“In the six hundredth year of Noah’s life…all the wellsprings of the great depths burst forth…” (Genesis 7:11).

  The Zohar compares the flood of Noah’s generation to the flood of scientific knowledge that began in the eighteenth century CE: “In the six hundredth year of the sixth millennium (since creation  of the world–starting at around 1740 CE) there will be an opening of …the lower wellsprings of wisdom preparing the world for [the final redemption]…”[1]

What are the “lower wellsprings of wisdom”?  They are secular knowledge, especially science and its offspring, technology.  From electronics to antibiotics, airplanes to automobiles, from plastic bags to computers, all of the items we rely on were invented or impacted by inventions dating from the mid-1700’s. Whether we will drown in this flood of stuff or rise above it depends on wisdom of a higher sort.

As stated in Part 1 of this preface (Let there be…Electrons!) electrons and light are connected as closely as body and soul.  The modern scientific view of light grew out of nineteenth century experiments with electricity.

The study of electricity received a giant boost in 1800.  Prior to then, scientists had no easy way to get a steady,

reliable source of electricity.  They could risk their lives to capture lightning in insulated glass jars called Leyden jars. Or they could build their muscles spinning a glass sphere against a cloth, to gather the sparks produced by friction into those same jars. [Fig. 1]

But in 1800, Allessandro Volta announced his invention of the “Voltaic pile.”  This was a precursor to today’s electric batteries.[2]  Soon, physics laboratories bristled with wires and with the chemicals for making Voltaic piles.

Science is a collective effort.  Each researcher builds on the findings and ideas of earlier researchers.  Nevertheless, a few stand-out discoveries lie directly on the path to understanding light.  I’ll present only those that, in my opinion, are most relevant to the question: “What is light made of?”

 

Fun with Magnets!

In April of 1820, Professor Hans Christian Oersted was lecturing about electricity to a group of university students.[3]  On the table before him lay equipment for demonstrations.  These included an electric circuit, a source of electric current, and a nearby compass.  The pointer of a compass is a magnet.  It lines up with Earth’s natural magnetism, to point to the magnetic North Pole.

Perhaps Oersted intended to demonstrate the effect of electric current on a compass.  Or he may have noticed it incidentally during this lecture.  Historical reports vary.  However, all agree that when he sent electric current through the wire, the compass pointer moved off its North/South alignment.[Fig.2]  Investigating further, Oersted found that the compass pointer always lined up perpendicular to the current-carrying wire.  That meant that the wire was acting like a magnet.  It was strong enough to overcome earth’s natural magnetism.

The effect of Oersted’s findings on contemporary scientists was, to say the least, “electrifying.”   Scientists hurried to duplicate Oersted’s results and to find out what else this “electromagnet” could do.  Electromagnets could be made stronger than any permanent magnets known at that time.   Paired with an iron needle they could be used to measure the strength of electric currents or the voltage.  Besides their role in research, electromagnets are workhorses of modern industry to this day.

Was the reverse of Oersted’s discovery also true?  Could magnetism produce electric current?  Many scientists tried to find out.  Their setups varied, but Fig. 3 shows the essential features. The circuit on the left is the magnet, typically an electromagnet.  (The electromagnet is coiled to concentrate more length of wire, hence more magnetism, into a smaller space.)  The circuit on the right is simply a loop of metal wire.  It’s placed near the electromagnet, but not touching it or connected to it.  It has no source of electric power.  A meter is attached to measure electric current.  If no current flows in the loop, the meter points to zero.

And zero was where the meter stayed, despite many variations of this basic design.  A few researchers may have noticed the pointer twitch while the electromagnet was being turned on or off.  But the movement was slight and short-lived.  If researchers noticed it at all, they dismissed it as irrelevant.

Enter a rare genius, a brilliant experimental scientist, Michael Faraday.

Faraday was born to working-class parents in Victorian England.  The strict class-conscious of that society barred him from the advanced education needed for a career in science. By then physicists were already expressing their ideas using calculus and higher mathematics.  Faraday’s formal schooling hadn’t gone beyond arithmetic.  But the disadvantage may have worked in his favor.  Apprenticed to a book binder, he read many of the books he worked on.  This self-education enabled him to “think outside the box.”

