What is a white hole? – with Carlo Rovelli

The speaker invites the audience to envision a journey towards the black hole Sagittarius A*, using either a spaceship or a more whimsical method like a broomstick. The choice of Sagittarius A* is justified by its visibility due to its size and location at the galaxy's center. The audience is led to imagine the view as captured by radio telescopes, which is a circle formed by light bending around the black hole, a phenomenon predicted by Einstein's theory of general relativity.

The speaker elaborates on the optical effects observed when approaching a black hole, such as the matter spiraling and heating up due to friction. The concept of time dilation is introduced, where time appears to slow down as one gets closer to the black hole. This effect is illustrated by the example of messages being received from Earth at increasing frequency as the hypothetical journey continues to descend into the black hole, thus experiencing the future relative to Earth.

The narrative continues with the audacious step of entering the black hole. The speaker reassures that initially, nothing dramatic happens upon crossing the event horizon. Inside, the black hole is described as having an unexpectedly vast volume, with layers of spheres and an immense depth. The speaker likens the experience to a fairytale where a small entrance leads to a large, hidden space, highlighting the distortion of space as predicted by general relativity.

Despite the initial calm, the speaker reveals that the interior of the black hole is dynamic, with space stretching and compressing over time. The black hole grows over time, but the focus shifts to the quantum phenomena that must be considered as objects within the black hole are squeezed to extreme densities. The speaker emphasizes the need for a quantum theory of gravity to understand the black hole's interior fully, as general relativity becomes insufficient at such scales.

The speaker discusses the implications of quantum mechanics on the nature of space, suggesting that space is granular, with a minimum size for its constituent parts. This granularity prevents the black hole from shrinking indefinitely, implying that a singularity, as traditionally conceived, may not exist. The speaker also touches on the concept of quantum jumps, suggesting that a black hole might transition to a white hole through a quantum leap, a hypothesis grounded in the principles of quantum mechanics.

Building on the previous discussion, the speaker hypothesizes that black holes may eventually transform into white holes through a quantum tunneling effect. This transition is described as a moment where the known physics (Einstein's equations) no longer apply, and quantum mechanics takes over. The speaker illustrates this with the metaphor of a bouncing ball, suggesting that just as a ball bounces back up after hitting the ground, a black hole might 'bounce' and become a white hole, a concept that aligns with Einstein's equations in a reverse scenario.

The speaker explores the concept of time distortion within the black hole-white hole transition. They propose that while the process may take billions of years from an external perspective, for an observer within the black hole, the journey through the quantum jump and emergence from a white hole could take mere minutes or seconds. This dramatic time discrepancy is likened to a shortcut to the future, offering a unique perspective on the nature of time within the context of general and quantum relativity.

The speaker contemplates the detectability of white holes and their potential role in explaining dark matter. They suggest that white holes, due to their minuscule mass at the time of transition, might exist in abundance and contribute to the gravitational effects attributed to dark matter. The speaker also reflects on the history of black hole research, highlighting the skepticism and eventual acceptance of their existence, and suggests that white holes might already have been observed but not yet recognized. They emphasize the iterative and sometimes serendipitous process of scientific discovery and the importance of asking the right questions.

In the final paragraph, the speaker reflects on the importance of imagination in scientific inquiry, drawing parallels between the thought experiments conducted by historical figures like Anaximander, Hipparchus, and Copernicus, and the 'mind travel' required to conceptualize complex phenomena such as black holes and white holes. The speaker argues that science, like poetry, relies on the ability to envision reality from different perspectives and to challenge established frameworks. They advocate for a balanced approach that acknowledges the limits of current knowledge while remaining open to new ideas and discoveries.