cc  >> Used to be easier than this.  So I'd like to welcome you all  to Space Place tonight.  This is a second Tuesday of the  month so tonight we have a guest  speaker.  And I'll tell you about him in  just a moment.  I want to mention first we have  a usual raft of things that are  available here at Space Place.  I just handed out the Eyes on  the Skies.  If you didn't get one of those  there's a few more there and  there maybe a few more out  front, and it summarizes then  what's coming up.  And we also have an  extraordinary, that is an out of  the ordinary, speaker that I  want to mention.  Second Tuesdays are the guest  speaker nights, but in June this  month we're going, I mean next  month in June on the first  Tuesday we're going to have a  speaker and that's on the  occasion of a conference that's  being held in Madison on the UW  campus.  It turns out that the UW  Astronomy Department has an  expertise in supermassive black  holes. 

 Which maybe you've heard about,  those giant monstrous black  holes at the centers of  galaxies.  So they're having a conference  here on the subject of these  supermassive black holes.  And Space Place is hosting a  public lecture by one of the  researchers who will be coming  into town to do that.  So that is on Tuesday June 2nd,  and that will be right here at  7:00 PM on the first Tuesday  in June.  And then some of you may know  that we have recently, we being  Space Place and some partners,  we've recently installed a scale  model solar system in Dane  County.  So I decided that it was time to  talk about scale models and why  they're interesting, and so on  the following Tuesday then, on  the 14th, I mean the 16th sorry,  yeah the 16th, of June on the  second Tuesday, I'll be giving  the talk, sorry?  >> (inaudible)  >> Pardon me?  >> Second Tuesday would be the  9th.  >> Oh, right.  I was going two weeks.  Sorry, yes, thank you the 9th.  On June 9th, Tuesday June 9th  I'll give a talk about scale  models, not about calendars. 

 And we'll talk about scale  models and how they're useful in  understanding the universe.  Scale model solar systems and  other sorts of scale models.  So that's what coming up then in  June, other things there in the  Eyes on the Skies.  So tonight we're going to go to  the other end of black holes.  Instead of black holes and the  absence of light, we're going to  talk about light.  It's the international year of  astronomy and in planning guest  speakers for this year I thought  we ought to have a talk about  light because light is the  predominant way that astronomers  learn about the universe.  It's not the only way but if you  start thinking real hard there  aren't very many others.  They can pick up meteorites off  the ground and learn a little  about extraterrestrial matter.  But it's really light that has  been the medium through which we  learn about the rest of the  universe.  But what is light?  And that has been an important  issue for philosophers all the  way along. 

 So as background here to what it  means for astronomers to try to  understand the universe through  the use of light, I thought we  ought to have a talk about how  people, how scientists and  philosophers have tried to  understand what light itself is.  So I turned to my friend  Dr. Peter Sobol who is an  historian of astronomy,  historian of science I should  really say.  Who has given the talk, probably  more talks than anybody else has  here at Space Place since we,  over the years.  And tonight Peter is going to  talk to us about the history of  ideas of the nature of light.  So Peter's going to talk on  illuminating the history of  light.  Peter?  (applause)  >> Can you hear me?  I didn't push the button.  Buttons, buttons, buttons.  And God said let there be light.  And there was light.  And God saw that the light was  good and God separated the light  from the darkness. 

 God called the light day and the  darkness he called night and  there was evening and there was  morning, one day.  This passage has occasioned much  puzzlement over the last 2500  years, in part because according  to the creation story from which  it is drawn, the sun and the  stars are not created until the  fourth day.  So what is this light that's  created on the first day.  The effort to discover the  answer to that question has a  long and interesting history.  St. Augustine,  5th century AD,  believed that it referred to the  creation of the angels whose  creation is referred to nowhere  else in scripture.  But the subject for this evening  is not that light that Yahweh is  busy pushing away from the  darkness there.  Instead, the subject for tonight  is the light that allows us to  see.  The light that does interesting  things in shiny surfaces, clouds  and crystals, that Galileo and  astronomers ever since have  collected in their telescopes.  That transcends the categories  that we use to describe most  other things in our world, and  that behaves in more astounding  ways the closer we study it.  It's the kind of light you can  do science on. 

 And this evening I would like to  take you on a whirlwind history  of that science.  It begins in the ancient world  where the question was really  not so much the nature of light  right away but the nature of  vision.  How is it that we see things?  What goes on in that medium  between a visible object and the  eye of the observer?  One idea was that something  arrives at your eye from a  distant object.  This was called, by historians  of optics, intromission and  there's an image of Lucretius,  the first century BC atomist.  He believed that thin films were  constantly wafting off of things  we can see and approaching our  eyes, sort of like this.  So there's a ghostly image of  Theophrastus getting closer and  closer to the observer.  This theory has the benefit that  it does retain the coherence of  the visual image, but it is open  to several criticisms.  One is that by the time it  reaches the eye ball only a tiny  little bit will fit there.  How is it that we can see  distant mountains?  Another criticism offered in  those days was why don't these  things crash into each other in  the medium?  If there were a bust of someone  else up there why wouldn't a  mass of gray arrive at the eye  of the observer?  Okay, well maybe something comes  out of your eye when you see.  Maybe when you open your eyes  something flies out and goes all  the way to the stars in an  instant or an apparent instant. 

