:05
when we examine nature on a large distance scale. Space and time can, given the concentrations of mass one finds in outer space, close up. We can create
03:22
bubbles in space-time which are regions in which phenomena are completely removed from our own universe. There is an event horizon. I refer of course to what we popularly know as black holes. Recently there have been very exciting
03:46
advances in this field. There has been made a connection between quantum electrodynamics and general relativity which seemed to me to be a close
04:03
parallel to the advance we heard about from the lips of its inventor Professor Dirac when he connected quantum theory with the more limited special relativity of Einstein. We know what advances followed from that link
04:23
and it seems to me that corresponding advances in the understanding of the large-scale universe may follow from the link between quantum physics and general relativity which is now being made by such scientists as Stephen
04:43
Hawking. These are features of large-scale physics which I'm sure will be most fruitful and we shall be hearing much about these in the years to come. However in my talk today I wish to be more humble and to
05:01
consider problems involved when matter reaches conditions under compression which make it far different from the matter we know in the laboratory. Cosmic forces the gravitation one can achieve inside massive stars is
05:25
to compress material until atoms the simple atoms are literally crushed out of existence and we achieve a new state of matter in which the density
05:40
can reach values as large as 1,000 million tons per cubic centimeter. The possibility of condensed matter in this state was first considered as early as as 1932 soon after the discovery of the neutron particle by Chadwick. To my
06:08
understanding the first discussion was made in in Copenhagen when news of this discovery reached the Russian scientist who was there at the time Landau and
06:23
Landau first proposed condensed matter of this kind in a discussion with with Niels Bohr and and Rosenfeld. However it was some years later that the possibility was extended and the astronomers Bader and Zwicky
06:45
predicted that perhaps one would find matter of this sort in space in the debris left behind from the explosion of a supernova. Theorists attempted to construct models of what a star would be like if atoms were crushed out of
07:04
existence and we had what was essentially a ball of material in which the density approximates that of the atomic nucleus. These models were discussed in the in the 1930s by such scientists as as as Oppenheimer, Volkov
07:26
but I think the subject really was of academic interest until quite recently because there was no evidence that such material really did exist. The
07:40
first evidence that stars of this kind could occur was obtained when we made the discovery of pulsars in in Cambridge in 1967. Now just to remind you of the of the basic features which lead us to the conclusion that
08:04
matter of this enormous density is a reality I should just like to run over the basic evidence which the pulsars give us. May I have the first slide please. The pulsar phenomenon as you all know is that one receives in the
08:27
radiation at radio wavelengths with a radio telescope one can receive from certain objects in space a regular succession of pulses as you see displayed there. Radiation of this kind was of course totally unpredicted and it
08:45
seemed at first very strange that there should be objects in the sky which produce radiation in a very regular succession of pulses typically one second apart and of duration a few hundredths of a second. The remarkable
09:04
feature of these of these sources which was realized very quickly was the incredible accuracy with which these pulses are maintained. Careful measurements show that in all cases except perhaps one or two where the
09:24
extent of the effect is not large enough to detect in in almost all cases one can measure a slowing down in time of the pulse rate. It takes generally a long time for anything noticeable to happen but if you were to wait in the
09:44
case of this source I display on the slide if you were to wait approximately 100 million years and make observations again the pulse rate would be a roughly half what it is today. Now in most cases one detects these strange
10:04
objects with radio telescopes but there is one example a famous example where a pulsar can actually be seen and I would just like to show you that because after all seeing is believing. May I have the next slide. This object
10:22
that you see is one of the most famous supernovae known to astronomers. What you see is the remains of a star which has exploded and that explosion was witnessed by Chinese astronomers it was witnessed and documented in the year 1054 when the star was visible in broad daylight. What we see now is an
10:46
expanding cloud and the object of interest is this star here near the center of the nebula is a pair of stars and it is the bottom right-hand star which is is the pulsar. That star has been known for many years to
11:03
astronomers but only after the pulsar discovery was it realized that this star here is flashing its light with great regularity at a rate of 30 cycles per second. If one puts a stroboscope inside a telescope one can display this
11:23
effect and may I have the next slide with a stroboscopic technique and this is an enlarged picture. These are the two stars at the center of the of the nebula here. The bottom right-hand star is is the pulsar and and and we see here one frame another frame taken a few milliseconds later this
