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Lisa Randall

Lisa Randall studies theoretical particle physics and cosmology at Harvard University. Her research connects theoretical insights to puzzles in our current understanding of the properties and interactions of matter. She[…]

A conversation with the professor of theoretical physics at Harvard.

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Question: What do particle physicists do on a day-to-dayrnbasis?

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Lisa Randall: So, particle physicists, we’re trying tornunderstand the substructure of matter; what’s really out there, what’s at therncore of matter, how it interacts, what are the fundamental forces.  And as you go to deeper and deeperrnscales you learn different things. rnSo the sort of two-pronged approach, we have some very theoretical workrnthat goes on and some very experimentally oriented work.  We’re trying to see what we can findrnout at the collider that will turn on again next week, we hope.  The collider that will be running forrnthe next couple of years, the Large Hadron Collider, that will collide togetherrnprotons of high energy and tell us what’s out there.  We also try to combine it together with theoretical ideasrnwe’ve been working on.  Some ofrnthem generated from more high energy theoretical ideas beyond what we can seernwith experiment and we’re trying to put those together. 

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On a day-to-day basis, we could be thinking about many differentrnthings.  We could be thinking aboutrnwhat the experiments will do.  Werncould be thinking about the theoretical idea of how they fit together, doingrnthe calculations to see what the consequences of some ideas, which we callrnmodels, would be.  There might bernsome speculations or hypotheses for what’s out there and we try to derive whatrnthe consequences would be and test whether those ideas work, test these ideasrnthat might synthesize the various phenomena that we’ve observed, but we don’trnyet understand.

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Question: What insights do you hope the LHC will yield?

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Lisa Randall: So, the LHC stands for Large HadronrnCollider.  It’s large; it’s 27rnkilometers in circumference.  It’srna big tunnel where two beams of protons will circulate around it and then collidernat various points within the tunnel. rnWhen it collides, it will collide together at high energy, about sevenrntimes the energy of the highest energy collider we have yet had on earth.  And when it does that, the idea is forrnthe mass of the particles converts into energy.

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The energy that’s created in the collision can then convertrninto new particles.  Thosernparticles can have heavier mass than anything that’s been created before, E=MC2,rnso having bigger energy means that we can create particles that have biggerrnmass.  It also will have what’srncalled high luminosity, which is to say, there’s a high collision rate.  So the hope is that by having these newrnparameters, this high rate of collision, these high energies, we’ll be able tornstudy matter in ways we haven’t been able to study it before. 

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In the process, we want to understand the answers tornquestions like; where do the masses of fundamental particle come from?  That might sound like an odd questionrnsince mass seems like an intrinsic property to matter, but it turns out that inrnthe simplest versions of the theories you’ve ****, particles don’t havernmass.  And we know that it’srnactually only possible for particles to have mass because of something like arnphase transition that happened **** universe, something called the HiggsrnMechanism, named after the physicist, Peter Higgs, who thought of it.  And the idea is that particles at highrnenergy might seem not to have mass, but at low energies they do.  And the question is; how did that comernabout?  If it’s a simple HiggsrnMechanism implementation, we’ll spy something called the Higgs Boson.  It’s a particle that tells us that thisrnis really what happens and really puts it all together.

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The standard model works to an incredibly high degree ofrnprecision.  We believe that it’srnright.  The standard model ofrnparticle physics tells us what are the basic elements of matter?  What are the forces through which theyrninteract?  It’s been tested to arnhigh degree of precision.  But itrndoesn’t answer questions like; where do the particle masses come from, thernelementary particle masses come from? rnAnd the other question it doesn’t answer is; why are those masses aboutrnthe scale that they are?  Whyrnaren’t those masses much bigger? rnThat has to do with something what we call the hierarchy problem ofrnparticle physics, which is connected to the question of why gravity is sornweak.  And those are the types ofrnquestions we hope to answer at the LHC. 

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If we’re really lucky, we might also get some insight intorndark matter.  The matter thatrndoesn’t emit light that we know is out there in the universe.  So, there’s a number of questions thatrnwe hope the LHC might help us answer. 

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Question: What is dark matter?

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Lisa Randall: As far as dark matter goes, we could be luckyrnin the sense that it seems that if you look at just the amount of dark matterrnthat’s out there in the universe, we can figure out what is called the relicrndensity is given a sort of type of interaction.  That is to say, how much stuff should be left over now thatrnthe universe is big and cool.  Andrnit turns out that if particles had mass about the scale it’s going to be probedrnat the LHC, the Large Hadron Collider, that it turns out to have about thernright relic density.  Now, thisrnmight be just a coincidence, or it might be something deep and fundamentalrntelling us that the Large Hadron Collider might actually be able to produce therndark matter particle.  It might notrnproduce it directly, but it could produce something that decays into the darkrnmatter particles.  So there is somernhope that by probing this energy, that the LHC will study, we can learnrnsomething about dark matter.