 Maybe it somehow pushes water  out of the way when you see  things that are submerged.  So although this too, this  theory will retain the coherence  of the image it has its own  problems.  There was a variation of this  which I'm calling the  coalescence theory, and there's  an image of the second century  physician Galen who believed  that out of the eyes of  observers came something that he  called optic numa, a very fine  subtle material, which would go  out and fill the medium between  the observer and the object but  on its own it could not do  anything.  It relied on illumination, and  as soon as the optic numa in the  medium was illuminated, by  sunlight in this case, it became  an extension of ourselves, in  Galen's words.  And then there was another  theory, that of Aristotle who we  have to mention because he was a  major authority figure for many  centuries afterwards.  Aristotle was impressed with the  role of the medium itself in  vision.  He did not believe that anything  was emitted from the eye.  He believed that when the medium  was illuminated that its  potential to allow visible  qualities to reach the eye in an  instant was actualized.  So I've crudely done this with  an ellipse, there's nothing in  Aristotle that says this has to  be an ellipse.  I just mean to show that under  the influence of illumination  the medium between the eye and  Aristotle's bust there becomes  able to instantaneously send to  the eye information about that  forms of grays, forms of shape,  things like that. 

 So the eye can see the bust.  Moving right along to visual  rays.  There were some people who  attempted to talk about vision  without really committing  themselves to whether things  were going into the eye or going  out.  And there were some things that  you could discover about the  visual process without getting  mired in the metaphysical aspect  of it.  And these are two images here of  the Greek mathematician Euclid.  And here are some rays.  Oh, dear, this worked when I  tried it before.  You'll have to imagine those  rays all still being there.  Euclid probably did believe that  rays came out of the eye.  But you don't really need to  think that way for most of what  Euclid and Ptolemy and other  geometrical oculists believed. 

 With these rays gone it's going  to be kind of hard to make this  point.  But were they all there you  would see that there's a  narrower angle that allows you  to see from about his eyebrow to  about the second button of his  shirt.  It's much narrower when he's  farther away than when he's up  close.  So far things subtend to much  smaller angle if you use these  rays to figure that out.  Euclid appears to have believed  in discreet rays so that they  separate as they travel from the  eye, if we take his emission  seriously.  And this would go some way to  explain why distant things are  not seen as well because they  don't intercept as many rays as  things that are close.  Ptolemy believed not in discreet  rays but in a solid cone of  vision.  So there were these minor  differences.  Euclid and Ptolemy both talked  about reflection.  They knew that the angle of  incidence and the angle of  reflection measured, with  reference to this vertical  normal line, that those two  angles are equal. 

 Really there are arrows on here  but it wouldn't matter which  way, whether a visual ray was  coming out of the eye and was  bouncing or a ray was bouncing  off a visible object and  reaching an eye.  The object would appear to be  down here at the end of this  dashed line.  And here's refraction where if  you think of the upper layer as  being optically rare and the  lower layer as being optically  dense, the ray is bent toward  the normal, the visual ray would  be bent toward the normal and  the object would appear to be at  the end of this dashed line.  Ptolemy illustrates this with  this little parlor trick where  this is a cross section of a  bowl, and where this observer is  located she cannot see the coin  or trinket that's down in this  bowl.  It's below the lip of the bowl  there but fill the bowl up with  water and the ray is bent and  the coin will now appear to be  here.  The coin will have been revealed  just by adding water to this  bowl.  The science of ancient Greece  was received with enthusiasm by  the scholars of Islam.  This is a postage stamp from  Yemen commemorating the  philosopher Al-Kindi.  Al-Kindi was an emissionist.  He believed that rays, visual  rays were emitted from every  spot on the cornea. 

 This dense line here is meant to  represent a cornea.  And from every spot on that  cornea, rays go off in all  directions.  Al-Kindi was not bothered by  whether they would collide; he  just assumed that this was stuff  that couldn't suffer collision.  These lines are meant to  represent where these rays will  cover.  So this line here which is  tangent to the very end of the  cornea up there, that means that  from this point out, from this  line out in this direction, that  area is all covered by rays that  are emitted from here.  Just so, this line indicates the  area that's covered by rays  emitted from this end of the  cornea and from the middle of  the cornea, this vertical line  shows the backward limit of the  area provided with rays from  there.  And we will see the stamp of  Al-Kindi very well because all  three areas overlap right here.  A later Islamic scholar Ibn  Al-Haytham, known to the west as  Alhazen, did not believe in  emission.  He was an intromissionist.  He believed that things enter  the eye when you see. 

 Why would bright lights be  painful if your eye reached out  to see them.  Why would we see after-images?  So he believed that but he also  retained Al-Kindi's idea that  things just radiate, things send  something of their nature out  into the medium.  So this image of Alhazen is  sending rays out all over the  place.  And over here we have an eye  ball with the lens located right  there.  It was known in those days as  the glacial humor.  Arabic scholars were very  interested in ocular anatomy and  did many studies of the anatomy  of the eye.  So the problem for this viewer  now is there's too much  information coming toward it.  Not as with the old intromission  theory where just a little bit  reached, now we'll have too  much.  Here's a pair of rays from each  of three spots on this image.  And yet to make a coherent image  you want only one of those rays  to get through and represent  that point on the image, and  what Alhazen does is realize  that only one ray from any given  point on that object will be  perpendicularly incident on that  lens. 

