Interpreting Physics Research

Recently I was reading a very interesting paper that discusses information and complexity in physical measurement processes. This is Inferring Statistical Complexity by James P. Crutchfield and Karl Young. There was just one problem, I didn’t understand a thing. While trying to get a foothold in the material, I found myself using several strategies for building my comprehension of the material.

Science publications have their own unique quirks and languages. I will share some tips that helped me understanding complex academic material. While my own specialization is physics, this can apply to many different academic fields.

Break it down

Don’t try to understand the entire paper all in one go. Tackle one section at a time. If the paper isn’t already organized into sections, skim the paper and try to break it down yourself.  Identify sections that contain what you need to know. Usually all the preceding sections are needed to understand that material, but sometimes you can get away with skimming sections when you have a specific purpose (Example: skimming the experimental apparatus when studying the theoretical implications). Always make sure you understand the introductory section before moving into the body of the paper.

Know the Language

Most papers have an introduction that provides some higher-level discussion of the foundational physics. Usually this includes more general topics that most readers will have some experience in. As the paper goes on, the language becomes more and more specific to whatever topic or sub-field this paper discusses. As you read this section, jot down any concepts you don’t understand, and jot down a few that you do understand. It is important to understand at least the base definitions of terms used in this sub-field. Watch out! Some terms that you know in a general sense might have a very particular meaning in a sub-field.

Read the Cited articles

If you aren’t already actively performing research in a sub-field, you probably aren’t up-to-date with all the existing research and techniques. Most papers don’t spend the time to fully describe all the foundations approaches and techniques. Naturally there is only so much space for detailing mathematically derivations or previously derived conclusions. Regardless, these are important pieces for understanding research. So as you read the paper, find areas that you don’t understand and dig up the articles that are cited for that area.

For example, when reading that paper on statistical complexity, I discovered that one of the citations was an extremely long and detailed thesis on the material that one of the authors had written . Reading the thesis not only familiarized me with the models they were using, but also the underlying assumptions of the research.

Draw Your Own Picture

Sometimes you need to draw your own picture of what is happening. As much as the authors of the paper try to make things clear, they are usually targeting their writing at researchers and academics. What they might consider to be a clear picture might be completely incomprehensible to anyone without a masters in the field. Instead of relying on their own presentation of the material, draw your own. As you read through the paper, built a visual map of interrelated concepts and factors in the research. Even if details aren’t explicitly stated, you can work out the gaps by looking at what is missing in your visual picture.

Learning is a skill, and you can develop your learning abilities to learn new concepts and skills more efficiently and effectively. While these methods work for me, they might not all be the approach that everyone takes.  As you read and learn, you’ll develop your own techniques as well to suit your learning style and way of thinking.


The Holographic Principle

Mathematical relationship or physical reality? Does the difference even matter?

A particular buzzword that flies around physics articles these days is holography. When most people think of holograph it conjures images of science-fiction movies with shimmering images.  In the movies, the hologram is a 3 dimensional illusion projected from a 2 dimensional screen. The physics concept is a bit different.

In general, holography refers to some higher-dimensional phenomena (a 3D image in this case) that is ‘encoded’ somehow into a lower-dimensional form (a 2D projector).  In physics, this general idea is applied to reality itself with the holographic principle.

Black Holes at the Surface

During the early studies of black holes, it became clear that we could need to understand how they work not only in terms of their gravity, but in terms of their thermodynamics. In particular, it was important to determine the entropy of a black hole. Entropy is often referred to as the disorder of a system but it also reflects the density of information. Jacob Bekenstein developed a model where black holes contained the maximum possible entropy for anything with the same volume.


S here represents the entropy for the entire volume.  k and l are constants. A is the surface area of the black hole.

In the course of understanding this they found that this maximum entropy was directly proportional to the surface area of the black hole. In other words, you only needed information about the surface of a black hole in order to calculate the entropy of the entire volume. Since higher dimensional information (the entropy of a black hole) is completely defined by a lower-dimension system (the surface), this is a form of holography. This observation has inspired physicists to ask why it is holographic? what does it mean? And are other physical systems holographic?

What does holography mean in Physics?

