Summary
Overview
Michael Stevens and Hannah Fry explore how we could theoretically observe Earth's past by using celestial objects as telescopes. They discuss using black holes as mirrors, the sun's gravitational lensing, and Earth's atmosphere as natural telescopes to view historical events, examining the physics, engineering challenges, and incredible possibilities of looking backward in time through space.
Looking Into Our Past Through Space
Michael introduces the fascinating concept of viewing Earth's history not through traditional astronomy, but by using celestial objects as telescopes to see our own past. He explains how light traveling through space creates an ever-expanding sphere of photons that represent moments frozen in time. If we could intercept these photons after they've traveled great distances, we could literally watch historical events unfold, from ancient civilizations to more recent history.
- Everything we see is actually in the past due to light travel time - even face-to-face conversation has a nanosecond delay
- A light nanosecond ruler demonstrates that light travels 30 centimeters in one billionth of a second
- When looking in a mirror, you never see yourself as you are in that exact moment
- Photons leaving you create an ever-growing sphere that contains images of your past self
" We only have one way to look and it's a go. And it's really trippy to think about what that means. It means that there is a sphere of light growing with me as the center at all times that contains photons that touched me sometime in the past. "
Measurement Systems and Scientific Communication
Throughout the episode, Michael and Hannah engage in playful discussions about measurement systems, sharing fascinating trivia about how units relate to each other. Michael reveals that the inch is legally defined as 2.54 centimeters, meaning the US system is actually built on top of metric. They discuss various mnemonics, including 'five tomatoes' for remembering 5,280 feet in a mile, and explore how 50 inches equals exactly 127 centimeters. These lighter moments illustrate how even basic measurements carry interesting historical and mathematical stories.
- The mnemonic 'five tomatoes' helps remember there are 5,280 feet in a mile
- In the US, the inch is legally defined as 2.54 centimeters, making it dependent on the metric system
- 50 inches equals exactly 127 centimeters, the closest integer alignment on a ruler
- 'Give them an inch and they'll take a mile' originally was 'give them an inch and they'll take an ell' (a cubit-length measurement)
" Just remember five tomatoes. Five tomatoes. 5,280. Five two eight O's. 5, 2, 8, 0, 5 tomatoes. That's how many feet are in a mile. "
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