 So he believes that the lens  only allows those rays to pass.  Those will be the strongest rays  that pass.  They will be the ones that  contribute to the image formed  at the back or perhaps carried  through the optic nerve into the  brain.  There was lots of scholarship  done in Islam and that sort of  in the same way Islam got the  science from the Greeks, the  Latin west got science from both  the Greeks and the Arabs.  And it came in little by little  beginning in the 12th century,  beginning of the 13th century,  there was enough there to begin  to make up the curriculum in the  new universities in Europe.  And one of the first people to  see these works was a man named  Robert Grosseteste, the Bishop  of Lincoln.  Died in 1253.  He was very interested in light.  He thought light was quite basic  to the structure of the  universe.  In fact, he wrote a little  creation story which reads very  much like a big bang if you look  at it now. 

 That the very beginning of  creation was this spot of light  that spread out in all  directions.  And in his little treatise on  the rainbow he has the following  passage which I've always  enjoyed so I'm going to read it  to you.  He will refer in here to a  science called perspective,  perspectiva.  And that was what medieval  scholars called the whole  science of vision beginning with  how things arrive from visible  objects to the eye, what goes on  in the eye and then actually  what goes on in your brain.  It's all part of perspectiva.  So Grosseteste writes, "And so  this part of perspectiva, when  perfectly understood, shows us  how we may make things far away  appear very close.  And how we may make large things  that are close appear very  small.  And how we may make small things  far away appear as large as we  like so that it would be  possible for us to read small  letters from incredible  distances or to count sand or  grains or seeds or any small  things."  So there it is.  He anticipates telescopes and  microscopes.  He doesn't build them but he  knows that they'll be possible  just from looking at the ray  diagrams of the perspectivists. 

 This is Roger Bacon, a statue of  Roger Bacon somewhere at Oxford  University.  And Roger Bacon was very  interested in how we see.  He did not believe that  something was carried to the  eye, something traveled to the  eye, the way the old  intromissionists did.  So here's a little animation of  a gray dot flying over to the  eye of this observer.  If you blinked you missed it.  That's not Bacon's theory at  all.  Bacon wrote about what he called  multiplication of species, that  there's some quality in the  visible object that can impress  itself on the medium if the  medium is prepared for that.  And that bit of the medium can  impress that quality on the next  bit of medium.  So here we have a string of  contiguous bits of medium.  That first one picks up the form  of grayness from the statue and  like a movie marquee sort of  translates that gray through  subsequent bits of the medium  until it reaches the eye. 

 And that was the multiplication  of species.  Bacon believed that many other  senses also used them.  But I'm going to rush ahead now.  Rush way ahead, skip a lot of  fun stuff, unfortunately, and  jump right to the 17th century  and Galileo.  We've seen that Aristotle  believed that light traveled  instantaneously and Galileo,  early in his career, also  thought so.  In his book "The Assayer" he  writes as if light is a stream  of particles that travels  instantaneously, but by the time  he comes to write he discourse  on two new sciences in 1638 he  has suggested an experiment to  measure the speed of the light  which would involve two people  with lanterns, one on one high  place and one on another.  I've sketched a couple mountains  here.  So let's assume that Galileo is  standing there.  He opens in lantern, light  travels to his accomplice on the  other mountain who then seeing  that light opens his own lamp  and Galileo is waiting to see  that light come back.  And that would be a round trip  which he has been timing in  some, we hope, precise way. 

 And whether he ever carried this  out, it's not clear.  Even if he had tried it it  wouldn't have been a very  accurate result.  He could never have convinced  Kepler to be his assistant on  the other mountain.  Kepler was convinced that light  was instantaneous.  Galileo, of course, built his  own telescopes after learning  about their design.  Other people had only been using  them to spy on their neighbors,  and Galileo looked in the sky  and saw mountains on the moon  and moons around Jupiter and sun  spots.  Kepler also built a telescope,  he modified the design somewhat.  You can see the rays near the  eye piece are bent in a  different direction.  Jim assures me that these  diagrams contained woefully  exaggerated angles of refraction  and I believe him.  Here is Rene Descartes, we're  already in the 17th century, and  there is the universe that he  envisioned in a book called "The  World or Treatise on Light."  That's how important these  things were. 

 That's how important light was  to him.  He was working on that in 1633  and getting ready to publish it  when he heard what happened to  Galileo for Galileo's support of  heliocentricity, and he decided  to suppress "Le Monde," his book  on the world, because this is a  universe full of swirling  vortices of matter and here's  our vortices right up here where  the S is.  You may even be able to make out  the astrological signs of  planets here.  Descartes believed that the  universe consisted of these  contiguous swirling cells called  vortices, and the reason that  the sun looks so bright to us,  that the center of our vortex  looks so bright to us, is that  light is really nothing more  than pressure, an outward  pressure.  As this stuff swirls around it  creates this sort of centrifugal  tendency.  But there's no possible --  motion because for Descartes the  universe is a --, it is full,  there is no space without matter  in it.  So one of the only ways you can  get motion is to swirl things in  a circle.  So you create this outward  pressure in these spheres.  Here's Descartes drawing of the  spheres.  Descartes believed that matter  sorted itself out soon after the  creation into three basic  shapes. 

