A profoundly light matter

It is time to think again.

I began my essays with a quote from one of last century’s top ten physicists and my favourite science writer, Richard Feynman. I would like to continue this essay with another quote, from the second chapter, or lecture, of his book The meaning of it all: “I agree that science cannot disprove the existence of God, and that a belief in science and religion is consistent. I know many scientists who believe in God. It is not my purpose to disprove anything.”

In this essay I want to get closer to metaphysics, but I want to leave out the religious aspects entirely.

We should all agree that when we look at a star that is a thousand light years away, we see the image of that star as it was a thousand years earlier; that is we are looking back into the past. As we stand on this earth we are seeing objects around us that are nearly in our present. For instant we see the sun as it was eight minutes and twenty seconds earlier. At our present time we can see stars as they were in the past. You don’t need to be an Einstein to understand this; I could easily accept this as a child. Now here is my point; if there were a universal observer, who could see throughout the universe, that observer would see us in our present and at the same time see the past image the distant star beside us. Half way between us and the star, the universal observer would see the star as was five hundred years ago and as it will appear in five hundred years time on our earth. It follows that any universal observer who is spread throughout the universe must be effectively outside of time as we understand it.

You might ask can a universal observer see into the future? I don’t know. Such a universal observer is beyond my comprehension. However, Einstein and others have assured us, that for us humans on earth, it is physically impossible to visit or see into our future.

The strange electron

There are two fundamental particles that almost completely determine what we experience and feel as we live on this earth. These are electrons and photons.

Electrons are minute identical particles; each has the same electric charge and the same mass but as things stand we cannot give electrons a volume. They also have a number of properties that we cannot explain with our classical physics (think of quantum numbers).

From our common point of view, using sensible classical physics, we understand that electrons can generate the light we see, as well as all other forms of electromagnetic radiation. They control the chemistry that shapes us and the world around us and all the technological devices that mark the twentieth century as the “age of electricity”.

You may be tempted to ask: Why can’t we measure the volume of an electron? I can give a classical explanation but the problem arises from our inability to understand or comprehend quantum behavior. Any plausible explanation on my part would leave you with the false impression that it is at present too technologically complex to measure an electron’s volume, but someday soon our clever scientists will find a way of doing so.

Let me give you a simpler example of how we simplify our explanation of the behavior of electrons. It is well believed and taught that electric charges flow through a metal wire when a switch is closed or a voltage is applied. When we apply our classical theories of electron flow through a conductor our calculation tells us that the current will flow somewhat slower than one millimeter a second. According to this model, when we turn on a bedroom light at night we will need to wait many hours before the light appears. The more sophisticated classical model suggests that the electrons don’t flow; rather an electromagnetic field travels outside the wire at the speed of light and when this field reaches the light bulb, other electrons are induced to activate the light bulb. While this latter explanation is more correct, it isn’t taught to school pupils or trade-persons because this more complicated model is of little practical value and doesn’t help practical minds to understand the nature of a closed circuit. It is often expedient to grasp a useful but wrong, or excessively simple, solution rather than labour for ever to find the real truth! At the same time it is obviously ignorant or arrogant to suppose you know the real truth

The stranger photon

Photons are minute particles that always travel at the speed of light. They have no charge, nor do they have a resting mass; obviously if they are always moving at light speed we cannot stop one to weigh it, as it would no longer be a photon. However photons have an inertial mass because they generate forces when they collide with matter (think solar sails). Photons also have a weight; the observation that starlight “falls” under gravity as it passes close to the sun was the first experimental proof of Einstein’s general theory of relativity.

Even before it was shown that light travelled as photons, it had been shown that light must come from quantized sources. This is no classical explanation for this. The new quantum theory gave the energy of a photon as being directly proportional to its measurable frequency. (The momentum is proportional to the reciprocal of the wave-length.)

Einstein considered the way in which light freed electrons from their host metal atoms. It had been found experimentally that photons with too low energy (frequency) could never free electrons no matter how intense the light. Further, very weak light had very few photons but if these had sufficient energy then this weak light would still free electrons into the metal. Thus the light carried energy, not as waves, but as packets, or particles, of light energy. The electrons binding atoms could only gain a quantum (or packet) of energy from individual photons on a one to one basis; that is as particle to particle. Thus it is that photons with the appropriate energies are absorbed and emitted by the electrons in atoms or molecules.

In grappling with these strange and initially confusing facts we have to leave our intuition behind and start to create “unreal” theories. Even Einstein, himself, had problems accepting the “spooky” aspects of quantum theory. He did of course have a profound understanding otherwise.