11:46
this star here as has totally vanished its radiation is completely extinguished so that star is we know flashing at a rate of approximately 30 flashes per second. Now how does one account in broad terms for for this phenomenon? Well in
12:05
the early days and I won't bore you with early theories there were many ideas but now the only theory which has stood the test of time is that this radiation can only be explained if we have a star which is really acting like a
12:21
lighthouse beacon a star which is rotating rapidly on its axis and producing a well-defined beam of radiation. May I have the next slide please? The type of phenomenon which we believe accounts best for the pulses is the situation perhaps like this you have some star which produces a
12:44
well-defined beam of radiation that beam as the star rotates circulates around the celestial sphere and any observer who lies in the right belt of latitude with respect to that star will observe a flash each time the star
13:02
rotates. This is a rather simplified model it is probably more realistic to suppose that the star really is producing a beam along some axis which perhaps does not coincide with the rotation axis and that beam produces flashes for any observer in two regions of space. Now the basic problem is to
13:25
find some star which can spin sufficiently fast to account for the observed flashes. This is the fundamental problem and the difficulty is to find a star which will hold itself together at the enormous rate of
13:42
rotation which this phenomenon demands. All stars are bound by gravitational forces and if you spin a star on its axis too fast then it simply flies to pieces like an exploding flywheel. And until the the pulsar discovery the
14:00
most compact star known to astronomers was the white dwarf star that's a star approximately as large as the earth and stars of such matter stars of such a kind can rotate once every few seconds without disrupting but they cannot spin fast enough to account for the most rapid pulsars
14:24
that we observe. The only possible candidate is a star in which one finds matter in the neutron state which I shall be discussing and such a star can spin at any speed up to several thousand revolutions per second the
14:42
gravitation is sufficiently strong to hold it together and that is the only known star which theoretical star which which could produce the pulsar phenomenon. Well there is much evidence and I won't go into this in detail there is much evidence to confirm this conclusion which was reached it was
15:00
indeed suggested in the pulse when the pulsar discovery was made that such a star was responsible but there were many other possibilities too. The conclusion that the star must be a neutron star was confirmed within roughly two years of the discovery and this theory is now generally accepted. I want you to understand something about this strange behavior of
15:25
matter when it reaches the densities one finds in a neutron star and to do this I would like to come back to some elementary physics because the properties of matter under extreme compression can really be predicted with
15:41
some precision from what we already know of the behavior of fundamental particles. May I have the next slide? I have sketched on this slide very schematically the behavior of matter common matter under compression and we start with matter as we know it perhaps a lump of iron or something of
16:04
that kind and on a simple model we know that atoms are composed of a electrons moving in the region surrounding that nucleus moving under the under the laws of quantum mechanics. This phenomenon is well known to radio
16:30
astronomers it is the kind of difficulty we have in our experiments and that noise I have frequently heard on a radio telescope and it can come
16:42
from agricultural machinery at a distance of 20 miles. To return to common matter the distance at which one finds electrons from the nucleus is
17:02
determined by the most elementary property of fundamental particles that particles need to be associated with a quantum wavelength and in the very first suggestion by de Broglie of course we remember that the quantum wavelength depends upon the speed at which the particle is moving. The
17:24
wavelength of a particle is determined by Planck's constant divided by the momentum of the particle. Now in ordinary matter the electrons are at a distance roughly speaking of one angstrom from the from from the
17:40
nucleus and this defines the density of common material around us in in in the world. The atoms in a solid are roughly as close together as the electron orbits will permit and it is the value of of Planck's constant which essentially determines the state of matter as we see it around us and it is perhaps hard
18:04
to believe that the common matter is virtually empty space there is an enormous amount of of empty space within an atom because the fundamental particles are a great distance apart. Now what happens when we put such matter under some compression and force it force it together. Now these
18:24
experiments one can only do on a very small scale in a in a terrestrial laboratory but if you if you allow cosmical forces to apply an ever increasing pressure to ordinary material one can bring about some remarkable changes. As you squeeze the particles as you squeeze the atoms together then of
18:45
course the electrons have less space in which to move and from quantum mechanics we see that we have to shorten the wavelength of those particles to fit them into the available space and as we shorten the
