Aharonov-Bohm Effect

Since I was busy last week and I’m feeling ill this week, my good friend Michael Schmidt has agreed to write a guest post for me this week. Mike has a masters degree in physics from the University of Colorado, an interest in teaching, and a passion for math and physics.  You can find out more about him on his personal website or read more on his blog, duality.io.

So, without further ado, here’s Mike’s article.

Force Vs. Energy

When we teach physics, usually force is one of the first concepts. Force is easy to understand. I can have you imagine riding in a car riding around a curved road. As the car accelerates, the seat pushes you along. When the car turns you can feel the seat push you in the direction of the curve. In fact, force is such an understandable notion we often neglect to ask what force is or if there may be a better way to talk about the world.

What is force?

Newton’s notion of force is the method which physically exchanges momentum. If two objects interact, they change each other’s momentum. Think of a two billiard balls bouncing off each other. If you placed your finger between between the balls you could feel a considerable force (don’t really do this, it will hurt). The billiard balls feel force due to the other and bounce off each other.

Now, this is how we speak about interactions for the most part. We draw force diagrams and use them to create equations we can solve. This, however, is not always so simple.

Let’s consider two similar examples where a ball bearing rolls (frictionlessly) down a slide: one where the slide is a straight slope and the second the slide is curved. Now suppose you want to find out how fast the ball will be moving when it gets to the bottom of the slide assuming it was nearly stopped at the top. In the first example at every point on the slide the effective force on the ball is constant. This is due to the slope being the same, what is true for one part is true for any other. Since the force is constant we can use the constant force equations to solve this.

Two similar cases of a ball bearing sliding down a slide, one a simple straight slide, and a more complicated slide second.

Now, the second case. This situations is substantially more difficult, we need to recompute the force for every point along the slide.
We can’t use any convent equations we have to derive them. This can certainly be laborious and is not preferred.

Wondrously, there is a better way: energy.


Energy is a strange notion; unlike force, you can’t feel energy.
The rules of Newtonian mechanics can be used to create two quantities: kinetic energy (or KE) and potential energy (or PE). Energy, unlike force which has a strength and direction, is just a number. Kinetic energy, roughly, is how much work it takes to accelerate an object up to some speed, whereas potential energy is the capacity for an object to acquire kinetic energy. In other words, potential energy is energy that can become kinetic energy in the future.

Energy may flow between each of these types of energy but their total must always remain the same. To illustrate this imagine a spring fixed to a table on one end and let there be a weight on the other.

An oscillating weight on a spring. (From Wikipedia En:User:Svjo)
An oscillating weight on a spring. (From Wikipedia En:User:Svjo)

If you pull the spring to one side, stretching the spring, and release the weight it will move back and forth. When the weight is at the resting position of the spring, the weight will be under no force and will be traveling as fast as it can go, since as it continues to move it will be slowed again by the spring. It’s at this point the weight has it’s maximum kinetic energy and it’s minimum potential energy since the weight will not be sped up anymore. In contrast to this point, at both ends of the oscillation, the weight will stop. Here, we say the kinetic energy is zero and the potential energy is maximum.

If we use the notion of energy, we can make any situation like the bearing on the ramp nearly trivial to solve. This works since energy is allowed to be either kinetic or potential and the total must always be same. For the ball bearing on the slide example, the ball has only potential energy at the top of the slide and only kinetic energy at the bottom. We can represent this in an equation by

    \[PE + KE = C\]

    \[mgh + \frac{1}{2} m v^2 = C\]

Since C is an constant, you can make both sides equal for the beginning and end:

    \[mgh = \frac{1}{2} m v^2\]

We can then solve for the v and we would know the final speed of the ball. This method is has some obvious advantages, but all it seems we have done is find a quantity which hides the force.

What is Energy?

Potential and kinetic energy seems just to be abstractions of force. In other words, energy isn’t real, the force is. We just made up energy to make the math easier.

This certainly seems like the right answer, especially in the light of how we can actually feel force and energy can only be referred to in equations. Of course, I would not have said that if it were so simple. Quantum mechanics seemed to turn the scientific world on it’s head but could the notion of force be false too?

The Aharonov-Bohm Thought Experiment

Double Slit Waves/Interference
From Wikipedia’s en:User:Lacatosias and en:User:Stannered

This experiment begins with the double slit experiment, which shows the wave-particle duality of electrons. The double slit experiment has three elements to it: an electron emitter, a solid panel with two parallel cuts or slits in it, and a phosphorescent screen all arranged in this order. The setup is shown in the following image:

Double Slit Experimental Setup

As the particles move away from the emitter they pass through the slits and interact to create the interference pattern show here:

From Wikipedia's ￼ en:User:Lacatosias and en:User:Stannered
From Wikipedia’s ￼ en:User:Lacatosias and en:User:Stannered

The additional element added by the Aharonov-Bohm experiment is a very long solenoid encased within an impenetrable shell. The solenoid is place between the screens and the slits. A diagram for this is here:
Double Slit Setup (solenoid)

This solenoid will create a magnetic field inside itself but not outside. This means under our view of things that there ought to be no changes to the setup outside the solenoid as the magnetic field cannot possible be exerting forces on the electrons. Interestingly, as you change the magnetic field strength the interference pattern on the screen will move. This effect is named the Aharonov-Bohm Effect after its discoverers. How could this be though, there is no force on the electrons! In fact there is no magnetic field anywhere the electrons are. The answer is there is another field present, the vector-potential. The vector-potential is a way to abstract the notion of a magnetic field and it is non-zero outside the solenoid. If it were just a mathematical trick, we would say it being non-zero outside is a side-effect of the math and is inconsequential. However, as we see the strength of this field has a direct impact on our observable world.

