Category Archives: QED

Location or Momentum

Space Guy

Bruster Rockit: Space Guy!                           by Tim Rickard

A key element of quantum mechanics is Heisenberg’s uncertainty principle, which forbids the simultaneous measurement of the position and momentum of a particle along the same direction, as so aptly illustrated by Tim Rickard above. 

E = c \, p \!  for a photon, where E is the energy, c is the speed of light and p is the momentum.    So the momentum of a photon is equivalent to the energy of the photon divided by the speed of light or p =  E/c  where E is also related to the frequency of the photon by Planck’s Constant E = hf.   h is Planck’s constant and f is the frequency assigned to the photon.   f is also related to the wavelength of the photon by f = c/λ.  

So E = hc/λ = cp       Therefore    p = h/λ

But we know the values for both h (6.26×10^-34 joules sec.) and for λ if we know the color of the photon.  Usually if we are dealing with coherent light (red laser for example) then we know the wavelength λ very accurately.   Thus we know the momentum very accurately.

There is another factor in this equation – spin angular momentum of the photon which is independent of its frequency.  Spin angular momentum is essentially circular polarization for a photon.  Angular momentum is ±h/2π.   It is the helical momentum of the photon along its flight path.   In order to pin down the momentum we also need to know its angular momentum, but it is a constant that is either spinning one way or the other, no half spins no quarter spins just +h/2π or -h/2π.   

The key for this discussion is that we know the momentum for any photon if we know its wavelength.   p = h/λ and the direction of its spin ±h/2π.   According to Heisenberg’s principle we cannot know the location of the photon if we know its momentum.  Since we do know its momentum we are at a loss to try to pin the location to a particular spot such as through a narrow slot or pinhole.  

Whenever we try to fit a photon through a slot, we are trying to pin down the location as it goes through the slot.  The narrower we make the slot the closer we are trying to pin it down.   Nature resists by causing havoc with our measurements – fuzzy behavior/weird effects.

Pair Production

Pair production is a possible way for nature to slip one by us – putting a photon through both slots simultaneously, thus confounding our measurements completely.   When a photon hits an obstacle such as the thin barrier between the two slots, it melds through the slots around the barrier as in my earlier posts or possibly down-converts to a lower frequency pair of photons (or up-converts to a higher frequency) through pair production (conserving energy by the frequency change).  These pairs recombine on the far side of the barrier through an up (or down) conversion process causing an effective interference due to jiggling in the conversion process. 

Our barrier strip knocks the photon silly, and it responds by splitting up, zipping through the two slits independently, then recombining in a way that looks like interference.

Virtual Photons 

Another type of pair production would be through creation of a virtual photon – a pair with one real and one virtual as also mentioned in an earlier post.   The scenario is the same – barrier knocks photon silly, virtual photon forms, passes through other side, then effectively recombines while interfering with the “real” one.   The original and virtual photons could actually be down converted or up converted photon pairs that recombine by up or down conversion causing interference-like behavior.

In either case, blocking one slit or the other would prevent melding and also prevent pair production as well as the formation of virtual photons.

Pair production through down/up conversion and/or virtual pairs would fit better with particles with mass acting like waves that cause interference when passed through slits.  Even bucky balls and cats could potentially form virtual pairs if moving close to the speed of light.   Well, again, maybe not cats.

 Oldtimer

Quantum Weirdness – A Matter of Relativity? Part 3

Quantum Weirdness

A Matter of Relativity?

Copyright 2006/2007 James A. Tabb

Part 3: Polarized Light Weirdness

Figure 7 shows calcite crystals in which the light is split into two parts, a horizontal (H) and a vertical (V) channel. If we 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.

Polarized crystals

Figure 7. Photon in Calcite 

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 45degrees right or left. If we orient the input to 45 degrees, tilted right (+45), 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 or vertical polarized, no longer polarized at +45. 

Reversed Crystals 

Figure 8. Reversed Crystals

Now comes the weird part as shown in Figure 8. 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 H or V polarized photons we expect only H or V polarized photons out of the second crystal.

However, if we test the polarization of the output, we find that the photons coming out are oriented to +45. Individual photons go in at +45 at the input, become individual H or V oriented photons in the middle, but come out oriented +45 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.

Relativistic Effects Again

When you apply relativistic effects to this scenario the effect is exactly the same. The calcite crystals and all the paths are initially zero length as the photon approaches them.

The crystals and the paths expand as the photon enters them until the photon spans them end-to-end of the entire experiment. The two paths are separated by zero distance and have zero length and can be treated as if they are only one path. The experiment has no depth.