19:01
wavelength we must increase the velocity of the particles that is the elementary fact of quantum mechanics so as we squeeze these atoms together the electrons will move faster if we squeeze the matter until it has a density which we can never achieve in the laboratory but say a density of one
19:22
ton per cubic centimeter at that density the electrons require such a small wavelength to fit into the available space that they are moving so fast and they are that they are no longer trapped in orbit about particular nuclei the electrons move freely amongst the nuclei all the
19:44
electrons in any atom one can consider they are dissociated from one particular nucleus and this state of matter we call degenerate matter a few electrons degenerate electrons we understand in in in the physics of ordinary metals but under this compression all the electrons will become
20:03
degenerate and that when matter has reached a density of of 1 million tons per cubic centimeter then we have the state in which all the electrons move freely through the material now the electrons are need need to move very fast to do this the electrons have speeds which are approaching the
20:23
velocity of light itself and if we compress the material still further then a remarkable change takes place the matter here is largely composed of positive and negative charged particles with of course some neutrons present also the fundamental particle the neutron is basically composed of in
20:48
a simple model a proton and an electron with opposite charges when one creates free neutrons in a reactor such as the reactor we heard about at Grenoble earlier this week when one creates neutrons in a reactor the
21:05
normal condition is that that neutron will rapidly change within a few minutes into two charged particles the proton and the electron plus other particles to maintain the balance now this lifetime as I say is short a few
21:24
minutes only but in matter which is sufficiently compressed it is energetically more favorable for protons and electrons to combine to form neutrons in other words the reaction proceeds from left to right nature always chooses if it can a state of minimum energy and it is not
21:46
difficult to calculate that the energy of compressed material is less when you combine the protons and the electrons into neutron particles this is because the high energy that you require to fit electrons into a small volume
22:04
causes the energy on this side to be greater than the energy on this side unless you have the conversion of protons and electrons to neutrons so progressive compression of this material will eventually lead you to an
22:22
equilibrium state in which matter contains mainly stable neutrons always of course one has a fraction of charged particles present but that fraction will be small in general now the neutron material has a density
22:40
which is approximately that of the atomic nucleus and so we reach finally a density of 100 or perhaps even a thousand million tons per cubic centimeter and that is what that is the density of this material we find it hard to to visualize this this number physicists are quite quite happy
23:03
to juggle large numbers and of course they don't usually try to imagine what the numbers mean it's not it's not a physical it's not a sensible physical thing to do but in explanations one likes to know roughly how big things are and I can only say that if one had a spoonful of neutron star
23:23
material a spoonful of neutron material of this kind would contain enough matter to build all the ships that are currently sailing the ocean now this is elementary physics essentially and is the result of an idealized experiment where we might just simply compress material on a laboratory bench
23:44
now we witness phenomena of this kind when we look into the sky this basic physics has great relevance to the evolution and the life cycle of common stars may I have the next slide please in a slightly oversimplified way one can
24:05
sketch quickly what is likely to happen to most of the stars that we see in the sky if we take a common star like the Sun which is sketched at the
24:27
reactor in which we are fusing the simple nuclei to more complex ones with the release of energy the star has the size we see because it is a balance between the pressure generated inside by the fusion reaction which tends to
24:44
expand the star and the force of gravity which contends to which which tends to compress the star so a star is a battle is a battleground between the outward pressure and the inward force of gravity and that determines the size at which we see it as the reaction proceeds and the fuel is depleted
25:01
eventually we are left with the ashes of the nuclear fusion process and gravity the force of gravity will then squash the star irrevocably and we end up in the case of a light star with a small object in which we have degenerate matter with a density of about one ton per cubic centimeter and
25:23
this is the white dwarf star and we see many such stars in the heavens but if the star is heavier to begin with than the Sun we cannot have such a peaceful passage to to old age and and retirement in the case of a heavy star what happens is that within the interior where of course the action
25:43
takes place as the fuel is depleted and gravity compresses it we can have that conversion of electrons and protons into neutrons and when this takes place we remove suddenly the cause of the pressure within the star
26:01
