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

(Will R) writes:

As long as we are on the topic of FTL signal propagation, maybe you can
help me answer a question that has been stuck in the back of my head.

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?

Classic question, covered in the Physics Relativity FAQs,
http://www.math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#4,
and the answer is unequivocally "No."

Such a "signal" would be transmitted as a longitudinal acoustic wave
propagating down the rod, and therefore cannot exceed the speed of sound
in the "Super Deluxe Unobtanium," which must necessarily be slower than light.
(Proof: The forces between the molecules of the "Super Deluxe Unobtanium"
that transmit the ascoustic wave are themselves limited to the speed of light,
so they cannot possibly transmit sound faster than they themselves propogate.
As a corollary, Special Relativity implies the _ABSOLUTE IMPOSSIBILITY_
of a "perfectly rigid substance." No matter how "Super Deluxe" your
"Unobtanium" is, the speed of sound in any physically possible material
must be limited to less than the speed of light.

You might ask, "well, what if the intermolecular forces can move _faster_
than the speed of light." Well, first of all, then you wouldn't need the
rod, you would just use the FTL forces; however, that would lead to violations
of Einstein causality, implying the possibility of communicating with the past
unless one postulates an "aether" or other prefer reference frame or temporal
foliation of spacetime. As I've already argued, it would be virtually
impssoble to accommodate such a "preferred" spatial or temporal structure
into physics and still construct a quantum field theory consistent with
observation. Second, it can be shown that such matter would be "exotic,"
and "exotic" matter can be used to violate the Laws of Thermodynamics,
in addition to allowing FTL travel and signalling backwards in time;
while we cannot _prove_ that the Laws of Thermodynamics _can't_ be broken,
they is no evidence that it _is_ possible to break them, and so much
evidence in favor of the Laws of Thermodynamics that very few physicists
or engineers take the idea that they could be broken AT ALL seriously.


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...

Essentially correct, although the concept of "relativisticly variable mass"
is now considered physically misleading and no longer useful, since there
is no sense in which the "relativisticly variable mass" acts like a "mass,"
see the FAQ: http://www.math.ucr.edu/home/baez/physics/Relativity/SR/mass.html.

You might also see the FAQ entry on the "Rigidly Rotating Relativistic Disk,"
http://www.math.ucr.edu/home/baez/physics/Relativity/SR/rigid_disk.html.


Remember, this is a Super Deluxe Unobtanium rod, so we can safely assume
it will withstand the stresses being so large...

Again, just as Special Relativity implies that the speed of sound in any
material must be less than the speed of light, the material must also have
a finite elastic limit.

Note also that elastic strain energy necessarily also contributes to the
mass/energy of the rod. To be consistent with Einstein causality, the
energy density of any material must always exceed the pressure or tension
in that material in order to ensure that the speed of sound will not exceed
the speed of light. Material that violates this condition would be "exotic,"
and would allow one to violate both causality and the Laws of Thermodynamics.


-- Gordon D. Pusch

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

"Keith F. Lynch" writes:

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.

....However, there is no evidence that the "preferred frame" represented
by the microwave background exerts any _physical_ effect on an otherwise
isolated observer (beyond a slight warming effect and a minute amount of
viscous drag if you are ploughing through it); nor is there any evidence
that the microwave background provides a "preferred clock" governing any
other physical phenomena. Finally, there is no a priori reason why the
"preferred frame" in which the microwave background is isotropic should be
correlated with the "preferred frame" represented by a cosmologically
"co-moving" observer in the sense of a General Relativistic cosmology,
and there are perfectly physically valid solutions of GR in which the
microwave background and co-moving observer frames can be "tilted"
relative to each other (where "tilted" here means something different
from what it means in "inflation theory" circles). Most would consider
such a "tilted" cosmology to be quite un-aesthetically asymmetric and
thermodynamically unlikely, but it is NOT RULED OUT by the physics of GR!


Maybe signals can travel at any finite positive speed relative
to that frame of reference.

One would need to find a plausible physical mechanism why the "microwave
background" or whatever should exert such an effect on temporal processes,
yet otherwise produce no "aether drag' effects on microphysics. So far,
no such physical mechanism even exists, let alone a physically plausible one.

