could we shine a laser on this new solar system and detect something in 180 years?

(Gordon D. Pusch) writes:

(Tony Rusi) writes:

[...] "Einstein causality," Chiao says, "rules out the propagation of any
signal faster than light, but it does not limit the group velocity of
electromagnetic propagation."

The first half of Chiao's statement is correct. The second half is wrong.

Furthermore, it is not even the "group velocity" that the correct
definition of "signal velocity" in a dispersive medium --- a fact
that later bit Chiao in the @$$ when he published his "gain-assisted
FTL propagation" claim a few years later.

Correction: This claim was originally made by L.J.Wang, A.Kuzmich and
A.Dogariu, not Raymond Chiao (although Chiao later collaborated with
Wang et al on some equally wrong work on this same subject...).


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

(Gordon D. Pusch) wrote in message ...
(Tony Rusi) writes:

"Christopher M. Jones" wrote in message ...
We can't even shine a laser on Pluto from Earth and get
a return. This Solar System is roughly a kajillion (to
use a technical term) times farther away.

So no.


What about quantum teleportation? Can that be used for communication?

http://www.sciforums.com/archive/33/2002/06/4/8294

I thought Bell's theorem said that "spooky action at a distance" is a
reality.

I thought tachyons were possible too. Nothing can travel "at" the
speed of light but nothing prohibits FTL communication.

(Tony Rusi) writes:

What about quantum teleportation? Can that be used for communication?

So-called "teleportation" (which in reality is NO SUCH THING) is neither as
robust nor as reliable as a "classical" communications channel, and moreover is
STILL LIMITED BY THE SPEED OF LIGHT.

The only major advantage "quantum" channels have over "classical" channels
is that in principle they are "un-tappable:" Any attempt to intercept the
message will destroy the intelligibility of the message.


I thought Bell's theorem said that "spooky action at a distance" is a
reality.

Whether one considers it a "reality" is a matter of opinion, but one thing
even Bell and his supporters agree on is that such "spooky actions at a
distance" CANNOT BE USED TO COMMUNICATE =ANY= FORM OF INFORMATION.


I thought tachyons were possible too.

The fact that they have been _theoretically_ considered does not mean
they are "possible" --- only that they are not inconceivable. However,
should tachyons exist, it would imply that violations of both the Law
of Cause and Effect and the Laws of Thermodynamics would be possible.
Hence, only a handful of physicists takes the idea of tachyons seriously
--- and the rest of the physics community does not take those physicists
seriously.


Nothing can travel "at" the speed of light but nothing prohibits FTL
communication.

On the contrary --- FTL communication would necessarily also allow one to
communicate BACKWARDS IN TIME, which would violate the laws of cause and
effect --- which is about the most fundamental physical law there is !!!

The only way out of such paradoxes would be to hypothesize some form of
"cosmic censorship" that forbids communication with the "absolute past" ---
and the very existence of such an "absolute past" would in turn imply
the existence of a "preferred cosmic reference frame" (AKA an "aether"),
which would in turn imply that Einstein's Relativity would be false.
Again, VERY few physicists take such an idea seriously, and the rest
of the physics community does not take _them_ seriously.


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

In article ,
Tony Rusi wrote:
What about quantum teleportation? Can that be used for communication?

No. It can enhance other forms of communication in interesting ways, but
by itself you cannot use it to communicate. (When you measure your
particle, and the other guy measures his, the results match up, but each
result is otherwise entirely random -- there is no way for you to
*influence* his results.)

I thought Bell's theorem said that "spooky action at a distance" is a
reality.

No. It says that *either* quantum-mechanical weirdness is real, *or* the
particles can talk to each other at FTL speeds. The evidence on this one
is quite clear by now: the weirdness is real. The basic ground rules of
the weirdness say that FTL communication is not possible with it. Only if
you insist that the underlying reality *has* to be non-weird is there a
requirement for FTL communication in it.

I thought tachyons were possible too. Nothing can travel "at" the
speed of light but nothing prohibits FTL communication.

Oddly enough, I'm told that if you analyze the behavior of tachyons very
carefully, it appears that they would not provide FTL communication.

In any case, there is nothing in current physics that *prohibits*
tachyons, but that does not mean they exist in the real world. The
"totalitarian principle of physics" -- "everything not forbidden is
compulsory" -- suggests that if they could exist, they do, but while said
principle has been a useful guide in the past, there is nothing that
actually requires Mother Nature to obey it.