which maintains it at its at its equilibrium size so it is rather like having a balloon and pricking it with a pin in the case of a heavy star as the conversion to neutrons takes place there is a dramatic decrease of internal pressure and the inside of the star can simply collapse rather suddenly
26:25
it collapses from the size of a white dwarf to something much smaller in a time of the order of one second or less so the final evolution of a star can be quite dramatic the the collapsing fusion products will only reach
26:42
equilibrium when the material has become almost pure neutrons at a density of 1,000 million tons per cubic centimeter in that condition it occupies only a small volume very close to the center of the star and since the material has really fallen from the edge of a star to the center
27:04
it is moving very fast under the scale of the collapsing core in a star of the size say 10 times as massive as the Sun the collapsing material is falling inwards at nearly the velocity of light and when it collides as it were at
27:22
the center here the situation can can be quite complex but there is an enormous release of energy and of course that it generates a shock wave which propagates outwards from the center and will blow off into space the remaining material which has not yet had sufficient time to collapse now this
27:42
is believed in very brief outline to be the theory behind exploding stars the supernova process so we expect neutron material to be left behind at the center of a stellar explosion that's one possible end to a fairly massive
28:00
star stars that are heavier still can have even more dramatic evolutionary cycles and finally can collapse into these bubbles of space-time that I mentioned the the black holes of general relativity but in that case the star would collapse until it disappeared from view entirely and it is
28:22
only some quantum effect of the type which is now being considered which I mentioned which can save matter from collapsing to to zero volume this is a region of physics which is not yet well understood this however is where we are concerned at present we expect to find a neutron star a neutron material
28:42
left at the center of a stellar explosion and you can regard this ball of neutrons as the ashes of one type of stellar evolution the material will be completely inert and it will simply retain the physical properties which it had at the moment of formation but there is nothing left
29:03
there to burn any longer there is no possibility of further fusion reactions well we can imagine what a cold neutron star might be like may I have the lights for a moment if we take a neutron star containing roughly
29:24
the mass of a body as large as the Sun then when it collapses to the neutron star configuration it will have a radius of some 10 kilometers only it will be an extremely small object and the most dramatic feature that one first
29:40
considers is of course the intense gravitation which will surround such a region of space if that represents the surface of a neutron star then we have that the the gravitational acceleration which I'll call G neutron the
30:00
gravitational acceleration is approximately 10 to the 11 times stronger than the gravitational acceleration at the earth this would be very noticeable if one got close enough to the neutron star to make experiments if you lift an object through a height of one centimeter and allow it to
30:25
fall then it will reach the surface of the neutron star here the velocity after falling one centimeter is going to be something like 400,000 kilometers per
30:42
hour the weight of a tiny object like a feather would be hundreds of tons the effects of space curvature which are normally only just detectable in the laboratory become quite prominent it's not of course very safe to to walk about on on a neutron star even if it's totally cold because the gravitation
31:04
will cause your weight to be several millions of tons and so of course you're spread very thinly over the surface but we can perhaps attempt to escape such effects by being in orbit in a spacecraft around around a neutron star
31:25
we customarily consider astronauts as as weightless when they are following their geodesic in four-dimensional space and are in orbit about some mass well this is a possibility the spacecraft would have to orbit a neutron
31:42
star roughly 1,000 times a second in order to maintain a stable orbit but we can also consider the effects of space curvature on the astronaut himself the fact we tend to forget is that suppose we have our astronaut here
32:02
supposing he's just got outside his spacecraft to look around as of course has been done already in orbit around the earth what we have to remember is that it is only the center of mass of the spaceman which is which follows this weightless geodesic now the effects of space curvature are quite
32:21
noticeable because in an extended object there is a different gravitational acceleration at your head and at your feet and the effect of this is to put a force between your head and and your toes this is this is just this is just one phenomenon of space curvature and in the particular example of a
32:42
neutron star and a typical specimen of humanity here then this this force is something like 300,000 tons so space curvature becomes not a marginal effect but extremely unpleasant well I'm not joking entirely here because
33:04
in the early days of neutron star physics it was considered that there might be remnant particles of the original system in orbit about the neutron star and this might have led to some of our pulsing phenomenon it is not of course possible to have large regions of ordinary material