Much like the case of the double slits, if the paths are complete such that the edges of the photon can “feel” itself through the crystals when it hits them, it partially separates and then melds together at the front to recombine without ever becoming two parts while maintaining its integrity, in this case +45.  If the paths are not complete, it is forced to choose one path or the other. When it is able to meld around a path, the recombination restores the wave polar orientation.  When it can’t, the recombination does not occur and the polar orientation is destroyed.

Once again, the effect is due to the relativistic effects of time dilation and length distortion/contraction for the photon.

Next:  Photons that hit tilted glass weirdness explained. 

Quantum Weirdness – A Matter of Relativity? Part 2

Quantum Weirdness

A Matter of Relativity? 

 

Copyright 2006/2007 James A. Tabb 

Part 2 – Double Slit Weirdness

When 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 and/or bent by the edges of the slit as shown in Figure 3.

Single Slit Diffraction
Figure 3. Single Slit Diffraction

 If 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.

 Double Slit Diffraction

Figure 4. Double Slit Interference

The result is an interference pattern (light and dark bands) on the screen as shown in Figure 4. If you block either of the two middle slits, the interference pattern disappears. If a photographic film replaces the screen and the intensity is reduced so that only a few hundred photons are sent through the double slits before the film is developed, the interference pattern will be made up of individual dots organized in a pattern that duplicates the interference pattern. Keep the film in place long enough and the patterns become more complete. Put a cover over one of the slits and the film still shows dots, but no interference pattern, only a diffraction band. Put a detector in one of the slits and the interference pattern also disappears.

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.

Part of the current explanation is that the photon goes both ways, but any 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. How do we explain this quantum weirdness?

A Matter of Relativity

There are two processes going here. One process is the real time that our experimenter sees, about 1 nanosecond per foot of photon travel. The photon is traveling through the experiment with real and measurable delays from the emitter to the first slit and from there to the double slits and from there to the film. The other process is that the photon’s relativistic path is zero so it is in contact with the film and the emitter at once and all of its paths in between are of zero length and require zero time. All paths that can lead to the same path are conjoined. Time of flight and distances for the photon expand only as it passes through the setup. The photon and the observer see simultaneous events differently. All the events are simultaneous to the photon, but none are to the experimenter.

All the elements of our experiment have no depth and seem to be congruent as if they were paper cutouts that have been bonded together with the emitting source. As observers, we can’t see it. As the photon leaves one element of our experiment, such as the first screen with one slit, the double slits are squeezed down to a point and plastered across its nose. The photon easily fits across both slits of the second screen as the distances to them are zero and thus the distance between them is also zero. Indeed it fits across the entire second screen, but the edges are less distorted. Since the photon is also plastered across the slits, everything behind the slits is also plastered there – the entire path is available at one instant as in Figure 5 a. The photon is able to take all paths (even simultaneously) that lead to a common point because they are all in front of it as it enters our experiment, and zero distance separates all the paths. No amount of fiddling with flipping mirrors or detectors will fool the photon into disclosing its path because the mirrors and detectors are also plastered to the photon’s nose throughout its (instantaneous) flight. The mirrors and detectors are in place when the photon makes its decision or they are not. The result is path shut or open.

As the photon moves from the first screen to the second, the second screen moves with it (attached to its nose) until it reaches its normal (real world as we see it) dimension and then expands as the photon moves into the slits as in Figure 5 b. Portions perpendicular to the path of the photon become normal size and atoms from the edges again buffet the photon.  Everything behind the photon is of no consequence, gone – vanished.

Relativistic Double Slit

Figure 5. – Relativistic Double Slit

 From the relativistic point of view, the photon has a number of crisis points such as within the first slit. As it passes through the first slit, the atoms at the edge of the slit buffet it and the photon’s path is randomly diffracted from the original path.   The slit has grown to normal size (perpendicular to the photon’s travel) but now the photon is virtually attached to the entire screen containing the double slits in the background that represent the next crisis point or wakeup call. If neither slit is blocked, it has an opportunity to go through both.

Photon Recombining

Figure 6. Photon Recombining

I see the photon as being a packet of energy that obeys the laws of conservation of energy. It flows around the barrier between the two slits only if it can recombine on the other side without ever completely breaking into two separate pieces. It behaves almost like a perfect fluid and leaks through where it can, but unlike a perfect fluid, it cannot separate into multiple “drops”.

