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Authors: Lawrence M. Krauss
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wished? To date, we do not know the answer. Some
specific time machines such as Gšdel's, and the cosmic-string-based systemhave been shown
to be unphysical. While wormhole time travel has yet to be definitively ruled out,
preliminary investigations suggest that the quantum gravitational fluctuations themselves
may cause wormholes to self-destruct before they could lead to time travel.
Until we have a theory of quantum gravity, the final resolution of the issue of time
travel is likely to remain unresolved. Nevertheless, several brave individuals, including
Stephen Hawking, have already tipped their hand. Hawking is convinced that time machines
are impossible, because of the obvious paradoxes that might result, and he has proposed a
“chronology-protection conjecture,” to wit: “The laws of physics do not allow the
appearance of closed timelike curves.”
I am personally inclined to agree with Hawking in this case. Nevertheless, physics is not
done by fiat. As I have stated earlier, general relativity often outwits our naive
expectations. As a warning, I provide two historical precedents. Twice before (that I know
of), eminent theorists have argued that a proposed phenomenon in general relativity should
be dismissed because the laws of physics must forbid it:
1. When the young astrophysicist Subrahmanyan Chandrasekhar proposed that stellar cores
more massive than 1.4 times the mass of the Sun cannot, after burning all their nuclear
fuel, settle down as white dwarfs but must continue to collapse due to gravity, the
eminent physicist Sir Arthur Eddington dismissed the result in public, stating, “Various
accidents may intervene to save the star, but I want more protection than that. I think
there should be a law of nature to prevent a star from behaving in this absurd way!” At
the time, much of the astrophysics community sided with Eddington. A half century later,
Chandrasekhar shared the Nobel Prize for his insights, which have long since been verified.
2. Slightly over 20 years after Eddington dismissed Chan-drasekhar's claim, a remarkably
similar event ocurred at a conference in Brussels. J. Robert Oppenheimer, the
distinguished American theoretical physicist and father of the atomic bomb, had calculated
that objects called neutron starsleft over after supernovae and even more dense than white
dwarfscould not be larger than about twice the mass of the Sun without collapsing further
to form what we would now call a black hole. The equally distinguished John Archibald
Wheeler argued that this result was impossible, for precisely the reason Eddington had
given for his earlier rejection of Chandrasekhar's claim: somehow the laws of physics must
protect objects from such an absurd fate. Within a decade, Wheeler would completely
capitulate and, ironically, would become known as the man who gave black holes their name.
The Physics of Star Trek
CHAPTER FOUR
DATA
Ends the Game
For I dipt into the future, far as human eye could see, Saw the Vision of the world, and
all the wonder that would
be.
From “Locksley Hall, ” by Alfred Lord Tennyson (posted aboard the starship
Voyager,)
Whether or not the Star Trek future can include a stable worm-hole, and whether or not the
Enterprise
crew could travel back in time to nineteenth-century San Francisco, the real stakes in
this cosmic poker game derive from one of the questions that led us to discuss curved
spacetime in the first place: Is warp drive possible? For, barring the unlikely
possibility that our galaxy is riddled with stable wormholes, it is abundantly clear from
our earlier discussions that without something like it, most of the galaxy will always
remain beyond our reach. It is finally time to address this vexing question. The answer is
a resounding “Maybe!”
Once again we are guided by the linguistic
specific time machines such as Gšdel's, and the cosmic-string-based systemhave been shown
to be unphysical. While wormhole time travel has yet to be definitively ruled out,
preliminary investigations suggest that the quantum gravitational fluctuations themselves
may cause wormholes to self-destruct before they could lead to time travel.
Until we have a theory of quantum gravity, the final resolution of the issue of time
travel is likely to remain unresolved. Nevertheless, several brave individuals, including
Stephen Hawking, have already tipped their hand. Hawking is convinced that time machines
are impossible, because of the obvious paradoxes that might result, and he has proposed a
“chronology-protection conjecture,” to wit: “The laws of physics do not allow the
appearance of closed timelike curves.”
I am personally inclined to agree with Hawking in this case. Nevertheless, physics is not
done by fiat. As I have stated earlier, general relativity often outwits our naive
expectations. As a warning, I provide two historical precedents. Twice before (that I know
of), eminent theorists have argued that a proposed phenomenon in general relativity should
be dismissed because the laws of physics must forbid it:
1. When the young astrophysicist Subrahmanyan Chandrasekhar proposed that stellar cores
more massive than 1.4 times the mass of the Sun cannot, after burning all their nuclear
fuel, settle down as white dwarfs but must continue to collapse due to gravity, the
eminent physicist Sir Arthur Eddington dismissed the result in public, stating, “Various
accidents may intervene to save the star, but I want more protection than that. I think
there should be a law of nature to prevent a star from behaving in this absurd way!” At
the time, much of the astrophysics community sided with Eddington. A half century later,
Chandrasekhar shared the Nobel Prize for his insights, which have long since been verified.
2. Slightly over 20 years after Eddington dismissed Chan-drasekhar's claim, a remarkably
similar event ocurred at a conference in Brussels. J. Robert Oppenheimer, the
distinguished American theoretical physicist and father of the atomic bomb, had calculated
that objects called neutron starsleft over after supernovae and even more dense than white
dwarfscould not be larger than about twice the mass of the Sun without collapsing further
to form what we would now call a black hole. The equally distinguished John Archibald
Wheeler argued that this result was impossible, for precisely the reason Eddington had
given for his earlier rejection of Chandrasekhar's claim: somehow the laws of physics must
protect objects from such an absurd fate. Within a decade, Wheeler would completely
capitulate and, ironically, would become known as the man who gave black holes their name.
The Physics of Star Trek
CHAPTER FOUR
DATA
Ends the Game
For I dipt into the future, far as human eye could see, Saw the Vision of the world, and
all the wonder that would
be.
From “Locksley Hall, ” by Alfred Lord Tennyson (posted aboard the starship
Voyager,)
Whether or not the Star Trek future can include a stable worm-hole, and whether or not the
Enterprise
crew could travel back in time to nineteenth-century San Francisco, the real stakes in
this cosmic poker game derive from one of the questions that led us to discuss curved
spacetime in the first place: Is warp drive possible? For, barring the unlikely
possibility that our galaxy is riddled with stable wormholes, it is abundantly clear from
our earlier discussions that without something like it, most of the galaxy will always
remain beyond our reach. It is finally time to address this vexing question. The answer is
a resounding “Maybe!”
Once again we are guided by the linguistic
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