33:22
anywhere near a neutron star space curvature simply disrupts the material by immensely strong tidal forces well on a slightly more scientific level may I have there's something evil about this spot may I have the next slide please now the state of matter under extreme compression is of course an
33:45
extrapolation of the field of solid-state physics and solid-state physicists have been very busy in attempting to work out all the possibilities of of the structure of neutron stars now there's still much to be learned here but the general picture seems fairly clear we have we
34:04
have a ball of neutrons which is which is roughly 10 kilometers in radius but of course near the surface the gravitational compression is not yet strong enough to compress all the material into the neutron configuration and we would expect near the surface a shell of the most stable nucleus
34:23
known which is Fe 56 we expect therefore a skin of iron around the neutron star as we descend through that skin we come then to a shell of initially degenerate material where the density is still not high enough for
34:41
many neutrons to be present we have a very rigid lattice of material here with probably a melting point of greater than 10 to the 10 degrees Kelvin a very rigid material of degenerate electrons flowing between a rigid lattice of positively charged nuclei as we as we descend lower down
35:03
more and more neutrons are formed the nuclei become neutron rich the nuclei eventually become unstable and as one proceeds far enough down in into the material one then reaches virtually pure neutrons now the properties of
35:21
this of the of this neutron material are interesting because neutrons are particles we call fermions but it is believed that they will behave at any temperature below roughly 10 to the 9 degrees Kelvin very much like helium at at room temperature I'm sorry like at helium near the absolute zero of
35:43
temperature the fermions will pair up to form bosons in much the same way as in superconductors and we shall get a quantum fluid a neutron fluid in which the neutrons have essentially paired and we get then a liquid which has the quantum properties of of liquid helium but it does have also
36:04
the enormously high density now that is generally understood as one goes further into this into the center and the pressure rises the problems multiply and we do not really seem to understand yet the potential the neutron neutron potential sufficiently well to predict in detail what is going
36:23
to happen further in there is the possibility that the neutrons themselves form form a rigid lattice there is the possibility also that rather more exotic states of matter are found the pion field can become highly coherent one can have what is called a pion condensate in this region here
36:43
but these possibilities at the moment one can barely distinguish because one doesn't understand fully the neutron interaction near the center one gets graver problems hyperons maybe long-lived particles near the center of a neutron star these are of course the exotic particles of high-energy physics
37:03
which normally last a negligible time in a terrestrial laboratory it is conceivable that they would be long-lived particles at the center of a neutron star a sufficiently massive neutron star the quantum state of these particles is problematic because they are overlapping to such an extent that
37:22
the meson fields surrounding the fundamental particles are overlapping and interfering and the general quantum state of such a situation of course is quite unknown well for the moment then the important parts of a neutron star are its rigid shell which is fairly light because the material is not yet too dense and then the liquid region inside and this model is generally
37:44
believed to be correct now I have the next picture please now of course a neutron star is formed from a real astronomical body and it is not made ideally in under controlled conditions in the lab so it is endowed already with with with stellar properties which it had originally and if we
38:04
collapse a star say as with properties something like the Sun where we have a magnetic field of roughly say a 100 gauss 10 to the minus 2 Tesla radius of 10 to the 6 kilometers and a rotation rate of roughly one revolution per month if
38:22
we collapse such a star to the neutron configuration then it is probable that there will be no leakage of flux the magnetic flux in astrophysics is a quantity which is usually conserved so that we shall arrive finally with an extremely powerful magnetic field at the surface
38:42
of the star perhaps 10 to the 8 Tesla and if the star maintains also its angular momentum if that is conserved in the collapse are not given to escaping debris from the explosion then the star can be spinning at up to 10 to the 4 revolutions per second these are the properties which must
39:02
maintain a neutron star and make it detectable because as I said earlier a neutron star is not is not burning it is only endowed with the energy of collapse which is largely turned into rotational kinetic energy and its magnetic flux so we expect this lump of matter to cool down steadily in
39:23
space well there is some evidence that this general model is more or less correct from observation may I have the next slide please occasionally one observes in certain pulsars that the rate of the pulses slow shows a characteristic jump this is the famous pulsar in in the Crab Nebula the