Since the early studies of black hole physics, new theories have adopted what we call the holography principle, the idea that a volume of space can be described by a lower dimensional boundary. Black holes were are early example, but now the concept is applied to string theories, quantum gravity and more. Some theories suggest that entire universe itself is actually caused by some behavior on a two dimensional boundary.


Where holography is concerned, The behavior on the boundary is directly linked to what occurs in the volume.

Let’s unpack this a little bit because it isn’t entirely clear what all this means. A major dispute in discussions of relationships of holography is how to interpret it. The literal interpretation is that the higher dimensional reality doesn’t actually exist and that it is illusion created by interactions on some lower-dimensional structure. This goes against our intuition that 3 dimensional structures exist as whole entities that are more than just their bounding surface.

Another interpretation is that the holography principle doesn’t tell us what dimensions really exist or not, but instead it tells us about the relationships and structure of these system. Saying that we can describe something using a bounding surface doesn’t imply that the volume isn’t real, it is just that the information in that volume is bound by certain relationships that cause the total behavior to be describable by the surface. This approach has some interesting implications for how information itself flows in our universe.

Regardless of how you interpret it, the holography principle has many applications to physics. One of the major areas of research is in what we call AdS/CFT correspondence. This is a approach to quantum gravity where two theories are able to be connected through holography. Both theories produce the same results, however one theory acts on the volume (AdS) and one theory acts on the boundary (CFT).

These kind of correspondences is called a duality. That is where different theories and approaches end up being simply two ways of describing the same thing. It seems fitting that all the varied approaches to studying the universe might lead to different perspectives on the same reality.
VIDEO: The World as a Hologram,  Leonard Susskind
A different approach that focuses on the black hole information paradox when discussing holography. A very interesting watch.

PDF: The Holographic Universe, Jean-Pierre Luminet
A very readable and accessible introduction to the approaches to holography and their implications.

COURSE: String Theory and Holographic Duality,  Prof. Hong Liu
A free graduate course that introduces string theory and the AdS / CFT correspondence in detail.  Includes pdf readings and videos.

Quick Intro: Group Theory

A group is an abstract description of how a system can transform.

Now and again in higher-level Physics you might run into terms like this: SO(3), Lie Group, U(2), etc… These sound like strange and disconnected terms but they are all part of how we categorize systems into groups.

What is a group you might ask? A group is an abstract description of how that system can transform. Specifically, a group consists of a set of elements that create our group space. With this group there is an operator that allows us to combine any two elements into a third element. The operator is usually represented by a plus + symbol so that if A, B and C are group elements, a statement like A + B = C makes sense. You might notice that this looks a lot like simple algebra, and groups do form their own algebra in a sense, but they aren’t limited to the math operators that we learned in primary school.

Since a group is completely abstract, it can have different ‘representations’ that express the group. A cyclic group can be represented by the way we rotate an object, so that if you add two rotations, you might end up rotating the object 360 degrees and returning to the original position. Another example of a group representation would be how we can combine simple twists of string to form complex braids (the group in this case is called the braid group).


One example of a group is the ways that we can flip a mattress. This helpful article explores group theory in that scenario:

While there are infinite representations of groups, we try to reduce each group to it’s simplest form, known as the irreducible representation. This allows us to compare different systems in terms of their groups.

Breaking systems down into their fundamental groups allows us to predict transitions that appear in the group before we can even observe them. Group theory can also be used to understand symmetry, this perspective can not only be applied to the physical sciences, but the study of art.

As far as physics goes, here are a few common groups that come up a lot:

SO(3): The special orthogonal group in three dimensions. This group consists of simple rotations in 3 dimension space around a fixed point.

U(2): The unitary group of degree 2. This group consists of all 2×2 matrices that are unitary.

SU(2): The special unitary group of degree 2. Like the unitary group, but the determinant of each matrix is equal to 1.

There are a few ways of discussing different types of groups. Many groups are characterized by some kind of symmetry, which is defined by a parameter of the system that doesn’t change during a transformation. For example matrices in the special unitary group always have a determinant equal to 1, or the special orthogonal group doesn’t change the length of a given radius as it rotates.

