Given the 71st Lindau Nobel Laureate Meeting is dedicated to chemistry, Donna Strickland and Brian Schmidt’s presence on the agenda was a surprise, albeit a welcome one. Strickland and Schmidt received their Nobel Prizes in Physics in 2018 and 2011, respectively. The former received hers alongside Gérard Mourou “for their method of generating high-intensity, ultra-short optical pulses,” The latter received his alongside Saul Perlmutter and Adam Riess “for the discovery of the accelerating expansion of the Universe through observations of distant supernovae.”
Since then, they have both been regular and passionate contributors at the Lindau Meetings, with Schmidt even taking up the mantle of spokesperson for the Mainau Declaration 2015 on Climate Change, signed by a host of laureates at the 65th Lindau Nobel Laureate Meeting.
It was telling of how deep their passion to engage with young scientists at Lindau is, then, that both physicists delivered their lectures despite challenges due to COVID-19 restrictions.
Building a Laser Hammer
Awaiting a negative PCR test, Strickland’s Lecture on Monday 27 June titled ‘Generating High-Intensity, Ultrashort Optical Pulses’ was delivered from her hotel room, a stone’s throw away from the Inselhalle. Although Strickland was not physically present, it was an instant classic, condensing 150 years of progress in our understanding and manipulation of light into a short talk, while also explaining Strickland and Mourou’s landmark contribution.
Where Strickland herself – only the third woman to receive the Nobel Prize in Physics – came into the story was perhaps the most interesting part. After gaining her Bachelor of Engineering from McMaster University in 1981, Strickland headed to the US for her PhD at the University of Rochester.
“My PhD project had two parts: one, make a cold plasma of twice ionised nickel, and two make a really intense laser pulse,” she recalled. “Now, I have to tell you, that the first part of this project never worked… But luckily, the other part turned out to be pretty successful.”
Strickland explained the basic mechanism she and her doctoral supervisor Mourou developed for amplifying laser pulses (or as she referred to it, building a ‘laser hammer’): “We start with our short pulse and we simply stretch it and make it a long pulse so that the power is low,” she said. “We can then safely amplify it up to the maximum power that we can and then compress it back to the application at the end.”
Chirped pulse amplification (CPA), as the technique is known, generates ultrashort optical pulses of terawatt intensity. Such pulses are used in myriad applications today. For example, CPA femtosecond lasers have been successfully wielded in 24 million eye operations since 2001, as Strickland demonstrated in a rather gruesome video of a real-life surgery.
For some, if their very first paper as a PhD student had led to a Nobel Prize, it could change them for the worse. But looking back on her early-career accomplishments, Strickland remains humble. “Why was I the one that got to do this work? It was because I was lucky enough to be the first in [Mourou’s] group to try to use short pulses to make high powered pulses,” she said. To the young scientists in the audience, she had a simple message: “I hope some of your PhD projects are just as lucky.”
Looking to the Stars
Schmidt, who was also at an early stage of his career when he made his Nobel discovery, also had a message for the young scientists in his Lecture ‘Astronomy in 2022’ on Tuesday 28 June: “When I finished by PhD, I said ‘I’m going to measure the future of the universe’, and it turns out you need to be bold and stupid to do that when you’re 27,” he joked. “That’s why it’s really important to have bold and stupid people here today like I used to be.”
Having participated remotely in the Panel Discussion ‘Trust in Chemistry, Trust in Science’ from a hotel room in Austria on Sunday due to still being in recovery from COVID-19, Schmidt’s solo talk was delayed by a day to give him a chance to make it to Lindau in person.
The delay was worth it. “We live in a universe that contains a bunch of stuff we call dark energy and dark matter,” he said in his opening remarks. “I will try to convince you that there’s good reason to believe this stuff exists even if it is very ethereal.” To do this, Schmidt took the audience on a journey through time and space, from Copernicus to the latest astronomical facilities, from the Big Bang to the eventual demise of the universe, and everything in between.
How Schmidt, Riess and coworkers, and Perlmutter’s team independently, came to the conclusion that the universe’s expansion is accelerating was particularly fascinating given the technological limitations they faced in the mid-1990s.
Schmidt recalled how he wielded the biggest, most advanced telescopes of the time to produce thousands of 8 MP images per night. “We would take 50 GB of data in a night, but the hard drives we had for storing data were 1 GB, so I had to daisy chain every computer on the mountainside.” Then, he had to sift through the images to find supernovae. “My software to find these things used early versions of machine learning, but it was terrible and it required a lot of human intervention,” he recalled.
Despite these and other challenges, between them, the rival teams found more than 50 supernovae whose light, based on calculations about the Big Bang, was dimmer than expected – strong evidence that the expansion of the universe is accelerating.
Though rightly proud of his discovery, Schmidt looks back on those times with more than a tinge of discomfort: “My one regret is that we could have had women on our team,” he said. With women still underrepresented and facing barriers in physics, this was a stark reminder that although physics has taken tremendous strides in expanding our knowledge since the 1990s, it still has a long way to go in representing the society it serves.