Category Archives: photon

Quantum Weirdness in Entangled Particles

Entangled Particles

Selecting which atom we use with careful attention to its excitation states can create entangled particles.  Some atoms emit two photons at a time or very closely together, one in one direction, the other in the opposite direction.  These photons also have a property that one spins or is polarized in one direction and the other always spins or is polarized at right angles to the first.  They come in pairs such that if we conduct an experiment on one to determine its orientation, the other’s orientation becomes known at once.   They are “entangled”.

Link to image EPR 

Figure 10 – Entangled Particles   

All of this was involved in a famous dispute between Einstein and Bohr where Einstein devised a series of thought experiments to prove quantum measurement theory defective and Bohr devised answers. 

The weirdness, if you want to call it that, is the premise that the act of measurement of one actually defines both of them and so one might be thousands of miles away when you measure the first and the other instantly is converted, regardless of the distance between them, to the complement of the first.   Action-at-a-distance that occurs faster than the speed of light?

Some would argue (me for instance) that this is more of a hat trick, not unlike where a machine randomly puts a quarter under one hat or the other, and always a nickel under a second one.  You don’t know in advance which contains which.  Does the discovery that one hat has a quarter actually change the other into a nickel or was it always that way?  Some would say that since it is impossible to know what is under each hat, the discovery of the quarter was determined by the act of measuring (lifting the hat) and the other coin only became a nickel at that instant.   Is this action at a distance? 

It is easy to say that the measurement of the first particle only uncovers the true nature of the first particle and the deduction of the nature of the second particle is not a case of weirdness at all.   They were that way at the start.

However, this is a hotly debated subject and many consider this a real effect and a real problem.  That is, they consider the particles (which are called Einstein‑‑ Podolsky‑Rosen (EPR) pairs) to have a happy-go-lucky existence in which the properties are undetermined until measured.   Measure the polarization of one – and the second instantly takes the other polarization.

A useful feature of entangled particles is the notion that you could encrypt data using these particles such that if anyone attempted to intercept and read them somewhere in their path, the act of reading would destroy the message.

So there you have it – Weird behavior at a distance, maybe across the universe.

Next:  Some Random Thoughts About Relativity

Quantum Weirdness for Tilted Glass

  Photons That Hit Tilted Glass

Individual photons directed at tilted glass have an option of being reflected or going through.  They can’t do both because they can’t be divided, or so we are told.  Yet some experiments seem to imply that they sometimes take both paths unless a detector is in place. 

Tilted GlassTilted glass acts like a sort of beam splitter.  It either goes through or bounces off  (or sometimes absorbed). 

QED can easily compute the probability dependent on the angle.  Some go through and some reflect and the angle makes the difference.  You can adjust the angle to get a 50-50 chance of reflect or go through.   

If you use other beam splitters to put the two beams back together you can get an interference pattern, not unlike the one depicted in the double slit experiment.   The beam goes both ways, but one path is longer and so when they come back together, they interfere with each other.

However, if you turn the light down so only one photon at a time goes through you still see the same effect, implying the photons go both ways.   If you leave the single-photon-at-a-time beam on long enough and have a good film in an exceptionally dark room, the outcome will be a well defined interference pattern. 

How can single photons being emitted minutes apart interfere with each other?   How can a photon that can only go one way or the other interfere with itself?   QED cannot explain this quantum weirdness for single photons.  It can predict the pattern but cannot explain it.  Every indication is that when no detectors are present, the individual photons somehow split.

There are some very sophisticated delayed choice experiments involving beam splitters.  There are super fast detectors that can be switched into the photon beam after it goes through the splitter. In other words, spit a photon at the splitter, calculate when it reaches it (about 1 nanosecond per foot of travel) and then switch the detector into the path behind the splitter.

The idea is to try to trick the photon into “thinking” there is no detector so it is ok to split, then turning on the detector at the last moment and try to catch the photon doing something it is not supposed to do, breaking laws along the way.  If it arrived at a detector in the reflected path and was also seen by the detector behind the splitter, some law has been broken and the mystery solved – figure out a new law. You do this randomly. If the photon goes both ways, it can be caught by the detectors.   It never does.

The physics says that if you try to make the measurement, it will disturb the experiment. And so every test seems to verify that fact. Whenever a detector is present there is no interference pattern. Whenever the detector is absent, the pattern reappears.

There is an argument that the photon must go through the glass whole since the photons transmitted through the glass are actually retransmissions within the glass, not the same photon that impacts it.  That argument then says that the other path has to either have had no photon or a whole one also (creation of energy not allowed).   It also says that the photon must retain a whole packet of energy.   Yet single photons seem to interfere with each other.  QED cannot explain why.   I hope to do so.