This questions our assumption that force is the most primitive or basic of interactions. Perhaps our mathematical trick is the real thing. There is much debate about this and there is likely no simple answer. The notion of force isn’t useless but it does have it’s limits. Maybe at some point a future experiment will help out understand more. For now, we’re stuck without an easy answer.

Further Reading

If you would like to learn more about quantum mechanics Jonah has written a number of articles you might find interesting.

Jonah has a three-part series on quantum mechanics:

  • In the first part, he introduces particle-wave duality.
  • In the second part, he describes matter waves using the Bohr model of the atom.
  • In the third part, he describes how one should interpret matter waves.

Jonah also wrote a post describing quantum tunneling.

More recently, Jonah wrote a two part series exploring the relationship between particles and waves.

  • In the first part, he shows how you can build a particle up from a bunch of waves.
  • In the second part, he shows that this isn’t always possible.
Posted in Physics, Quantum Mechanics, Science And Math | Tagged , , , | 1 Comment

Spin and the Stern-Gerlach Experiment

Figure 1. A plaque at the University of Frankfurt commemorating the Stern-Gerlach Experiment. (Source)

The word “quantum” means a single share or portion. In quantum mechanics, this means that energy comes in discrete chunks, or quanta, rather than a continuous flow. But it also means that particles have other properties that are discrete in a way that’s deeply counterintuitive. Today I want to tell you about one such property, called spin, and the experiment that discovered it: the Stern-Gerlach experiment.

(The goal of the original experiment was actually to test something else. But it was revealed later, after the discovery of spin by Wolfgang Pauli, that this is in fact what Stern and Gerlach were measuring.)


The Stern-Gerlach experiment involves magnetic fields. So before I tell you about the experiment itself, I need to quickly review some of the properties of magnets.

As you probably remember, the north pole of a magnet is attracted to the south pole of other magnets and repelled from their north pole, and vice versa—a south pole is attracted to north poles and repelled by other south poles. In other words, opposites attract.

Suppose we generate a very strong magnetic field (say, with a very big magnet or with a solenoid) and put a small magnet in the field, as shown in Figure 2. What happens to it? The north pole of the big magnet will attract the south pole of the small magnet, and the south pole of the big magnet will attract the north pole of the small magnet. Since the north and south pole of the big magnet are are equally strong, these attractions will be equal and opposite, and they’ll cancel each other out so that the little magnet feels no net force. As a result, it doesn’t move up or down—it just hovers in place.

Figure 2. We put a little magnet in a big magnetic field and see what happens.

Now suppose we create a big magnet whose north pole is more powerful than its south pole, as shown in Figure 3. (It’s not actually possible to make a magnet with a stronger north pole than south pole. However, we can create the same effect by using multiple smaller magnets.) What happens now?

Figure 3. If we create a magnetic field where the north pole is much stronger than the south pole, the behavior is different.

To answer this question, we must understand that the strength of a magnetic force depends on the distance between the interacting poles; the closer the poles, the stronger the force. This means that the net force the little magnet feels depends on its orientation, as shown in Figure 4. If the south pole of the little magnet is close to the north pole of the big magnet, the little magnet will be pulled upwards. If, on the other hand, the north pole of the little magnet is close to the north pole of the big magnet, the little magnet will be pushed downwards. If the poles of the little magnet are the same distance from the poles of the big magnet, the little magnet will feel no force. And of course, anything in between is possible. A little magnet whose south pole is just barely closer to the big north pole will feel a weaker pull than a little magnet whose south pole is very close to the big north pole.

Figure 4. If we put a little magnet in a magnetic field that’s stronger near the north pole and weaker near the south pole, the force on the little magnet depends on its orientation.

The Stern-Gerlach Experiment

The Stern-Gerlach experiment, performed by Otto Stern and Walther Gerlach, tested whether subatomic particles behaved like little magnets. To do this, Stern and Gerlach created a magnet with a bigger north pole than south, just like the one described above, and shot a beam of electrons with random orientations through the resulting magnetic field. If electrons behaved like little magnets, then the beam would be spread out by the magnetic field, as shown in Figure 5. Some electrons would be pulled upwards, some would be pushed downwards, and some wouldn’t change direction, depending on the orientations of the individual electrons. But if electrons didn’t behave like magnets, then none of them would be affected by the magnetic field, so they would all just fly straight through.

Figure 5. What we expect to happen in the Stern-Gerlach experiment  if electrons behave like little magnets.

Surprisingly, although the electrons were affected by the magnet, they didn’t spread out as in Figure 5. Instead, the electrons split cleanly into two beams, as shown in Figure 6.

Figure 6. What actually happens when the Stern-Gerlach experiment is performed on electrons. The electron beam doesn’t spread out like a fan, but splits neatly in two!

That’s very weird! It implies that electrons behave like little magnets, but only sort of. A magnet can be oriented any way it likes. But an electron can only have two orientations: either aligned with the big magnet or aligned against it. So the electron can travel up or down, but it can’t stay in between. This is a distinctly quantum phenomenon—the electrons behave like magnets fixed into a pair of discrete orientations, or states, as opposed to a continuum of possible orientations. An electron’s spin is what describes which of those two states it’s in.


A Cool Video

Here‘s a cool video I found on Wikipedia that shows what I just explained.

Where Does Spin Come From?

I won’t discuss it in detail here, but we can understand spin as emerging from the structure of the underlying quantum field theory that describes the behavior of a given particle. For those of you who know the lingo, it has to do with whether the underlying field is a vector or scalar field, and how large that vector is. (Among other sources, see Quantum Field Theory in a Nutshell by Anthony Zee.)


