HOW TO BUILD A TIME MACHINE     

Paul Davies

Viking, 2001, 130 pp.  

Paul Davies is an internationally known theoretical physicist and author. He
is known for explaining the intricacies of science to ordinary people. In
this volume he takes a tongue-in-cheek look at the possibilities of building
a time machine.

"Today the scientific community is unanimous that 'time is relative' and the
commonsense notion of an absolute time with a universal 'now' is a fiction."  
"The theory of relativity implies that a limited form of time travel is
certainly possible.."  "The universe is not only queerer than we think, it
is queerer than we can think."  (4)

"Time is elastic.  It can be stretched and shrunk.  How?  Simply by moving
very fast."  (6) "The exact duration of time between two specified events
will depend on how the observer is moving."  (7)  "To get a really big
effect you have to move very fast.  The benchmark here is the speed of
light."  "Physicists call the slowing of time by motion the time dilation
effect."  (8)  "With a high enough speed, you could 'jump' to any date in
the future you like.  The year 3000 could be reached in less than six months
by traveling at 99.99999 percent of the speed of light."  (13)

"Speed is only one method of warping time.  Another is gravity."  (14)
"Gravity slows time."  "Time runs faster in space."  (15)  "If you could
magically squash the Earth to half its diameter (retaining all its mass),
its surface gravity would be twice as big, and so would be the timewarp. Go
on compressing, and the effect rises.  When the radius reaches a critical
value of 0.9 cm, time 'stands still.'  Nothing can escape!"  (17)

"A clock on a typical neutron star would tick about 30 percent slower than
one on Earth [because of the greater gravity]."  "If you could look back at
Earth from the surface of a neutron star, you would see terrestrial events
speeded up, like a fast-forward video show."   (21)

The equation E=mc2 "tells us that mass and energy are related; that is,
energy has mass and mass is a form of energy."  "This means a little bit of
mass is worth an awful lot of energy.  For example, one gram of matter,
converted into electricity, could power an entire city for several days."
"Mass is a source of gravity.  As energy has mass, it must gravitate too."
(23)

As a particle gets nearer the speed of light, it becomes heavier - that is,
puts on mass.  This makes the particle harder and harder to speed up.  More
and more of the energy goes to making the particle heavier, less and less to
increasing its speed.  (25)

When a range of events happen at distant places, "which particular events
are judged to be happening at the same moment . will depend on just how the
observer is moving.  "For a star on the other side of the galaxy, events
happening at the same moment as an event on Earth today might lie anywhere
in a time span of 100,000 years."  (27)

"There is no universal 'now.'  We have to accept that time at a faraway
place must extend somewhat into our perceived past and future."  "Some
events that you judge to be in the past will be regarded by someone else as
lying in his or her future or present-and vice versa."  (28)  "So the future
is out there all right, and it can be visited.  All you need as an effective
time machine is a spaceship that can travel very close to the speed of
light.." (30)

If an observer went into orbit around a cylinder of infinite length that was
rotating fast enough, he could return to the starting point before he left.  
(32)  Faster-than-light can mean backward in time.  Rotation does not enable
a person or particle to break the light barrier, but it does affect the
motion of light itself.  (34)

Black holes are dense, dark bodies in space that suck in everything around
them.  (36)  The pull of gravity becomes overwhelming inside large stars
that run out of fuel and can no longer sustain their internal pressure. The
core abruptly implodes, in a fraction of a second, leaving behind a region
of empty space-hence, 'hole.'  (37)  Where does it all end?  With the entire
contents concentrated at a single point at the center.  This dot of matter
would have infinite density, and its gravity would also be infinite there.  
This is referred to as a singularity.  "When infinity looms in a physical
theory, it is an alarm signal, suggesting that something drastic happens,
but in this case nobody is quite sure what." (38)

"So the light from a shrinking star gets lower and lower in frequency as the
escalating timewarp retards is oscillations."  Eventually, the last of the
light from the star gets out; after that, all is black.  Thus, the region of
space around the collapsed object is both black and empty-hence, black
holes."  (39)

"The formation of an infinite timewarp around an imploding ball of matter
leads to an arresting conclusion: A black hole is a one-way journey to
nowhere.  You can't fall into one and come out again, because the region
inside the black hole is beyond the end of time as far as the outside
universe is concerned.  If you did somehow manage to emerge from a black
hole, you would have to come out before you fell in.  Which is another way
of saying that you would be projected back in time.  So there's a clue here.  
A black hole has an entrance but no exit; it is a one-way fast track to the
end of time."  (40)

"What if there existed something like a black hole, but with an exit as well
as an entrance-a wormhole?  Maybe it could be used to reach the past." (40)

The fun part of time travel is considering the baffling paradoxes that can
be imagined.  Science fiction writers have exploited these for years.  In
the movie Back to the Future, Marty McFly becomes embroiled in the love life
of his mother, risking interfering with her marriage plans, as a result of
which he teeters on the edge of obliteration.  (95)  In other stories, the
individual travels into the past and murders an ancestor.

Another absurdity is when the time traveler meets his younger self, for then
there would be two of him.  You could carry a gold bar with you and give it
to your younger self to keep until that self travels in time.  You can
effortlessly double your investment.  (100)

Or what about plucking knowledge out of thin air?  A professor goes forward
in time to 2010, goes to a library and finds a splendid new theorem in a
current journal.  He returns and summons a clever student and outlines the
theorem.  The student writes a paper and publishes it in a journal.  It was,
of course, in this very journal that the professor read the paper in 2010.  
Where did the theorem come from?  (102-3)

[Still with us?  Then you'll want to get the book.  Otherwise you've
probably not made it this far in the notes.  dlm]