 One was big chunks like planets.  But the other was these small  spheres which he believed filled  what we would call space, and  there were even smaller little  chips that could fill in the  space between these spheres.  So light is pressure is it?  Well, not when it comes to  explaining various optical  phenomena like reflection and  refraction.  In his discourse on method in  1637 Descartes appended three  essays where he wanted to show  how you could use the method set  forth the discourse on method  and one of them was a method on  optics.  And here he treats optical  phenomena as if they are the  result of projectiles, not the  result of pressure.  He sort of waves it off and  says, well, the tendency of  motion is the same sort of thing  as motion itself.  But it's not the case.  Reflection is easy if you  imagine that this horizontal  line is a hard surface and here  comes a projectile, bang, it  bounces off.  Equal angles, no problem.  If you imagine that the top is  air and the bottom is water, and  if you've ever skipped stones  you know that the same kind of  thing can happen with a  projectile. 

 In fact Descartes writes that he  has tried this with canons.  He says, regarding this sort of  reflection that you've just  seen, this we have sometimes  experienced with regret when  firing artillery pieces toward  the bottom of the river for  pleasure we have wounded those  who are on the other side of the    shore.  So let's call this air and call  that water and put a projectile  up there.  What does at Descartes say will  happen when this hits the water?  It will go that way because in  terms of time Descartes believes  that the vertical component of  this motion will now take more  time but that the horizontal  component will have all that  time to keep going its merry  way.  So there will be more horizontal  motion than there was in the  other medium.  If you swap these around then  it's pretty clear that going  into the less dense medium the  projectile will now approach the  normal.  This required Descartes to  believe that light moved more  quickly in dense media.  Which many people thought was  rather counterintuitive.  But it didn't matter using this  analysis of refraction allowed  Descartes to come up with a sine  law, the sine law, that shows  how the angle of incidence and  angle of refraction are related.  There are ratio that's equal to  a constant which depends on the  refracted indices of the two  things involved. 

 We call this Snell's law.  The Dutch scientist Willebrord  Snell had thought this up in  1612 but never published it.  There were people who believed  that Descartes plagiarized this  but that is inconclusive.  So Descartes believed that light  is pressure and people take that  seriously in spite of the fact  that he uses projectiles for  occasions like this.  Pressure is instantaneously  passed through contiguous  spheres.  But does he have any sort of  proof for this?  And he does try to argue that  light is instantaneous for the  following reason.  Now here we have the sun, this  arc according to Descartes,  would be the orbit of the earth  and this line down here is,  let's say, the distance that  light can cover in half an  hour.  He thought he was being quite  generous with that.  Okay, let's put the earth there.  At this moment no light is  continuing on past the earth. 

 But as soon as the earth moves  out of the way light can now  continue on that line, and half  an hour later, if we take this  speed seriously, light will have  reached right about there.  Okay, let's put the moon there.  Now that light is reflecting to  the earth and it won't reach the  earth for a while yet, maybe  half an hour, probably longer  because now this is a longer  distance.  So people on earth will see the  moon come out of its eclipse  when they can measure an angle  this big.  And Descartes' point is we don't  see any angle like that.  When the moon comes out from the  lunar eclipse, for all intents  and purposes, the earth and the  moon and the sun are in a  straight line.  So light must be instantaneous.  He should have said light must  be very fast, but he said light  must be instantaneous.  Now the problem with pressure as  a means of conducting light is  that if two people are looking  at each other that means that  pressure is going both ways.  And it was hard for people to  imagine what that would really  mean. 

 Can anything really be  transferred with pushing at both  ends of a stack of spheres, as  Descartes seems to want to say.  That didn't make sense but what  did make sense was that pulses  could be transferred.  You've probably all seen the  old, I haven't seen one of these  in a long time, a toy to put on  your desk called Newton's Cradle  which is these five ball  bearings that are hung from  strings, and you can pick them  up and they go clack, clack,  clack, clack like that.  If you pick up the two at the  end and let them go they'll go  clack, clack, clack, clack.  So there is some pulse being  transferred through the  stationary spheres there.  And pulses began to make sense  to people who wanted to  understand how light was  conducted.  Let's talk about Ole Roemer.  He has been puzzled by many  years of study of the moons of  the Jupiter especially the first  moon of Jupiter, Io.  People were observing that moon  and trying to time how long it  takes for Io to come out from  behind of shadow of Jupiter, go  around and come out again.  And there were very inconsistent  results with attempting to time  this. 

 And Roemer had the idea that  maybe, suppose the earth is here  at L when Io is at D just coming  out of the shadow, and by the  time Io goes around and comes  out again the earth has moved to  K.  And then over here let's measure  when it goes into the shadow  since it would be hard to see  over there.  At F we, look the moon  disappears, Io disappears, it  goes around and when it goes  into the shadow again the earth  is at G let's say.  That means that on this side the  light has had to cover this  extra distance and on this side  when it goes back into the  shadow it has had this much less  distance so cover.  And it looked as if that would  account for the discrepancy in  the times measured of Io  spending the time that Io spends  going around Jupiter.  Roemer did not try to calculate  a speed of light from this.  Other people have tried to do  that but to get a speed of light  you'd have to have a pretty good  grasp on what the diameter of  the earth was.  And he didn't attempt that,  other people did.  One of those other people was  Christiaan Huygens.  Looking very dapper there. 

