Tag: physics

the Ummm… Drive

figure19

So, there is now a peer-reviewed paper on the fabled EmDrive, which empirically measured a statistically significant thrust. The important results are in Figure 19 up above, and here is what the paper has to say about it:

Figure 19 presents a collection of all the empirically collected data. The averaging of the forward and reverse thrust data is presented in the form of circles. A linear curve is fitted to the data and is shown with the corresponding fitted equation. The vacuum test data collected show a consistent performance of 1.2±0.1uN/kW

It’s not clear if the fit was to the averaged data or the raw data. I suspect the averaged, because looking at the raw data, at no time did thrust exceed 130 uN, even when power was increased from 60 to 80 kW. In fact the data at 80 kW points averages out to the same thrust as at 60 kW, and the error bars are a textbook example of the difference between accuracy and precision.

These results are peer-reviewed, and there is a “statistically significant” linear fit to the data that does demonstrate a correlation between the input power and the observed thrust, but this data does not show that the EmDrive actually works. As Chris Lee at Ars Technica put it, the drive still generates more noise than thrust:

The more important point is that the individual uncertainties in their instrumentation don’t account for the variation in the thrust that they measure, which is a very strong hint that there is an uncontrolled experimental parameter playing havoc with their measurements.

Lee also points out that there are a lot of experimental questions left unanswered, including:

  • Why are there only 18 data points for an experiment that only takes a few minutes to perform?
  • Where is the data related to tuning the microwave frequency for the resonance chamber, and showing the difference between on-resonance mode and an adjacent mode?
  • What is the rise-time of the amplifier?
  • What is the resonance frequency of the pendulum?

on that last point, Lee elaborates:

The use of a pendulum also suggests the sort of experiment that would, again, amplify the signal. Since the pendulum has a resonance frequency, the authors could have used that as a filter. As you modulate the microwave amplifier’s power, the thrust (and any thermal effects) would also be modulated. But thermal effects are subject to a time constant that smears out the oscillation. So as the modulation frequency sweeps through the resonance frequency of the torsion pendulum, the amplitude of motion should greatly increase. However, the thermal response will be averaged over the whole cycle and disappear (well, mostly).

I know that every engineer and physicist in the world knows this technique, so the fact that it wasn’t used here tells us how fragile these results really are.

This is really at the limit of my empirical understanding, but it’s a question that the authors of the paper (not to mention anyone over at /r/emdrive) should be able to field with no worries.

Basically, this paper doesn’t answer any of the substantive questions. But it does at least validate the notion that there is something going on worth investigating. But let’s be real about the outcome – because we’ve seen this before:

For faster-than-light neutrinos, it was a loose cable. For the BICEP2 results, it was an incorrect calibration of galactic gas. For cold fusion, it was a poor experimental setup, and for perpetual motion, it was a scam. No matter what the outcome, there’s something to be learned from further investigation.

and that’s why we do science. It’s not as if scientists are fat cats out to protect their cash cow. (Seriously. I wish it were so). Maybe we are on the verge of another breakthrough, but it will take a lot more than this paper to convince anyone. And that’s as it should be.

MAGNIFICENT – string theory FTW and loop quantum gravity FAIL

Calabi-Yau for the win!!
I’ll freely admit that I am comprehending only about 10% of the argument, but this is still a magnificent post about why string theory is right and why loop quantum gravity is wrong.

And incidentally also reveals that the science writers on Big Bang Theory really are on top of the game. Sheldon’s snort of derision here is utterly justified.

“I’m listening. Amuse me.”

I really need to start watching the show. Hulu or Netflixable, I assume…

make war, not bosons

I try to keep things apolitical around here, and its not my intent to change that policy. But this is an issue of science funding as a national priority, so I feel it is relevant: Fermilab funding ends in September.

U.S. researchers will soon abandon their search for the most coveted particle in high-energy physics because of a lack of funding.

Researchers working at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, had wanted to run their 25-year-old atom smasher, the Tevatron, through 2014 in hopes of spotting the so-called Higgs boson before their European counterparts could discover it with their newer, more powerful atom smasher. But officials at the U.S. Department of Energy (DOE), which funds Fermilab, informed lab officials this week that DOE cannot come up with the extra $35 million per year to keep the Tevatron going beyond September.

“Unfortunately, the current budgetary climate is very challenging and additional funding has not been identified. Therefore, … operation of the Tevatron will end in [fiscal year 2011], as originally scheduled,” wrote William Brinkman, head of DOE’s Office of Science, in a letter to Melvyn Shochet, chair of DOE’s High Energy Physics Advisory Panel (HEPAP) and a physicist at the University of Chicago in Illinois.

Fermilab is, as far as I am concerned, a national treasure like the Hoover Dam or Mount Rushmore. It’s about 50 miles from my home growing up and I still remember a childhood visit there 20 years ago.