The Stern-Gerlach experiment reveals a dramatic difference between the quantum world and the world we’re used to. It’s not possible for a particle to have any old orientation; it must be oriented either with the external magnetic field or against it.

But what if there is no external magnetic field? How is the particle oriented? Somehow the act of measuring the system changed how it behaves, or at least how we perceive it. These are questions that physicists struggled with in the early twentieth century as quantum mechanics was being discovered. Indeed, to some extent, physicists are still struggling with them.

In the next few weeks, I’ll address some of these issues. Next time, I will talk about an extension of the Stern-Gerlach experiment that helps us explore, if not answer, some of these questions.

Related Reading

This is only the latest in a number of articles that I’ve written about quantum mechanics. For example, I wrote a three-part introduction to the field:

  • In the first part, I describe some of the experiments that first revealed particle-wave duality.
  • In the second part, I use the Bohr Model of the atom to explain how packets of energy emerge from the wave nature of matter.
  • In the third part, I describe how we can interpret matter waves as probability waves.

More recently, I wrote a pair of posts exploring particle-wave duality.

I’ve also written a number of stand-alone articles on quantum mechanics:

  • Quantum mechanics uses complex numbers, so I wrote a short explanation of imaginary and complex numbers here.
  • I explain the Feynman path integral, which is a way of understanding quantum mechanics, here.
  • I use particle-wave duality and matter waves to explain quantum tunneling here.
  • I use quantum mechanics to describe how atoms form covalent bonds here.

Further Reading

Here are some additional resources on the Stern-Gerlach experment:


Thanks as always to Alexandra Fresch for her line-editing.

Recently I’ve had a lot of discussions on Google+  about the interpretation of quantum mechanics. (In particular, I’ve spent a lot of time talking to +Charles Filipponi and +David R.) This article was partly inspired by those conversations. Thanks, guys!

Posted in History, Physics, Quantum Mechanics, Science And Math | Tagged , , , , | Leave a comment

The Universe Is an Inside-Out Star

the universe is an inside-out star!
The universe as an inside-out star. The stellar surface is the cosmic microwave background. Behind it is plasma. And behind that, the center of the star is the Big Bang. (Thanks to Douglas Scott for generously providing these images.)

No, not really. But as we’ll see, it’s a useful analogy. Today we’ll learn about sound waves in the sun and how, if we imagine that the universe is the sun but inside-out, these are the same as the sound waves that filled the early universe.

DISCLAIMER: This is a pedagogical exercise only! I am not claiming the universe is ACTUALLY an inside-out star or that scientists think of it as one.

Sound Waves in the Sun

I’m sure you won’t be surprised when I say that the sun is a complicated beast. A nuclear furnace burning at tens of millions of degrees powers a burning ball of turbulent hydrogen gas and plasma. All sorts of crazy things happen in the sun. Magnetic fields reconnect and plasma flows on the surface, neutrinos fly out of the nuclear reaction in the core, et cetera. But let’s ignore all that for now. Let’s say that the sun is “just” a gigantic ball of superheated hydrogen gas.

But hydrogen gas is… well, a gas. And if something makes a noise, sound can travel through it. Moreover, how the sound travels,  and the frequencies that make up the sound, can tell us a lot about the interior of the sun. Fortunately for us, lots of things in the sun make sound. For example, if a bit of gas is hotter than its surroundings, it will create a pressure wave through the sun. And this pressure wave is nothing more than a sound wave.

But if we want to use these sound waves to understand the interior of the sun, we have to measure them. How on Earth do we measure sound in the sun?

Wiggles Beget Wiggles

Fortunately, we don’t need to measure the sound waves directly. All we need to do is measure the color of the light coming off the surface of the sun. A sound wave is just a fluctuation in the velocity of the particles that make up a gas. So, as a sound wave reaches the surface of the sun (called the photosphere), it will accelerate the atoms in that area. This in turn slightly changes the color of the light these atoms emit, thanks to something called the Doppler effect. (I’ve spoken about the Doppler effect before in the context of the expanding universe.) Atoms moving toward us emit light that is more blue than it otherwise would be, while atoms moving away emit light that is more red. Since not all light coming from the sun is emitted at the surface, the change in the color of the sunlight that reaches us is small but measurable.

Therefore, all we have to do is look at the surface of the sun and measure the changes in the color of the light emitted from different points on the solar surface. These changes in color correspond to the peaks and troughs of a sound wave traveling through the sun. The scientific field that studies the sun’s interior using the color fluctuations on its surface goes by the awesome name of helioseismology.

The Universe

So what does all of this have to do with universe at large? Well, as I’ve remarked before, we know that the early universe was filled with an extremely hot plasma—so hot that atoms and molecules couldn’t form. And this plasma glowed incredibly brightly. As the universe expanded and cooled, atoms and molecules formed, but the glow remained. It still remains today in the form of a bath of microwave radiation filling the universe, which we call the cosmic microwave background, or CMB for short.

That’s one way to look at things. But there’s another way, too.

Looking Back in Time

The speed of light is finite. Indeed, it’s the speed limit of the universe. This means that the light from a star four lightyears away from us is four years old. In other words, when we look out into space, we look into the past. And greater distances take us further back in time.

As we peer away from Earth, things are mostly empty for a while. Stars and galaxies are incredibly far apart, after all. But eventually we peer far enough away, into the extreme past, that we see the hot plasma of the early universe. The plasma is opaque, though, so we can’t see inside it. What we can see is the point when the plasma cools enough for atoms to form. The distance at which we see this happen is called the surface of last scattering. The corresponding time in the history of the universe is called recombination.

