Radiolab Episode Summary: "Super Cool"

Podcast: Radiolab (WNYC Studios)
Original Air Date: December 5, 2017
Hosts: Jad Abumrad and Robert Krulwich
Special Guest: Walter Murch

Episode Overview

In "Super Cool," Radiolab explores the science and mythology of supercooling—how water can remain liquid below its freezing point and then transform into ice instantaneously in the right conditions. The episode is sparked by a chilling wartime story involving horses and a lake that supposedly flash-froze around them. Through debates, experiments, and insights from scientists, the episode unpacks both the plausibility of the story and the underlying physics, before leaping into the ways these phenomena provide metaphors for the birth and structure of the universe itself.

Key Discussion Points and Insights

1. The Story That Started It All: Horses Frozen in a Lake

• Walter Murch introduces a haunting tale purportedly from WWII: During the siege of Leningrad, Soviet horses, fleeing a forest fire, rush into a lake—which then instantly freezes around them, trapping their heads in a "sculpture garden" of ice.
  • “In the blink of an eye, the lake snaps them shut. It just freezes suddenly, turns to ice with a bang.” — Walter Murch [05:33]
  • Both hosts react to the story's power and strangeness—Jad is fascinated, Robert is skeptical.
• The group quickly moves to determine if this story could possibly be true or is merely metaphorical.

2. Investigating the Science: Supercooled Water and Nucleation

• The team visits Rockefeller University to try a supercooling experiment with Alexander Petrov.
  • Using ultra-pure water supercooled to -20°C, a plastic horse is dropped in, causing the water to freeze instantly.
    • “Because the moment that little plastic horse hit the water, the water slammed into ice.” — Jad Abumrad [09:46]
  • This dramatic demonstration shows catastrophic, near-instantaneous ice formation, but only in controlled conditions.

3. What Actually Makes Water Freeze?

• Virginia Walker, biologist, explains water's need for a "nucleator"—a seed particle (like dust or bacteria) that triggers freezing.
  • “The only reason that water freezes normally at 0°C is that there’s something there that makes it freeze. We call that a nucleator.” — Virginia Walker [10:33]
  • Without nucleators, water can be cooled far below freezing without solidifying—until something disturbs it.

4. How Plausible Is the Lake Story?

• Multiple experts, including glaciologist Erin Pettit, debunk the instant-lake-freeze premise for a real-world lake full of impurities and warm-blooded horses.
  • “When you’re talking about freezing an entire lake, you’ve got a lot of problems to consider… the water would never have been pure enough to supercool.” — Stephanie (paraphrasing Erin Pettit and Virginia Walker) [21:10]
  • Summary answer: The story is metaphorical at best, not possible under natural circumstances.

5. The Bacteria Connection: Microbes as Cloud Seeding Engineers

• Fascinatingly, certain bacteria serve as highly effective ice nucleators—helping water in the clouds form ice crystals, which become snowflakes.
  • “If you melt each snowflake, you’ll find a little bacterium inside it.” — Virginia Walker [20:01]
  • Bacteria’s evolutionary reason for this ability may be to hitch a ride to new places via snowflakes—turning winter weather into a method of transportation.

6. From Icy Lakes to Star-Birthing Clouds: The Cosmic Parallel

• The conversation shifts to how supercooling and nucleation provide a metaphor for the early universe.
  • Janna Levin, physicist, describes how tiny density fluctuations (“seeds”) in the young, rapidly cooling universe led to the large-scale structures—galaxies, clusters—that we see today.
  • “Amazingly, the largest structures that we know about in the universe have their seeds in these tiny fluctuations.” — Janna Levin [25:23]
• Jad notes that supercooling and seeding happen many times during cosmic evolution; perhaps more, as the universe continues to cool.

Notable Quotes & Memorable Moments

• On the emotional power of the story:
  “You have hundreds of horses bolting through this flaming... heading towards the open space ahead, which was the lake.” — Walter Murch [05:12]
• On the scientific skepticism:
  “I told you at the time, don’t trust this story. It’s not scientifically possible.” — Robert Krulwich [07:16]
  • “Did somebody actually see this? What is the actual evidence?” — Stephanie [21:42]
• On discovering the bacterium at the heart of every snowflake:
  “If they make ice, they can get back down, and they can get back down in a different place and start a new colony of bacteria somewhere else. And so by this way, they get dispersed around the whole Earth.” — Virginia Walker [19:44]
• On the cosmic scale:
  “Like these beautiful structures that you’re describing are like the snowflakes around the little bit of D.” — Janna Levin [26:11]

Timestamps for Important Segments

• Intro & Setup: [01:03-01:45]
• Malaparte’s Freezing Lake Story: [02:28-07:13]
• Experiment with Supercooled Water: [07:57-10:11]
• Explaining Nucleators & Ice Formation: [10:15-12:34]
• Key Science: Nucleators in Lakes and Bacteria: [17:17-20:09]
• Examining Story Authenticity, Scientific Rebuttal: [20:19-22:23]
• Cosmic Metaphor: Phase Transitions and Universe Formation: [22:26-27:13]
• Credits and Further Listening: [27:27-end]

Summary Takeaway

Radiolab’s “Super Cool” is a masterful blend of storytelling and science: a haunting, possibly apocryphal wartime tale leads to a real laboratory demonstration of supercooling, a primer on nucleation (from dust to bacteria), and finally a grand cosmic metaphor explaining the birth and clustering of matter in the universe. The episode ultimately debunks the legend while marveling at the awe-inspiring phenomena that inspired it.

Further Exploration:

• Watch videos of supercooling experiments at: Radiolab.org
• Read Walter Murch's translation of Malaparte's writings: The Bird that Swallowed Its Cage