A Lie group is any group that is defined by a continuous transformation. A particle might have discrete transformation between one energy state of another, but rotations can be made continuously.

There are so many different ways of looking at groups. Even if it doesn’t become your field of study, it can make for a very interesting way of looking at different systems. Here are a few sources where you can learn a little bit more about groups:

PDF/BOOK: Introduction to Group Theory for Physicists – Marina von Steinkirch
This book breaks gives us the mathematical formulation of group theory and looks at a number of groups that have special significance for Physics.

VIDEO: Group Theory – Robert de Mello Koch
The first in a series of lectures that cover the study of group theory and representations. A great place to start if you have some experience with Quantum Mechanics.

Path Integrals

Because of the statistical nature of quantum physics, there is a degree of choice in how a particle moves, as a result there are a infinite number of possible paths that could reach a destination.

Today I’m going to talk about what we call Path Integrals in quantum physics. The last two articles on the principle of least action and propagators have provided some conceptual background that will help us along. Continuing on those concepts, I’ll introduce an important approach that extends the principle of least action to the quantum world and gives us a way of using it to calculate propagators.

Quantum Paths

With the details from last week details under our belt, we can start looking at quantum propagators from a different angle. In particular, the Hamiltonian approach that we discussed last time was a little troubling.  The problem stems from the fact that our formula includes specific references to time and the hamiltonian (a quantity that can be loosely interpreted as the total energy).


The Hamiltonian propagator suffers from being incompatible with relativity. Both time (t) and the hamiltonian (H) change when we look at different reference frames

Time can appear differently in different reference frames due to relativistic effects, and as a result, the hamiltonian also varies based on our perspective. We want a formula that is lorentz invariant, meaning that when relativistic effects come into play, this formula is still valid.  While this form of the propagator isn’t lorentz invariant,  it does come in handy for deriving a relativistic propagator that uses the Lagrangian instead. With the Lagrangian approach, we won’t have to worry about about relativity because we can work with lorentz invariant lagrangian functions instead of hamiltonian functions.


While energy is always conserved, it is different in different reference frames, and thus is not lorentz-invariant.  A ball moving in one reference frame would appear to have a different velocity in a another reference frame, leading to different observations of energy.

The main thing that we need to do is to somehow express the transition between our two points in spacetime. We can call them point A and B. The hamiltonian method just needed the difference in time to work properly, however the lagrangian is associated with the path. We can no longer just get by with the difference, in time, we need to account for different possible paths to a destination now.

Here is another way of looking at it.  In classical mechanics, any particle with a given energy and position will follow a single path to move to its destination. Because of the statistical nature of quantum physics, there is a degree of choice in how a particle moves, as a result there are a number of possible paths that could reach a destination. As it turns out, there is actually an infinite number.  This means that we need to consider the contribution of each possible path to calculate the propagator. This approach is referred to as the sum-over-histories.


In classical physics, a particle has a single possible path that it can take from one spacetime point to another, this is considered to be a deterministic system. In quantum physics, there an infinite number of paths that a particle can take, and the outcome appears to be randomly determined. This is considered to be a stochastic system.

You can break the hamiltonian propagator down through a method called time-slicing. By splitting the overall time into segments of size delta-t, you get a ton of small time evolutions each moving the particle slightly further forward in time.



By splitting it up into a number of tiny points in time, you can use rewrite the equation as a series of smaller propagators on a variety of paths. The overall integral indicates that the propagator is comprised the the sum of all possible paths between the two points.


Note that I’m deliberately omitting the more complex calculations in this description, in order to focus on the overall picture. Skipping past a few steps, the action emerges from the hamiltonian. With the action, we just need to find some way of combining all these paths into the propagator. It turns out that you can perform a special kind of integral across all the possible paths. This is called the path integral and it looks something like this:


This is the general formula for the path integral. The D[q(t)] means that this is a functional integral, that integrates over a changing function instead of a variable. In this case it is integrating over all the possible functions for the action (which are related to each individual path).  The overall picture here is that this formula is adding the contributions of an infinite number of ‘mini-propagators’ for each individual path. Remember that because the particle oscillates as it travels, the principle of superposition applies and it can either add constructively or destructively and some paths will cancel each other out.