39:45
only pulsar to emit visible light and here you see the steady slowing down represented as an increase of period with time calendar time well schematically what has been observed is this type of phenomenon where the
40:02
pulse rate suddenly shows a small increase over a few days and then relaxes to its previous slightly slower value now how does one understand this kind of phenomenon I should emphasize that the this is grossly exaggerated on the slide when the pulse rate changes by perhaps one part in 10 to the 8 this
40:24
is a notable event for pulsar observers the pulses are normally so irregular that the slightest change becomes a noticeable noticeable phenomenon now how does one explain a sudden change in the pulse rate like this well clearly you can't suddenly speed up a star which is spinning on
40:41
its own in space there must be some rearrangement of the material within it to cause this effect may I have the next slide the type of model and I'm not saying this is exactly correct but the type of model that's been put forward to account for these phenomena is sketched schematically here when the neutron star is spinning rapidly you expect it to be an oblate spheroid
41:03
because of the outward forces at the equator so you have a rigid shell of material surrounding a liquid core and the whole thing will be spinning as a solid body although the interaction between the quantum fluid and the
41:23
quantized magnetic field lines which thread the star and transfer momentum from one part of the star to another the momentum transfer probably takes place by the scattering of electrons the residual few few small percentage of electrons from these quantized vortex field lines as the star spins more
41:43
slowly when it loses energy it wants to revert to a more spherical shape and since the crust is rigid it can only do this when the crust suddenly cracks like the shell of an egg so progressively from time to time as the store as the stored elastic energy overcomes the strength of the material
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then the crust will crack and suddenly become more spherical and when it does this the outer crust will spin a little faster to conserve its angular momentum but finally there will be a coupling of momentum between the outside and the inside and the star will spin again at its approximately its original speed
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well effects of this kind are detectable and give us some confidence that the solid-state physics which has defined our model of a neutron star is mainly is mainly correct but this is a complex subject and I cannot spend too long on it perhaps the most serious problems are that we don't really
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understand the plasma physics of the space surrounding a pulsar it would be nice to know after all what it is that generates the radiation that makes these neutron stars detectable and this is a major problem which we as yet understand about which we understand very little could I have the next slide
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speaking as a physicist what we need to do of course is to solve Maxwell's equations around a rotating magnetized sphere and that rotating magnetized sphere might initially have commenced its life in a total vacuum it scarcely
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needs to be stressed that a normal atmosphere surrounding a neutron star one would expect to be non-existent the scale height of the atmosphere in the roughly one centimeter for some reasonable temperature so that one really
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expects no atmosphere one would expect a neutron star to be surrounded virtually by a vacuum but a vacuum does not generate electromagnetic radiation very easily and we must have some type of plasma surrounding this star well the basic ideas I can only sketch because as yet there is no
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exact solution to this problem when one spins a magnetized sphere in vacuo that of course on a laboratory scale is a purely classical experiment the the magnetized sphere if it is spun in the simplest symmetrical fashion with the magnetic with the magnetic axis aligned with the rotation axis will
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generate polarization charges within itself and charge will redistribute and the sphere will be surrounded by what we call a quadrupole electrostatic field that is what is well understood but when we try to extend the simple
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solution to a neutron star the numbers get slightly out of control the electric field between the pole and the equator of a neutron star is obtained by by integrating a field strength of perhaps 10 to the 14 volts
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per meter one can then have something like 10 to the 11 megavolts electric field between the pole and the equator of a neutron star and this means that the surface electric effects at the star will produce forces on any charged particles one has which can exceed even the immense
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gravitational fields which which I've discussed so the possibility is that particles are literally wrenched from the surface of a neutron star and flung into space by electrostatic forces when this happens one runs into the usual astrophysical situation which is that charged particles in space tend to move