Since we can’t see inside the plasma, it might seem impossible for us to learn what happened before recombination. But it’s plausible that the plasma fluctuated and moved… and maybe sound waves even traveled through it. Fortunately, we can measure that! The fluctuations in the pre-recombination plasma change the color of the light in the cosmic microwave background!

And now we’re at the punchline. One way to understand this is to imagine that the universe is an inside-out version of the sun, as shown in the figure. As we look away from the Earth, backwards in time, there’s empty space. Then we reach the surface of the universe-sun, which is nothing more than the surface of last scattering. Behind it is the plasma which makes up the interior of the universe-sun. The sound waves in the interior change how the atoms and molecules on the surface (the surface of last scattering) move and thus change the color of light that’s emitted and eventually reaches us!

And thus, by measuring the fluctuations in the CMB, we can measure the dynamics of the very early universe!

The Big Bang Wasn’t a Point

One thing I like about this analogy is that it takes the center of the sun, which is a single point, and smears it out so that it becomes the surface of a very large sphere, one with the same radius as the observable universe. I like this because it reverses a common misconception.

People usually imagine that the Big Bang, the beginning of the universe, was  a single point from which everything emerged. This is completely wrong. The beginning of the universe happened about fourteen billion years ago at every point in space. So, in our inside-out sun analogy, the smeared stellar center is the Big Bang.

(Of course, there may not have been a Big Bang if, for example, cosmic inflation is correct. But that’s a story for another time.)

Related Reading

What I described in this post is a weird and crazy way of looking at the cosmic microwave background. But I’ve discussed the more “standard” understanding of the CMB several times. Most recently, I described the nitty-gritty of how cosmologists measure the CMB and how this is related to the failed BICEP2 “primordial gravitational waves” measurement.

I also wrote a three-part series on the early universe:

  • In the first post, I describe how the cosmic microwave background helped convince scientists of the existence of the Big Bang.
  • In the second post, I describe some problems with the Big Bang theory.
  • Finally, in the third post, I describe how the model of cosmic inflation fixes the problems with the Big Bang.

Further Reading

  • This post is inspired by—and borrows heavily from—a pedagogical paper by Crowe, Moss, and Scott, which you can find for free here. It’s very readable, even for the layperson, so I recommend checking it out if you’re interested.
  • Astrophysicist Brian Koberlein has a beautiful (pun intended!) blog post on how we probe the interior of the sun, in which he describes helioseismology and some other techniques. You should definitely check it out.
  • There’s a nice piece in Scientific American on the CMB here.
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Tidbit: Radio Waves Bouncing Off of an F-15

A simulation of radio waves bouncing off of an F-15 fighter jet. Left: the fighter jet is constructed out of many tetrahedra, inside each of which lives many waves. Right: the electric field on the surface of the jet. (Source: Gottlieb and Hesthaven, Spectral Methods for Hyperbolic Problems)

I’m afraid I don’t have time to write very much this week. So instead, I leave you with a little hint of the sort of thing I’m thinking about. The above picture is from a paper I just read. It shows a simulation of radio waves bouncing off of an F-15 fighter jet. The simulation was effected by first building the jet out of many tiny pyramids linked together at the faces (shown on the left). Then, a set of five waves or so was allowed to exist inside each pyramid. When you take all of these waves together, you get the radio wave that’s hitting the jet (shown on the right).

I’m working on taking this technique and using it to simulate relativistic astrophysics, like black holes and supernovae.

I’ll have a lot more to say on this eventually, but for now back to work!

Posted in Computer Related, Mathematics, numerical analysis, Physics, Science And Math | Tagged , | Leave a comment

The BICEP2 Result Was Just Dust, and That’s Okay

Planck on Earth
Figure 1. The Planck satellite being prepared for tests. (Image due to the Planck Collaboration and the European Space Agency.)

You may remember that about this time last year, there was a big hullabaloo because a research group from Harvard claimed that they had discovered primordial gravitational waves using BICEP2, their telescope in the South Pole. This was very exciting because, if true, the result would be extremely good evidence for a model of the early universe called cosmic inflation. (Cosmic inflation is mostly accepted by the scientific community, but it has some philosophical problems and is thus still a little bit controversial. The BICEP2 results would have ended the controversy once and for all.) Even better, the precise strength of the primordial gravitational waves measured by BICEP2 was much greater than anyone had expected, implying that there might be some exciting new things we could learn by studying them further.

Unfortunately, the BICEP2 team’s results were quickly called into question. A competing, space-based experiment, Planck, released some of their own data indicating that what the BICEP2 team had observed was simply cosmic dust. But the analysis wasn’t definitive. The jury was still out.

The beautiful thing about science, though, is that scientists collaborate. The BICEP2 team and the Planck team got together, shared data, and worked through a joint analysis of their measurements. This analysis took several months, but it’s finally been released.

Now we know definitively. The BICEP2 measurement was indeed cosmic dust, not primordial gravitational waves. But the jury is still out on the existence of primordial gravitational waves. It’s just that, if they exist—I personally think they probably do—then they’re as weak as (or weaker than) we originally thought, not as strong as the BICEP2 measurement indicated.

But why did BICEP2 get such a strong false positive? How did their measurement go so wrong? Well, hold on to your hats, ladies and gentlemen, because I’m going to explain.

This is a follow up to my post from last March on the BICEP2 result. If you haven’t read that yet, I suggest you go check it out before continuing.