The method of solving this particular equation is fairly complex and can differ depending on the form of the lagrangian, so we’ll leave the mathematical formulation for now.

What does it mean?

Now that we have a look at the general mathematical description, we can start looking at the implications of this formulation.

One particular feature or working with path integrals is that some of the paths can end up moving backwards in time for a portion of the path. This may seem pretty nonsensical but particles moving backwards in time are interpreted as anti-particles moving forwards in time. So these time switching paths would be observed as particles and antiparticles pairs being created and annihilating. This paints an interesting picture of the way particles can move on the quantum scale.

Another consequence of those infinite paths is that some of these paths can end up in extremely unlikely places.  In classical physics, we can devise some kind of impassable barrier that a particle could never cross, however in quantum physics we have to account for paths that go around the barrier, or even travel through time to circumvent this barrier. This leads to a non-zero propagator for passing through the barrier, so some particles will actually cross the barrier. This phenomena is called quantum tunnelling because in experiments it appears as if the particle as crossed the barrier through some kind of invisible tunnel.

As far as the sum-over-histories approach itself, the physical interpretation is divided. One possibility is that the particle only takes one possible path from one point to another, however there are some people who argue that the particle takes every single path to its destination. It is also equally possible that all of this is just a clever mathematical abstraction that can’t be interpreted in human terms, but where is the fun in that?

These are only a couple of the perspectives that open up through our formulation of path integrals. Here are a couple resources for those of you who want to learn more.

VIDEO: PSI 2016/2017 Quantum Field Theory II – Lecture 1,  Francois David
This provides a introduction to the general method, and provides a thermodynamic analogy that shows how this method can apply to more than just quantum physics.

BOOK: Quantum Field Theory for the Gifted Amateur, Tom Lancaster,  Steven J. Blundell
I’ve drawn heavily from the derivation in Chapter 23 of this great book. I must admit that some of the equations in this post are right out of this chapter. Overall this book is a great introduction to Quantum Field Theory and I’d recommend it to anyone looking to get started in this kind of material.

Learning Resources For Physics – Part 1: Video

Everyone learns in different ways and takes different paths to nurture their understanding

Learning is the bread and butter of science, we could be studying some esoteric equations, attending class, or even just looking at the myriad patterns in your everyday life.  The tricky part is that everyone learns in different ways and takes different paths to nurture their understanding.

It helps to have lots of different tools in our search of knowledge so that we can explore are these avenues of learning. I’ve found many people are interested in theoretical physics, but find it inaccessible and daunting. With that in mind, I’ve begun to compile a few helpful resources so that people can have more avenues of learning.  In the spirit of inclusivity, I will only include resources that are 100% free.

Today, I’ve gathered a few video learning resources to help people along at every step of their learning.

Khan Academy
This is a staple of online learning sites. It covers all the STEM fields from high-school up to at least second-year university level. It provides a reliable and accessible entry point into the foundation material of any scientific study.

MIT OpenCourseWare
MIT has hosted course content from hundreds of their past courses. This includes a few pretty solid courses on physics. You can even filter the courses to see the ones that include video content.

Perimeter Institute Recorded Seminar Archive
The Perimeter Institute has made recordings of all their lectures openly accessible for anyone to watch. If you have a solid grasp of undergraduate physics, these provide a doorway into graduate level material. The lectures are well laid out and cover a huge variety of topic material. Make sure to check out their seminars too.

  • 2014/2015 Course lectures:

  • Main video archive:

CERN Document Server
This site includes not only video lectures on cutting edge particle physics, but also foundational topics taught by some of the leading physicists and engineers working there. This is a treasure trove for anyone interested in particle physics.

These are by no means the only resources out there. Just the ones that I tend to use. You can find even more resources on other lists like these:


Propagators and Time Evolution

On the quantum scale, particles no longer travel in well-defined paths or states, new tools are needed to describe the uncertain states of reality.

This week I’m going to quickly introduce a couple more foundational concepts in quantum physics. This discussion is a little brief, and one of these days probably end up writing something in a little more detail on some of these topics. Regardless, this hopefully gives a  starting point in some of these concepts.