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Question: What is dark energy?

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Lisa Randall: There are various cosmological measurements,rnthat’s to say, by studying how things have evolved in looking out at what’s outrnthere in the sky, we can determine what type of matter it out there. It turnsrnout that the kind of matter we see is only about 5% of the universe.  That is to say, stuff made up of protonsrnand neutrons, the kinds of stuff we’re made of.  It seems only to be about 5% of the energy.  Then another 25% seems to be somethingrncalled dark matter, which is matter that’s just like matter in the sense thatrnit clumps together.  If we look atrngalaxies, there’s lots of dark matter. rnBut it doesn’t interact with light in the same way that the matter wernknow about does, which is why it’s dark and it’s hard to see, where I meanrnliterally mean see with light.  So,rnwe know it’s there because the gravitational effect.  And there’s various ways we’ve seen those gravitationalrneffects, and there’s various ways we’ve seen those gravitational effects, butrnwe don’t yet know exactly what it is. 

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In addition to that, more recently we became aware of thernfact that most of the energy density in the universe consists of somethingrnthat’s become known as dark energy. rnSo that is very different than dark matter.  Dark energy is just energy that permeates the universe, it’srnnot associated with any particular type of particle, any type of matter, it’srnjust out there.  So if there wasrnnothing, no particles around, there could still be this kind of energy.  And in addition to that, that energyrndoesn’t clump and it doesn’t dilute as the universe expands.  So it’s a very special type of energyrnthat Einstein first thought could be around.  It turns out it was there in the amount that Einsteinrnthought, but there seems to be this amount, this residual dark energy that’srnleft that seems to dominate the energy density of the universe today.

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Question: How did the universe evolve to where it is today? 

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Lisa Randall: We have a pretty good idea of how the universernhas evolved to today and remarkably, in the last however many years, we’vernreally been able to test the evolution of the universe, the cosmology model wernhave in much greater detail then we have ever had before.  So it’s really made cosmology into arnprecision science in a way that it hadn’t been earlier on in this century. 

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We now have something which we call the Big BangrnTheory.  Now the Big Bang Theory isrna little bit funny because it’s called the Big Bang Theory, but the Big Bang isrnthe one thing we don’t know about. rnWhat the Big Bang Theory tells us is really how things evolved at laternstages.  How the universe givenrnsome energy density which could consist of matter, radiation, dark energy.  Given those energies, how is thernuniverse evolving, what’s happening? rnSo, Big Bang Theory tells us how it evolves and we have really nice experimentalrnconfirmation.  We’ve seen that thernuniverse is expanding, we’ve seen the Hubble expansion, we’ve see the relicrnradiation left over from the early time. rnIn the beginning, it was all hot and dense, but as the universe expandedrnthe radiation has cooled down significantly, so now we see this 3 degreernmicrowave background in the sky.  Andrnin addition to that, we even have detailed predictions for the abundance ofrnvarious nuclei, which is very much connected with what happened as the universerncooled. 

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So there are really good tests of the Big Bang Theory andrnthis theory of expansion.  Thernthing is, it doesn’t tell us about the early stages.  And particularly, there was some mysterious features when wernlooked at this background radiation, and I’m using “we” very loosely, the otherrnpeople when they looked out at the sky. rnAnd it was seen that radiation was very uniform and was homogeneous andrnisotropic, it looked the same in all different directions in the sky to a very,rnvery high degree of precision; like one out of ten thousand with very smallrntemperature deviations as you looked out at the sky.  Which was very surprising because there were pieces of thernsky that didn’t even have enough time, according to the standard Big BangrnTheory, to talk to each other. rnThey’re so far away that light could not travel between them in enoughrntime.  So, some how all theserndifferent regions of the sky had to have originated in some common place.  And the theory of cosmologicalrninflation combined together with Big Bang Theory seems to be what reallyrnworks. 

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So for cosmological inflation, the idea is that early onrnthere was actually an exponential expansion of the universe driven actually byrnsomething like dark energy except in a much greater energy density, and itrndrove this exponential expansion of the universe.  Actually, later on in our evolution, we will also have anrnexponential expansion, but it’ll be a much slower exponential expansion.  But early on there was this very rapidrnphase of exponential expansion where the universe kept doubling its size, andrnduring that time, the universe grew an enormous amount, and then after itrnstopped that whatever it was that drove the expansion might have decayed, butrnsomehow the stuff that becomes our matter was created.  So, in some sense, that is really thernbeginning that we can trace.  Thernfact that there was some expansion, some inflationary stage where itrnexponentially expanded, then afterward this more conventional Big Bang Theoryrntakes over. 

