Sunday, March 26, 2017

Demo Day 2017

Yesterday, I participated in a demo day for elementary school-aged girls, hosted by FEMMES.  I always have tons of fun showing off the department's cool physics demos, and I thought I'd talk a bit about the one I was showing this time: A levitating superconductor.

Most materials fall into two categories: conductors, which allow electric current to flow through them; and insulators, which don't.  Even though conductors allow current flow, there's always some resistance – The electrons lose energy as they move.  Superconductors, however, allow charge to move with zero resistance, which leads to some interesting effects.

For the purposes of the demo, we used a "high-temperature" ceramic superconductor:
The temperature where it transitions to a superconducting state is only high compared to other superconductors – It needs to be cooled to 77 Kelvin, or about -321°F.  Luckily, that's the temperature of liquid nitrogen, which is a common tool in science labs.

This is a type-II superconductor, which means that when it is cooled to the proper temperature, it can "pin" the magnetic flux passing through it.  In simpler terms, it remembers the magnetic field it was in when it was cooled.  That means if we set up a magnetic track like this,
we can make the superconductor levitate!  On the left is a styrofoam cup holding the superconductor in liquid nitrogen.  Underneath the cup are magnets arranged in a pattern that matches the one on the track to the right.  When the superconductor is set on the track, it settles into the exact field it was in when pinned, resulting in levitation:
Big thanks to the Demo Lab for providing the equipment, and FEMMES for organizing the event!

Monday, March 13, 2017

LVC Meeting 2017

This week I'm at the LIGO-Virgo Collaboration Meeting!  Every March, we get together in Pasadena, CA to talk about what we've been working on, and trade insights.  Similar to the APS Meeting earlier this year, I'll be updating this page with my experiences as the conference unfolds.

I arrived late in the evening, zigzagging my local time through +1 hour for daylight savings followed by -3 hours for Pacific time.  While waiting for my shuttle to the hotel, I listened to an announcement repeatedly detailing the use of the various lanes, not unlike this scene from Airplane!.

The first two days of the meeting are devoted to parallel sessions for the various search groups.  The one I belong to, Continuous Waves, has about 30 members.

One speaker, Lilli Sun, presented work on a search for waves from the Scorpius X-1 binary system.  The system consists of a neutron star that sucks up mass from a neighboring sun.
An artist's impression of the Scorpius X-1 LMXB system. (Courtesy of Ralf Schoofs)
In a binary system, the two bodies revolve around a common point between them, usually quite rapidly.  This causes a Doppler shift in the gravitational waves that oscillates between two frequencies:
At one of my first meetings with the LIGO group at Michigan, this shape was compared to the Tower of Sauron, from Lord of the Rings.  In that moment, I knew I was with my people.

An interesting bit of vocabulary I can give you from the Scorpius talk is "torque balance limit".  As you may know, torque is the rotational equivalent of force – Applying a torque makes things rotate faster or slower.  We can observe systems like Scorpius X-1 slowing down their rotation, which implies that there is a torque acting on them.  The torque balance limit says, "If all the slowdown we see is due to gravitational wave emission, how strong would those waves have to be?"  That lets us set a threshold for how good our detectors need to be to pick up waves from these sources.

Another group of talks (including mine!) was on noise sources in the detectors, and our efforts to eliminate them.  One big source of noise turned out to be an extra network cable in one of the end stations, where the laser is reflected back.  Any wire is essentially an antenna that can send out signals that interfere with other electronics.  The precision of our measurements require total electromagnetic silence.

I'll leave you with this image from the poster gallery:

Today opened with an interesting talk on pulsar glitches.  Normally, pulsars emit continuous electromagnetic waves (and theoretically gravitational waves) with a frequency that gradually drops over time.  However, two known pulsars, Crab and Vela, have been observed to suddenly increase in frequency, before continuing the downward trend.  Any change in EM frequency would also appear in gravitational waves, so the presenter, Greg Ashton, was discussing ways to account for glitches in gravitational wave searches.  This sparked a discussion among the Michigan LIGO members over lunch about whether our searches could account for glitches – The whole point of meetings like this!