 Huygens was one who believed  that pulses could explain many  behaviors of light.  He sometimes described as a wave  theorist and his diagrams do  look like modern wave fronts  explaining how this light  propagates away from that source  there, A.  But he doesn't say much about  amplitudes or frequencies of  wave lengths or anything like  that.  It's a very sort of qualitative  wave.  So some people call it a pulse.  Huygens believes that Roemer is  correct and that light is not  instantaneous.  It's very fast but it's not  instantaneous.  So it can't be conducted through  inelastic spheres.  Whatever this medium of  conductance is, it's got to have  some give to it.  There has to be something about  this medium that is inelastic. 

 His treatise published in 1690,  the treatise on light.  Actually the whole title is very  interesting, Treatise on Light  in Which are explained the  causes of that which occurs in  reflection and refraction and  particularly in the strange  refraction of Icelandic crystal.  This is called double refraction  and that's a piece of Icelandic  crystal.  It's a particularly transparent  form of calcite.  There's a horizontal line drawn  behind there but if you look  through the crystal you see two  lines.  And if you look at the diagram  over there on the right, that's  Huygen's diagram.  And a ray of light comes in at  A/B and is split.  Notice on the left side the  light ray is coming in  perpendicular to that top face  of that Icelandic crystal and it  goes straight through.  In a regular refraction  situation if you're exactly on  the normal there is no  refraction.  But look at that other ray, that  B/C ray. 

 It gets split off.  On the right side you'll see  that a light of ray that's  incidence on that top face  parallel to the other face, to  the sides, goes straight through  but there's still an  extraordinary ray that breaks  off.  Those were his terms.  The ray that underwent normal  refraction or passed straight  through was the ordinary ray and  this weird thing was the  extraordinary ray.  Why does this happen?  What's going on here?  And he takes some guesses, he  doesn't really know.  He says it seems one is obliged  to conclude that the waves of  the light after having passed  through the first crystal  acquire a certain form or  disposition of virtue which will  affect the way they behave when  they go into the crystal again.  So this, as you may have  recognized, is called  polarization.  This was the first instance of  polarization.  And explaining it is a problem  for wave theorists because  here's where I think our high  school physics classes let us  down.  Most of us learn about waves. 

 We saw water tanks and these are  transverse waves, they go up and  down like this.  But for these early wave pulse  theorists remember they're sort  of still in the Cartesian world.  They think we're immersed in a  medium that conducts that pulse.  So it's not a transverse wave,  it's a, let's see I think I have  that in here some place, but  I'll get to it, it's a  longitudinal wave, like a  slinky.  So how to you explain  polarization type phenomena with  a slinky type wave?  It's not easy to do.  In fact, Huygens could not do  it.  Okay, here's Isaac Newton  looking rather bemusedly over at  this other picture of him on the  right there where he's  performing his famous experiment  with the prism except that  that's not how it happened at  all.  Here's Newton's own drawing,  done many years later, of what  happened.  There was a big distance between  the prism and the place where  the dispersed light was  projected.  And that was crucial to his  noticing that something was  strange. 

 The opening where the light  comes in is quite small, this  spectrum is all spread out.  And he notices that that's  because the blue light is bent a  whole lot more than the red  light.  And then he performed what he  called the experimentum crucis,  the crucial experiment, and you  can see here there's kind of a  board here and this is probably  the blue light and this is  probably the red light, he can  send whatever color he wants  through a particular hole and  put this second prism behind it  because some people might say  well a prism just does that.  Prisms do that, they'll take  whatever goes into them and  spread them out and you'll get  another rainbow type thing.  But by doing this with the  second prism he found out that  red light is bent just as much  as it was in the first prism and  you can see this very narrow  beam.  You can't change it anymore than  it's already been changed.  So it's not really the prism  that's doing anything to light.  Prism is merely dissecting that  beam of sunlight.  Now this would explain why in  telescopes like Galileo's you do  get this color fringes because  when you send light through a  lens it would be differentially  refracted, the different colors  would be differentially  refracted.  So if you look in the background  of this picture again, where is  it, there, see this funny tube  thing here?  That's a reflecting telescope. 

 Which Newton didn't, this  probably took place in 1666,  Newton's reflecting scope was  built in 1669.  And avoids the problem of what  we now call chromatic  aberration.  Here's an interesting phenomena  which we still call Newton's  rings.  If you take a flat piece of  glass and a curved piece of  glass, a spherical curved piece,  and push them down and shine  light through them you see this  interesting pattern of what we  would now call interference.  But Newton was hard-pressed to  explain this.  In fact, he ended up proposing  that light rays from their  source had something called fits  in terms of one thing fitting  into another.  And that somehow in these little  spaces the fits would act to  make the beams interact with  each other.  I can't claim that I ever really  understood what Newton meant by  these fits.  It's almost as if the ray had a  wave kind of built into it.  But in terms of an ether, Newton  plays with the idea, throughout  the life he says yes there's an  ether, no, there isn't, yes,  there is. 