Next:  Entangled Particles

Polarized Light Weirdness

  Polarized Light Weirdness

Polarized CrystalsThe same weirdness problem arises when we pass light through polarized devices as in the figure at the left.  The devices are calcite crystals in which the light is split into two parts, a horizontal (H) and a vertical (V) channel.  If we try to send individual photons through, they go through only one channel or the other, never through both, and those that come out of the H channel are always horizontally polarized, those that come out of the V channel are always vertically polarized as we might expect.  

It is possible to orient photons to other angles at the input.  One such arrangement is to adjust them polarized so that they are tilted 45 degrees right or left as illustrated in the same figure.   If we orient the input to 45 degrees, tilted right, we get half of the photons coming out the H channel and half out of the V channel, one at a time, but these are always horizontal and vertical polarized, no longer polarized at 45 degrees right.  

Reversed Crystal setupNow comes the weird part.  See the figure at the left.  If we put a second calcite crystal in line with the first one, but reversed so that the H channel output of the first goes into the H channel of the second and the V channel output of the first goes into the V channel of the second, we expect the output to consist of one photon at a time (and it is), but since the first crystal only outputs horizontal or vertical polarized photons we expect only horizontal or vertical polarized photons out of the second crystal.  

 Quantum Weirdness at work.

However, if we test the polarization of the output, we find that the photons coming out are oriented to 45 degrees right, exactly like the input.  Individual photons go in at 45 degrees right at the input, are still individual photons but horizontal or vertical oriented in the middle, but come out oriented 45 degrees right again at the output!  Somehow the two channels combine as if the individual photons go through both channels at the same time, despite rigorous testing that detects only one at a time.  Quantum Weirdness at work. 

The polarization problem, like the double slit problem, is often called a quantum measurement problem.  An often-quoted theory is that the photon does go both ways, but any attempt to detect/measure one of the paths disturbs the photon such that the measurement results in a change in the path of the photon.

My theory reafferms the idea that it does go both ways, but in a manner you would not expect.  We will get to that later.  Next I want to mention  Quantum Weirdness in Glass

Double Slit Weirdness

Double Slit Weirdness

Single Slit ExperimentWhen a proper light source (coherent – light from a single source all at the same frequency) is placed in front of a screen with a narrow slit, the light is diffracted (spread out) as it goes through the slit and appears as a shaded band centered on a screen or photographic film.  The light is scattered by the edges of the slit as shown in the first figure.

Single Slit Diffraction. 



Double Slit Setup



Double Slit Setup




Double Slit InterferenceIf we add two more slits located side by side between the first slit and the screen, the light passing through each of the new slits is diffracted again such that the photons from each slit are bent across each path and combine to reinforce or cancel each other where they strike the screen.    The result is an interference pattern (light and dark bands) on the screen. 


If you block either of the two middle slits, the interference pattern disappears.  The diffracted light from the forward slit is diffracted again by the open slit in the second screen, but no interference pattern emerges because the other slit is blocked.

It can be shown that when both slits are open, the interference pattern seen can be duplicated by drawing the photon as a wave at the frequency of the source at each of the second screen slits and then combining the resulting waves by addition and subtraction of the waves where they mix behind the slits.  This addition and subtraction of the waves into the interference pattern seems to prove that the light is a wave.   Indeed, if the light is a particle, it would imply that their interaction cancels particles in the dark areas or at the very least, they bump each other into specific areas on the screen.   We know that does not happen. 

However, if you replace the screen by a photographic film and reduce the intensity so that only a few hundred photons are sent through before the film is developed, you will find that you can see the individual places the photons hit as dots on the screen.  You can also see that the individual dots are organized so that they fall into the bands of the interference pattern and duplicate it.  Keep the film in place long enough and the patterns become more complete.  Put a cover over one of the slits before you start and you will find that the film still shows dots, but no interference pattern, only the diffraction band. 

The dots seem to prove that the light is indeed a particle, not a wave, but yet they seem to interfere with each other like a wave when both slits are open.   A mystery, but not necessarily weird.

Quantum Weirdness at Work

Now if the light source is reduced in intensity enough to send only one photon at a time, a weird result can be seen if the photographic film is left long enough (days or even months in a very dark box) where both slits are left open.  The interference pattern continues to develop on the film, even though there is no possibility of interference (or even photon bumping) unless the individual photons go through both slits somehow. 

A test for this is to remove the film and put a detector behind the slits.  But then we find that the detectors only detect one photon at a time and only through one slit or the other, never both.   The interference pattern never develops if either slit is covered.  Quantum Weirdness at work.

There have been some very inventive tests such as using super-fast mirrors behind the double slits that switch in and out of the path between the time the photon leaves the slit and before it arrives at the screen.  The results are the same.   When either slit is covered, the interference pattern disappears but when both are open it reappears, even if the photons arrive only one at a time.