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And what’s really remarkable is that there’s sort of anrnimprint of this state of exponential expansion in the sky.  If you look at details at the cosmicrnmicrowave background, it’s very much in agreement with the predictions ofrncosmological prediction.  So itrnreally does seem like this happened and it explains some features; explains thernflatness of the universe, the fact that the universe doesn’t – we can talkrnabout the geometry of it, even the three-dimensional spatial universe and itrnseems extremely flat.  And again,rnthat’s because if you think about a balloon, if you blow it up, if you blow itrnup enough even though we know over a large scale, it looks sort of round orrnspherical or oval, if you look at a small region, it looks very flat.  So, if you blow up a balloon enough atrnany given region, it looks flat, and the same way maybe we blew up the universernenough that it looks large and flat. rnSo, inflation seems to explain many features of the initial conditionsrnof how things started before this Big Bang Theory took over.

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Question: What does this research into the universe’s earlyrnhistory tell us about its ultimate fate?

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Lisa Randall: Well, what really tells us about the ultimaternfate of the universe is what we see today.  What we see today seems to be this dark energy thatrndominates.  And as I said earlier,rndark energy doesn’t dilute as the universe expands.  Radiation dilutes, matter dilutes, so although we know rightrnnow we have about 30% matter if we include dark matter and our matter.  That matter, as the universe grows,rnthat matter will become more and more dilute.  And dark energy is going to just take over at which point wernwill again have exponential expansion of the universe.  At that point, we will cease to havernmatter creation in the way we do today; star formation in the way we do.  Eventually we might have all the matterrnin black holes, but those black holes will radiate away.  So we will ultimately have, if this isrnright, a large essentially empty universe that will just keep growing.

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Question: When do you predict this will happen?

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Lisa Randall: This is not going to be the dominant thingrnthat affects any of our lives. rnThis is a projection based on what we observe today of what would happenrnbillions of years from now.

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Question: What might the LHC tell us about the existence ofrnextra dimensions of space?

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Lisa Randall: The LHC will hopefully find the HiggsrnParticle; it hopefully will answer the question about the weakness of gravityrnand why there are different mass scales in the universe.  What the answer will be, we don’t know,rnalthough we’ve conjectured what it might be.  One of the possibilities that I and others have worked on isrnthe idea that there could be extra dimensions of space other than the ones wernsee the forward, backward, left, right, up, down.  There could be other dimensions and they could be connectedrnto explaining some of these features that we can’t otherwise understand.  We don’t know that that’s right.  Other people conjecture, in fact I’vernworked on it too, the idea that it might be something called supersymmetry,rnwhich have doubled the amount of particles in the universe, but again has somernchance of explaining why there could be this huge hierarchy of massrnscales. 

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So there could be various possibilities that we conjecture.  I don’t think anyone feels 100%rnconfident, probably there are some people that do, but we really don’t knowrnwhat’s there.  That’s why we dornexperiments.  All these theoriesrnhave good aspects and bad aspects and so until we actually see how the universernworks, sometimes the universe is far more clever than anything we thinkrnof.  So these were allrnpossibilities, there’s some finer probability for all of them.  And we certainly want to make sure thatrnthe machine will test these ideas that it is capable so we’ve made the right –rnwe tell them what the predictions are, what they should be looking for. 

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One thing to keep in mind is the experiments at the LargernHadron Collider are very difficult because they are very messy.  There’s many billion events per secondrnand you’ve got to narrow that down and be able to look at the small things thatrnthe small predictions have a small probability of what could happen and be ablernto pull those out.  So, you have tornhave a very clear idea of what it is that you are looking for.  And that’s why we want to make modelsrnand tell them what they should be looking for.  What are the unique features that will identify somethingrnthat’s new and can tell us what’s really going on at these energy scales?

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Question: What is supersymmetry?

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Lisa Randall: Well the term – to really understandrnsupersymmetry requires you to know something about quantum mechanics.  Supersymmetry connects togetherrndifferent kinds of particles known as bosons and fermions.  Bosons are particles that like to be inrnthe same place, fermions are particles that don’t like to be in the samernplace.  And the conjecture is,rnsupersymmetry is that for every particle that we know about is a partner that’srnthe opposite in the sense that if the Higgs boson is a boson there will be arnHiggs Zeno, a partner that’s a fermion. rnIf there are Gath Bosons there are Gath Genos, if there are fermions,rnthere are partnered bosons.  So itrnsort of doubles the number of particles which, if you think about it from thernpoint of view of what you’ve seen experimented, is quite dramatic.  If supersymmetry is right and explainsrnwhat we call weak scale phenomena, this questions about masses and about thernhierarchy problem, then we’d be able to see a whole school of new particles whenrnthe LHC is running at full capacity.

