Introduction of Stephen Hawking:
Stephen Hawking, Lucasian Professor of Mathematics at Cambridge University, is welcomed to the second of three Hitchcock Lectures. He is regarded as the world’s foremost authority on gravitational physics and has won numerous awards, including the Wolf Prize for Physics. Despite facing personal adversity, Professor Hawking’s achievements are remarkable.

Recognition of Hawking’s Courage:
Dr. Forbes Norris, Vice President and Clinical Director of the San Francisco ALS Research Foundation, speaks of Hawking’s courage and valor.

Presentation to the Lawrence Hall of Science:
The ALS Foundation for Research presents a bust of Professor Hawking by Marjorie Fitzgibbon to the Lawrence Hall of Science. Dr. Glenn Seaborg accepts the bust on behalf of the Lawrence Hall of Science. The bust symbolizes the importance of public understanding of science and the work of the Lawrence Hall of Science in this area.

Public Understanding of Science:
Sir Stephen Hawking is a symbol of the best efforts to help the public understand science and their place in the universe. The Lawrence Hall of Science is dedicated to promoting public understanding of science and its significance in the modern world. Visitors are invited to come and see the bust of Professor Stephen Hawking on display at the Lawrence Hall of Science.

Black Holes:
Black holes are regions of spacetime with such strong gravitational forces that nothing, not even light, can escape them. They were introduced by John Wheeler in 1967 as a suitable name for the concept discussed earlier. They are often depicted with the mythology of science fiction, which stimulated scientific research on the topic.

Escape Velocity and Black Holes:
The escape velocity is the critical speed at which an object can escape the gravitational pull of a celestial body. On Earth, the escape velocity is about seven miles per second, and for the sun, it is about 100 miles per second. Mitchell suggested that if a star is massive enough and sufficiently small, its escape velocity could be greater than the speed of light. This would make the star invisible as light would be unable to escape its gravity.

General Relativity and Spacetime:
According to the general theory of relativity, space and time are combined to form a four-dimensional structure called spacetime. Spacetime is not flat but is distorted or curved by the presence of matter and energy. Objects move on paths called geodesics, which are the closest to a straight line in curved spacetime. Light travels on geodesics, but its path appears bent due to the curvature of spacetime, as observed during eclipses.

Fate of Matter in Black Holes:
Under the extreme gravity of a black hole, matter would be torn apart and crushed out of existence. However, particles making up the matter may persist in another universe.

Further Research:
The second part of the lecture presents recent work that has not yet been widely accepted but has generated interest and excitement among scientists.

Bending of Light by Massive Objects:
Light passing near massive objects like the Sun is bent due to curved spacetime. With extreme curvature, light can be trapped, forming a region called a black hole.

Event Horizon:
The boundary of a black hole is called the event horizon, where light fails to escape and hovers on the edge.

Contraction of Stars:
Stars initially resist gravitational collapse due to heat and pressure from nuclear fusion. However, they eventually run out of fuel and start to contract.

White Dwarfs and Neutron Stars:
Stars less than twice the mass of the Sun may become white dwarfs (radii: few thousand miles, densities: hundreds of tons per cubic inch) or neutron stars (radii: about 10 miles, densities: millions of tons per cubic inch).

Discovery of Neutron Stars:
Jocelyn Bell and Tony Ish discovered pulsars (rotating neutron stars) in 1967. Initially mistaken for alien signals, they provided observational evidence for the existence of neutron stars.

Formation of Black Holes:
Stars with masses more than twice that of the Sun can collapse into black holes if they don’t shed enough mass through explosions. The gravitational field of a black hole bends light so much that it cannot escape.

Observational Evidence for Black Holes:
Observational evidence suggests the existence of black holes. One notable example is Cygnus X-1, where matter from a normal star falls onto an unseen companion, emitting X-rays. The companion is likely a black hole.

Bet with Kip Thorne:
Hawking initially bet Kip Thorne that Cygnus X1 didn’t have a black hole as insurance against the failure of his black hole research. Now, he concedes the bet based on compelling evidence.

Reasons for Black Holes:
Observations in Cygnus X1 suggest that the mass of the companion of a binary system exceeds the maximum mass for a white dwarf or a neutron star, indicating the presence of a black hole.

Black Hole Properties:
Objects entering a black hole encounter a region of extreme light bending, preventing their escape. The laws of physics allow for hypothetical objects called white holes where matter emerges but cannot enter.

Space Travel Through Black Holes:
Initially considered possible based on solutions to Einstein’s equations. However, subsequent studies revealed that these solutions were unstable, making such travel impractical.

Black Holes and Quantum Mechanics:
Hawking explored the effects of quantum uncertainty on black holes. His findings showed that black holes emit radiation and particles at a steady rate, contrary to their previous perception as completely black objects.

Initial Skepticism:
Hawking’s initial announcement of his results was met with disbelief and criticism. However, subsequent independent studies confirmed his findings.

Uncertainty Principle & Particle Escape:
Black holes give off radiation due to the uncertainty principle, allowing particles to travel faster than light for a small distance. It enables particles and radiation to escape the event horizon.

Energy Conservation & Black Hole’s Lifespan:
Only the energy of what falls into a black hole is conserved, not the matter itself. As it emits particles, it loses mass, shrinks, and sends out particles faster. Eventually, it disappears completely.

Parallel Universes & Black Hole Travel:
Objects falling into a black hole enter a separate, self-contained “baby universe.” This baby universe may rejoin our spacetime, appearing as another black hole. Particles from one appear as emitted particles in the other, enabling hypothetical black hole travel.

Imaginary Time & Complexities:
“Imaginary time” is a mathematical concept crucial for quantum mechanics and the uncertainty principle. It’s perpendicular to our subjective sense of time, not related to aging or gray hair. However, the baby universes containing particles from black holes exist in this imaginary time.

Real-Time Consequences for Astronauts:
In real time, astronauts falling into black holes face a grisly end, torn apart by gravitational forces. Even their particles wouldn’t survive, ending their histories in real time at a singularity.

Immortality in Imaginary Time:
In imaginary time, the histories of the particles persist, passing into the baby universe and reemerging in another black hole. In a sense, the astronaut’s particles are transported to another universe, but greatly altered.

Think Imaginary:

The motto for anyone entering a black hole should be “think imaginary.”

Black Holes and Baby Universes:
Stephen Hawking considers the fate of particles that fall into black holes. These particles do not disappear but emerge in baby universes via a process of “evaporation.”

Challenges of Space Travel Through Black Holes:
Hawking highlights the impracticality of using black holes for space travel. The process involves imaginary time travel and unpredictable destinations.

Unpredictability in Unified Theories:
Hawking discusses the implications of baby universes for attempts to develop a complete unified theory of the universe. These universes introduce uncertainty in predicting physical quantities.

Baby Universes and the Cosmological Constant:
Hawking suggests that baby universes may explain the observed value of the cosmological constant, which is much smaller than expected. This value is crucial for the existence of beings like us.

Hawking on the Escape of Radiation from Black Holes:
Radiation cannot escape from a black hole because the escape velocity exceeds the speed of light. However, the uncertainty principle allows particles to temporarily travel faster than light, providing a mechanism for radiation to escape.

Hawking on the Need for a Creator:
Questions arise about whether the universe requires a creator or if it simply exists without one. Hawking does not directly address this question, leaving it open for discussion.