Unless Special Relativity is grossly wrong, by the way, any form of FTL
communication causes disastrous problems for causality. It permits things
like sending messages back in time. Some very fundamental rules of
physics would then need major overhauls. Whether this "prohibits" FTL
communication is a matter of opinion, but many physicists would say so.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

In article ,
Roger Stokes wrote:
An astronaut has a "magic rocket" that can travel at twice the speed of
light using some currently undiscovered principle of physics. He climbs
aboard, travels 1 AU, looks back with a telescope and sees himself walking
towards the rocket 4 minutes before launch. He then returns to Earth, and
finds 8 minutes has passed.
This doesn't violate causality...

This *particular* thought experiment doesn't violate causality, but there
are others (somewhat more complex, involving high sublight velocities as
well as the magic rocket) which do. Any FTL communication system permits
them.

...All the "currently undiscovered principle
of physics" has to do is allow 2c travel while being consistant with all
other laws of physics (which might be asking a lot, but is concievable).

It cannot be consistent with both special relativity and causality.

Special relativity is very solidly established, verified to quite high
precision in many different experiments.

There have been suggestions that a more complex and sophisticated notion
of causality is eventually going to be needed. General relativity seems
to be full of ways to build time machines. (Although people have rather
less confidence in GR than in SR, not least because it appears to be
fundamentally inconsistent with quantum mechanics.)
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

(Henry Spencer) writes:

In article ,
Roger Stokes wrote:
An astronaut has a "magic rocket" that can travel at twice the speed of
light using some currently undiscovered principle of physics. He climbs
aboard, travels 1 AU, looks back with a telescope and sees himself walking
towards the rocket 4 minutes before launch. He then returns to Earth, and
finds 8 minutes has passed.
This doesn't violate causality...

This *particular* thought experiment doesn't violate causality, but there
are others (somewhat more complex, involving high sublight velocities as
well as the magic rocket) which do. Any FTL communication system permits
them.

...All the "currently undiscovered principle
of physics" has to do is allow 2c travel while being consistant with all
other laws of physics (which might be asking a lot, but is concievable).

It cannot be consistent with both special relativity and causality.

More precisely, it cannot be consistent with Special Relativity and
_EINSTEIN'S_ definition of "causality" in terms of light-cones, which is
intimately and inextricably intertangled with Special Relativity.

Stokes has already indicated by his problem statement that he is willing
to abandon Special Relativity, but has not indicated what he is willing
to consider in its place. From his problem statement, it appears he may
be suffering from some rather "Newtonian" conceptions about "absolute"
cosmic time, or is willing to postulate some sort of "aether" that would
provide some "absolute" standard of "rest" relative to which time is measured.

He should be aware that Einstein's revised definition of "causality" in
terms of "light cones" is now wired very deeply into to must fundamental
levels of physics (for example, all of Quantum Field Theory is based on it),
and a retrenchment to an "absolute time" would almost certainly screw up
all of high-energy quantum physics very, VERY badly --- it's one reason
why almost no physicist (except for a few recidivist crackpots) is still
willing to take such an idea seriously.


Special relativity is very solidly established, verified to quite high
precision in many different experiments.

Likewise, certain parts of Quantum Field Theory, such as QED
(Quantum Electrodynamics, the best-tested physical theory we have!)
are experimentally established to a truly MIND-BOGGLING number of
decimal places. As I observed earlier, Einstein's notion of "causality"
(and therefore Special Relativity) is almost inextricably hard-wired into
such theories, and virtually no one takes seriously the notion that an
"aether" theory could reproduce these results to umpty-ump decimal places.


There have been suggestions that a more complex and sophisticated notion
of causality is eventually going to be needed. General relativity seems
to be full of ways to build time machines. (Although people have rather
less confidence in GR than in SR, not least because it appears to be
fundamentally inconsistent with quantum mechanics.)

....However, there is a theorem by Yorke that any such General Relativist
"time machine" (or "space warp" FTL drive) must NECESSARILY contain
"exotic" matter. While the apparent observational evidence for exotic
"Dark Energy" has caused physicists to become somewhat less skeptical
about such possibilities, it should be noted that =ANY= form of "exotic"
matter NECESSARILY implies that the Second law of Thermodynamics can be
violated --- and even the few remaining "Newtonian Recidivists" are (mostly)
given pause by such a deep and disturbing revision to the Laws of Physics...