When developing our understanding of that Quantum world, it is easy to see that things in the classical world never quite work in the same way. On the quantum scale, instead of working with simple particles travelling in well-defined paths, you usually get particles with paths and properties that can best be described as ‘smeared’ across spacetime. Most of the time you can’t categorize them as either a discrete particle or a continuous wave.  Particles can be considered to simultaneously existing in different states and locations at the same time. Given these difficulties, physicists tend to work with probabilities and statistical descriptions instead of focusing on some absolute truth of some singular reality.

One important function in quantum physics is the propagator. The propagator is a function that takes an observed position and state, and tells us the likelihood of observing that same particle at a another position and state at a different time. Propagators help us predict how a particle will behave on quantum scales, so calculating them is an important part of any quantum theory.


If a particle exists at spacetime point A, the propagator can tell us how likely we will observe it at another spacetime point B. Note that only one of many possible paths is shown,

The tricky part is actually calculating the propagator. You may be recalling my last post on the principle of least action.  ‘Hey!’ You might say, ‘Can’t we just find use the action again and find a minimum/maximum path to predict how the particle moves?’ That would be a good approach, however there is no absolute certainty in the quantum scale so the best we can do is find a probability. Luckily, we can tweak our approach of using the action just a little and it’ll be able to find a probability instead.

Our solution to this problem hinges on our understanding on how a particle moves through time on a quantum scale. Since particles are ambiguously wave-like (see wave-particle duality), they also travel like waves.  This means that they oscillate in a way that is most conveniently described by imaginary numbers. The full derivation involves exploring the Schrodinger equation and something called the Hamiltonian, and we’ll touch on that a little.


So without dipping too much into the details, I’ll lay out some of the technical formulation of how this works. We can represent the transition along a path between two spacetime points like this:


In this equation, |x-A> represents our initial spacetime state.  <x-B| represents our final spacetime state, and e^-(iH(tB-tA))  is our time evolution operator. tB-tA represents the change in time between the two spacetime points. G is our propagator.

For those unfamiliar with this kind of exponential with an imaginary number, it means that the state is going through a complex (or imaginary) rotation. It isn’t easy to describe in relatable physical terms, but it is heavily related to the concept of superposition. When two different things are in sync with one another, they tend to add up and cause constructive interference. When they are oppositely aligned, they cancel out and cause destructive interference.


Waves in sync can combine to larger waves through constructive interference. Waves out of sync with produce smaller waves through destructive interference. Image courtesy of Joe’s waves revision page:

As mentioned earlier, the value of H is the Hamiltonian, a quantity that is closely related to the Lagrangian. The Hamiltonian probably deserves its own article so I’ll be pretty brief here. In most cases the hamiltonian can be interpreted as the total energy of the system. So this time evolution operator tells us that the particle undergoes a complex rotation through time, and that rotation is related to the total energy of the system (ie: the particle in this case).

What does it mean?

The time-evolution operator and the propagator are both used to describe and understand the nature of quantum physics.  While these are both human tools used to describe a physical realm the defies our intuitive understanding, examining them can reveal a lot about the nature of quantum reality.

The imaginary rotation through time evolution is more than just an abstract mathematical description, it is what tells us that particles can undergo constructive and destructive depending on a difference in some kind of phase. From this equation, we know that the progression of this wave-like behavior is related to energy, and from here we can begin to ask why? What would energy be connected to the passage through time in this way?

The propagator gives a basic tool to start working with the uncertainties of the quantum world. Through it we can start working with complex interrelations between particles in Feynman diagrams or we can used them to describe the odd phenomena of tunneling. Possessing these tools moves us away from the assumptions of the classical world and empowers us to tackle much more difficult problems in quantum physics.

Many of these tools are central to the field of quantum mechanics. Here are some resources for anyone who wants to learn more:

BOOK: Modern Quantum Mechanics, J. J. Sakurai
One of the most commonly cited textbooks on Quantum Mechanics. Develops all the concepts in a detailed approach. Chapter 2 tackles some of the topics here.

