The Ghostbusters trailer contains an Easter egg hidden in the whiteboard filled with physics equations. This Easter egg goes by quickly but if you pause at just the right moment, you will see one "equation" on top is a URL that spells paranormalstudieslab.com. Fans will recognize this is a nod to the original movie as The Paranormal Studies Lab was the name of the laboratory run by Dr. Peter Venkman (Bill Murray) at Columbia University.
Eagle-eye fans who picks up on this and visits the URL, will come across a hidden web page on the physics of the Proton Pack, as explained by MIT particle physicist, James Maxwell. James curiously shares a name with James Clerk Maxwell, the Scottish physicist famous for formulating the laws of electromagnetism. The laws of electromagnetism and special relativity play an important part in the physics of the proton pack.
In practical terms, the Proton Pack is a miniature particle accelerator that ensnares negatively charged ectoplasmic entities inside a positively-charged proton beam. The term proton pack is not used in the original movie at all and was originally described as a positron collider. It was not until the subway tunnel scene in the second movie that the word is actually used. So, let's take a look at the physics behind the Proton Pack and see whether it is possible to build one.
The Original Proton Pack: The Cyclotron
As Maxwell points out, the orginal Ghostbusters Proton Pack, as designed by Dr. Egon Spengler (Harold Ramis), was a cyclotron. This was one of the earliest types of particle accelerators ever developed. The device was invented by Ernest O. Lawrence in 1932 and works by accelerating charged particles along a spiral path. Back then, cyclotrons were the most powerful particle accelerators around until they were replaced by the more advanced synchrotons in the 1950s though they are still used today in the first stage of some particle accelerators.
Inside the cyclotron, a charged particle is injected into the middle of the chamber where it is accelerated between two D-shaped electrodes or "dees." In the case of the Ghostbusters' Proton Pack, that charged particle is a positron or positively charged electron. The magnetic field passing through the dees bends the particle's path, making it travel in a circle, while the electric field between the dees accelerates the particle, giving it a "kick" to make it go faster. When the positron beam has enough energy, it strikes a metal target to release a beam of protons.
The kick the particle gets from the electric field is very similar to a parent pushing a child on a swing. If a parent times the push at just the right moment, the child gains more energy and swings higher and higher. Instead of having one parent pushing from behind, there is also an adult in front who pushes in the opposite direction. If the parent in front also pushes at the right moment (and does not get hit in the face), the child will gain more energy and continue to swing higher and higher with each push.
Just as the child on the swing gets two pushes, the positron gets two kicks. It gets one kick as it passes through gap. As it circles back and passes through the dee in the opposite direction, the electric field switches direction to give it another kick.
The cyclotron was a significant advance over previous particle accelerators. Previous accelerators, or linacs, just accelerated particles along straight lines. Inside the cyclotron, the magnetic field bends the beam, making it curve back on itself so it could pass through the gap, and be accelerated multiple times. As the particle gains more energy, it spirals outward.
When the particles reach the rim, they exit the dees through a small gap, and hit a metal target located at some point beyond the rim of the chamber. This creates secondary particles, in this case protons (the positively charged partice found in the nucleus of an atom), which may be guided outside of the cyclotron and into instruments for analysis.
Proton beams look nothing like in the movie. Proton beams are (usually) collimated, i.e., they travel in straight lines. In the movies, they are seen to whip about and undulate wildly though, this is probably done for visual effect.
Proton Pack Upgrade: The Synchroton
To attain even higher particle energies, the Ghostbusters reboot's Proton Pack, possibly designed by the team's particle physicist Erin Gilbert (Kristen Wiig), uses an even more powerful particle accelerator: the synchrotron. While a larger cyclotron and, by extension, proton pack, can accelerate charged particles to much larger kinetic energies but there are limitations to how far it can do so. The size of the accelerator is not the only practical consideration!
The spiral path of the cyclotron needs to sync up with the electric field's "kicks," or the cyclotron resonant frequency. The cyclotron resonance is the time it takes for the particle in a circle and depends on the strength of the magnetic field, the electric charge, and its mass.
If the particles goes fast enough, its mass starts to increase as relativistic effects come into play. This changes the cyclotron resonance and the beam goes out of phase with the oscillating electric field; the particle beam no longer gets the kicks at the right time. The classical cyclotron is therefore only capable of accelerating particles up to a few percent of the speed of light.
To get even higher energy particles, the synchrotron solves this problem by varying the strength of the magnetic field. As the speed increases, so too does the magnetic field. Instead of moving out in a spiral, the changing magnetic field exerts a greater centripetal force on the faster moving particle to keep it along the same circular path.
This means that the Proton Pack seen in the reboot is a far more powerful accelerator (Go, Erin!) but the science does not stop there. To make all this possible, the Ghostbusters needs to find a way to generate much stronger magnetic fields to keep a particle moving in a circular path about the size of a back pack. To do this, they turn to superconductors and cryogen systems to generate these magnetic fields.
And Powerful Magnet to keep it Compact
To get the intense magnetic fields to make a synchrotron work, the Proton Pack uses superconduting magnets. The resistance of all metals becomes lower as they as cooled but superconductors are special. At a certain critical temperature, their resistace falls to zero. An electric current flowing through a loop of superconducting wire can persist indefinitely with no power source. This allows superconductors to handle much larger currents than metallic conductors and produce more intense magnetic fields better than metallic conductors.
Superconducting magnets need to be kept cold, at cryognic temperatures, to work. The new Proton Pack needs a cryogen system, possibly using liquid Helium needed to keep the superconductors at approximately minus 269.15 degrees Celsius or minus 452.47 Fahrenheit. This will keep the superconductors at the temperatures they need to handle the large currents needed to generate the intense magentic fields for the synchrotron.
Crossing Streams: Does the Science Measure Up?
In an interview, Maxwell said that the movie's director, Paul Feig, has made a lot of effort to make the science in Ghostbusters seem as realistic as possible, as we can see from the Proton Pack's blueprints. Maxwell points out in the video that, while the science behind the Proton Pack is sound, there are limitations. For one, it's not possible to actually build a synchrotron particle accelerator into an actual back pack. The technical considerations, e.g., getting the intense magnetic fields to make things work, are not practical.
One of the interesting challenges for him, as one of the show's science advisers, was figuring how actual science and #technology would influence the prop design. As a fan of the original Ghostbusters, he admitted that seeing scientists play the role of heroes, and saving the day, was one of the things that piqued his interest in the sciences as a child. Hopefully, layering real physics into an awesome science based ghost-fighting weapon will do the same when the first Ghostbusters came out, and capture a new generation of scientists like it did Maxwell.