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

----- Original Message -----
From: "Gordon D. Pusch"
Newsgroups: sci.space.tech
Sent: Wednesday, July 23, 2003 3:11 AM
Subject: could we shine a laser on this new solar system and detect
something in 180 years?


(Henry Spencer) writes:

In article ,
Roger Stokes wrote:
An astronaut has a "magic rocket" that can travel at twice the speed of
light using some currently undiscovered principle of physics. He climbs
aboard, travels 1 AU, looks back with a telescope and sees himself
walking
towards the rocket 4 minutes before launch. He then returns to Earth,
and
finds 8 minutes has passed.
This doesn't violate causality...

Stokes has already indicated by his problem statement that he is willing
to abandon Special Relativity, but has not indicated what he is willing
to consider in its place. From his problem statement, it appears he may
be suffering from some rather "Newtonian" conceptions about "absolute"
cosmic time, or is willing to postulate some sort of "aether" that would
provide some "absolute" standard of "rest" relative to which time is
measured.

I wasn't postulating some reversion to absolute space and time, I think that
has been pretty much discredited now - the figures were merely to give scale
to the thought experiment, they weren't intended to be measurements. In any
case advances in physics usually are based on augmentation of predecessor
theories, not abandonment of them.

And that was the point of my post (which did not appear in this newsgroup).
There are a set of mathematical models that most people adhere to today
(including SR,GR,QM) because they are very successful, but that doesn't mean
that they are TRUTH, or that one MUST NOT discuss extending them to
encompass phenomena not currently allowed by the theories.

Thus current theories should not be defended as if they were inviolable
dogma - maybe most people who propose revisions and augmentations are
ignored by the mainstream, but that doesn't NECESSARILY prove they are
wrong, or are crackpots.

The final point of the OP was to point out that 19th century physicists
almost certainly viewed the Newtonian model as being "truth", yet there were
clues (Michelson-Morley etc) that modifications were needed. My question was
what clues exist pointing to areas of current theories where modifications
might be needed?

There have been suggestions that a more complex and sophisticated notion
of causality is eventually going to be needed. General relativity seems
to be full of ways to build time machines. (Although people have rather
less confidence in GR than in SR, not least because it appears to be
fundamentally inconsistent with quantum mechanics.)

This is obviously one of the clues I referred to

...However, there is a theorem by Yorke that any such General Relativist
"time machine" (or "space warp" FTL drive) must NECESSARILY contain
"exotic" matter. While the apparent observational evidence for exotic
"Dark Energy" has caused physicists to become somewhat less skeptical
about such possibilities, it should be noted that =ANY= form of "exotic"
matter NECESSARILY implies that the Second law of Thermodynamics can be
violated --- and even the few remaining "Newtonian Recidivists" are
(mostly)
given pause by such a deep and disturbing revision to the Laws of
Physics...

Is this a clue, and if so in which direction is it pointing? BTW I have read
that some VSL theories violate conservation of mass-energy, yet still are
achieving a consistent description of "reality" - there were no details
however.

--Roger

"Crackpots of the world unite, you have nothing to lose but
your...er....um..."

"Henry Spencer" wrote in message
...
In article ,
Roger Stokes wrote:
There are a set of mathematical models that most people adhere to today
(including SR,GR,QM) because they are very successful, but that doesn't
mean
that they are TRUTH, or that one MUST NOT discuss extending them to
encompass phenomena not currently allowed by the theories.

However, it does mean that they are not to be blithely discarded without
good reason. It also means that extensions must be consistent with the
existing theory... which means that understanding the existing theory is
essential to being able to propose sensible extensions. You don't get a
viable extension by just saying "well, let's assume that one of the basic
principles of the theory is a little bit false".

In particular, it is really, really hard to extend SR to accommodate FTL
anything. The theory adamantly resists it, and that theory is immensely
successful in explaining a huge variety of experimental evidence, and is
implicit in several other very successful theories. (E.g., it's implicit
in Maxwell's equations, which actually pre-dated relativity.)

Thus current theories should not be defended as if they were inviolable
dogma - maybe most people who propose revisions and augmentations are
ignored by the mainstream, but that doesn't NECESSARILY prove they are
wrong, or are crackpots.

However, that is the way to bet. Most of those people are ignored by the
mainstream because their ideas are so vague and so poorly presented that
they aren't even wrong -- they're just meaningless noise.