The worst thing about this is how science is a victim of political climate. As others have pointed out, even the reduced spending on Afghanistan as we draw down there still means we spend more in six hours there than we’d need to keep Fermilab funded through 2014. I’m not saying we shouldn’t spend the money in Afghanistan (which puts me at odds on my other blog communities, as some of you are aware). But I am saying that maybe in the grand scheme of things, with a deficit in the trillions anyway, we shouldn’t be penny wise and pound foolish.

end rant.

star-trekkin’ across the universe

Sometimes there’s a visible gulf between geekdom and academia, despite the stereotype of these two realms being congruent. I am reminded of this gulf by this odd story about a paper by William Edelstein, a senior and distinguished physicist (in my own field of MRI research), who has calculated the lethality of interstellar travel:

Interstellar space is an empty place. For every cubic centimetre, there are fewer than two hydrogen atoms, on average, compared with 30 billion billion atoms of air here on Earth. But according to William Edelstein of the Johns Hopkins University School of Medicine in Baltimore, Maryland, that sparse interstellar gas should worry the crew of a spaceship travelling close to the speed of light even more than the Borg decloaking off the starboard bow.

Special relativity describes how space and time are distorted for observers travelling at different speeds. For the crew of a spacecraft ramping up to light speed, interstellar space would appear highly compressed, thereby increasing the number of hydrogen atoms hitting the craft.

Worse is that the atoms’ kinetic energy also increases. For a crew to make the 50,000-light-year journey to the centre of the Milky Way within 10 years, they would have to travel at 99.999998 per cent the speed of light. At these speeds, hydrogen atoms would seem to reach a staggering 7 teraelectron volts – the same energy that protons will eventually reach in the Large Hadron Collider when it runs at full throttle. “For the crew, it would be like standing in front of the LHC beam,” says Edelstein.

The spacecraft’s hull would provide little protection. Edelstein calculates that a 10-centimetre-thick layer of aluminium would absorb less than 1 per cent of the energy. Because hydrogen atoms have a proton for a nucleus, this leaves the crew exposed to dangerous ionising radiation that breaks chemical bonds and damages DNA. “Hydrogen atoms are unavoidable space mines,” says Edelstein.

The fatal dose of radiation for a human is 6 sieverts. Edelstein’s calculations show that the crew would receive a radiation dose of more than 10,000 sieverts within a second. Intense radiation would also weaken the structure of the spacecraft and damage its electronic instruments.

All well and good and I have no reason to doubt Dr. Edelstein’s calculations (we medical physics types do have a professional interest in radiation dose and shielding, after all). But clearly Dr. Edelstein is not a fan of Star Trek, because even the most newbie of Trekkies knows about the Navigational Deflector Array. In addition, Starfleet vessels also have Bussard Collectors on the warp nacelles, which are the sci-fi-ified version of the Bussard ramjet.

My point is, physics geeks and sci fi geeks clearly aren’t as overlapping sets as I had assumed. But where a medical physicist might see errant hydrogen atoms as dose, a different kind of physicist might see them as fuel. In a way we scientists do bring our own biases to the table…

many worlds

I have a bit of bias towards the Overcoming Bias blog, because of the way in which Eli pretends his Bayesian worldview is utterly pristine without a trace of dogma. That said, his recent forays into explaining quantum mechanics are superb… that is, until he revealed that his aim was to set up a tension between Science and Bayes. In a nutshell,

Science-Goggles on: The current quantum theory has passed all experimental tests so far. Many-Worlds doesn’t make any new testable predictions – the amazing new phenomena it predicts are all hidden away where we can’t see them. You can get along fine without supposing the other worlds, and that’s just what you should do. The whole thing smacks of science fiction. But it must be admitted that quantum physics is a very deep and very confusing issue, and who knows what discoveries might be in store? Call me when Many-Worlds makes a testable prediction.

Bayes-Goggles on: The simplest quantum equations that cover all known evidence don’t have a special exception for human-sized masses. There isn’t even any reason to ask that particular question. Next!

And just like that – Science is dismissed. Many-Worlds must be true, after all, it makes the most sense and is the simplest possible explanation!

Intriguingly, Eli has often dismissed the idea of God even though one could argue that God too is the “simplest” answer to any number of great Questions. Likewise, he often defends his robust atheism with an analogous assertion to “you can get along fine without supposing [God exists], and that’s just what you should do.”

This is the sort of abuse of science that drives me crazy. People approach issues with a-prioris, such as “God doesn’t exist” or “The Singularity exists” or “Many-Worlds is True” and then contort poor physics and math into supporting positions that are purely situational.

I don’t pretend to be overcoming bias myself. I am a scientist, I am deeply religious, and I think Bayes’ Theorem is useful in certain situations (ie, when measurements are not independent), but is hardly enough to build an entire worldview on.

I am not implacably against the Many Worlds Interpretation, mind you. I enjoyed Tegmark’s article in Nature which really made the case more fairly. I just think that a false dichotomy between Bayes Theorem and Science as an institution serves only to muddle things rather than help us understand.

As an aside, I wonder if Eli would be willing to prove his commitment to Bayes Theorem, by performing quantum suicide?