If the packet can meld behind the slit spacer as in figure 6, it does so before it separates in front of the spacer. The melding process takes place an integral number of wavelengths from the slits and results in a change in path that leads to an impact in the interference pattern, a pattern that can be calculated using the methods of QED.  As soon as the melding takes place, the photon separates in front of the slit spacer and begins joining the rest of the body already melded together, so that the photon is always a full packet of energy

If melding does not take place because of a blocking detector or some other shield, then the photon pulls itself into whichever slit passed the bigger portion of its packet and slips through that slit whole.  If it is the blocked slit, it is destroyed there.  If it is the unblocked slit, it comes though whole but does not interfere with itself because it did not meld around the slit due to the blockage in the other slit.  It may also be destroyed by the slit itself.   The photon is destroyed in the blocked slit or on the film behind the open one, never both. It makes no choice. In the case of a blocked slit, there is no recombination. The side with the larger energy pulls the photon through an opening if there is one and if that opening has a detector or blockage, it dies there.

The answer to the weirdness of photons seeming to interefere with itself is that it is due to the forshortening of the experiment due to the effects of relativity.

Next:  Polarized Light Weirdness Explained

Quantum Weirdness – A Matter of Relativity? Part 1

Quantum Weirdness

A Matter of Relativity?

Copyright 2006/2007 James A. Tabb

Part 1: Introduction and Photons In Glass

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.

Photons and Relativistic Effects:

I suggest that most of the difficulties we have in addressing the various weirdness phenomena at the particle level can be traced to relativistic effects. It all comes down to the two different simultaneous viewpoints: The one we can see and measure, and the one the photon experiences. Relativistic effects rule the photon world and our life experiences rule ours.

Consider that photons travel at the speed of light and thus experience relativistic effects. What are these effects? Einstein gave us some tools to work with to describe the various space-time relativistic changes as shown in Figure 1. There is a mass equation also, but the mass increase is not a factor here, since we know that the photon has no rest mass.

Relativistic Equations

Figure 1.   Relativistic Effects at c

The photon’s clock stops because the time between clock ticks becomes infinitely long at c. Similarly, the distance traveled becomes zero because the photon’s unit inch becomes infinitely long and stretches to the end of its journey in one bound. In other words, the entire path is foreshortened to zero length, and everything in its path is compressed to a dot.

We, on the other hand, see the photon from our experimental perspective. Photons move at speed c, take a nanosecond to go about a foot, take centuries to go from a nearby galaxy to earth, all of which we can measure or calculate with confidence and confirm with experiments.

The Photon’s Go-Splat World

The photon lives in a “go-splat” world. The clock of a photon completely stops the instant it is emitted and stays stopped throughout its journey. The distance traveled by a photon becomes zero as compared to the distance measured by the stationary observer. It may take a photon a billion years to cross from a distant galaxy to our telescope from our perspective, but for the photon, as soon as it is emitted, it arrives – splat; there is no time elapse in the photon world. In effect, the space and time between the photon’s emission and its destination are severely warped.

Therefore, the photon’s world is flat and stapled together, front-to-back, between its start point and its end point. In effect, the photon is touching its emitter on one end and our eye on the other with zero depth of field. Whatever phase it has at the time of emission, it has when it hits our telescope because it is all frozen in time. Physicists call the time experienced by the photon null time and the path the null time path.

It is this stapled together, zero time world that I believe explains much of the quantum weirdness we experience. Our life and experimental experiences are so strong that we can’t easily get our minds around the relativistic phenomena.

What the photon would know of the experimental setup, whatever it is, consists of wake-up calls at various edges or medium changes and eventually wherever it is absorbed in our screen or detector, all zero distance apart. This is vastly different from our perspective where everything is so carefully laid out, separated, calibrated with finite distances and photon flight times.

From our perspective, if it is going across a table, it moves about a foot every nanosecond. If it is going across the universe it takes years, even millions or billions of years to get from there to here. However we see it or calculate it, the time it takes for the photon’s lifetime is always zero. Go-Splat! As soon as it leaves on its journey, it arrives.

Quantum Weirdness in Glass

One of the weird aspects of photons involves reflection from glass of varying thickness. Send a laser pointer beam perpendicular to a pane of glass and about 4% of it will reflect back, on average, but, by carefully selecting glass of various thicknesses, the reflections vary from 0% to 16%. Glass a foot thick can be slightly adjusted in thickness to not reflect at all! All the light goes into the glass – perfect transmission. QED easily shows how this works for light beams. Rays from the back of the glass interfere with the rays coming in the front so as to cancel the reflection if the wavelength is a multiple of ½ wavelength.