There is no better way for a young scientist to make a name, and possibly
a Nobel Prize, for himself than to prove a major existing theory wrong.
But he needs *evidence*, not just speculation. Even purely theoretical
work needs ironclad internal consistency, full understanding of the theory
it is modifying, solid agreement with existing data (which means knowing
what that data is), and clear, testable predictions before it will be
taken seriously.

The bar is set high when proposing changes to successful existing
theories, and quite rightly so.

The final point of the OP was to point out that 19th century physicists
almost certainly viewed the Newtonian model as being "truth", yet there
were
clues (Michelson-Morley etc) that modifications were needed.

Michelson-Morley was a 20th-century experiment, actually. A much more
vexing problem in the late 19th century was the spectrum of black-body
radiation, which was inconsistent with classical theory, but that
particular hint pointed to quantum mechanics rather than relativity. (And
when Planck solved the problem with quantized oscillators, he thought this
was just a mathematical abstraction -- reportedly he was horrified when
Einstein's experimental work on the photoelectric effect demonstrated that
quantization was real.)

Michelson-Morley experiment was first run in 1887 in the States, but
Michelson did an early version of the experiment in 1881 in Berlin
(Encyclopedia Brittanica online and various other sources, by way of
Google).

Murray Anderson

There really wasn't much of anything that pointed to special relativity,
until the devastating negative results from the Michelson-Morley work.
The ether certainly had strange and counterintuitive properties, which
bothered some people, but there wasn't anything out-and-out wrong with it,
and observational facts like the aberration of light seemed to require it.

My question was
what clues exist pointing to areas of current theories where
modifications
might be needed?

When it comes to SR, precious little. It's not like people haven't been
looking for these things. The one faintly disturbing note is the QM
prediction of FTL propagation of light itself in a Casimir-effect gap, and
that effect is so tiny that it is almost certainly unmeasurable (and
definitely of no practical use).

GR is more problematic, both because of all those time machines and
because of its stubborn resistance to quantization.

The main problem with QM/QED/QCD/etc. -- with the caveat that it's an area
I don't follow closely -- is its persistent inability to explain some very
suspicious coincidences. E.g., the observed value of the cosmological
constant is strangely small: the contributions to it from undiscovered
particles apparently cancel out the ones from known particles to better
than 40 decimal places. The theory places no constraints that would
explain these things, and many people think they call for an explanation.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

In article ,
Will R wrote:
Suppose we built a large rod of Super Deluxe Unobtanium that stretched from
Earth to Pluto. If I yank on the end of that rod (With a motor built of
unobtanium, so as to pull it quickly), could the "signal" propagate faster than
light?

No. The signal propagates at the speed of sound in the rod, typically 5-6
orders of magnitude slower than light.

On a side note, suppose I started spinning the rod. Could the end get going to
the speed of light? I assume that the mass would just increase, and it would
get progressively harder to spin, preventing me from spinning it fast enough to
go FTL...

Basically, yes. One can argue about the terminology, but you have the
right bottom line: as the tip speed approaches the speed of light, it
gets harder and harder to add the next 1RPM.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

Henry Spencer wrote:
This *particular* thought experiment doesn't violate causality, but
there are others (somewhat more complex, involving high sublight
velocities as well as the magic rocket) which do. Any FTL
communication system permits them.

It's possible that FTL is possible without violating causality. There
could, for instance, be a preferred frame of reference. Perhaps the
one in which the cosmic microwave background is maximally isotropic.
Or perhaps the one in which the Big Bang was the same amount of time
ago. Maybe signals can travel at any finite positive speed relative
to that frame of reference. This would mean signals can go back
through time in other frames of reference, but not in a way that could
let you relay a message back to your present location before you sent
it. Maybe you could send a message in 2003 that will get to Alpha
Centauri in 1999, but no reply could get back to us until 2004.

Another way to get FTL without violating causality is if there's a
preferred *direction* in space. Maybe it's possible to send signals
at any finite positive speed in that direction, but no other. This
too would allow FTL without violating causality.

Another way to get FTL without violating causality is if there's some
way of increasing the speed of light over a finite volume of space.
If there's a limit to how quickly this can be done, and if the speed
of light falls off gradually back toward its usual speed as you leave
the region rather than there being any sharp edges, this would also
allow FTL without violating causality. In a sense, gravitational
waves do exactly this, albeit to too small and too temporary a degree
to be useful.
--
Keith F. Lynch - - http://keithlynch.net/
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