VIDEO AND READINGS: Quantum Entanglements, Leonard Susskind
These lectures provide a great introduction to Quantum Mechanics. Each lecture contains video and written material. Lectures 8 and 9 are related to what I have discussed in this post.


The Principle of Least Action and Lagrangian Mechanics

The principle of least action is a idea that helped transition physics away from Newtonian methods and toward a more general description of physical systems.

Lately I’ve been studying methods of calculating path integrals, and while I would very much like to write about that, I think I should start with a more foundational topic. The principle of least action is a idea that helped transition physics away from Newtonian methods and toward a more general description of physical systems. It has been so important that it is still used across many different areas of physics today.

Principle of least action

To start, we should discuss what the principle of least action actually is. In the 1600s, Pierre de Fermat discovered that light always takes a path that takes the least amount of time to traverse. Knowing the initial point, and the final point of light, you would know what path it took by finding that path that minimized the time taken by the light. This would become known as Fermat’s principle or the principle of least time.


The light ray moves slower when passing through a material. As per Fermat’s principle, the ray is deflected in such a way that minimizes the light takes to pass from beginning to end.

This was a novel idea, since older approaches of physics first considered the present state of a system and then apply laws to determine what further point it will transition to. In Fermat’s principle, you consider the endpoints of the system (beginning and end of the light path), and then you could derive the laws based on some kind of constraint (ie: minimizing the time that light travels).

Lagrangian Mechanics

As it turns out, light isn’t the only place you can apply this kind of approach. In Lagrangian mechanics, you could define the starting point of a system, and the end point of that system, then you’d try to minimize and maximize a particular quantity known as the action (S). The action is an abstract value that depends on the path the system takes; if find the path that minimizes or maximizes the action, that path corresponds to the real path that the system takes. This sounds pretty abstract but there is a well defined system for calculating the action for a path, and it comes from formula you can build for your system called the Lagrangian function or just the Lagrangian (L).


 The Lagrangian is defined at each point in time. Integrating along the path of the system provides the action for that path.

So by deriving the Lagrangian function and applying the principle of least action, you can describe almost any physical system. This has been a very successful approach and was later adapted to a similar technique in what we call Hamiltonian mechanics.

There are some conceptual oddities of Lagrangian mechanics. Particularly, the idea of both the past and the future influencing the behaviour of the present can be a little discordant with our idea of causality. Regardless, it has been proven as an effective approach to understanding the behaviour of a large variety of scenarios in physics.

What does it mean?

The principle of least action is an extremely handy concept that lets us tackle some problems that would otherwise be extremely difficult in physics. A commonly used example is the problem of coupled oscillators. In this problem, a series of masses are connected by springs and strung between two walls. Deriving it using tradition newtonian methods can be extremely tedious for a large number of masses. On the flip-side using lagrangian mechanics can vastly simplify the problem. If you are interested in lagrangian methods for harmonic systems, this problem can illustrate can illustrate the power of lagrangian methods:

Coupled Harmonic Oscillators by Irina Yakimenko

Keep in mind that this approach can offer more than just mathematical power, it is conceptual change in how we look at interactions in the universe. Instead seeing the universe as of a series of sequential events, we can see a system of relationships across time and space that emerge from the optimization of a few constraints.

The principle of least action also provides interesting implications for our perspective of time. As mentioned earlier, since the lagrangian derives our equations of motion from past and future constraints, it violates our intuition about causality. Some have argued that it implies an eternalist universe where the past, present and future all exist simultaneously and are separated only by the dimension of time. This is largely philosophical and as such, is difficult to demonstrate with any certainty.


Here are a few resources for getting a handle on Lagrangian Mechanics and the principle of least action:

PDF: Introduction to Lagrangian and Hamiltonian Mechanics, Melanie Ganz

A short pdf that introduces the main concepts and equations of Lagrangian Mechanics

BOOK: Mechanics: Volume 1, Landau and Lifshitz

The go-to book on classical mechanics. Uses Lagrangian methods from the beginning and covers many different methods applying them to mechanical situations.

VIDEO: A Special Lecture: Principle of Least Action, Kenneth Young

A fantastic introduction to the material and great speaker.