Furthermore, the Hughes, the Drever, and the Eotvos Experiments impose
stringent experimental limits on =ANY= "preferred frame" effect that
couples to "normal" matter (see below).


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.

Most physicists find this notion of such a "tachyon corridor" even more
un-aesthetic than a preferred reference-frame or a preferred time ---
and again, one must ask why it apparently has _NO OTHER OBSERVABLE
PHYSICAL EFFECT_. In particular, one must ask why such a spatial
"preferred direction" does not show up in the Hughes and Drever
Experiments, which establish spatial isotropy of inertia to the
unprecedented accuracy of better than one part in 1e20.

To see why the observed experimental isotropy of space is in serious
conflict with the idea of a "tachyon corridor," consider that in order
to be useful for communication purposes, the tachyon or whatever type
of particle mediates the FTL communication must be capable of interacting
with some form of "normal" matter. If the tachyon or whatever is capable
of interacting with some form of "normal" matter, quantum mechanics demands
that those "normal" matter particles must be surround by a cloud of virtual
tachyons (or whatever) that contribute to its inertial mass --- and since
by hypothesis, spatial isotropy has been broken, in general there is no
reason why the virtual particle distribution should be the same for a
particle moving parallel to the "tachyon corridor" versus perpendicular
to it, and as Dicke has shown, this would lead to forces that violate the
Equivalence principle. Yet the Hughes and Drever experiments and the Eotvos
experiment both show that the inertia of all forms of matter tested are
isotropic and the equivalence principle is satisfied to a truly astounding
degree --- arguing strongly against the existence of a "tachyon corridor."

Finally, since every particle is surrounded by a cloud of virtual particles
mediating every force they couple to, and those virtual particles in turn
interact with every particle that _they_ couple to, it would appear that
the Hughes, the Drever, and the Eotvos Experiments rule out any sort of
"preferred frame" coupling to =ANY= form of matter that couples to
"ordinary" matter at all, to high precision. Hence, if the "tachyon
corridor" exists, it must not couple to ordinary matter, rendering it
useless for communications purposes, or it must couple to ordinary matter
so weakly that it is utterly impractical for communications purposes.

A more subtle version of this argument also argues strongly against the
existence of a preferred reference frame, as the cloud of virtual tachyons
or whatever would lead to an anomalous variation in particle energy with
velocity, and this variation would in principle show up in particle physics
experiments, allowing on to determine one's absolute velocity relative to
the "cosmic frame" --- yet no such effects have been observed.

Since such anomalous direction or velocity dependences of mass have =NOT=
been observed, one must conclude that either "physically preferred frames"
allowing FTL communication do not exist, or that they couple to "normal"
matter so weakly that they MIGHT AS WELL not exist.


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.


Actually, this commonly proposed notion doesn't work, since the "light cone"
is what _DEFINES_ the "speed of light." People who have attempted to work
on the so-called "Variable Speed of Light" (VSL) cosmologies run into this
problem all the time: Careful analysis shows that, since the speed of light
is a "dimensionful" quantity, it can only be compared to other quantities
that have the _SAME UNITS_ . Hence, it is physically meaningless to talk
about the speed of light "varying" unless physics provide a "preferred
system of units" --- and there is no evidence that such a "preferred system
of units" exists.


In a sense, gravitational waves do exactly this, albeit to too small
and too temporary a degree to be useful.

There is =NO= sense in which gravitational waves do this in GR !!!

In General Relativity, the propagation of gravitational waves is governed by
the VERY SAME LIGHT-CONES that determine the "speed of light" for every other
force, and for every other form of matter !!! (This is at the heart of the
controversy over Kopeikin's "speed of gravity" claim, and why Clifford Will
maintains that all Kopeikin has done is measured the "speed of light" by a
perverse and inaccurate method.)

Gravitational waves can only "change the speed of light" in NON-GR THEORIES,
e.g., Rosen's "bimetric" theory (which is STRONGLY ruled out by experiment).


-- Gordon D. Pusch

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

Roger Stokes wrote:

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?

Quantum gravity theory, or the inability of such to be found? By present
trends it would appear that we will arrive at the result that the folded
up extra dimensions needed by string theory do not exist.