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with the magnetic field the situation of the frozen in magnetic field which one so frequently finds so that charged particles which are in space surrounding the neutron star are essentially tied to the magnetic field
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lines and will spin with it now if one attempts to solve this problem one can write down the equations of electrodynamics and leave out of the equations any effects of the inertia of the mass of the system this is a reasonable approximation close to the star and one results with the sort of
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situation I have here one one generates a plasma surrounding the star in which the charges are virtually separated into two regions depending on the sense of rotation one can have a region of positive charge near the equator one can have a region of negative charge around the poles and this generates a
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plasma with which we're not familiar when we create plasma in the laboratory the charge balance is always perfect or nearly perfect the idea of a plasma in which the charges are completely separated is is rather unusual to us well we have this atmosphere in which the charges as I
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mentioned are as it were tied down to the magnetic field like beads on a wire and as the star rotates the atmosphere must rotate with it but one soon reaches a grave problem because in the case for example of the neutron star in the Crab Nebula at something over 1,000 kilometers from the star one
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reaches a region where the material tied to the magnetic field is trying to sweep through space at the speed approaching velocity of light now this runs into relativistic physics and the situation is not understood but
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probably the kind of effect that happens is that at this critical radius which we call the the velocity of light cylinder material can no longer be tied to the star and must escape so one probably has an efflux of particles as a stellar wind probably at a speed close to the velocity of light beyond
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that distance and within that distance one has material which co rotates spins with the star and forms a closed system well somehow within this type of model we have to explain the the generation of of the radiation which
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makes these these these objects detectable just one feature while we have the slide present some of these magnetic field lines intersect the velocity of light cylinder and they are the field lines which come from the magnetic poles from here particles can always escape from the star because they
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are they are threaded on lines which which never return to the star so near the polls one can have escaping particles along the so-called open field lines which intersect the velocity of light cylinder well now we
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have a wealth of information available in fact we have too much information available there are too many facts about pulsars to be explained and I would just like to show you some of the features which observers can feed into this this situation in an attempt to to understand the physics of this complicated relativistic plasma may I have the next slide this is an
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expanded version in time of a succession of pulses from the first pulse are to be discovered the timescale here is some tens of milliseconds from one side to the other and you see successive flashes of radiation are displayed one beneath the other and you see the profile of
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the flash of radiation from this particular neutron star it is like a lighthouse in which somebody is tampering with with the mechanism each time it goes round one gets a flash of slightly different character and the
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situation can change dramatically from one second to to another second these peaks here in the in the emission are reminiscent of an alpine range and this kind of irregularity is typical of all pulsars could I have the next slide please one can measure also such features as the polarization of the
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radiation and an important feature which shows up clearly in this example of a pulse here you see a very rapid pulse are in the southern skies the total pulse lasts a few milliseconds only and here is shown the direction of the
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electric vector in the in the radiated field and from the leading edge of the pulse to the trailing edge it shows a very characteristic rotation the radiation in this case at radio frequencies is approximately 100% linearly polarized and that of course gives us some strong clues as to
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the kind of radiation process which must be taking place in the plasma close to the close to the star the properties of pulsar radiation change so quickly that attempts have been made to display in a more elaborate way that the character of the of the pulses may I have the next slide please