About 13.8 billion years ago, the universe was extremely hot, so hot that matter couldn’t form at all… it was just a chaotic soup of charged particles. Hot things (and accelerating charges) glow. And this hot soup was glowing incredibly brightly. As time passed, the universe expanded and cooled, but this glow remained, bathing all of time and space in light.

(The reason for why the universe was so hot in the first place depends on whether cosmic inflation is true. Either it’s because the Big Bang just happened or it’s because, after cosmic inflation, a particle called the inflaton dumped all of its energy into creating hot matter.)

Even today, the glow remains, filling the universe. As the universe expanded, the glow dimmed and its light changed colors (due to gravitational redshift), until it became microwaves instead of visible or ultraviolet light. This ubiquitous glow is called the Cosmic Microwave Background, or CMB for short, and if you turn an old analogue TV to an unused channel, some of the static you hear is CMB radiation picked up by your TV antenna.

Since its discovery, the CMB has been one of our most powerful probes of cosmology. It lets us accurately measure how fast the universe is expanding, the relative amounts of normal stuff vs dark energy and dark matter, how the density of matter fluctuated in the early universe, how the Earth is moving relative to the expansion of the universe, and much more.

Measuring the CMB

One amazing thing about the CMB is that all of the light that reaches us is the same color, to an incredible degree. However, the color does fluctuate a little bit…in a special way that’s independent of position in the sky or direction. These tiny deviations from the norm are primarily what we’d like to measure.

So how does a measurement work? How can we measure something that’s literally everywhere? On Earth, we can see things in three dimensions because we have two eyes separated from each other. But on the scale of the CMB, which fills the entire universe, the whole Earth looks like a single eye–in other words, from our perspective, the sky is two-dimensional.

Figure 2. Although light from the CMB (red) fills the universe, when we observe it on Earth, it looks like it’s been projected onto a two-dimensional screen (blue) above our heads. (Earth courtesy of NASA.)

This means we observe all of the light from the CMB as if it were projected onto a spherical screen above our heads, as shown in Figure 2. Looking from the outside in, the result is something like Figure 3, which plots the wavelength of the light across the sky. (The differences in the wavelengths have been enhanced by about a factor of a million.)

the CMB on our sky
Figure 3. The cosmic microwave background projected onto our sky, looking from the outside in. (Image made by Damien P. George with data from Planck.)

Of course, although three-dimensional models are easiest to visualize, they’re not great to actually work with. So we usually map the CMB onto a flat surface, the same way we map the Earth. This is what gives rise to the famous “all-sky” maps like the one shown in Figure 4.

Figure 4. The measured CMB mapped on a flat surface. (Image due to the Planck collaboration.)

There’s a lot of information hidden in Figure 4 that you can’t see unless you do some serious math. In fact, you could learn almost everything I’ve told you so far just from looking at the CMB! And there’s more to learn as we make new and increasingly precise measurements.

Planck Vs. BICEP

It’s at this point that I need to provide a clarifying comparison. The images I just showed you were generated by the Planck satellite, which is a small satellite that lives just beyond the moon’s orbit. As Planck orbits the Earth (and as the Earth orbits the sun), it makes measurements of the CMB in small segments of the sky. Over the course of a year, it can build up a map of the CMB in the entire sky, as shown in Figures 3 and 4. (Planck also takes measurements of several different wavelengths of light and aggregates the data. This is important and I’ll get back to that.)

BICEP2 (shown in Figure 5),  on the other hand, is a single telescope near the South Pole. The BICEP2 people chose to measure a small patch of the sky extremely precisely and they only measured one wavelength of CMB light.

BICEP2 sunset
Figure 5. The BICEP2 telescope at the South Pole. (Courtesy of the BICEP collaboration).

What Went Wrong?

If sending a satellite into space or pointing a telescope at the sky were all that was required to precisely measure the CMB, the BICEP2 team never would have mistaken dust for gravitational waves. So what went wrong?

Well, I told you that the fluctuations in the CMB are very very small. This means that they can be drowned out by the many other sources of microwaves in the universe. Jupiter, the sun, black holes, pulsars, cosmic dust…tons of things produce microwaves. Collectively, all this other stuff is called foreground.

To screen out the foreground, cosmologists build an extremely detailed map of non-CMB sources of microwave radiation in they sky, called a mask, and subtract it from the map of microwave light that the instrument actually measured. After the subtraction, you get something like Figure 4. The mask used to remove known sources in the Milky Way is shown in Figure 6.

Planck Galaxy Mask (for the Power Spectrum)
The mask that the Planck collaboration used to screen out galactic microwave sources from their measurements of the CMB. (Source: the Planck collaboration)

But mask-making is tricky business. To build a map, cosmologists use previous measurements of the sky and computer simulations. The Planck collaboration uses an additional trick: they can detect several different wavelengths of microwaves. The only microwave source that will look the same in every wavelength is the CMB, so by comparing the measurements in different wavelengths, Planck can remove unexpected sources of noise.

But BICEP2 only measured one wavelength of light, and this is what killed it. The computer models the BICEP2 people used to make a mask for their little corner of sky didn’t predict that it contained as much spinning cosmic dust as it does. Planck, with their multi-wavelength detector, wasn’t fooled in the same way.

(I should emphasize that the BICEP2 team’s mask was flawed. The team based their dust estimates on older measurements and made a mistake when estimating how much the radiation from the dust would change when you looked at a different color of light. But these are subtle errors, and having several colors of light to look at would have been a fail-safe against them.)

BICEP2 Didn’t Do Anything Wrong

It’s tempting to say that the BICEP2 collaboration failed in some way—their data analysis was poor, they designed their experiment badly, etc. But they couldn’t have known that this cosmic dust would have been a problem. It’s easy to see what to do in hindsight…not so much when you’re planning a multimillion-dollar project years (or even decades) in advance.