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Question: What was the greatest “Eureka!” moment in yourrncareer?

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Lisa Randall: I mean, for any problem you work on there’srnsome moment where there’s sort of maybe it’s a moment where no one else caresrnabout, where it all fits together and I found some simple way of doingrnsomething.  There are also thernmodels that people have heard about such as the models about extra dimensions,rnsomething else that we did having to do with a different way of communicatingrnsupersymmetry breaking.  There wererna lot of eureka moments in that and making it work out and the fact that werncould all tie it together.  So,rnit’s hard for me to pull those out because a lot of the time when you’rernworking on something there’s that one thing that you get excited about.

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Question: Have you ever been completely surprised by thernresult of an experiment?

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Lisa Randall: I think one thing that surprised us, Bramanrnand I, when we were working together was this existence, or the possibility ofrnthe existence, of an infinite extra dimension.  People had actually thought there were theorems that yourncouldn’t have an infinite extra dimension, that gravity would just look wrong,rnthat there was no consistent way to do it.  And it was really interesting because we came across this byrnreally following the consequence of our equations to the end and there was justrnno mistaking that it was allowing the possibility of an infinite extrarndimension.  And it was interestingrnto go back and see what assumptions have been made that made us think, or madernother people think that it was impossible, and just to see how the theory wasrnalmost smarter than we were and worked it out.  That was kind of fun.

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Question: What excites you most about your current research?

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Lisa Randall: Right now I’m enjoying thinking about whatrnwe’ll see at the Large Hadron Collider. rnWhat is it that they could find that they’re not looking for yet?  Making sure that it’s what we have andrnmaking sure it does as much as we can. rnAlso thinking about what we can see in dark matter experiments.  Making sure, again, that we’vernconsidered all of the possibilities and we’ve looked for how to search and whatrncould be out there and to make sure that we can look for other candidates forrndark matter, you know, what can it be? rnSo, right now I’m thinking really because we are in this era where thesernexperiments are turning on, so it’s important to really make sure that we use themrnfor all they’re worth.

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Question: Apart from science, what’s an area of intellectualrninquiry that fascinates you?

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Lisa Randall: rnWell, right now, I’m interested in art and music.  I wrote a libretto for a small, what werncall a projective opera, which was kind of a different kind of way to communicaternideas to the public.  It’s not arnlinear story where we’re explaining things, but just to communicate why we carernabout exploration discovery.  Forrnme it was just an interesting experience to see what it’s like to do somethingrnthat is connected with performance. rnIt was performed at the Pompidou Center; it was performed at the OperarnHouse in Barcelona, and that was a very exciting and interesting thing forrnme. 

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I think right now it’s also a little bit interesting tornfollow what’s going on with the economy, what’s going on with various politicalrnthings and by fun -- kind of disturbing sometimes, but I think there are a lotrnof interesting questions that are lurking there that are fun to think about.

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Question: How can scientists reach out to the public morerneffectively?

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Lisa Randall: I think, I’ve obviously made the choice tornspeak to the public at some level and it’s in part because I enjoy writing andrnI enjoy being able to think creatively in different ways.  I don’t think that any one particularrnscientist is obliged to do this, to speak to people.  But I do think that as a community, if we are expectingrnresources, it’s very difficult material and I think it’s important that therninformation is out there.

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When I wrote my book “Warped Passages,” I really had in mindrnan interested audience.  I thinkrnit’s very easy to try to pander to a large audience – not easy, but – or tornwrite technical things for your audience, but there’s sort of this in-betweenrncategory of people where they’re really smart and really interested, but theyrnjust don’t know the physics yet, and I think it’s important that if they wantedrnto know what we’re doing and why it’s important and what the full implicationsrnare, then that information is out there in a way that they can access.  And I just think, generally, it’srnimportant to be able to communicate among different fields.  We live in an era where science isrnimportant to the decisions we make. rnIs what happens at the LHC going to determine our policy toward it?  Not necessarily, but being able tornthink like a scientist on some level we’ll be able to think about predictionsrnand risk and probabilities. rnThere’s some basic science training that might help people, but there’srnalso just curiosity about the world which everyone has and understanding why werncare about these questions, what it is that we’re looking for and why it’srngoing to tell us these deep fundamental properties of the universe at both arnsmall scale and the large scale. rnScales that we can directly access just by looking, that we needrntechnology for and saying what is it that we’re after?  And I think those are – a lot of peoplernwant to know that, and it’s important that they have access to that information.

Recorded on February 17, 2010
Interviewed by Austin rnAllen

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