
Is Faster-Than-Light Travel or Communication Possible? - bhavin
http://www.desy.de/user/projects/Physics/Relativity/SpeedOfLight/FTL.html
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geuis
I'm going to be the odd man out here. There are several statements made in the
article that are incorrect, and then other arguments are based on them.

In particular, the tying together of speed and time.

"By 'world line' we mean a curve traced out in the four dimensions of space-
time". Time is not a dimension. Time is a human concept. Clocks don't measure
a physical dimension or force called time, they repeat mechanical motions at
regular intervals that we label as time passing. Same for atomic clocks. We
measure the spin rates of cesium atoms but that isn't any different than
mechanical clocks. Time is not a force that figures into physics equations.

Further, time doesn't exist. Not in the classical sense. What we experience as
time is the entropy of energy in the universe winding out. We remember things
that happened before and imagine things in the future, but since there is no
physical "time" dimension there is no traveling forward and back.

If you are on a ship traveling close to c, the rate of entropy in the matter
and energy on your ship is lower than the outside universe. That's why time
seems slower.

There's a classic thought experiment that you can build a time machine by
putting one end of a wormhole on a ship, send it out for 20 years near c, then
bring it back. At this point you would have a wormhole with ends in two
different times. It doesn't work like that though. Passing through it will
just put you wherever it opens to, and you'll just end up in whatever local
entropy rate is going on.

~~~
Chirono
_Further, time doesn't exist. Not in the classical sense... If you are on a
ship traveling close to c, the rate of entropy in the matter and energy on
your ship is lower than the outside universe. That's why time seems slower._

Not trying to be facetious, this is a genuine question, but rate of change in
entropy with respect to what?

~~~
felipemnoa
I suspect with respect to the frame of reference from where the measurement is
being done. If the frame of reference is the ship itself then you will not
notice any change in entropy. Hence why you won't be able to tell that time is
actually slower. However, from a frame of reference outside the ship you will
notice the difference in rate of change.

So really what you are really measuring is the difference in change of entropy
from your local frame of reference to the frame of reference of the ship.
There is no such thing as absolute rate of change. All is relative.

With respect to time, if you could somehow reverse all the motions of every
single subatomic particles in a particular frame of reference then you would
essentially be moving back in time in that specific frame of reference. To
reverse time you have to reverse the motion of every single particle, sub-
particle. Is almost the same thing as simply playing a movie in reverse. Now
the real issue with this methodology is that the particle movements are not
being recorded anywhere as far as we can tell. So we need to first find a way
to record the movement of all the particles in a frame of reference and then
find another way to run the entire recording in reverse. I wonder if exceeding
the speed of light would actually reverse the motion of particles. That would
imply that the motion is somehow being recorded? Who knows, just thinking out-
loud.

Another really good question that I've been wondering about is why does
entropy decreases when you move faster? What is it that causes entropy to
decrease? Is it some sort of "friction" with space-time? Anybody have any good
suggestions? It may have to do something with conservation of energy. Or
conservation of something. The faster it moves the slower the particles move.
Something is being compensated for.

~~~
geuis
In regards to your last question, I have had similar thoughts. I no longer
like using the phrase "speed of light" because it's most commonly used to
reference the maximum speed anything observed can attain. It's very clear this
has nothing to do with light itself but is derived from the medium itself,
space-time.

One analogy that occurrs to me is movement through a fluid. There is almost
always a terminal velocity. We can transmit waves via a fluid, and particles
through it as well.

I would like to know what vacuum looks like at the Planck scale. Perhaps
entropy increases slower or faster based on interactions at the smallest scale
between light/matter and whatever space-time is. Movement through the medium
at higher speeds decreases these interactions, maybe by skipping over them.
Less interactions, slower entropy, slower apparent time.

Perhaps c is the terminal velocity of space-time. The air/water analogy breaks
down easily, since we can travel faster than terminal velocity in air. But
it's a different way of looking at the question.

~~~
felipemnoa
Terminal velocity is a really good analogy. Light is just a ripple in space.
The ripple will continue forever until it gets absorbed by other matter.
Space-time does seem to be an actual substance. Perhaps the proponents of the
Ether were right except that now we call it space time. Space-time seems to be
made up of super tiny particles were all other particles ride on.

One more thing, if we were to discover that light actually accelerates to c
when it is first released by an electron then that would be strong evidence
that the upper limit on light velocity is just something intrinsic of space-
time. Nothing is ever instantaneous and I have a feeling that neither does
light go from zero to c in zero time.

~~~
dkersten
If we were to discover that light accelerates to c when it is first released,
the next step would be to discover that c isn't actually the maximum possible
velocity of light at all, but rather the maximum _naturally occurring_
velocity (terminal velocity) and that through some as yet undiscovered means,
it actually _is_ possible for light to be artificially accelerated above the
value of c.

The terminal velocity analogy makes sense to me. It would be pretty
interesting is it turned out to be more than just an analogy.

------
VladRussian
Special Relativity states (and so far all evidence confirms it ) that speeds
faster than "c" is impossible relative to (i.e. _through_ ) local spacetime
("aether") . The mathematical model of SR (smooth manifold with static metric
(and we know that our real world doesn't have static metric, so real
applications of SR approximate it by considering only sufficiently local
regions of space during short periods of time)) doesn't cover and thus doesn't
preclude or contradict with other types of relative "motions" (for the lack of
better term) which are known to exist and have speed faster than "c" :

\- quantum entanglement (we don't know the machinery behind it, very-very
possible that the SR's smoothness of space is also only a rough approximation
of the real world.) Obviously any attempts to find the "signal" between the
entangled particles in the conventional sense (something traveling through
smooth static spacetime) have so far been and will continue to be futile.

\- relative motion of the parts of the (our expanding) Universe that are far
enough from each other (observed, space expansion, described by General
Relativity, at this scales of spacetime the SR's condition of static metric
just isn't valid anymore )

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raganwald
I am constantly reading that information cannot travel faster than light, and
I accept each of the explanations as to why the various methods for attempting
to send information faster than light do not work.

What I do not understand is whether sending information faster than light
would cause a paradox of any sort. It may be that it happens to be impossible.
But is it _necessarily_ impossible?