This is how science is done. We make a prediction, we design an experiment, we measure something in the world, and we invariably mess up. But by keeping our minds open to our own fallibility, we give ourselves the opportunity to try again and eventually get it right. That’s what happened here. BICEP2 made an erroneous conclusion, took the opportunity to collaborate with Planck, and they figured it out.

That’s fantastic. That’s what I call science.

(Parenthetical note. There are other ways that the BICEP2 team deserves criticism. Before submitting their article to peer review, the team held a huge press conference and generated a lot of publicity. Given that the conclusion wasn’t yet vetted by the scientific community, this kind of behaviour can and probably did detract from the credibility of science in the public’s eye.)

Further Reading

  • If you’d like to read the joint Planck-BICEP2 press release, you can find it here. In the press release, there’s an link to the scientific paper that the two collaborations wrote together, which is currently undergoing peer review.
  • Planck periodically releases their measurements and statistical tools, including software, masks, and all-sky maps, to the scientific community at large. This is where I got the galaxy mask I showed. If you’d like to browse, you can find all the data and documentation explaining how to use it here.
  • If you’re very brave, you can read this review paper on how foregrounds are removed from CMB measurements. This article describes in great detail how masks are generated.
  • If you want to look at the CMB in its full spherical glory, Damien P. George created this webapp. It’s pretty awesome.

Related Reading

  • This article is a follow-up to my previous article on BICEP2, which you can find here.
  • If you’d like to know how we know that the universe is expanding, you might want to check out my article on exactly that.

If you’re confused about the Big Bang or this whole “inflation” thing, you might want to check out my three-part series on the early universe:


Thanks to Alexandra Fresch for proof-reading and editing and thanks to Sara Simon for making sure I get the cosmology right.

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Sometimes a Particle Isn’t Possible

That blob turns into two!
Figure 1. That blob splits into two! This animation shows what happens when we try to construct a single particle (with the given shape) and make it bounce between two mirrors.

Last time, I showed you how you could construct a photon, a light particle, in a configuration of mirrors called a ring cavity. This time I’ll show you that sometimes, you can’t make just one particle—they only come in pairs. And sometimes, the notion of a particle doesn’t make any sense at all. (This post relies heavily on last week’s post, so if you haven’t read that, I recommend you do so.)

Disclaimer: What I’m about to describe is only the simplest case, and I make simplifications for the sake of exposition. It is possible to capture and manipulate single photons between two mirrors for short times if you play tricks. In fact, that work recently won the Nobel prize.

Last time, I showed you what happens when you arrange three mirrors to make a ring. Now let’s see what happens when we bounce light between two parallel mirrors, as shown in Figure 2. This is called a Fabry-Perot cavity.

A Fabry-Perot Cavity
Figure 2. Light (yellow) bounces between two mirrors (light blue).

We’re going to put waves into our Fabry-Perot cavity and see if we can make just one particle. (Spoiler alert: it won’t quite be possible!)

As I’ve discussed before in some detail, light is an electromagnetic wave made up of electric and magnetic fields. To draw a parallel to our example last week, the strength of the electric field can very roughly be thought to correspond to the probability of measuring a photon. However, there are complications; for example, the quantum-mechanical wavefunction can be imaginary. It’s an experimental and theoretical fact that electric fields are zero inside conducting materials like metals. (This isn’t quite true…the field actually falls off slowly, based on something called the plasma frequency. But we’re making approximations.) Therefore, the electric field that makes up a photon must be zero at the metal mirrors.

This means that if we put a wave of light between the two mirrors and look at the strength of the electric field (which wiggles), it has to go to its center position at the mirrors, as shown in Figure 3. This restricts the type of wave that can fit in between the mirrors. (On our plot, the field is zero when it’s smack dab in the middle of the plot. Above the center line, it’s positive. Below the center line, it’s negative.)

Figure 3. If you put a light wave (red, green, blue) between two mirrors (dark blue), the wiggles have to end at the mirrors.

Last time, when we added a wave to our ring cavity, the wave travelled uniformly to the right with some speed. That’s not what happens now. Now the wave can’t travel, so the height just grows and shrinks. Let’s look at the longest possible wave that can fit in the cavity, shown in Figure 4. This is called a standing wave.

Fabry-Perot with one wave
Figure 4. Our Fabry-Perot cavity with the largest possible light wave sitting in it. The wave just grows and shrinks in time.

(There are complications, of course; I’m completely ignoring what the magnetic field is doing. But for explanatory purposes, this is enough.)

Now we can add additional waves to the cavity. If we add the first five that fit (in special amounts based on a mathematical calculation using Fourier analysis), we get a plot that looks something like Figure 5.

Figure 5. The first five largest waves in the Fabry-Perot cavity.

We seem to have some complicated wave motion here! Let’s add even more waves! If we add nine waves to the cavity, we get something like Figure 6.

Nine waves
Figure 6. Nine waves in a Fabry-Perot cavity.

If we add nineteen, we get something like Figure 7.

Figure 7. The Fabry-Perot cavity with 19 waves in it.

Now what’s happening is beginning to become clear. If we extrapolate to Figure 1, we see this:

We attempt to put a wave with a particle-like shape into our cavity, but it splits into two waves which fly apart, reflect off of the mirrors, pass through each other, and continue reflecting for all eternity.

In this case, it’s not possible to put just one particle in between the mirrors.

