What is Light?

According to Chazal, the light of our world is but a flickering candle compared to the illustrious blaze of creation. As the Kli Yakar writes, the Torah describes the light in the opening verses as ohr, light, while the luminaries later are described as me’oros. This is because only a spark of the original light breaks through them to illuminate the world.

Nonetheless, it has taken mankind centuries to penetrate the secrets of our earthly light, and who knows what surprises still await us? As Isaac Newton wrote about discoveries in general: “I don’t know what I may seem to the world, but, as to myself, I seem to have been only like a boy playing on the sea shore, and divert-ing myself in now and then finding a smoother pebble or prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.”

Reflections or Feelers

The first investigators to unravel light’s secrets were the ancient Greeks whose fundamental argument of how light worked persisted for centuries. While Pythagoras believed that we see objects by viewing light reflected from them, Plato and others maintained that the eyes work like a ray gun, sending out ephemeral “feelers” that detect objects far away. A major objection to the second theory was – if vision depends totally on the eye, why don’t we see things at night?

A reflection of the latter theory is the description, found in certain works, of birds that hatched their eggs not by brooding on them as most of our feathered friends do, but by glaring at them with their eyes.

Yet  another  discovery  of  those times was Aristotle’s observation that rainbows form from light shining through myriad raindrops. The Ram-ban mentions this idea in next week’s parshah where (Bereishis 9:12) Hash-em promises after the flood, “This is the sign of the covenant that I place between Myself and you, and be-tween every living soul that is with you for all generations. I have placed my bow in the cloud and it will be a sign of a covenant between Me and all the land.”

“Since this is a sign,” the Ramban begins, “it would seem that the bow in the cloud was not in existence from the act of creation, but now, Hashem created something new to make a bow in the sky on a cloudy day…”

“However,” he continues, “we are forced to believe the words of the Greeks that the rainbow arises naturally from the rays of the sun in damp air, since the appearance of a rainbow can [also] be seen in a vessel of water [placed] before the sun. If you examine the words of the verse again, you can understand so, because it says ‘I placed in the cloud,’ and does not say’I place in the cloud.’ It does not say “I will place’ in a cloud as it said, ‘This is the sign of the covenant that I place,” and [also], the word ‘my rain-bow’ indicates that He had the rain-bow already.

“Therefore, we explain the verse [as follows] – the rainbow that I placed in a cloud from the day of creation will, from this day onwards, be a sign of covenant between Me and you…”

In later generations, scientists in-ferred something else from the rainbow – that light is not one unit, but many distinctive colors that blend into one. This was a small observation compared to the later discovery that visual light is only a minuscule part of the much larger “electromagnetic radiation” family, whose rays, whether x rays, gamma rays, light rays, or radio waves, consist of the same basic blend of electricity and magnetism.

Fortunately, we are almost blind to the huge electromagnetic spectrum that ranges from dangerous gamma rays a billionth of a meter long, to radio waves that stretch out for meters. Visible light takes up only a tiny range of one thousandth of one per-cent, ranging from red with waves 400 billionths of a meter long, to violet whose waves measure 700 billionths of a meter.

This is just as well. As an eminent author once said, “There are two kinds of light – the glow that illuminates, and the glare that obscures.” If we could view the entire electromagnetic spectrum, every time we opened our eyes to study a daf Gemara, we would be distracted by the babble of an innumerable radio programs.

To make things worse, we are not only blind to most of the electromagnetic spectrum, but also to most of visible light. The Muslim scientist, Ibn al-Haytham (d. 4800/1040) was the first to theorize that we can only see light rays that strike our eyes head on. For us, any light passing in front of us obliquely is as good as non-existent.

To illustrate this idea, imagine you are standing in a sunny room early one morning. Although light is streaming in through the window, you can only see it after it bounces off the walls, floor, and furniture and flies straight into your eyes. To see the actual light beams, you would need to shake out a dusty blanket, whose dust mites would bounce the beams towards your eyes.

Al-Haytham’s concept came in handy decades ago after it was claimed that Neil Armstrong and his crew never landed on the moon and that the photos of his landing on the moon were cunningly faked on earth. Clear proof, people asserted, was that the sky in the moon photos is pitch black with nary a star in sight. If the photos were taken on the moon as NASA claimed, the skies should have been blazing with stars as in a dark night on earth.

The answer to this objection is simple. The truth is thousands of stars were shining overhead. However, since it was daytime, the camera had to be adjusted for the sun’s brightness, which made the stars too dim to register.

But if was daytime, you may ask, why was the sky in space pitch black?

The answer is what al- Haytham said hundreds of years ago. The only light we see is that which comes straight to our eyes. In the same vein, the only light visible in space is that of the sun, and whatever bounces to-wards you from the moon, planets, stars, and whatever else is floating up there. The light traveling between these celestial bodies is as good as non-existent, resulting in a space that is pitch black even at midday.

This is clearly nonsense, you might challenge. If it were true, why do we have a beautiful blue sky overhead on earth? No one knew the whole answer to this question until Einstein came along and made it one of his minor discoveries.

When sun-rays hit our atmosphere, he explained, blue light rays (which are less energetic) are deflected by air and water molecules and scatter in every direction. Some of these scattered rays fly to your eyes in a straight line, resulting, overall, in a sublime blue dome stretching from horizon to horizon.

Wave or Particle?

In 5705/55 BCE, the Roman Lucretius wrote that light consists of tiny particles: “The light and heat of the sun are composed of minute at-oms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove.”

Newton adopted this idea in his Op-ticks of 5464/1704, which amazingly led his contemporary, Pierre-Simon Laplace, to postulate the existence of what we now call black holes. If light consisted of particles, Laplace argued, stars so massive might exist that even light cannot break free of their crushing gravity. This would result in a lightless, black star.

Laplace withdrew this brilliant hypothesis when another contemporary, Robert Hooke, persuasively argued that light is not a particle but a wave, and it took over a century before Einstein explained that light is actually a particle and wave simultaneously.

In 5685/1924, Louis-Victor de Broglie went even further, claiming that not only light, but all matter, has the dual quality of being both a particle and wave at the same time. This includes the atoms we are made of. We are simply too large to notice that on a sub-microscopic level we are like waves breaking on the world’s shores.

Even without Broglie’s idea we are not much more than that, as the atoms in our bodies consist of well over 99.9% of empty space! Perhaps this is just as well. If our atom constituents were as tightly packed as neutron stars are, one teaspoon of human flesh would weigh well over a billion tons!

Torah is Light

Another concept of light gives new meaning to the verse of Mishlei (6:23), “For a mitzvah is a lamp, and the Torah is light.”

Not everyone knows that there are two theories of relativity. The first was of Isaac Newton who discusses a man in a ship cabin sailing at sea. For him, the cabin is at rest in a moving sea, while for someone on shore it’s the ship that seems to be moving. Everything is relative.

Unlike Newton’s theory where everything is relative to the observer and there is no place in the universe you can drive nails into, Einstein’s special theory of relativity gave the universe a new benchmark – light. Scientists were puzzled to discover that light always moves at a constant speed regardless of whether you are moving towards it or away from it. This contradicts everyday experience. Obviously, if you run towards an approaching car it reaches you faster, and if you run away it comes slower.

This led to Einstein’s strange conclusion that if there are two people, one of them running towards the light, and the other one standing still, time moves slower (and space stretches out) for the running person. The speed of light remains constant for them both. In other words, time and space are relative; the only constant is the speed of light.

In a similar vein, we can add, everything in the world is temporal and inconstant; the only absolute value is the light of Torah.

 

This entry was posted in Uncategorized. Bookmark the permalink.