Why cant a person travel faster then the speed of light

As you approach the speed of light, your mass increases without limit.
 Therefore it would take an infinite amount of energy to accelerate to
the speed of light.  This is an impossibility.

Ansel's answer is correct. To understand this in detail requires an
understanding of special relativity. It follows directly from the
seemingly innocuous seeming proposition that the speed of light in a
vacuum is always the same for any observer.

Curiously, one can imagine particles that can ONLY travel faster than
the speed of light. These "tachyons" are hypothetical, but slow down
if you push them in the direction of motion and speed up if you push
in the opposite direction. It would take an infinite amount of energy
to slow a tachyon down to the speed of light.

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What is known about tachyons, theoretical particles that travel faster
than light and move backward in time? Is there scientific reason to
think they really exist?
		
John Manahan
Annapolis, Maryland
		
Raymond Y. Chiao is professor of physics at the University of
California, Berkeley. He replies:

"Briefly, tachyons are theoretically postulated particles that travel
faster than light and have 'imaginary' masses.

Editor's note: imaginary mass is a bizarre theoretical concept that
comes from taking the square root of a negative number; in this case,
it roughly means that a particle's mass is only physically meaningful
at speeds greater than light.]

"The name 'tachyon' (from the Greek 'tachys,' meaning swift) was
coined by the late Gerald Feinberg of Columbia University. Tachyons
have never been found in experiments as real particles traveling
through the vacuum, but we predict theoretically that tachyon-like
objects exist as faster-than-light 'quasiparticles' moving through
laser-like media. (That is, they exist as particle-like excitations,
similar to other quasiparticles called phonons and polaritons that are
found in solids. 'Laser-like media' is a technical term referring to
those media that have inverted atomic populations, the conditions
prevailing inside a laser.)

"We are beginning an experiment at Berkeley to detect tachyon-like
quasiparticles. There are strong scientific reasons to believe that
such quasiparticles really exist, because Maxwell's equations, when
coupled to inverted atomic media, lead inexorably to tachyon-like
solutions.

"Quantum optical effects can produce a different kind of 'faster than
light' effect (see "Faster than light?" by R. Y. Chiao, P. G. Kwiat,
and A. M. Steinberg in Scientific American, August 1993). There are
actually two different kinds of 'faster-than-light' effects that we
have found in quantum optics experiments. (The tachyon-like
quasiparticle in inverted media described above is yet a third kind of
faster-than-light effect.)

"First, we have discovered that photons which tunnel through a quantum
barrier can apparently travel faster than light (see "Measurement of
the Single-Photon Tunneling Time" by A. M. Steinberg, P. G. Kwiat, and
R. Y. Chiao, Physical Review Letters, Vol. 71, page 708; 1993).
Because of the uncertainty principle, the photon has a small but very
real chance of appearing suddenly on the far side of the barrier,
through a quantum effect (the 'tunnel effect') which would seem
impossible according to classical physics. The tunnel effect is so
fast that it seems to occur faster than light.

"Second, we have found an effect related to the famous
Einstein-Podolsky-Rosen phenomenon, in which two distantly separated
photons can apparently influence one anothers' behaviors at two
distantly separated detectors (see "High-Visibility Interference in a
Bell-Inequality Experiment for Energy and Time," by P. G. Kwiat, A. M.
Steinberg, and R. Y. Chiao, Physical Review A, Vol. 47, page R2472;
1993). This effect was first predicted theoretically by Prof. J. D.
Franson of Johns Hopkins University. We have found experimentally that
twin photons emitted from a common source (a down-conversion crystal)
behave in a correlated fashion when they arrive at two distant
interferometers. This phenomenon can be described as a
'faster-than-light influence' of one photon upon its twin. Because of
the intrinsic randomness of quantum phenomena, however, one cannot
control whether a given photon tunnels or not, nor can one control
whether a given photon is transmitted or not at the final beam
splitter. Hence it is impossible to send true signals in
faster-than-light communications.

"I refer interested readers to our paper 'Tachyonlike Excitations in
Inverted Two-Level Media' by R. Y. Chiao, A. E. Kozhekin, and G.
Kurizki, Physical Review Letters, Vol. 77, page 1254; 1996, and
references therein.

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Recent articles in Scientific American have talked about traveling
faster than light. But I always thought that Einstein's theory of
relativity forbade that. What gives?
		
Jorge,
New York, NY
		
Physicist Thomas Roman of Central Connecticut State University offers
the following response:

Einstein's special theory of relativity predicts that nothing can
exceed the speed of light. But special relativity applies when
spacetime is flat. When spacetime is curved, the theory applies only
"locally"--that is, over regions of spacetime small enough to be
considered flat. Consider the analogy of a plane that is tangent to a
sphere. The flat geometry of the plane is a good approximation to the
geometry of the sphere when the size of the plane is very small
compared to the sphere's radius of curvature.

FASTER-THAN-LIGHT TRAVEL, depicted in series such as Star Trek, is not possible
Animation: KENNETH JONES
FASTER-THAN-LIGHT TRAVEL, depicted in series such as Star Trek, is not
possible--except for maybe inside a spacetime or Alcubierre warp
bubble.

In curved spacetimes, when we compare two observers at large
separation, we can no longer use the "locally flat" approximation. In
the plane-and-sphere analogy, this situation would correspond to
comparing two observers on the sphere separated by a distance
comparable to the sphere's radius of curvature. Although each observer
could approximate the geometry in his or her local region as a plane,
there is no single plane that would be applicable to both observers.
Consequently, the two observers in curved spacetime can each apply
special relativity in their own local region, but not globally.

