
Warp speed travel is theoretically possible, says astrophysicist - chris-at
http://www.sciencealert.com/warp-speed-travel-is-theoretically-possible-according-to-top-astrophysicist?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+sciencealert-latestnews+%28ScienceAlert-Latest%29
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zamalek
Interestingly a short while back White's lab did notice a statistically
relevant effect regarding one of their candidate warping devices. However,
they aren't ready to make any conclusions about the effect that they saw - so
far as I understand it's merely a very weak correlation right now. Something
interesting, but nobody should get their hopes up.

Either way I seriously recommend some thorough Googling about this subject as
that article is unbelievably out of date.

 _I play Elite: Dangerous once in a while and one thing that game really
teaches you is that even 'c' is really slow in astronomical terms. You start
to realize that for space colonization to become truly viable we'd have to
hope for some real analog of "hyperspace," as "warp" simply won't cut it in
the long run._

~~~
w0000t
Well that is clearly not true. Thanks to relativity, colonization is very
possible in a short time span.

~~~
zamalek
> relativity

Good point. Let me rephrase that: a long time from the perspective of the
people doing the colonization :)

~~~
w0000t
If by those you mean people who are actually traveling in the star-ships, then
it is a short time. It is only a long time for people staying stationary
relative to the travelers.

~~~
zamalek
Am I severely misunderstanding something? Doesn't the warp drive move the ship
at e.g. 20c according to the ship's frame of reference? Given that, at 20c it
would take the inhabitants of the ship 53 days to reach Alpha Centauri (4
light years). Multiply that by 18000 for the other side of the galaxy (72000
light years): 261 years. You most likely couldn't make that in "one hop" so
you'd have some crazy multiplier depending on the maximum travel distance of
your drive. Even if you didn't want to travel colonize the other size of the
galaxy you'd have to search some 400 billion stars for a habitable planet.

That's what I'm saying: once you move past the "local neighborhood" things
become quite ridiculous.

~~~
w0000t
I was referring to sub c speeds and what happens by using real confirmed
physics. I didn't realize you were talking about warp drives in that case.

As far as I understand from popular science, warp drive doesn't actually move
the occupants of the vehicle, it moves the space around the ship so in that
case there is no time dilation. It takes the same amount of time for people,
in and outside, the ship. If your warp drive is limited to 20c then yes it is
a problem, but I don't see why would it have to be limited in the first place.
Make it go 10^10c and the problem is solved.

~~~
zamalek
> I was referring to sub c speeds and what happens by using real confirmed
> physics.

That much so much more sense. In addition to the velocity there would also be
time spent completely outside of a gravity well. However, the article is
really talking about warp.

> Make it go 10^10c and the problem is solved.

Too much energy. From what I understand it's exponential - i.e. more energy
would be used getting to Alpha Centauri at 10^10c than at 20c. We don't know
_exactly_ how to get to 1c in the first place, but we do know that if we could
then e.g. 100c would likely be something that would be out of reach.

Keep in mind that before White optimized the Alcubierre metric, getting to
Pluto (AFAIR) with a warp drive would have required the mass of Jupiter.
Getting to Alpha Centauri would have required the mass of the observable
universe (which is now down to something with "the mass of Voyager 1"). That's
the amount of energy we are talking about even at a pitiful 20c.

~~~
w0000t
> Too much energy.

Right, you would need those rare, stable, dilithium crystals.

~~~
zamalek
> Right, you would need those rare, stable, dilithium crystals.

The spice must flow.

All of this stuff is a massive longshot, whether or not we can do it it's
still interesting.

------
rootbear
The article is badly edited, speaking of both "negative density energy" and
"negative energy density", when the latter is what was meant.

Even though, as I understand it, General Relativity doesn't forbid FTL
explicitly, it doesn't get you out of the causality problems of FTL. At least,
I've never seen an explanation of a "warp" style FTL method that doesn't allow
for causality violation. I'm all ears, if someone wants to convince me that
warp drives can't violate causality.

~~~
w0000t
You assume that warp drive actually moves you trough space, it doesn't.
Problem solved.

Here is a practical example: I decide to travel to Alpha Centauri, and I'm
there. Time passed for everybody is the same. You cannot cause any problems.

Note that you are not allowed to change reference frames, by doing this. This
does sound illogical, since you move to some other location, but if you assume
you didn't move, like we did, then it isn't. Actually the space moved, and
that might cause some other problems we don't know about.

~~~
drdeca
I think the alcubierre drive still violates causality if you go ftl.

Iirc, the ftl violates causality thing isn't from going faster than light, but
rather because where you end up is outside of the future light cone of where
you started.