Most of the other presentations today were practice runs of plenary talks to be given to the whole collaboration tomorrow.  I probably won't see the real ones, since I'll be at the LAAC (LIGO Academic Affairs Council) Tutorials that introduce parts of LIGO research I don't get to see in my continuous wave bubble.

As I mentioned yesterday, I spent most of today at the LAAC talks.  It started with a panel on finding academic career opportunities.  The main takeaway seemed to be "Send out LOTS of applications, and don't get discouraged!"  The usual path after graduate school is 2-4 years of postdoctoral positions before applying to faculty positions.  The panelists emphasized the importance of showing you're able to communicate the purpose and requirements of your work, since even if you're not looking for a teaching position, you need to explain why you should be funded.  They suggested finding opportunities to give talks, and participate in public outreach.

During lunch, I went to a presentation on the Pegasus tool, which acts as a wrapper for the program we use to run jobs on our computing clusters.  It had some nice display features for giving info on the status of jobs, but I decided it would probably be too much trouble to switch over the infrastructure I have now.

I took the evening off to spend time with my Aunt Kaylyn, who was kind enough to drive out for a visit!

Final day of the meeting!  Lots of plenary talks today, summarizing the work each group has been doing.  One, by Vuk Mandic and Florent Robinet, addressed the topic of cosmic strings, which I hadn't heard too much about before.  Reading the summary from Wikipedia makes them sound a bit like the "crack in time" that Doctor Who revolved around a few seasons ago, but sadly for sci-fi fans, the talk was about how our observations had mostly ruled them out.

During lunch, there was a diversity open-mic, where people were invited to discuss issues they had seen within the LVC.  The main subject was gender pronouns, and our ombudsperson, Beverly Berger, made a great point about why people shouldn't complain about new ones: Around the turn of century, we started using Ms. as a way to leave the marital status of a woman unspecified.

The afternoon included talks about the improvements that have been made to the detectors.  At the Hanford, WA detector, they could sometimes get 20 mph winds that would move the building enough to knock the equipment out of its delicate alignment.  Some precision engineering led to the detector staying "locked" in January, and collecting some important data I'm not at liberty to discuss!

I always enjoy these meetings, since it gives me a chance to see the scope of the collaboration I'm part of.  I hope to have a long career in this field!

Saturday, March 4, 2017

It's a Trap(pist)!

I didn't get around to posting last week, since I was busy moving in with my amazing fiancee, but there was some big news from the astronomical community!  In case you missed it, a number of groups confirmed the existence of seven Earth-size planets in the habitable zone of the star Trappist-1.  I thought I'd talk a little bit about how these planets get detected.

Isolated stars are generally too small and too distant for us to see them as more than single points of light, even with powerful telescopes, so you might wonder how we can find the significantly smaller planets that orbit them.  The key is when the planet passes between the star and our line of sight, it casts a shadow on the star:

We can figure out the total light blocked from the two bodies' apparent size:
where d is the diameter of the body, and D is the distance from Earth to the body.  Since the 39 light-years to Trappist-1 is a lot bigger than any orbit the planets would have, we can assume D is the same for the planets and the star.

Closer to home, the apparent size of the sun and the moon from Earth are approximately equal, which is how we can get a total solar eclipse, even though their diameters differ by a factor of 400.

In the case of Trappist-1 though, the amount of light blocked is approximately the ratio of the cross-sections:
where Rp is the radius of the planet, and Rs is the radius of the star.  That leads to data that look like this:
From Nature paper
That plot shows the drop in intensity as each of the seven planets passes in front of the star.  The height of the drop tells us how big the planet is, but we can also use the duration of the drop to find the orbit.

Assuming a circular orbit, we can find the angular velocity of the planet from Newton's gravitational equation:
where G is Newton's constant, M is the mass of the star, and R is the orbital radius of the planet.  To figure out how long the planet takes to cross in front of the star, a picture helps:
The angular distance over which the planet is crossing the star is
Combing this with the previous equation, the transit time is
We measured the transit time from the dimming of the star, and we can figure out the radius and mass of the star from stellar dynamics, so this gives the orbital radius of the planet!

If you want to know more about the discovery, I highly recommend my friend Josh Sokol's piece at New Scientist.