The first easy problem

For most of what follows I will be sketching details from Richard Feynman’s book of lectures on QED the strange theory of light and matter. I have elsewhere referred to his first exposition of these matters in the opening chapters of the third volume of The Feynman lectures on physics. The QED lectures came about twenty years later after he had constantly refined and refreshed his discussion. If you can read Hawking or Davies, you should cruise through this simple but authoritative text.

It is well known that if light of shines perpendicularly onto a flat transparent glass surface, then about 4% of the light is reflected back. This result is predicted by the classical theory, but there is one theoretical assumption that should be reconsidered. The calculation assumes that the electromagnetic fields reflect from just the abrupt surface of the glass. I have elsewhere mentioned that about one million, millionth of this glass is solid matter the rest is vacuum with tiny electrons somehow holding the glass together. This means the incoming light encounters an area which is one hundred millionth of solid material and the rest is just empty space with a few tiny electrons. We cannot suppose the classical model of electromagnetic fields bouncing from a discontinuous surface gives a real description of what happens; even though it gives the correct result. The scale of the encounter is more like a meteorite entering our solar system. Sooner or later, somewhere near the glass surface, an electron will attract and absorb a photon and then either send the identical photon back the way it came, or resend the photon in the direction it was travelling.

Now how does this look from a quantum point of view? Well if a hundred photons reach the surface of the glass then for some unknown reason four photons will reflect and the other ninety six will continue into the glass. There is no way we can predict ahead of time how any particular photon will respond; nor can we trace any distinctive history of this event. This dilemma is the origin of the now famous statement attributed to Einstein; “The good lord doesn‘t play dice.” At a more complex level it indicates the origin of the famous Schrödinger paradox; which in short says, if you open a box and find a live cat, you cannot prove the cat was alive before you opened the box.

Before continuing I would like to add some clarifying remarks. Flat means ‘optically’ or perfectly flat, no bumps or cracks etc. Parallel will mean precisely parallel. Transparent means that any absorption of photons need not be considered. The number in 4% is not a precise integer and if a hundred photons go into a glass surface then three or four or five might be reflected; four represents the long term average of many events rounded to the nearest whole number.

Another side to the story

When light shines perpendicularly onto a flat transparent surface, of a parallel sheet of glass most of the photons pass through the transparent glass. Some photons are reflected by the first, front surface and as expected some photons are reflected from the inner, back surface between the glass and the outside air. The classical calculations suggest that the percentage of photons reflecting at the back surface is also 4%.

So far so good, but here is the problem: Depending on the thickness of the glass photons of a given colour/wavelength do not reflect back from the glass sheet at all. This effect is the basis of the light spectrum that can be seen reflecting from the sheet of oil that forms as an oil slick on water.

Here is what is found experimentally: When light of one colour (monochromatic) falls as a perpendicular (normal) beam (or ray) on a parallel sided piece of glass, then for some thicknesses of the glass, no light is reflected at all. And yet for other thickness, the reflection can be up to 16%. This can be explained classically as interference of the light as it passes backwards and forwards through the glass.

Now consider what is happening using the quantum theory of photons: Identical photons hit a glass slab with a thickness that ensures no reflection. If there was no second surface, then 4% of these of these incoming photons would reflect. But, how can the arriving photons know that there is a second surface that prevents any of them from reflecting from the first surface? The same question arises when the thickness is adjusted so that 16% of the photons will reflect. Why do 16% of photons reflect without knowing the thickness of the glass? Photons can anticipate what lies ahead, or future events.

If you are thinking that there is some catch to what I am telling you, or I am trying to tease you; think again. I have isolated some of the problems that the best physicists have been grappling with for over eighty years. Nothing that I am presenting is new or controversial.

Interference

When moving waves, in a common medium and of the same wave length come from two synchronized sources and combine together, then interference may occur. Here is a textbook illustration showing the interference of water waves in a ripple tank.

Two sets of waves are generated and radiate out so that they meet and combine. Where the waves are opposing each other there is no disturbance of the water as indicated by the grey areas or bands; where the flattened waves meet the wall (at the top of the diagram) no disturbance occurs. When the waves reinforce each other, stronger waves are created and the waves will arrive and break against the wall.

When single coloured light shines from a common source through 2 very narrow parallel slits the image formed on a screen looks like this:

This experiment was done in 1805 and was considered as the definitive proof that light radiates as continuous electromagnetic waves and not particles. Clearly the light waves are combining in the same manner as the water waves interfere in the first picture.

