Title image courtesy NASA, STScI, and ESA
If you’ve followed science news for the past few days, you’ve no doubt seen an abundance of articles about the EmDrive–the prototype reactionless engine which converts microwaves into detectable thrust. Skeptics have been tearing into the experiments on the device for months, but the recent experimental results from a NASA Eagleworks team have thrust the device back into the fickle gaze of the internet memeplex.
Many other news articles and bloggers have already pointed out good reasons why the device probably doesn’t work, or that the results are suspect. Suffice to say, if the device does work, then it’s probably the most important invention of the past thousand years. That you don’t see universities all across the planet scrambling to reproduce the experiment might suggest that no one actually believes the device works, but the EmDrive apologist could also argue that the academic establishment is in a collective state of denial / is paralyzed by some sort of Bystander effect / is actively trying to sabotage the research.
So, instead of continuing to criticize the laudable efforts of the Eagleworks team, I thought I’d supply three short lists. The first is of cases when we thought we broke physics and we didn’t. The second is when we thought we broke physics and we actually did. And the third is a list of small effects that could be confounding the physical results of the Eagleworks team.
When we broke it, but we also fixed it
1. The “faster than light neutrino”: Just four years ago, the OPERA experiment (designed for studying neutrino oscillations) revealed they were detecting neutrinos that seemed to be moving faster than the speed of light. Most were skeptical initially, and the few that were still panicking eventually calmed down when the culprit behind the signal was revealed to be a loose cable.
2. The Pioneer Anomaly: Back when I fashioned myself an astrophysicist, I came across a paper on the Pioneer anomaly. Essentially, the Pioneer probes seemed to be accelerating ever so slightly faster than they were supposed to. This paper, and others, tried to use this slightly-faster-than-expected acceleration to bound contributions from dark energy to the accelerated expansion of space. A careful study was eventually done in 2012 which convincingly determined the cause of the acceleration to be from anisotropic thermal radiation. Simply, the pioneer probes radiated heat in some directions more than others. Because photons carry momentum, this anisotropy was enough to cause a small extra acceleration–no fun dark energy physics here.
3. The Papp perpetual motion machine: Wherein Richard Feynman causes a small explosion.
4. Steorn’s “Perpetual Motion Machine”: Because we clearly didn’t learn from Feynman, in the early 2000s the Irish company Steorn claimed to have invented a perpetual motion machine. It’s more fun to read their wikipedia page after reading the wikipedia page on the history of perpetual motion machines.
Bonus #5: Perytons at the Parkes Radio Telescope: A team that had been trying to track down a mysterious presumably extragalactic signal at the Parkes Radio Telescope finally determined the culprit: microwaves of “terrestrial origin”. Or, literally, they saw the anomalous signal when someone in the break room opened a microwave oven before it was done cooking.
When we broke it, but it was for the best
1. Dark Matter: In the 60s and 70s, Vera Rubin performed pioneering measurements on galactic rotation. Due to a wonderful theorem from kinematics called the Virial Theorem, we are able to calculate how a given system’s energy is partitioned between kinetic energy and potential energy. Rubin’s measurements, which were motivated by work from Jan Oort and Fritz Zwicky, suggested that galactic rotation either strongly violated the Virial Theorem, or that there was a huge amount of mass in galaxies that we just couldn’t see. These measurements led to the discovery of the still poorly understood Dark Matter.
2. The expansion of space, and the accelerated expansion of space: When Einstein first wrote down his eponymous Field Equations, he added a term to make universe static–that is, not expanding and not contracting. He did this almost entirely because it was aesthetically satisfying to him. Shortly afterwards, Hubble discovered that distant galaxies were getting farther from the earth, which has led some to call Einstein’s added term his “greatest blunder”. Then, in the 90s, for entirely different reasons, a term very much like the one Einstein added became necessary to explain measurements by the Supernova Cosmology Project and the High-Z Supernova Search Team which suggested that space wasn’t just expanding, but that it was accelerating in its expansion.
3. The anomalous precession of Mercury: In the late 1850s, it was discovered that Mercury was precessing around the sun slightly differently than Newtonian mechanics suggested it should. A flurry of effort went in to trying to figure out why the shift was there, but it wasn’t resolved until Einstein developed General Relativity, and more or less produced exactly the correct form of the shift.
4. CP Violation: Until the 1960s, physicists expected all interactions in nature to be symmetric with respect to exchange of particles and antiparticles (Charge symmetry) coupled with a form of mirror symmetry (Parity symmetry). In 1964, James Cronin and Val Fitch discovered CP violations in neutral kaon decays, which was crucial to the development of particle physics in the late 20th century. Many expect that CP violation is at least part of the reason for the huge matter/antimatter imbalance in the universe.
A tiny list of tiny forces
1. Knudsen force (the saddest Wikipedia page): A force arising from two materials that have a temperature gradient and are separated by about a mean-free-path’s worth of distance of an ambient air molecule. The newer experiments run by the Eagleworks team were done under vacuum, so this might be ruled out unless there was some ablative shedding or degassing in their instrument.
2. Lorentz force: Any current carrying wire experiences a force when exposed to a magnetic field. The Eagleworks team has a lot of electronics in a small area. It probably isn’t very hard to get a 50 micronewton force from a small current loop interacting with Earth’s magnetic field.
3. Radiation pressure: Photons carry momentum, and shedding enough of them can give rise to small but detectable force (see pioneer anomaly). This is likely too small of an effect to explain the Eagleworks measurement, though.
4. Casimir force: The force that people love to argue about. Chemists say it’s a Van der Waal’s force. Physicist plug their ears and whisper “vacuum fluctuation” until the chemists leave. Almost definitely not the culprit in the Eagleworks device, but still a fun little force.
If there’s any common thread in the big discoveries of physics–it’s one of careful scrutiny. When we encounter something we don’t understand, we poke it again and again. After a couple thousand years of poking things, we’ve come up with fairly stringent heuristics for what happens when things are poked. If dumping microwaves into a carefully designed cavity was all it took to invent warp travel, then I will be very happy, especially when I get my spaceship. But I’m not holding my breath.