It's also, I think, outside of your past light cone, so a single ftl trip in a
straight line might not have retrocausality, but because the region outside of
both the past and future light comes can be considered to be either before or
after the start timeplace, by choosing a different reference frame, if you
have two ftl trips, that is enough for causing past events.

I think this doesn't matter what happens during the trips, just the start and
end timeplaces.

~~~
w0000t
The light cone only matters if you actually move. The accepted idioms from
relativity don't apply here.

I think we are just not talking about the same "warp" drive. The version that
doesn't violate causality works like this. You are not allowed to accelerate(
using normal rockets ) to near c velocity and then warp to a stop to some
distant location. This would cause problems. Instead you will end up there
with all of your accumulated velocity before the warp. Then you have to come
to a stop the same way you accelerated.

In other words, warp will change your location but not your velocity in
spacetime.

~~~
drdeca
I am aware that the ship does not accelerate in space. No, I don't think we
are talking about different warp drives, but either way it doesn't matter,
(unless the one you are talking about has weird additional restrictions on
things like what directions you can go or you can only use it once and only in
a straight line, or something like that).

What I'm saying is the "how you get there" doesn't matter.

If you pick a point in spacetime, the future light cone of that point does not
depend on your reference frame, and neither does the past light cone.

And, no, I'm fairly sure that the light cone is still a thing even if you do
not accelerate.

So, pick a point X in space time, and a point Y which in some inertial frame
of reference is "after" that point, but not inside the future light cone of X,
such that your ftl device allows point X to influence point Y.

Then there is some other frame of reference where point X is still not inside
point Y's past or future light cones, but now is "after" point Y.

Then, the same ftl device would allow point Y to influence point X, or other
points in X's past light cone.

(to simplify, assume that the ftl device does not have a maximum "speed" This
implies that it can impact any future point. I'm pretty sure that the same
line of reasoning with the two points still applies when ftl device is limited
to a "speed" of twice c, but I am not well versed in relativity, so I am
treating the "speed" of the ftl device as unlimited. I am putting "speed" in
quotes to indicate that it isn't a literal speed, so much as a ratio of (I
think) distance in some reference frame compared to time in the reference
frame, within which the ftl device can reach.)

Note that I have not at any point in this talked about a path between X and Y,
or anything like that.

So, you can't get around this by noting that the ftl device never actually
goes faster than c in the space it is in, because none of the things deal with
the ftl device moving, just influencing things outside of its light cone.

~~~
w0000t
You are clinging to light cones because that is what you know about
relativity, but you don't understand it intuitively. I didn't get any new
information from you last comment, just what has already been said. Warp and
light cones clearly don't work. Does that mean that warp couldn't work at
least in theory, no.

Learn more about relativity( it is really interesting ), otherwise you will
always be stuck in your frame of mind.

~~~
drdeca
Ok, heres another explanation which does not refer to light cones.

Take some ftl causation (i.e. two points in spacetime such that one is a
causal influence on the other, and such that the spacetime interval between
them is spacelike).

Take the one which has a causal influence on the other to be first. Then, by
relativity of simultaneity for things which are separated by a spacelike
spacetime interval, there is another frame of reference in which the point
which is causally influenced by the other point occurs before the point which
causally influences the other point.

That is, A influences B, and B is before A.

Which is retrocausality.

Therefore, because any two points in mostly flat spacetime (so like, no black
holes or anything) which have spacelike separation can be taken to be before
or after eachother, if one has a causal influence on the other, there is a
frame of reference in which that would be retrocausality.

edit:

also, do you mean that I don't intuitively understand relativity, or don't
intuitively understand light cones?

I don't understand where you are getting the idea that "warp and light cones
clearly don't work". Light cones are still used in general relativity (at
least locally). Shifting things based on frame of reference still works in
general relativity.

Do you have anything to back up your claim that light cones "don't work" with
"warp"?

At the very least a good search term?

edit2:

further, consider this quote by alcubierre: ' Miguel Alcubierre briefly
discusses some of these issues in a series of lecture slides posted
online,[31] where he writes "beware: in relativity, any method to travel
faster than light can in principle be used to travel back in time (a time
machine)." '

[31]
[http://ccrg.rit.edu/files/FasterThanLight.pdf](http://ccrg.rit.edu/files/FasterThanLight.pdf)

So, you can see, Alcubierre (who, iirc, first showed the negative energy
density method of ftl mentioned in the article), also states that it could be
used for retrocausality (though there's a caveat based on the “chronology
protection conjecture").

So, I think my arguments are fairly well supported.

~~~
w0000t
I read the _real_ ( peer reviewed ) paper, and it contradicts your ideas.

Your final conclusion is a strawman argument. Those quotes are referring to
normal travel through space.