Sometimes Particles Just Don’t Make Sense

The example I’ve just described highlights a problem with the standard popular narrative of particle-wave duality. We’re told that particles sometimes act like particles and sometimes act like waves. But if this were true, a single particle would never split into two just because we dropped it between two mirrors. The truth of the matter is that everything is a wave. It’s just that sometimes, like in last week’s experiment, waves can be made to act like particles.

But this week’s experiment shows us that sometimes, waves can’t be made to act like particles–at least, not a single particle. And sometimes they refuse to behave like particles at all! What all of this means is that there are conditions where particles cannot exist. For example: We think that, about 13.8 billion years ago, the universe underwent a period of rapid inflation. During this expansion, for reasons that I promise to try to address in the future (see Mukhanov and Winitzky), the very notion of a particle broke down. In the inflationary period, the packets of waves that make up particles simply could not form.

Related Reading


I know I’ve been lazy about citing my sources on this blog and I should be better about it…even when the sources are not layperson-legible. So, for that reason, I offer the intrepid student a list of introductory texts he or she can use to learn more.

  • Introduction to Electrodynamics by Griffiths offers a comprehensive introduction to electromagnetic theory (e.g., how light behaves).
  • You can also find a simpler, more accessible introduction in the Feynman Lectures on Physics, which are available for free online.
  • A Fabry-Perot cavity can be approximated as a particle in an infinite square well. This problem, as well as particle wave duality and basic Fourier analysis, are all covered at an introductory level in the excellent text Modern Physics For Scientists and Engineers by Taylor, Zaphiratos, and Dubson.
  • A more advanced student may want to check out Introduction to Quantum Mechanics by Griffiths.
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What’s in a Particle?

photon identity crisis
This photon doesn’t know whether it’s a particle or a wave. (Unknown source.)

If you’ve read or heard anything about quantum mechanics, you’ve heard the phrase “particle-wave duality.” The common wisdom is that this means that particles sometimes behave like waves and sometimes behave like particles. And although this is right, it’s a bit misleading. The truth is:

Everything is always a wave. It’s just that waves can be made to behave like particles.

To see what I mean, let’s actually show how one can make a set of waves behave like a particle. Specifically, let’s show how a set of light waves can be made to behave like a photon, a light particle.

Light Goes Round

Just to be concrete, let’s talk about light that bounces between a special configuration of mirrors. It looks something like this:

a ring cavity
The configuration of mirrors we’re considering. The light (yellow) starts at mirror A (light blue), bounces off of mirrors B and C, and ends up back where it started. The process repeats forever.

The idea is to configure three mirrors (light blue) so that the light (yellow) bounces around in a loop so that it ends up in the same place that it started from. In optics, this is called a ring cavity. It’s often used to make a type of laser called a ring laser.

We can represent the path of the light (which lives in two-dimensions) as a position along a single line. All we have to do is demand that the lefthand side of the line be the same point as the righthand side of the line so that it wraps around to where it started, as shown below. This is called a periodic representation.

The ring cavity reduced to a single line so that the path of the light starts at the left and ends on the right… the left and right points represent the same point.

So, given our periodic plot of the path of the light in the cavity, what does a light wave look like? Since the wave has to wrap around to where it started, the wave on the left side of the line must look the same as it does on the right… In other words, it has to become itself after it travels around the cavity, as shown below:

sine it!
A sine wave in the ring cavity. The left side of the wave looks the same as the right because it must be periodic.

One important consequence of this periodicity is that the waves in the cavity can’t be whatever they want. Only certain waves fit. In the figure above, if the wave stopped a little earlier on the right (say at the peak, the highest point), as shown below, then the right and left sides of the wave wouldn’t be the same. This would obviously be a problem.

This wave doesn’t fit in our ring cavity because the left and right sides are not the same.


Now, given a bunch of light waves that fit in our cavity, we want to combine them in such a way that we get a particle travelling around the ring in the cavity. An important idea that we’ll need is the principle of superposition. 

The Superposition Principle

I’ve previously discussed wave interference and the superposition principle, so if you remember my previous discussions, feel free to skip to the next section.

Imagine waves as wiggles on a very stretchy string. If I try and push up on the string (make a wiggle that goes up) and you try and push down on the string (make a wiggle that goes down) at the same time, neither of us ends up moving the string as much as we intended. This is called destructive interference. Similarly, if I push up on the string at the same time that you push up on the string, we’ll probably stretch it quite a lot. This is called constructive interference. The process of overlaying one wave over another is called superposition.

Interference between two waves on a string
If we both try to make waves on a string, we may interfere with each other. If, as in the image on the left, we both try making the same wave, we may get a bigger wave than we intended. This is called constructive interference. If, on the other hand, we try to make waves exactly offset from each other, we may completely negate each others’ efforts. This is called destructive interference (source).

Light waves work the same way. If we make two waves overlap

What’s in a Particle

Okay, so let’s build our particle! As a first step, let’s put the largest wave that can possibly fit into our cavity, shown below. Note how the height of the wave on the left is always the same as the height of the wave on the right.

A single, low-frequency, wave travelling through a ring cavity.

Now, we add more, higher frequency, waves to the cavity. (By higher-frequency, I mean the waves have more wiggles in them.) If we take a wave that has exactly twice the number of wiggles as our first wave and add the two waves together, we get something like this:

Two waves superposed in the ring cavity. Already we start to see some particle-like structure.

Interesting! Now we get a big wiggle and a small wiggle. Perhaps that big wiggle will become a particle?

(Note that I added a wave of a specific height to the first wave. There’s a lot of math involved in knowing precisely what height to add. I’m going to completely ignore that detail for now.)