Since the photographic plates seem to prove that the light is a photon and never goes through both slits, the quantum weirdness problem arises and part of the current explanation is that the measurement (putting a detector in the path) always disturbs the measurement.  In fact a whole class of quantum theory has developed around the inability to make precise measurements due to the measurement disturbance problem.

Quantum Electrodynamics (QED) explains this behavior using simple diagrams to show the probabilities of where the photons will land.  It cannot predict where any one photon will land, but given enough photons, it can predict the pattern very accurately.   QED does little to explain the weirdness of it all.

I have a simple theory on how all this works that I will get to eventually.  Next I want to talk about Polarized Light Weirdness.

Introduction to Quantum Weirdness

Quantum Weirdness 

Quantum Electrodynamics (QED) theory has developed to be the theory that defines almost all of the understanding of our physical universe.    It is the most successful theory of our time to describe the way microscopic, and at least to some extent, macroscopic things work.

Yet there is experimental evidence that all is not right.  Some weird things happen at the photon and atomic level that have yet to be explained.  QED gives the right answers, but does not clear up the strange behavior – some things are simply left hanging on the marvelous words “Quantum Weirdness”.   A few examples of quantum weirdness include the reflection of light from the surface of thick glass by single photons, dependent on the thickness of the glass; the apparent interference of single photons with themselves through two paths in double slit experiments; the reconstruction of a polarized photon in inverted calcite crystals, among others.

This paper introduces some ideas that may explain some of the weirdness.

I want to introduce the subject in a way that appeals to the non-scientist public, but also introduce some ideas about what is going on, ideas that may explain some of the weirdness and include a few thoughts about the speed of light and relativity that should stimulate thought on the subject.  Hopefully a few physicists will look in and not be too annoyed with my thoughts.   This will not be a mathematical treatment other than some basic equations from Einstein that most of us are already familiar with.   The later chapters will be more theoretical, but easily understood if I do it justice.   I will include some experimental diagrams and discussion of results.

First let’s review a few facts about one of our most commonly known quantum objects.   Light is a quantum object.  When you see the light from a light bulb it is likely you do not realize that the light you see comes in very tiny packets called photons that are arriving in really huge numbers.   Your nearby 100 watt bulb emits around 250 billion billion photons a second!  A photon can travel unchanged completely across our universe from some distant star or across a few feet from a nearby lamp.   Once emitted, it continues until it hits something that stops it.  It lives a go-splat existence.

When we read this page, we are intercepting some of the billions of photons of light bouncing off the page, those that come off at just the right angle to illuminate rods in the back of our eyes.    Physicists tell us that photons are tiny bits of massless energy that travel at the speed of light.   These bits are indivisible; you can’t split them up into smaller pieces.   In transit they are invisible.

Here are some tidbits of information you will need to know later:

Every photon of a particular frequency has the same intensity (energy).     

If you make the light brighter, you are just making more photons, not changing the energy of the individual photons.  If you make the light very dim, only a few photons are being emitted.  Reduce intensity enough and you can adjust the source to emit one photon at a time, even minutes or hours apart.

The energy and frequency of blue light is higher than that of red light

The energy of each photon is dependent on the frequency of the light but not dependent on the intensity.   A brighter (more intense) light of a particular color is the result of more photons per second, not higher energy in the photons. 

Maybe I can illustrate some of the above this way.  Bird shot is a very small pellet load for a shotgun.  It is small and used for hunting birds.    If you drop a single bird shot pellet from a porch onto a pie pan below, it would make a small sound when it hit.  It would have a certain energy when it hit and every pellet of that size dropped from the same height would have the same energy.  The sound each makes at impact would have the same intensity.  If you dropped a hundred at a time, the energy of each pellet would be the same, but the combined impact and sound intensity would be much higher and louder.  Similarly all red photons hit your eyes with the same energy.  If you step up the current to the light source, the number that hits your eyes goes up accordingly, so you see a higher brightness as the number hitting the rods in your eye each moment is increased.  

Changing from a red photon (light) to a blue one is somewhat like changing from bird shot to buck shot, a much larger pellet.  The blue photon hits harder, as does the buck shot, no matter where it comes from.   

Regardless of color, if you make a light very dim, you can get it down to one photon at a time, sort of like dropping one pellet at a time.   Getting a photon down to one at a time is a bit tricky, much harder than getting a single pellet to pour out of a barrel of pellets, but not impossible.

Photons, unlike shotgun pellets have no mass, but they still have energy.  This energy is transmitted from whatever emitted it to whatever it finally hits.   Thus the photon is an energy carrier in a hurry, always moving at the speed of light.

Next I’ll tell you a little about an easily duplicated experiment using double slits that can be used to prove that light is a wave but also can be used to prove that light is a particle.  It is a good illustration of quantum weirdness.