 And ether was useful because he  could say that it hovered around  objects.  Knife-edged defraction.  We now know, and he knew this,  if you shine light at a knife  edge, you don't get a very sharp  shadow.  If you look very carefully at it  you'll see little bands.  Why would that happen if light  is, as Newton believed,  particles flying by?  Well, if there was a halo of  ether around that knife edge,  then some of the particles will  pass through that ether  gradient, that bit of ether that  sticks up above the actual edge  of the knife, and those  particles will be bent by that  ether and create those bands.  But he doesn't believe that  light itself is a wave.  In his book The Optics which  became sort of a lab manual for  many people in the 18th century,  he was very scrupulous always  about not including in his  published work things that only  hypotheses or conjectures.  But in "The Optics" he did tack  onto the end several things  called queries where he stated  what he probably was convinced  were true facts about nature but  he couldn't demonstrate them so  he phrased them as queries.  And in query 26 he proposes that  rays have sides.  That there's some sort of  symmetry to the ray that would  explain some how what happens in  that Icelandic --, that calcite  crystal. 

 In query 29 he says "Are not  rays of light very small bodies  emitted from shining  substances?"  He can't prove that  but he believes it.  Okay, I'm going to have to rush  along because the further we get  toward the present the more  interesting things happen.  James Bradley in 1728 has also  noticed that there are some  discrepancies with where certain  stars appear to be in the sky at  certain times of the year.  With some stars there's this  sort of annual variation in  their location and long story  short, he discovers that if the  earth is moving kind of sideways  the way that sort of crude arrow  is drawn, suppose the earth is  moving sideways with respect to  the direction of incoming light  then you would have to tilt your  telescope to catch that light.  Imagine that you've got a real  tall tennis ball can and it's  raining straight down but you're  on the roof of a moving car or  in the back of a pickup truck to  be safe, and you want to catch  rain drops.  You want them to hit the bottom  of the can, not hit the sides.  You're going to have to tilt  that can in just such a way that  as the truck carries you forward  the rain drop falls and hits the  bottom.  And that's what happening, the  stars appear to move because we  have to turn the scope to catch  the photons.  He didn't know that to catch the  light waves coming from that  star.  And I only throw this in here  because two very important  points are made with this  experiment. 

 First of all, it's experimental  proof that the earth is moving  which pretty much everybody had  believed by then, but there  wasn't any absolute good  evidence like this.  And the other thing that comes  from this is that light must  travel in time, it must.  If light were instantaneous you  wouldn't need to turn your  telescope.  It would go bang, right down in  there no matter how fast you  were moving sideways to the  direction of the incoming light.  The wave theory begins to catch  on especially with the work of  Thomas Young who did believe  that it would help explain lots  of things in nature, electric  effects, heat effects if there  was some medium, some ether.  The different colors that we see  might correspond to different  wave lengths, different  frequencies of vibration.  And he knows that waves will  interfere with each other.  And this is a demonstration like  the one we saw in high school.  This interference will explain  these kind of bands that you  might see.  Going to have to skip a bunch  here. 

 Let's go right to Fresnel.  Oh, here's the slide of waves  completely out of place, but you  can see here's the slinky, a  longitudinal wave and here's a  transverse wave.  It's becoming pretty clear that  light must have a transverse  component to deal with, to  explain those polarization  phenomenon.  This is a cute story.  Augustin Jean Fresnel, a civil  engineer but interested in  light, submits for a prize  offered by the royal academy can  you offer us a mathematical  explanation of defraction  phenomenon.  So in 1819 Fresnel submits his  paper.  And Poisson, who was one of the  prize committee judges, was a  Newtonian like many of the other  members of the royal academy at  this time and is convinced that  light is not a wave, light is  particles, that's what Newton  said.  He takes Fresnel's math and  shows that by Fresnel's own math  due to constructive interference  if you shine light at a small  sort of BB sized object by  constructive interference you'll  see a bright spot at the very  center of the shadow cast by  that BB if you put a screen  behind it.  Obviously absurd until the  experiment was actually done.  And there it is. 

 The bright spot does appear.  So more powerful evidence that  the wave theory is on to  something.  Rushing ahead to Michael  Faraday.  The book binder's apprentice  who, by several strokes of luck,  became assistant to Humphry Davy  and proved himself to be an  experimental genius.  On the right is a Chladni  figure, Ernst Chladni, a  physicist who figured out how to  demonstrate sound waves, sound  vibrations, you probably did  this is high school physics too.  You take a metal plate of some  not too great thickness,  sprinkle fine sand on it and bow  it with a violin bow.  And you'll get beautiful  patterns like this.  Faraday would demonstrate  Chladni figures for the royal  institution where he worked.  Faraday was also able to  polarize light and show in 1845  that a magnet will bend, will  twist, polarized light.  And this is still called the  Faraday effect. 

 In 1846 he and a scientist named  Weedstone, for who the bridge  was named, were scheduled to  give a talk at the royal  institution.  But apparently Weedstone was a  shy fellow and fled the chamber  in which they were both waiting  to go out and give their talk.  So Faraday had to go out himself  and sort of give an  extemporaneous talk which he  called "Thoughts on Ray  Vibrations," where he expanded  on his idea of lines of force.  The kind that you can see around  a magnet if you put the iron  fillings on top of the paper.  That these are all over the  place and that there must be,  actually Faraday wasn't so  interested the metaphysics of  it, he was interested in the  concept that there are these  lines of force the permeate  everything and vibrations in  these lines might be responsible  for many different natural  phenomenon.  To this day speakers who go to  the institution are locked in  the chamber.  (laughter)  It's a tradition  they've never given up.  Maxwell read Faraday and was  very appreciative of his work.  And chapter 20 of his magnum  opus, Treatise on Electricity  and Magnetism in 1837, he  writes, I'll say he points it  out that if both the  electromagnetic and what he  calls the undulatory of light  both seem to require a medium  then perhaps the medium is the  same for both.  And then here, and I'll quote  him, "If it should be found that  the velocity of propagation of  electromagnetic disturbances is  the same as the velocity of  light, we shall have strong  reasons for believing that light  is a an electromagnetic  phenomenon."  And, of course, that will  testify to the reality of the  medium. 