After we add five waves to the cavity and sum up the wiggle heights, we get:

ring 5
Five waves superposed in the ring cavity.

Now there’s one wiggle that’s clearly much bigger than the others. If we add enough waves to the cavity, we can make that wiggle all we can see:

20 waves superposed in a ring cavity.

Wow! That doesn’t even look like a wave! What is that? That, my friends, is a particle. The point where the peak is highest represents the average position of the particle and the width of the peak represents the quantum uncertainty in the position of the particle.

(Actually, the little wiggles around the peak are still there. They’re just too small to see in my plot. In principle, you can get rid of them entirely by adding up infinitely many waves.)

Enter Heisenberg

What if we wanted to make the peak narrower, thus making it possible to measure the particle’s position more precisely? Well, you might have noticed that our central peak got narrower as we added more waves to our ring cavity. This means that we would need to add more, increasingly wiggly, waves to the cavity to make the peak narrower.

How do we interpret that? We know that the number of wiggles in a wave determines both its energy and its momentum, meaning that the particle is not only made up of many waves, but has many different energies.

It has an average energy, of course, and an average momentum. But if we measure the particle, we might not measure that energy or that momentum. Instead, we’ll measure each energy some fraction of the time. The percentages look something like this:

The percentage of the particle that has a given energy.

This is a visible manifestation of the Heisenberg uncertainty principle. If we want to know the particle’s position better, we need to add waves with different energies to it, meaning that it has more energies and we know the energy (and thus the momentum) less well.

Particles Can Act Like Waves

So I’ve just described how we can make a bunch of waves like a particle. But, of course, a bunch of particles can wiggle to make a wave. This is, after all, what’s going on when you wiggle a string, since that string is made up of particles. So you might ask… can you make a wave out of particles, add a bunch of such waves together, and get a new particle?

Amazingly, you can! If you add up a bunch of sound waves travelling through a material (which is made of particles which are made up of waves), you can get a particle called a phonon!

So now we’re ready to state the principle of particle-wave duality one last time:

Everything is a wave. But particles can be constructed out of waves… and waves can be constructed out of particles.

Disclaimer: It’s Often Useful to Think in Terms of Particles

I just told you everything is a wave. But that doesn’t mean physicists always think in terms of waves. Often it’s more useful to think in terms of particles. For example, the width of the peak I constructed above can be very narrow on human terms and it can be very difficult to notice or measure uncertainty in the particle’s position. This is why we didn’t discover quantum mechanics until the early twentieth century.

But even at the quantum level, it’s often valuable to think in terms of particles. Richard Feynman’s diagrams, and Werner Heisenberg’s formulation of quantum mechanics, for example, treat particles as particles in some sense. This isn’t inconsistent, however, because one can prove that Feynman and Heisenberg’s formulations imply the wave nature of, well, Nature.

Alright folks, that’s all I have for now.

Play With it Yourself

I generated all of my animations in Python. If you’d like to play with them yourself, you can find my code here:


Related Reading

I’ve written about quantum mechanics before. If you enjoyed this post, you might also enjoy the following posts:

  • In this article, I introduce particle-wave duality by describing the experiments that convinced physicists that particles have to be waves.
  • In this article, I describe how Niels Bohr used particle-wave duality to unravel the mysteries of the atom.
  • In this article, I explain how we should interpret particles-as-waves.
  • In this article, I use particle-wave duality to explain quantum tunneling.
  • Everything I just described is based on something called Fourier Analysis. I discuss Fourier analysis in a very similar context in this article.
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The Most Important Scientist in my Life: My Mom

my mom






Here’s the interview:



J. Can you describe what heritability means?





A. Yes.












A. Oh, it’s definitely complicated.









J. Please do.


J. That sounds hard.


J. What strategies did you use?

A. I don’t remember…games, puzzles.



J. Surprised “good” or surprised “bad?”






J. That’s good advice. I’ve had that experience.


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Pope Francis says Evolution and the Big Bang are Compatible with Catholicism

Pope Francis (left) and Georges Lemaitre (right)
Pope Francis (left) and Georges Lemaitre (right). Source: Wikimedia Commons

You’ve probably heard, the news. Pope Francis has announced that Big Bang cosmology and evolutionary theory are compatible with Catholicism and “may even be required.”

This is, of course, wonderful news. It’s evidence that science and religion are not necessarily incompatible and that people of faith can modify their beliefs based on the evidence around them.

But it should have been this way all along. Indeed, it originally _was_ this way. One of the people who developed Big Bang cosmology, Monseigneur Georges Henri Joseph Édouard Lemaître was a catholic priest who believed that his studies of physics brought him closer to the mind of God. Indeed, Pope Pius XII completely accepted Big Bang cosmology when Lemaitre developed it, even going so far as to claim that it supported catholic beliefs.

At the same time, Pius XII declared that evolution was not at odds with Catholic beliefs. 

What happened?

Nothing went wrong! Francis was citing long-standing Church policy. The Catholic church has always accepted evolution, as I was surprised to learn.

Our Story Isn’t Over

The astute reader will remember that there may not have been a Big Bang. We now believe that instead, the early universe may have undergone a period of extremely rapid inflation. To learn about the discovery of the Big Bang and why we feel it might not be true, check out my three part series on the topic:

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I am buried in homework
I am buried in homework

Hi everyone. You probably haven’t heard from me in a while. This is because I have been completely overwhelmed by class work this semester, which has prevented me from doing the things I want to do, like blogging and doing my own research.

For the time being, I don’t think I can expect myself to blog every week, or even every other week. So I’m putting the blog on hold until the semester ends (which should be around the holidays).

As always, thanks for reading, everybody.

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