When it was found that light travels as photon particles, confusion reigned in the best minds of physics. It took physicists about twenty years to accept that light travelled as photons and these photons were manifest in wavelike patterns.

With this problem in mind, an obvious experiment was carried out. Why not do the 2 slit interference experiment with light-waves that are filtered down so that if photons exist then they would be going through either of the 2 slits one at a time. As photons can only go through one slit at a time would they still give an interference pattern? When the relevant experiment was done (it needed an exposure time of months) it was found that extremely weak light still produces an interference pattern. As Einstein had pointed out, even though the light was weak the photons still carried enough punch to knock electrons away from their atoms and trigger a detection system. As light detectors improved it was soon found that weak light could indeed be detected as individual photons arriving … click-click—click-click—click-click etc. Photons apparently go through one slit, but carry an awareness of the other slit; they also know that there are no other slits and they know the size of both slits. Photons have a complete knowledge of their surroundings.

Electrons too

As physicists grappled with explaining reality, the next obvious question was asked: If photons interfere, do electrons also interfere? By 1930 it was shown that they do indeed interfere, just like photons. If a beam of electrons with a common energy emerge from a single source and pass through two slits then an interference pattern is seen that is much like the pattern for light. The single electron at any time, 2 slit experiment was done by M. P. Silverman 1986 using a state of the art electron microscope. Here are two slides from Silverman’s video of the formation of the single electron, 2 slit diffraction pattern: After about 100 electrons had been recorded a screen showed:

After approximately 70,000 electrons had been recorded the screen showed:

Silverman’s result was not a proof, just a careful and graphic confirmation of a result that was brought to public attention by Feynman in 1964.

In case you are wondering why I have gone through all this, let me reiterate the problem we must come to terms with. Fundamental particles are sent towards two small openings in barrier and when they emerge they act as waves. In addition, they are aware of the details of the size and position of both openings. Thus electrons carry a complete knowledge of their surroundings.

We do not expect fundamental particles to break up send parts through each opening. If they could do this they would still need prior knowledge of how to break up and where to send the parts; this is unthinkable. The photons are travelling at the speed of light in straight lines, so they can’t poke around when they meet the barrier. The same applies to the electrons that are travelling at a significant fraction of the speed of light. In the cases of three or more slits the final pattern  will contain details of the number and size of the various openings. (This is the effect that leads to holograms.)

The standard explanation that I was given when I attended my undergraduate lectures is that each electron has a presence throughout the universe. Well, there is a finite chance that a particular electron will be found anywhere in the universe. All we can say is that there is a probability of finding an electron when we look for it. We can’t really say where it is, or, as we mentioned, how big it is. We also started out by saying that a “universal presence” cannot be fixed in time, so we can’t be sure where an electron comes from, or where it goes. Perhaps electrons teleport? As they are all identical it is not really meaningful to suppose we can track one particular electron. The same considerations apply to photons, as they have a similar set of quantum of properties.

If you are a little confused, don’t blame me, I didn’t create the universe!

Wait, there is more

You may be wondering why the particles spread out as they pass through the narrow slits. This is because of interference across the slit, more particularly called diffraction; this spread of a beam can be explained classically. It illustrates the Heisenberg uncertainty principle. If you try and squeeze a beam of particles into a narrow beam the photons simply spread out because of diffraction.

Again we can ask don’t the particles hit the sides of the slits? The answer is of course they do. But, if they do, they lose energy and consequently they have a wrong wavelength for interference; or they are delayed and are no longer synchronized with the other particles. Either way any particle interacting with the edges does not contribute to the interference pattern.

Feynman, himself discussed at some length the question of what would happen if we could record which slit each particle goes through. As you cannot extract any energy from a fundamental particle without changing the nature of that particle it is not really possible to do this, although modern technology approaches this sensitivity. From considering standard diffraction experiments it is clear that if any record is made of which slit was used then there will be no interference pattern, what appears will be the shadow outline of two slits spread out by diffraction. This result occurs whether or not we examine the record. Apparently nature will not let us pry too deeply into her workings!

Summary

The quantum world is counterintuitive.

  1. Quantum particles have a sense of the whole universe.
  2. Quantum particles are not bound in time as we are.
  3. We cannot completely determine the history of a quantum particle.
  4. Quantum particles have a knowledge of future events.
  5. The full nature of quantum particles is inaccessible to us.
  6. Return to the index here.