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will that focus a little here you see a plot in color of a succession of pulses and one is displaying different characteristics this slide was sent to me by the operators of the of the large radio telescope at Arecibo in the United States Puerto Rico and here is a sample of pulses just one under
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the other and the first slide shows you how the intensity varies the intensity is adjusted on on a color scale there with white being most intense and and violet the least intense so that this is essentially the pulse profile measured
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across here you then get various polarization parameters plotted out successively and you can see that it really is a very complicated situation apart from the changing intensity and polarization there are features in the radiation which appear to to drift it shows up quite nicely on this on this
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band of radiation here a polarization feature which in this case lasts a few milliseconds drifts across the pulse as if it were somebody in the lighthouse moving something across the lens which is which is forming the beam this is this phenomenon is well known it occurs in many many examples and it's
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phenomenon of drifting sub pulse features well we do not understand these at all all we can do is make some guesses as to the correct explanation and to finish I will just mention very briefly the kind of model which on
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which astrophysicists are currently working may I have the next slide please the kind of type the type of model which appears to fit a fair quantity of observational data is that we have a neutron star with a rotation axis which is vertical and a magnetic axis which is oblique as one rotates
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this system then the overall character of the plasma surrounding the star will be much the same as in my simplified sketch earlier but of course one has now a fluctuating magnetic field fluctuating at the rotation rate of the pulsar and this adds further complications but the kind of model
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which seems to fit the observation is that we have charged particles escaping along the field lines which can reach to infinity so those particles can escape from the star altogether and if some mechanism can be found to create particles in bunches then as they accelerate outwards along
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this curving magnetic field near the pole of the magnetic pole of the star they will launch radiation which is akin to the synchrotron radiation one is very familiar with in high-energy particle beams this will generate
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radiation which points beams of radiation which point along the general direction of the of the magnetic axis and this can lead to the kind of shapes which we certainly observe but pictures of this kind are drawn with much much imagination and we what we have to do is work out in some detail
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why the electric charges should be bunched and just how many one needs of course to make the radiation fields these sort of calculations are fairly straightforward and whether the radiation has the right polarization it turns out that this synchrotron type radiation does have the sense of
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polarization to account for the drifting angle of polarization which I mentioned earlier but some arrangement where electrons move extremely coherently as in a radio transmitter some arrangement of that kind is essential to explain our radiation as I said earlier pulsars are not burning
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any fuel they only have this they are as it were a freely running electrical dynamo and the radiation they emit must come from purely electrodynamic processes may I have the last slide please theories as always tend to become exotic and the kind of exotic idea which is being mentioned
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now is that one perhaps has a pair creation process electron-positron pairs near this process occurring near the magnetic pole of a star the sort of situation would be as follows the star is rotating about a vertical axis and charged particles in the magnetosphere are escaping from the polar cap region
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where such escape is possible the escaping particles can perhaps leave at a rate which is not easily maintained by a flux of further particles from beneath the star that is a statement which I could amplify but unfortunately
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it would take us too far from the from the general scope of this lecture but one might end with a near vacuum with zero charge and escaping charges higher up in this case one will build up across this gap the voltage which occurs between the pole and the equator of the star which as I
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mentioned can be something like 10 to the 10 10 to the 11 megavolts a very powerful field exists here well a stray particle in that field will be accelerated rapidly to relativistic speed it will emit gamma radiation and in the powerful magnetic field that exists here that gamma radiation can
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create positron electron pairs