 And of course Maxwell does  deduce the speed of light that  had been experimentally found.  This is sort of a mechanical  model of his ether.  There are things coming out the  screen at you that are sort of  rolling, six-sided things and --  balls in between them.  And the balls flow between the  hexagons going that way, that's  electric current.  Other flexes in this thing  produce other electromagnetic  effects.  He abandoned this model later on  and stuck to math.  He didn't try to make a physical  model.  But if there is an ether we  should be able to detect it.  And here is a diagram of the  famous Michaelson-Morley  experiment first conducted in  the basement of a building at  Case Western in 1887.  Light is emitted from this  device at E, it goes through a  beam splitter, goes up there and  is reflected, goes out there and  is reflected and then both beams  are collected here and an  interference pattern is noticed  and recorded. 

 And then the whole device is  rotated 90-degrees and checked  for a change in the interference  pattern.  Because if there whole apparatus  is moving through the ether and  if light is a wave in the ether,  then we should see a difference  if we rotate the arms of this  apparatus.  The whole thing was set in a big  vat of mercury, floating in  mercury they could move it  without upsetting it very much.  No change found.  No ether wind detected.  Not looking good.  William Thomson, later Lord  Kelvin, demanded that a  mechanical model of ether be  thought up some day.  It's rumored that at Glasgow  University there's still an  experiment that he started where  there's a block of wax halfway  down a big cylinder of water and  there are bullets slowly sinking  through the wax and corks slowly  rising up through it.  Which was apparently Thomson's  idea that the medium which can  conduct light at 186,000 miles a  second must be fairly stiff and  tense.  And yet planets go through it  all the time. 

 But, he reasoned, maybe  comparatively speaking planets  are going so slowly that the  necessary tension in this medium  is not going to resist the  planets noticeably.  They will still be able to pass  through and yet the medium will  still be able to pass these  vibrations of light and  electromagnetic phenomena.  Well, in 1900 Kelvin gave his  famous two cloud speech that  science is great, we're making  great progress, there are really  only two clouds on the horizon.  And one of them was the negative  result of the Michaelson-Morley  experiment.  Where is that ether?  We've got to find that ether or  explain why the  Michaelson-Morley experiment is  not detecting it.  And the other was something  called black body radiation.  Now, what should we deal with  first?  The ether of course was never  found by the Michaelson-Morley  experiment.  It was conducted again on Mt.  Wilson in 1905.  Up on a mountain because maybe  down in the basement of Case  Western there was a lot of ether  drag. 

 Maybe the ether that surrounds  the earth is kind of carried  along with it so you wouldn't  detect any.  If you put it up on a mountain,  maybe you're away from the ether  that's being dragged along with  the earth and you're actually in  the ether wind of the cosmos.  Still no positive result.  Black body radiation.  This is going to get difficult.  You think of a ball of soot,  black.  Any energy that comes through it  is absorb, nothing is reflected.  But as it gets warm it's going  to radiate energy itself and how  can we talk about that radiated  energy?  How does the frequency of  radiation coming off that ball  of soot relate to its  temperature.  Let's write an equation guys and  figure out how this works.  And it just wasn't working. 

 The blue line, the Wien's  formula, was actually just an  empirical attempt, it wasn't  derived from any more basic  physics.  And the gentleman on the left  here Max Planck didn't really  like it.  It turned out not to match the  experimental results anyway.  But even before they knew that  Planck didn't like this idea  because he wanted this equation  to be deducible, to be derivable  from prior physical commitments.  Now there was a law of classical  mechanics called the  equipartition of energy.  It wasn't really a law, I guess  it was an assumption.  And the Rayleigh-Jeans' formula,  which you can see taking off  into outer space there, they  cobbled this up to show that if  you assume this equipartition of  energy among whatever you  conceive of making up the ball  of soot at its smallest level.  If you assume this equipartition  thing it would be like having a  piano, this is George Gamow's  idea.  If you had a piano that had an  infinitely long keyboard and the  strings weren't too well damped  out, and you bang on a key like  that then all the strings  eventually would begin to  vibrate.  Gamow says that if this were  true sitting in front of the  fireplace you would be bathed in  gamma rays. 

 Because hot things would radiate  at all these possible levels of  radiation.  And of course that doesn't  happen.  And this gets called the  ultraviolet catastrophe but that  wasn't, Planck never heard that  term, it wasn't even coined for  many years.  Planck is trying to come up with  this equation and he ends up  having to sort of break his own  rule here and make an assumption  that he's not really comfortable  with.  He makes the assumption that  energy can only be emitted or  absorbed in discreet packets.  And once you make that  assumption you can make an  equation which matches the  experimental results on black  body radiation.  I don't know whether Kelvin ever  heard about this.  He wouldn't have been happy.  So energy is in packets.  Here's a young Albert Einstein. 