The stone which the builders rejected

became the chief cornerstone (Psalm 118:22)

In 1831, eleven years after Oersted had shown that electric current produces magnetism, Faraday succeeded at the reverse:  He produced electric current from magnetism.

The “cornerstone” of Faraday’s success was the very detail that previous researchers had ignored:  the meter’s slight motions while the electromagnet was being turned on or off.      Could changing magnetism be the source of electric current in a nearby circuit?  Faraday devised a set-up to keep the magnetism changing by constantly reversing the direction of current in the electromagnet.[4]  The resulting current in the loop also continually reversed direction.  In response, the pointer on the meter waved back and forth, from left to right.

This alternating current, AC, has some practical advantages.  It can be sent without losing power over much longer distances than direct current, DC.  That led to its world-wide adoption throughout the twentieth century.  Modern power plants produce AC; plug a device into a wall socket and AC is what you get.

With further improvements, Faraday made the alternating current move a wire around continuously in a circle.  Thus, he invented the principle of the electric motor!

Faraday’s curiosity led him to search for magnetism in unexpected places.  Iron responds to magnetic force, but what about other substances?  What about light?  In 1845, using strong electromagnets, he tested many different substances.  All responded to magnetism.  Most were repelled from the magnet; a few were attracted.[5]  That same year, Faraday tried a similar experiment with a beam of polarized light. [Fig. 4]   The magnets twisted the direction of polarization—showing that light, too, is at least in part magnetic!  (For an explanation of polarized light, click here.)

We started with the question, “What is light made of?”  Three discoveries, spanning twenty-five years, suggest an answer.  To summarize thus far:

• Electric current creates magnetism (Oersted, 1820).
• Changes in magnetism create electric current in a nearby wire (Faraday, 1831).
• Light is, at least in part, magnetic (Faraday 1845).

 

Fields…

One of Faraday’s most important contributions to science was not an experiment, but an idea, a new way of looking at an old, familiar observation.

To prepare yourself for Faraday’s new idea, here is something to try at home:

Take a magnet (preferably a bar or horseshoe magnet, but a refrigerator magnet will do).  Place a small iron object, such as a thumbtack or paper clip, on a flat surface.  Very slowly bring the magnet nearer to the iron object.  As the magnet gets closer, at some point the object will jump onto the magnet.  Now ask yourself:  How did the thumbtack know there was a magnet nearby?  How did it know which way to go?

This question isn’t a joke.  It’s a very real problem in physics.  It even has a name:  action at a distance.  Of course, thumbtacks don’t have brains or any organs for sensing their surroundings (at least as far as we know).

Scientists of Faraday’s era would have said that magnets give out an invisible force.  The force reaches across space to attract or repel an object some distance away.  But Faraday saw the matter differently.  He located the magnetic force in the space around the magnet.  Viewed that way, the thumbtack doesn’t have to sense a distant magnet.  Rather, it’s moved by a force that exists at its actual location.  That force has both strength and direction.  [Fig. 5]  Faraday’s idea evolved into what modern physicists call field theory.

The difference between field theory and action at a distance may seem hard to appreciate.  Both explain the same observation:  one object moves toward or away from another object some distance away.    Action at a distance imagines that an inanimate object (like a magnet) gives out force that travels across emptiness.  Field theory imagines that the inanimate object affects only the space immediately touching it.  That space affects only the space next to it, and so on, like a bucket brigade, each member handing off its burden to one further down the line.

The question isn’t which view is more plausible.  It’s which view has more success explaining known observations and predicting new ones.  In that, field theory is the definite winner.  One of its first successes was explaining how light travels from the Sun to Earth, through empty space.  If “empty” space contains fields, then empty space isn’t really empty!  It’s filled with fields of all sorts: magnetic fields, electric fields, gravitational fields, and others even more abstract and esoteric.

 

…and Waves

Everything in the world seems to be either particles or waves.  (The word “seems” is chosen advisedly here because we haven’t yet tangled with the weirdness of quantum mechanics.)  To which category does light belong?  Isaac Newton, probably the greatest scientist of the seventeenth century CE, believed light to be streams of particles.  He had evidence to prove it.  Christiaan Huygens argued for waves.  He, too, had evidence aplenty to make his point.  Each side had its scientific supporters.  In 1802, however, the controversy seemed resolved.  Thomas Young conducted his famous “double-slit experiment.” He passed a beam of light through two closely spaced narrow slits, so that they emerged as two identical beam of light.