~~~
chulan
Quantum entanglement seems to allow instantaneous info transfer over any
distance. Any Quantum Physicists on here?

~~~
rdtsc
Not _info_ transfer but it allows instantaneous collapse of the entangled
state into known states after the measurement is done on a entangled system.

Here is how I remember it (and I am just a comp sci nerd, not a physicist so
please correct me):

* you have 2 qubits Q1 and Q2. You entangle them. Now you have an entangled system Q1Q2.

* You separate Q1 and Q2 in space. Let's say you put Q2 on a spaceship and blast it into space, but leave Q1 in the lab on earth.

* After some time you measure Q1 to get its value. At that point the entanglement of Q1Q2 collapses. You now know the value of Q1 that you just measured and at the same time Q2 is forced to a known value too.

* At _appears_ as if you could send information this way but you cannot. Think about it. You don't know what value you'll measure in the lab for Q1. So you can't force Q2 to be a certain value either.

* Let's think about it another way -- suppose you tell the spaceship operator that if Q2 collapses to |0> then they should turn the spaceship immediately around and head back home and if it collapses to |1> they should arm their weapons and prepare for an alien attack. Now you are on earth in control of Q1, and you want to force Q2 to be measured to |1> because you know the aliens are coming. There is nothing you can do to Q1 to force Q2 to be measured as |1>.

~~~
aquark
OK, but can you detect in the spaceship when Q2 collapses? (I know no quantum
physics beyond a few articles0

If so why can't you simply modulate the signal on top of a series of
collapsing entangled pairs. Basically morse code with the timing of the
collapses, you don't care what value they collapse too.

~~~
incremental
Nice try, but no - basically, to use your phrasing, there's nothing to detect
when Q2 collapses.

------
lachenmayer
There is one sentence I have a problem with in this post: "Shadows and
spotlights suffice to show that there is no logic in this suggestion, because
they can certainly go FTL and still be seen." Now, I was only scanning over
the article and certainly missed some explanatory section, but can someone
explain to me how shadows and spotlights go faster than light?

~~~
raganwald
Shadows and spots can _appear_ to go faster than light. Example given in the
article: We shine a laser at the moon and wave it gently. The spot of light on
the moon moves across the surface faster than light would travel. However,
what we call the spot of light on the moon is not an object in any real sense.

If I am standing on the Earth with my laser and I place two observers on
either edge the moon, we may say colloquially that a spot has moved from one
observer to the other faster than light can travel, but in actual fact what
has really happened is that light has travelled from me to the first observer
and then to the second observer at the speed of light, and it is just that my
signal to the second observer arrived extremely soon after my signal to the
first observer.

------
Groxx
From the article:

> _Now consider the description of an EPR-entangled pair of photons:

(|↑↓> \+ |↓↑>)/√2

At first glance this looks very much like the single-photon case, except that
where before we had ΨU and ΨL we now have |↑↓> and |↓↑>, representing
respectively photon 1 being in the upper slit and photon 2 being in the lower
slit and viceversa. But this distinction is crucial because it turns out that
there is some notational sleight-of-hand going on here. First, |↑↓> is
shorthand for |↑>|↓>. Second, the arrow symbols have no semantic significance;
they are just compact mnemonic identifiers. We could just as well have written
|UL> and |LU> (which of course is shorthand for |U>|L> and |L>|U>) as |↑↓> and
|↓↑>. Finally, ΨU is just another way of writing |U>.

So if we employ alternative notation we get the following description of two
entangled photons:

(ΨU |U> \+ ΨL |L>)/√2_

As I have probably demonstrated other places, IANAQM. But I don't follow that
last transformation. If |↑↓> == |↑>|↓> == |U>|L> == |UL> and ΨU == |U>, how
does (|↑↓> \+ |↓↑>)/√2 == (ΨU |U> \+ ΨL |L>)/√2 and not (ΨU |L> \+ ΨL |U>)/√2
? Shouldn't the |U> and |L> be reversed?

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chicagobob
Something I've always wondered is -- of course going back in time is a bad
thing -- but is going "faster than light" necessarily going back in time? I
wonder why, say, instantaneous communication across the galaxy wouldn't be
possible, as long as it doesn't go back in time. (BTW: not necessarily from a
physics point of view, they would say that not only can't one travel back in
time, neither can you go faster than the speed of light, but I was wondering
more from a paradox / time causality point of view).

~~~
hugh3
See my comment elsewhere in this thread, but the answer is yes but I can't
explain why in anything shorter than an entire textbook on relativity. If
you've "always wondered" about this, then this is the universe telling you to
go and learn relativity (at least special relativity).

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phypi
Assuming that space and time collapse into/onto a one dimensional membrane,
time wouldn't be of consequence. Information would remain and it would be
"every where" at "every time." Question is, where are we in reference to that
membrane right now?

~~~
hugh3
_Assuming that space and time collapse into/onto a one dimensional membrane_

And why, exactly, would we assume a thing like that?

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asymmetric
anyone care to give a simple TL;DR for the rest of us?

~~~
pohl
[http://www.desy.de/user/projects/Physics/Relativity/SpeedOfL...](http://www.desy.de/user/projects/Physics/Relativity/SpeedOfLight/FTL.html#conclusion)

