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Black Holes: Seeing the Unseeable

World Science Festival・51 minutes read

Isaac Newton and John Mitchell laid the groundwork for understanding black holes, leading to Einstein's general theory of relativity, Schwarzschild's solutions on black holes, and Dolman's Event Horizon Telescope capturing direct images of black holes, showing the consistency of Einstein's gravity theory across different mass scales. The discovery and study of black holes continue to provide insights into physics, aiming to refine techniques for higher precision testing and exploring the substructure of black hole rings.

Insights

  • John Mitchell introduced the concept of escape velocity, which is the speed needed for an object to break free from a celestial body's gravitational pull, leading to the idea of black holes trapping light due to their immense mass.
  • The Event Horizon Telescope collaboration, spearheaded by Shep Dolman, successfully captured direct images of black holes, utilizing innovative techniques like very long baseline interferometry to create a virtual Earth-sized telescope, overcoming technical challenges to reveal detailed images of these enigmatic cosmic entities.

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Recent questions

  • What is escape velocity?

    Escape velocity is the speed required for an object to break free from a celestial body's gravitational pull and not return.

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Summary

00:00

Unveiling Black Holes: From Theory to Image

  • Isaac Newton formulated the universal law of gravitation, followed by an English clergyman, John Mitchell, who introduced the concept of escape velocity.
  • Escape velocity on Earth is approximately 11 kilometers per second, preventing objects from returning to the surface if launched at this speed.
  • Mitchell applied the escape velocity concept to light escaping from stars, suggesting that stars more massive than the Sun could trap light, leading to the idea of black holes.
  • Albert Einstein's general theory of relativity refined Newton's gravity theory, with Carl Schwarzschild proposing the existence of black holes by concentrating massive objects' gravitational pull.
  • Einstein initially resisted the concept of black holes due to mathematical challenges and the extreme nature of the idea.
  • Theoretical advancements in understanding black hole formation from massive stars and observational data supporting black holes' existence emerged over the years.
  • The Event Horizon Telescope collaboration, led by Shep Dolman, captured direct images of black holes, including one in the Milky Way's center and another in a distant galaxy called M87.
  • The telescope network includes sites worldwide, utilizing very long baseline interferometry to create a virtual Earth-sized telescope for imaging black holes.
  • Technical challenges, such as bandwidth limitations and equipment failures, were overcome through innovative solutions, leading to successful black hole image captures.
  • The process of obtaining and processing data from the telescopes is meticulous and time-consuming, involving data alignment, calibration, and imaging to reveal the final black hole images.

16:47

Harvard Teams Discover Black Hole Ring

  • Four teams at Harvard's Black Hole Initiative independently worked on the M87 image with the same data, isolated from each other to avoid groupthink.
  • Each team, using different techniques, revealed a ring in the image, causing excitement and awe among the researchers.
  • The history of black holes traces back to Albert Einstein's general theory of relativity in 1915, with Carl Schwarzschild providing the first mathematical solution suggesting black hole possibilities.
  • Schwarzschild's solution highlights special locations in the math, such as the event horizon where escape speed equals that of light, leading to a fundamental change in space-time metrics.
  • Einstein initially doubted the existence of black holes, considering them a strange mathematical construct without real-world application, contrasting with Oppenheimer and Snyder's work on stellar collapse.
  • The evidence for black holes in the universe slowly built over time, including observations of pulsars, x-ray binaries, and the massive centers of galaxies.
  • Andrei Ghez and Reinhard Genzel's work in the center of the Milky Way confirmed the existence of a 4 million solar mass black hole, observed through the orbits of stars around an unseen mass.
  • The discovery of a compact object in the Milky Way's center, possibly a black hole, led to the realization of the cosmos' immense and mysterious entities.
  • Ghez and Genzel's research, awarded the Nobel Prize, provided crucial evidence of a massive, unseen object affecting star orbits, hinting at a black hole's presence.
  • The ultimate goal remains to image the black hole at the Milky Way's center, turning it into a unique laboratory for studying physics and Einstein's theories.

32:26

"Black Holes: Gravity, Imaging, and Spin"

  • The stars near the black hole are a thousand Schwarzschild radii away, allowing for accurate tracing by Newton's gravity.
  • Indirect evidence supported the existence of a black hole, but direct observation was needed for confirmation.
  • The image of the black hole shows light bent around it due to the strong gravity, creating a ring outside the photon orbit.
  • The photon orbit is where light can orbit in a circle around the black hole, defining the ring seen in the image.
  • The image allows for testing Einstein's equations by comparing the observed size of the photon orbit with the predicted size.
  • The black hole in M87, 55 million light-years away, was imaged first due to its stability, unlike the rapidly changing Sagittarius A*.
  • M87's black hole is six and a half billion times the mass of the sun, while Sagittarius A* is four million times the sun's mass.
  • M87's image shows a bright bottom and faint top, indicating the black hole's spin, while Sagittarius A* is representative of most supermassive black holes.
  • The blotches in the Sagittarius A* image are due to its movement, prompting the need for dynamic imagery to refine observations.
  • Dynamic imagery will help determine the spin of black holes by tracking the motion of matter around them, providing insights into their orientation and other characteristics.

47:32

"Black Holes: Co-evolution and Einstein's Theory"

  • Supermassive black holes are found in most galactic cores, linked to the co-evolution of black holes and galaxies.
  • The process involves the formation of stars, small black holes merging into larger ones, sinking into gas clouds, and seeding more star formation.
  • Simulations show various phenomena occurring on a galactic scale, with smaller black holes nucleating galaxies that eventually merge.
  • Stellar mass black holes are too small to be directly imaged, unlike the millions and billions of times the mass of the sun black holes seen in the night sky.
  • Confirmation of Einstein's theories is seen in stellar mass and supermassive black holes, showcasing a wide range of mass scales.
  • Black holes, despite vast differences in mass, adhere to the same mathematical principles, demonstrating the consistency of Einstein's gravity theory.
  • Future work involves refining techniques to study black holes, aiming to test Einstein's theory at a higher precision level and capture the substructure of the black hole's ring.
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