A similar situation arises in an expanding universe. Here one should
not think of the galaxies as moving through space, but rather that the
space between the galaxies is expanding. Einstein's general theory of
relativity, on which such models are based, imposes no restrictions on
the rate at which the expansion of space can drive the galaxies apart.
But special relativity still applies locally, in the sense that a
particle chasing a light ray can never catch up to it. An analogy is
to imagine bugs crawling on a rubber sheet. By stretching the sheet we
can make the bugs recede from each other at arbitrarily high speeds,
but no bug can crawl across the sheet faster than a light beam.

In serious proposals for "warp drive," such as the Alcubierre warp
bubble, space is flat inside the bubble and special relativity
applies. In this region, nothing can travel faster than
light--relative to observers inside the bubble. Outside the bubble,
spacetime is also flat and no particle can travel faster than
light--relative to observers outside the bubble. But because of the
large expansion and contraction of the spacetime in the wall of the
bubble, the inside of the bubble can move faster than light relative
to the outside. This would also be true of light rays inside the
bubble; they would be carried along by the spacetime warp, too. What
causes this mismatch of the two flat spacetime regions is the large
spacetime curvature in the bubble wall that separates the regions.

Cosmologist Martin Bucher of the University of Cambridge adds this insight:

In the pre-Einsteinian conception of the nature of space and time,
there is no limit in principle to how fast an object can travel. But
in Einstein's special theory of relativity, the notion of
causality--of the past completely determining the future--would break
down if any type of matter, energy or signal were able to travel
faster than light.

In the pre-Einsteinian framework, time has an absolute character. The
time of an event--and thus its time ordering--is the same to all
observers; velocities add according to ordinary addition. For very
small velocities (small compared to the velocity of light), the same
holds in relativity, but for large velocities significant
modifications occur. Early in the 20th century the Michelson-Morley
experiment established that the speed of light is the same to all
observers whatever their relative motion. Therefore the law for adding
velocities must be modified. The relative velocity of two objects, one
traveling at the same of light and the other traveling at sublight
speeds, must equal the speed of light. When both are traveling at
sublight speeds, the relative velocity must be less than the speed of
light.
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If nature allowed them, wormholes would appear as spherical openings
to an otherwise distant part of the cosmos
Image: SLIM FILMS
WORMHOLE. If nature allowed them, wormholes would appear as spherical
openings to an otherwise distant part of the cosmos. This doctored
photograph shows a wormhole in Times Square that opens onto the Sahara
Desert one city block away.

One surprising consequence is that time loses its absolute character.
The times perceived by observers moving with respect to each other do
not coincide. But observers always agree on the ordering of events. If
we admit the possibility of faster-than-light speeds, some observers
would perceive one event as occurring before another, others would
perceive them as occurring simultaneously, and a third group would
perceive the reverse order. The time ordering is invariant only when
the two events can be linked by a signal traveling at a speed slower
than or equal to the speed of light.

In the context of an expanding universe, it is often stated that
widely separated points move apart faster than the speed of light. At
first sight this would seem impossible. But an expanding universe must
be considered within Einstein's general theory of relativity, a
generalization of the special theory of relativity. In general
relativity, motion relative to the speed of light is defined locally.
The separation between two distant points can increase faster than the
speed of light as a result of the swelling of the intervening
spacetime. Nothing can pass through the space faster than light, but
space itself can carry things apart superluminally.

The answer is simple.  "Is is possible for a person to travel faster
than light?"  No.  People are not made of tachyons (nor have they been
proven to exist), and people get obliterated into non-people when they
approach any scenario that would have them potentially move faster
than light.

You know that it takes more energy to move a "heavy" (massive) object
than it does to move a light one.  Everything we do in life is nowhere
near the speed of light, so we never notice this effect, but as things
approach the speed of light, their weight goes up... and then it
becomes harder and harder to move the object any faster, because it
keeps getting heavier and heavier.

However, there is one situation in which it can happen...  Along with
mass comes gravity.  In that case, the *heavier* an object is, the
faster it moves toward "the ground", or any other source of gravity. 
This becomes a "runaway condition", and eventually, if the object
doesn't smash into the object it's attracted to first, it exceeds the
speed of light.

Black holes do exactly this -- they're not moving fast, but they have
such a huge gravitational pull that anything that comes near enough
ends up moving faster than light.  Black holes have a certain spot
called the Event Horizon; at this spot, everything is moving toward it
at speed of light.

The odd thing about it is that as things approach the speed of light,
their time also slows down.  So when things hit this event horizon,
from our point of view, they get infinitely heavy, travel at the speed
of light, and freeze in time.

How can it be moving and not moving at the same time?  Actually, it's
more like it's being suspended in the air; it may have more room to
fall, but it's frozen in time.  With normal time, it'd be falling, but
because time has stopped for it, it's more like un-falling.. and
because the gravity is so high, it's un-falling, *really really fast*.

However, aside from all of the noise spewing out of the black hole,
life goes on as normal for the object.  The rest of the world
accelerates away at an infinite speed, and the object ends up
"rotated" in spacetime -- the dimensions switch around.  It has little
control over it's ability to rise or fall, just like we have no
control over our time, but it can move through time just like we can
walk left and right.

The universe follows rules similar to the black hole's event horizon
-- we have control over walking left and right, going up and down,
etc, but we are totally crushed by gravity into a single, specific
time.

Of course, all of this is simplified and analogy... which is the way
Quantum Science almost always is -- it's too bizarre to explain
directly, so the novice is given rather inaccurate, but sufficiently
vivid ideas.  As you learn more, you will hear new models that are
more bizarre but more accurate, and you'll be able to understand them
better.