 And the packetizing of energy  comes in quite handy to explain  something called the  photoelectric effect.  It had been noticed before that  certain metals or for every  metal you can shine light on it  and sometimes you'll get  electrons kicked out of it.  Every metal seemed to have a  threshold frequency.  If you shine low frequency light  on it you get nothing.  See here you're shining red  light, no electrons.  It doesn't matter how bright  that red light is, how intense  that red light is, you will get  no electrons out of that metal.  But as soon as you reach this  wave length, this higher  frequency, you will start  kicking electrons out of this  metal.  This is potassium.  Every metal has its own  threshold.  And notice here if you raise the  energy by sending higher energy  light, blue light, the electrons  are kicked out with more kinetic  energy. 

 You can't explain this with  waves.  It just doesn't work.  If the wave theory were correct  bright intense red light should  be kicking electrons out of this  metal.  It looks like Planck is right  and it looks like we're back to  talking about electromagnetic  things, like light, as if they  were little bits of things.  As if they were not waves at all  but particles.  Except there's no denying that  they have wave properties.  Here's John Wheeler, who died  last year, here's a thought  experiment of his called, and  this was sort of a generic name  for his experiments, they're  called delayed choice  experiments.  And we know that if you send  light at Thomas Young's old  two-slit experiment and put  photographic paper back there  you'll get an interference  pattern.  But if you block, well how shall  I put this, if you don't put  photographic paper back there  but put cups or detectors,  you'll detect photons as if they  went through one or the other.  You won't get an interference  pattern. 

 And the delayed choice  experiment has to do with not  deciding until the photons are  in area three.  They've already gone past where  the slits are.  You take away the screen and put  the cups there you'll get  particle phenomena.  If the cups are there when the  photons go through and you slam  the screen down you'll get an  interference pattern.  This was very strange.  Now this was only a thought  experiment but an equivalent  experiment has been done.  And then there's the quantum  eraser.  I'm amazed I have any time to  talk about this, and I may just  yet bail out because I'm going  to be brave and mention  something else that bothered  Einstein about quantum  mechanics.  And that caused him  to write a paper  called "Can Quantum Mechanic  Descriptions of Physical  Reality be Considered Complete?"  Actually, it was one of the grad  students that wrote the paper,  Einstein wasn't really  too happy with it apparently.  But it did contain some things  that bothered him. 

 There are some consequences of  quantum mechanics that Einstein  doesn't like.  He's often quoted as saying  "God doesn't play dice."  And there was aspects of quantum  mechanics that made it sound  like there were aspects of  nature at these very small  levels that were completely  probabilistic.  Well, one of the things that  quantum mechanics proposes is a  phenomenon called entanglement.  You can have a system that spits  out two electrons or two photons  that have complimentary  properties.  A spin or orientation in the  case of photons.  But according to quantum  mechanics that's really all one  system, it's not really two  photons, it's sort of one thing.  And you don't really know which  photon is oriented what way  until you test one of them.  And as soon as you know the one  the other one automatically  assumes the opposite property  even if it's on the other side  of the universe.  This violates what was called  locality.  A theory of physical reality,  according to Einstein anyway,  had to rely on locality. 

 So there must be something about  quantum mechanics that is  incomplete.  Since it has to violate  locality.  In order to retain locality we  have to discover what it is that  quantum mechanics is missing.  And he proposes an experiment  that would look at paired  photons flying away.  And no one could really do that  experiment at the time but with  some added math and some added  experimental talent the  experiment that was suggested in  the EPR, it's called the EPR  paper for Einstein, Podolsky and  Rosen, Boris Podolsky and Nathan  Rosen were grad students who  helped him, Einstein called this  spooky action at a distance but  lo and behold it happens.  And again it's quite something.  Just as like what happened with  Fresnel.  It's quite something when an  opponent of your theory devises  an experiment to say, ha, ha,  you're wrong, and it turns  supporting what you believe.  And several of these quantum  eraser experiments have been  done.  I found this on the web, some  grad student at the State  University New York Stony Brook  did this. 

 And I see I'm just about out of  time so I'm going to finish up  by trying to get through this  and explain it to you because  I'm not sure I understand it  myself.  You see here a crystal which is  abbreviated BBO and it is  acknowledged to produce paired  photons that are entangled.  One of them goes along path P to  the detector up there and one of  them heads for the double slit  down there.  And this detector can move side  to side, that's what that blue  arrow means.  The coincidence counter is there  to pair the photons and make  sure that we're not confusing  one photon arriving at one  detector with one from a  different pair arriving at the  other detector.  Now, what do you think the  pattern will look like that this  bottom detector sees?  It's going to look like this.  It's going to be an interference  pattern.  Because right now there's no way  to know which slit that photon  went through.  That's another delayed choice  experiment you can do by the  way.  You cover up one of the slits  after the photon has gone  through. 

 That wasn't done here.  Okay, the next set-up puts a  couple of quarter wave plates in  the path so that if a photon is  going to go through the left  side, the green one, it will be  rotary, its polarization will be  spinning to the left or the  right.  Now I'm not remembering exactly  but I don't think it matters.  The point is it would in  principle now be possible to  tell at the detector which slit  the photon went through.  In principle possible to tell.  What do you think the pattern  looks like now?  No interference.  Now how weird is that?  It gets even weirder.  But I won't try your patience  you've been very patient with me  up till now.  And that would be a good place  to end it.  It's just good to know that  because of things like these  pairing of photons that this  will never happen to you. 

 (laughter)  Thank you, very much.  (applause)