However, the cancellation at ½ wavelength also works for individual photons for thick glass, and there seems to be no answer other than “quantum weirdness”. How does an individual photon know how thick the glass is the instant it hits the front surface when the back surface is thousands of wavelengths away? The reflected photon would be six feet away before a copy could make a round trip through a foot thick piece of glass. (Two feet round trip at 1/3 speed of light in air)

Quantum Weirdness and Relativity

Lets look closer at our foot thick piece of glass. The photon is moving at c and from a relativistic perspective our piece of glass has zero thickness (our entire experiment has zero thickness) as shown in Figure 2a.

Photon in thick glass

Figure 2. Photon in Glass

Immediately after impact, a full  half wave of the photon fits completely into the glass (2c), no matter how thick. The photon’s wavelength in glass is only 1/3 of its air wavelength. If the thickness of the glass is a multiple of a half-wave of the (shortened) photon, the photon will go right on through without reflection. Otherwise, depending on the thickness, some percentage (0 to 16%) of them will reflect.  In effect, the glass collapses to zero thickness if it is an exact multiple of the half wavelength, and if not, there is an overhang on one of the collapsed thicknesses that determines the probability of reflection.  Thus the photon does not have to “wiggle” its way to the far side and back to make its decision. If it is going to reflect, the decision is immediate due to the glass being foreshortened to fit the photon. It is, in fact, relativistic foreshortening of the glass.

Note, although the surfaces in the drawing above and those that follow are drawn with straight lines and flat, they are shown that way only for illustrative purposes. At c, all the points in the direction of travel are pulled to one point at the nose of the photon because they are zero distance apart to the photon, and surfaces near the path are severely bent.

It should also be noted that, once within the lattice of the atoms of glass, the atoms to each side of the photon resume their normal spacing and are no longer foreshortened. This is because they are perpendicular to the direction of travel. Those atoms in front continue to be shortened to meet the photon. Thus the photon length and the glass thickness exactly match, regardless of thickness, if the glass is an exact multiple of a half wavelength.  In that case, the photon completely enters without reflection. If the thickness does not fit the wavelength of the photon exactly, there is a crisis due to a mismatch in which the glass is not quite zero thickness to the photon. The probability of reflection depends on the degree of mismatch, but the reflection decision is made while the photon is still at the front surface and just inside.

There are two effects going on simultaneously: The relativistic effects for the photon and the realistic effects for the observer. The photon fits within the entire experiment (zero thickness, no wiggle time due to no time elapse) while we, as the stationary observers, see the entire experiment where the photon is traveling at c and has to wiggle 130,000 times to get through the glass in a measurable time (about 3 nanoseconds for a foot of glass). One case of quantum weirdness explained by relativistic effects.

Next: Explaining Double Slit Weirdness

Random Thoughts About Relativity

Facts About Relativity  

In order to introduce some of my ideas, it will be good for the reader to become familiar with some of the weird behavior of particles traveling at very high speed, high enough to invoke relativistic effects.

As seen by a stationary observer:

1) The closer a moving object gets to the speed of light, the slower its moving clock gets.

At the speed of light, it is zero – to the moving object, everything is simultaneous.  Start, Splat. The moving object sees the outside world as distorted, getting shorter in length, and at c, the length from here to there is zero, no matter how far the stationary observer measures it.  Photons live in a go-splat world.

2) The closer a moving object gets to the speed of light, the shorter its length gets.

At the speed of light, it has zero length to the stationary observer, but normal length to the moving object.  Everything seems normal to the moving object  until it gets to c – the problem for the moving object at c is that there is no time to seem normal – everything is instantaneous.

3) The closer a moving object gets to the speed of light, the larger its mass gets due to kinetic energy increase (for objects that have mass).

At the speed of light, an object with mass would have infinite mass.   This rules out object with mass ever getting up to c.  Photons do not have mass so they can move at the speed of light.   Nothing with mass can go that fast.

4) The closer a moving object gets to the speed of light, the more energy you have to use to get it there.

You have to give more and more energy to the object to get it  closer and closer to the speed of light. Energy equals mass times speed of light squared.  At the speed of light, the energy required is infinite.  You can never push an object with mass that hard.

What is the equation that describes the way in which time slows down as you approach the speed of light?

The equation is known as the time dilation equation and is:

Δ t = Δ T/ √[ 1 - (v/c)²]    Time dilation

Where  Δ t is the moving object time ticks and Δ T is the stationary object time ticks, v is the velocity and c is the speed of light.

When the velocity approaches c, the term v/c becomes very close to 1 and then the term Δ t becomes very large because the right side is divided by a very small number approaching zero.  This means that the distance between clock ticks gets very long for the moving object.   Time begins to stand still as it reaches the speed of light because the distance between tics becomes infinite.