On a screen placed opposite the beams he saw an interference pattern.  [Fig. 6]  (To learn how waves can form interference patterns, click here.  For an explanation of Young’s double-slit experiment, click here.)

The dark bands occur where the crest of one wave and the trough of the other reach the same place on the screen.  They cancel each other out.  The bright bands occur where both waves reach the screen in the same phase–both as crests or both as troughs.  They reinforce each other.  This type of pattern is unique to waves.

Therefore, in good nineteenth-century spirit let’s assume light consists of waves.  That’s the model that Faraday and his contemporaries had to work with.  And for us in the early twenty-first century, we may know it’s not all right, but it’s also not all wrong.  (Ah, science!)

With that in mind, we’ll compare light with ripples moving across the surface of a pond.  Both move as waves.  Ripples form when the surface of the pond is disturbed.  A pebble falling into the water, for example, carries energy.  That is the energy of its motion.  Where the pebble hits the surface, some of its energy transfers into the water.  The energy spreads out quickly in circles of increasing diameter.  The pebble caused the ripples, but it does not move with them.  Instead, it sinks to the bottom of the pond.

A light wave starts as a disturbance in the electrical or magnetic field.  Disturbed, the field changes.  As an electric field changes, it creates a changing magnetic field.  This changing magnetic field creates a changing electric field.  This “new” electric field creates a new magnetic field.  On and on the two fields go, each creating the other, spreading out through space.  Like ripples in water, this wave carries energy outward from its source.

Which brings us to the climax, the purpose of this writing.

James Clerk Maxwell was a gifted mathematician and physicist.  He was a young man in Faraday’s old age, and greatly admired Faraday’s legacy.  Employing the field concept, treating light as waves, Maxwell developed equations describing how electrical and magnetic fields move through space.  They move together, each creating the other as they advance.  But what does this have to do with light?

The speed of light had been measured by in 1862 by Hippolyte Fizeau.[6]  Maxwell calculated the speed by which an electromagnetic wave would move through space.  It came out to be the same speed measured by Fizeau.  The conclusion was obvious.

In 1864 Maxwell reported the result of his calculations in a lecture to the Royal Society of London:[7]  We have strong reason to conclude that light itself—including radiant heat and other radiation, if any—is an electromagnetic disturbance in the form of waves propagated through the electro-magnetic field according to electro-magnetic laws.   The electromagnetic wave moves at the speed of light!

If it acts like light, and it moves like light, it must be light!

Faraday had suspected that light is both electrical and magnetic.  He lived to see Maxwell’s validation.  The question “what is light made of” finally had an answer:  Light is a wave of electrical and magnetic energy, the two types of fields braided together, inseparably.

[1] Zohar I, 117a

[2] Alexander Hellemans and Bryan Bunch.  The Timetables of Science.  New York:  Simon and Schuster, 1988, p.251.

[3] Gribben, Mary, and Gribben, John.  The Cat in the Box:  A History of Science in 100 Experiments.  New York:  Race Point Publishing, 2017, pp. 94-95

[4] To reverse the north and south poles of the electromagnet, reverse the connections to the positive and negative poles of the battery.

[5] Charles Byrne, A Brief History of Electromagnetism.  www.faculty.uml.edu/cbyrne/EMHIST.pdf, retrieved 9/10/2019.  The terms paramagnetic and diamagnetic are applied, respectively, to substances that are attracted or repelled by magnetism.  Based on which category a substance falls into, chemists can determine details of the arrangement of electrons in its molecule.

[6] Fowler, Michael, The Speed of Light,  http://galileoandeinstein.physics.virginia.edu/lectures/spedlite.html, retrieved 11/7/2019.

[7] Stark, Glenn, Light as Electromagnetic Radiation, Encyclopedia Britannica online, htt ps://www.britannica.com/science/light/Light-as-electromagnetic-radiation, retrieved 11/3/2019