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

Conversly, the real mechanisms of quatum mechanics might be different to
what they are thought to be. Similarily, macro-level causality might be
something the human mind invented, and the 'paradoxes' being the lack of
understanding of reality.


...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.

Temporary violations for tiny periods of time are possible, and have in
fact been demonstrated. So the need to violate it is not in and itself
a barrier to FTL, but a potential barrier to how long any given episode
of FTL can last.

So the usefulness would depend on what you can do say during the 10ns
spacewarp - but you need high multiplies of c, and travel (and not say
torpedoes or similar) needs you to be able to be able to produce lots
of such warps regularily. But once you can produce a warp, even for a
fleeting fraction of a second, and push for even a just slightly faster
than c, it all becomes engineering.


--Roger

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


Pots, of course 8-P

--
Sander

+++ Out of cheese error +++

Gordon D. Pusch wrote:

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.

Most physicists find this notion of such a "tachyon corridor" even more
un-aesthetic than a preferred reference-frame or a preferred time ---
and again, one must ask why it apparently has _NO OTHER OBSERVABLE
PHYSICAL EFFECT_. In particular, one must ask why such a spatial
"preferred direction" does not show up in the Hughes and Drever
Experiments, which establish spatial isotropy of inertia to the
unprecedented accuracy of better than one part in 1e20.

But this would not rule out something that was as weak as the weak
interaction - or would it? After all, localy, there are more neutrinos
moving away from Sun compared to any other direction.


-- Gordon D. Pusch

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

--
Sander

+++ Out of cheese error +++

[Posted and mailed.]

Gordon D. Pusch wrote:
...However, there is no evidence that the "preferred frame"
represented by the microwave background exerts any _physical_ effect
on an otherwise isolated observer (beyond a slight warming effect
and a minute amount of viscous drag if you are ploughing through
it); nor is there any evidence that the microwave background
provides a "preferred clock" governing any other physical phenomena.

I know. I was just pointing out that it's *possible* that FTL could
exist without violating causality. Personally, I think it more likely
that FTL doesn't exist. And that if it does, that it *does* violate
causality, perhaps by building bridges between "multiple worlds".

Finally, there is no a priori reason why the "preferred frame" in
which the microwave background is isotropic should be correlated
with the "preferred frame" represented by a cosmologically
"co-moving" observer in the sense of a General Relativistic
cosmology, ...

Which is why I listed that possibility separately.

Furthermore, the Hughes, the Drever, and the Eotvos Experiments
impose stringent experimental limits on =ANY= "preferred frame"
effect that couples to "normal" matter (see below).

But the limit isn't zero. For instance, since gravitational waves
have not yet been detected, it's possible that they travel slightly
faster than light.

Most physicists find this notion of such a "tachyon corridor" even
more un-aesthetic than a preferred reference-frame or a preferred
time ...

Yes. It's ugly, and GR isn't. But who says the universe has to have
beautiful or symmetrical laws just because we prefer them?

Actually, this commonly proposed notion doesn't work, since the
"light cone" is what _DEFINES_ the "speed of light." People who
have attempted to work on the so-called "Variable Speed of Light"
(VSL) cosmologies run into this problem all the time: ...

Yes, the new (1982) definition of the meter implicitly *defines*
the speed of light as constant. So if there's some process which
could have been interpreted as speeding up light, it would now be
interpreted as simply moving objects closer together. So, rephrasing,
perhaps there's some way to move Alpha Centauri closer to our solar
system, and to do so without making it look any brighter or have a
greater gravitational effect on us.

I have been puzzled by exactly what these VSL theories are trying to
claim. Sure, maybe the fine structure constant was different in the
past. But it seems to me completely arbitrary whether to attribute
such a change to a change in the speed of light, a change in Planck's
constant, a change in the charge of an electron, or some combination
of these. Are VSL theories really variable FSC theories?

There is =NO= sense in which gravitational waves do this in GR !!!

They make the distance between objects vary, without either object
experiencing acceleration. This could just as validly be interpreted
as a constant distance and varying speed of light as vice versa, if
you ignore the arbitrary choice in how the meter is defined. It's a
very small and temporary effect in either case.
--
Keith F. Lynch - - http://keithlynch.net/
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