What happens to space (in direction of motion)?

Δ x = Δ X/√[ 1 - (v/c)²]      Space distortion

Where  Δ x is the ruler mark as measured by the moving object and Δ X is the ruler mark as measured by the stationary object.

When the velocity approaches c, the right hand term approaches infinity.  essentially, a unit measure, such as an inch for the moving object would stretch millions of miles as measured by the stationary object at speeds near c.  

conversely, a foot long ruler moving near c would be invisibly short as seen by the stationary object – a term called foreshortening

Conversely again, the stationary object would seem impossibly close and impossibly short to the moving object near c.  At c, neither could see the other even with the best of instruments until they collide, which would be instantaneous for the moving object.  (To do so would imply that the image was moving faster than c.)

 So someone (very small and massless) sitting on a photon would think they see time normally, but the time of flight would seem to pass instantly from time started to time finished because no time would elapse (Δt very large).   Of course there would be no time to measure time (or even think about it) because the photon would instantly hit the other end of its path, no matter how far away that is.

Someone sitting and watching nearby would see time normally (from their perspective), but in their case, ΔT would be very short (time interval ticks near 0) and they would seem to age quickly compared to the someone riding on a photon.

The total time of flight might seem 100 years to an observer, but seem instantaneous for one traveling at the speed of a photon. The observer would age instantly according to the one moving fast, and the observer would think the one moving quickly didn’t age at all.   Weird isn’t it?  Weird but true.

Similarly, distance gets shorter as an object approaches c as seen by the observer and longer for the observer as seen by the object that is moving fast.

In other words, the time that passes in one time frame (Δ t) is the time that passes in another (Δ T) divided by the square root of 1 minus the velocity squared divided by the speed of light squared.

Enough of this – keep in mind that photons don’t have time to age, and photons arrive the instant they are emitted.  A photon emitted in the furthest star that we can see by telescope arrives the instant it is emitted.   (From the photon’s point of view).   They live an instantaneous “go-splat” life.

From our point of view it may have taken billions of years to get here.  Both viewpoints are valid.  That is the weird nature of relativistic speeds.  Time and space are distorted. 

One last thing:  Effect of speed on atoms:

Atoms are flattened in the direction of their motion.  Normally about 10 -8 cm in diameter they change from a sphere to a flattened disk as they approach the speed of light (from our stationary perspective only).

Particle accelerators have to be designed to account for both time dilation and space contraction in order to work. 

Atoms have mass so they can never reach the speed of light, but particle accelerators push particles, including atoms, to very high speeds that require design changes to keep them on track around their path – changes that involve the equations above.

Next – some of the quantum weirdness explained, example by example from the earlier posts.

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

Quantum Weirdness in Glass

  Perfect Transmission

One of the weird aspects of photons involves reflection from glass of varying thickness.  Send a laser pointer beam perpendicular to a pane of glass and about 4% of it will reflect back, on average, but, by carefully selecting glass of various thicknesses, the reflections vary from 0% to 16%.    Glass a foot thick can be slightly adjusted in thickness to not reflect at all!   All the light goes into the glass – perfect transmission. 

 QED easily shows how this works for light beams.   Rays from the back of the glass interfere with the rays coming in the front so as to cancel the reflection if the wavelength is a multiple of ½ wavelength.

However, the cancellation at ½ wavelength also works for individual photons for thick glass, and there seems to be no answer other than “quantum weirdness”.    QED cannot explain it for single photons.

Photon on Glass

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How does an individual photon know how thick the glass is the instant it hits the front surface when the back surface is thousands of wavelengths away?   The reflected photon would be six feet away before a copy could make a round trip through a foot thick piece of glass.  (Two feet round trip at 1/3 speed of light in air).

The photon has a number of options to reflect.  It can reflect at the front surface of the glass,  the back surface, somewhere within the glass, be absorbed or go on through.  The reflected and absorbed cases mean it does not exit the other side, and the latter one means it does exit.  

If the glass is exactly the right thickness, it does not reflect and shoots right through.   The rub is this: a photon that reflects off the front surface of a foot thick piece of glass has to make that decision at the front surface before it goes in.   As soon as it hits that surface, it begins to move away in the other direction.  The probability for reflection depends on the thickness of the glass.  However, the photon cannot know the thickness in advace.  Or can it?  I have a theory that tells how this works.

This is only one of the glass problems.   Next I will tell you about the weird effects for photons that hit tilted glass

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. 

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Double Slit Setup

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Double Slit Setup

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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. 

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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.