*Ta-Pei Cheng*

- Published in print:
- 2009
- Published Online:
- February 2010
- ISBN:
- 9780199573639
- eISBN:
- 9780191722448
- Item type:
- chapter

- Publisher:
- Oxford University Press
- DOI:
- 10.1093/acprof:oso/9780199573639.003.0008
- Subject:
- Physics, Particle Physics / Astrophysics / Cosmology

Black hole is an object so compact that it is inside its event horizon: a one-way surface through which particle and light can only traverse inward, and an exterior observer cannot receive any signal ...
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Black hole is an object so compact that it is inside its event horizon: a one-way surface through which particle and light can only traverse inward, and an exterior observer cannot receive any signal sent from inside. The Schwarzschild geometry is viewed in the Eddington-Finkelstein coordinates as well as in the Kruskal coordinates. Besides a black hole, the GR field equation also allows the solution of a white hole and a wormhole. The gravitational energy released when a particle falls into a tightly bound orbit around a black hole can be enormous. The physical reality of, and observational evidence for, black holes are briefly discussed. Quantum fluctuation around the event horizon brings about the Hawking radiation. This and the Penrose process in a rotating (Kerr) black hole comes about because of the possibility of negative energy particles falling into a black hole.Less

Black hole is an object so compact that it is inside its event horizon: a one-way surface through which particle and light can only traverse inward, and an exterior observer cannot receive any signal sent from inside. The Schwarzschild geometry is viewed in the Eddington-Finkelstein coordinates as well as in the Kruskal coordinates. Besides a black hole, the GR field equation also allows the solution of a white hole and a wormhole. The gravitational energy released when a particle falls into a tightly bound orbit around a black hole can be enormous. The physical reality of, and observational evidence for, black holes are briefly discussed. Quantum fluctuation around the event horizon brings about the Hawking radiation. This and the Penrose process in a rotating (Kerr) black hole comes about because of the possibility of negative energy particles falling into a black hole.

*John W. Moffat*

- Published in print:
- 2020
- Published Online:
- June 2020
- ISBN:
- 9780190650728
- eISBN:
- 9780197517383
- Item type:
- chapter

- Publisher:
- Oxford University Press
- DOI:
- 10.1093/oso/9780190650728.003.0005
- Subject:
- Physics, Particle Physics / Astrophysics / Cosmology

In 1935, Einstein and Rosen described what is now called the Einstein-Rosen bridge. Wheeler called this a wormhole, which could connect two distant parts of the universe. Thorne and Morris showed the ...
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In 1935, Einstein and Rosen described what is now called the Einstein-Rosen bridge. Wheeler called this a wormhole, which could connect two distant parts of the universe. Thorne and Morris showed the wormhole cannot be traversable unless exotic matter with negative energy props it up. Using the Penrose mechanism of superradiance, one can produce rotational energy from a black hole, which could be used to detect dark matter particles. Higher dimensional objects such as branes in superstring theory have been considered as sources of gravitational waves. Black holes have even been proposed to be giant atoms, related to Hawking radiation and black hole entropy. Bekenstein and Mukhanov postulated that black holes radiated quantum radiation. Many such speculative ideas have been put forth that could potentially be verified by detecting gravitational waves. Yet, many physicists work with mathematical equations, unconcerned with whether their ideas can be verified or falsified by experiments.Less

In 1935, Einstein and Rosen described what is now called the *Einstein-Rosen bridge*. Wheeler called this a *wormhole*, which could connect two distant parts of the universe. Thorne and Morris showed the wormhole cannot be traversable unless exotic matter with negative energy props it up. Using the Penrose mechanism of *superradiance*, one can produce rotational energy from a black hole, which could be used to detect dark matter particles. Higher dimensional objects such as branes in superstring theory have been considered as sources of gravitational waves. Black holes have even been proposed to be giant atoms, related to Hawking radiation and black hole entropy. Bekenstein and Mukhanov postulated that black holes radiated quantum radiation. Many such speculative ideas have been put forth that could potentially be verified by detecting gravitational waves. Yet, many physicists work with mathematical equations, unconcerned with whether their ideas can be verified or falsified by experiments.

*Dean Rickles*

- Published in print:
- 2020
- Published Online:
- April 2020
- ISBN:
- 9780199602957
- eISBN:
- 9780191844393
- Item type:
- chapter

- Publisher:
- Oxford University Press
- DOI:
- 10.1093/oso/9780199602957.003.0008
- Subject:
- Physics, Theoretical, Computational, and Statistical Physics

This chapter focuses on John Wheeler’s work on geons and geometrodynamics which would lead to many concepts and results that would be of importance to quantum gravity research - these projects, ...
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This chapter focuses on John Wheeler’s work on geons and geometrodynamics which would lead to many concepts and results that would be of importance to quantum gravity research - these projects, initially, were rather old fashioned, harking back to the classical ‘unified field theory’ work of Einstein. Moreover, we find that this work that we now tend to think of as foundational in quantum gravity---e.g., we often think of ‘quantum geometrodynamics’ as just another phrase for ‘quantum gravity’---had its roots firmly embedded in the quest for understanding the elementary particles. It wasn’t until after 1957 that Wheeler began to look seriously at general relativity and quantum gravity independently from concerns in particle physics, and this shift in fact coincides with a more general trend to treat gravitational physics as a worthwhile field in its own right.Less

This chapter focuses on John Wheeler’s work on geons and geometrodynamics which would lead to many concepts and results that would be of importance to quantum gravity research - these projects, initially, were rather old fashioned, harking back to the classical ‘unified field theory’ work of Einstein. Moreover, we find that this work that we now tend to think of as foundational in quantum gravity---e.g., we often think of ‘quantum geometrodynamics’ as just another phrase for ‘quantum gravity’---had its roots firmly embedded in the quest for understanding the elementary particles. It wasn’t until after 1957 that Wheeler began to look seriously at general relativity and quantum gravity independently from concerns in particle physics, and this shift in fact coincides with a more general trend to treat gravitational physics as a worthwhile field in its own right.

*Nikk Effingham*

- Published in print:
- 2020
- Published Online:
- April 2020
- ISBN:
- 9780198842507
- eISBN:
- 9780191878480
- Item type:
- chapter

- Publisher:
- Oxford University Press
- DOI:
- 10.1093/oso/9780198842507.003.0002
- Subject:
- Philosophy, Metaphysics/Epistemology

This chapter discusses the ways in which one might travel back in time. Of course, there are no actual, known instances of time travel, so the different modes are drawn from fiction, historical ...
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This chapter discusses the ways in which one might travel back in time. Of course, there are no actual, known instances of time travel, so the different modes are drawn from fiction, historical thought, and speculative physics: perhaps we could ‘teleport’, discontinuously, back into the past; perhaps we could travel back into the past in the same way we persist forwards, traversing the intervening instants between ourselves and the past; perhaps we instead warp spacetime to allow us to come back to where we began. The chapter ends by discussing two things that are not technically time travel—cases of frozen time and time being an illusion—which are nevertheless closely connected.Less

This chapter discusses the ways in which one might travel back in time. Of course, there are no actual, known instances of time travel, so the different modes are drawn from fiction, historical thought, and speculative physics: perhaps we could ‘teleport’, discontinuously, back into the past; perhaps we could travel back into the past in the same way we persist forwards, traversing the intervening instants between ourselves and the past; perhaps we instead warp spacetime to allow us to come back to where we began. The chapter ends by discussing two things that are not technically time travel—cases of frozen time and time being an illusion—which are nevertheless closely connected.