
Warp drive may actually be possible, NASA scientist says - kausikram
http://www.ksl.com/?sid=22195854&nid=1012&title=warp-drive-may-actually-be-possible-nasa-scientist-says&s_cid=queue-5
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Tossrock
> "A trip to Alpha Centauri would take about two weeks. Traveling as fast as
> we are now currently able, it would take tens of thousands of years."

This isn't true. Nuclear pulse drives built with currently available
technology could achieve .05C [1], meaning a 4 light year trip would take ~80
years.

[1]:
[http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsi...](http://en.wikipedia.org/wiki/Project_Orion_\(nuclear_propulsion\))

~~~
danielbarla
IIRC, that assumes we don't want to decelerate. Adding the fuel to decelerate
also increases the fuel needed for the original acceleration itself, which
then quickly becomes less feasible. In any case, I'm guessing they were
referring to "current, tried and tested" technologies.

Edit: I'm also having a slight chuckle at the estimated costs presented.
Somehow, I'd think that just getting 40,000,000 t of spaceship into orbit
would cost a bit more than 1 year of US GNP.

~~~
wcoenen
The mass added by the "fuel" of the orion nuclear pulse drive is negligible: a
nuclear bomb has very little mass compared to the impulse boost you can
extract from it.

You could literally lift a city-sized space ship to orbit on the cheap with
this technology. The problem is the nuclear fall-out for those left on the
ground.

~~~
jasonwatkinspdx
It's not that simple. You are vastly underestimating how much scaleup is
required to sustain that acceleration up until you get a reasonable trip time.

Here's the summary from NASA:

 _"Here are four examples [large graphic][1] of what it would take to send a
canister about the size of a Shuttle payload (or a school bus) past our
nearest neighboring star...and allowing 900 years for it to make this journey.

Well....If you use chemical engines like those that are on the Shuttle,
well..., sorry, there isn’t enough mass in the universe to supply the rocket
propellant you’d need.

So let’s step up to next possibilities, nuclear rockets with a predicted
performance that’s 10 to 20 times better!

Well...it’s still not looking all that good. For a fission rocket you would
need a BILLION SUPERTANKER size propellant tanks to get you there, and even
with fusion rockets you would still need a THOUSAND SUPERTANKERS!

Even if we look at the best conceivable performance that we could engineer
based on today’s knowledge, say an Ion engine or an antimatter rocket whose
performance was 100 times better that the shuttle engines, we would need about
ten railway tanker sized propellant tanks.

That doesn’t sound too bad, until you consider that we didn’t bring along any
propellant to let us stop when we get to the other star system...or if we want
to get there quicker than 9 centuries.

Once you add the desire to actually stop at your destination, or if you want
to get there sooner, you’re back at the incredible supertanker situation
again, even for our best conceivable rockets.

In conclusion, we’d really like to have a form of propulsion that doesn’t need
any propellant! This implies the need to find some way to modify gravitational
or inertial forces or to find some means to push against the very structure of
spacetime itself."_

The rest of the website[2] has some good information, even if the organization
is a bit disjoint.

[1]:
[http://www.nasa.gov/centers/glenn/images/content/84509main_w...](http://www.nasa.gov/centers/glenn/images/content/84509main_warp06.gif)

[2]:
[http://www.nasa.gov/centers/glenn/technology/warp/ipspaper.h...](http://www.nasa.gov/centers/glenn/technology/warp/ipspaper.html)

~~~
wcoenen
Your references are about rocket designs, which have limits to how fast they
can expel reaction mass before they blow themselves up. Orion isn't really a
rocket as it does the violent reaction outside of the spaceship. With Orion,
the limit is how fast the pusher plate can be cooled or how much momentum can
be handled by the shock absorbers.

Granted, even with fusion bombs there is still a limit to how much energy and
momentum you can extract from a certain mass, so the same concerns of
diminishing returns apply. But according to Dyson's numbers, a 133 year trip
to Alpha Centauri might be possible[1].

Even discounting interstellar travel, a trip time of only a few weeks to Mars
with current technology sure sounds interesting to me.

[1]
[http://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsi...](http://en.wikipedia.org/wiki/Project_Orion_\(nuclear_propulsion\)#Interstellar_missions)

~~~
jasonwatkinspdx
Orion most definitely is still an action/reaction rocket. It works via
radiation and particle pressure against the pusher plate. While its thrust is
high, its overall efficiency is low because the majority of the fission energy
result is not contained enough to influence forward motion of the spacecraft.
High thrust is pointless for interstellar travel. High isp is essential. This
is why fission or antimatter powered ion drives beat Orion. Orion is great for
interplanetary however.

If you read the through the material on that site you will see they are
familiar with Orion. While the public oriented site glosses over details, the
companion site will provide you with papers with rigorous detail.

There is no escaping it: for practical interstellar travel, we must develop
massless drive or some form of spacetime manipulation that changes the mass
requirements.

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ch0wn
Previous discussion: <http://news.ycombinator.com/item?id=4534359>

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WalterBright
One issue about warp drives that I never see mentioned is one has to be
careful one is not creating a perpetual motion device.

Consider a warp drive that transports you from, say, earth's orbit to pluto's
orbit. Now, you fall back towards earth's orbit. This means that the warp
drive, in order to not violate conservation of energy, must require at least
as much energy as the potential energy difference between the two positions.

~~~
yk
Conservation of energy may or may not be valid in general relativity, at least
in a naive understanding of conservation of energy. The technical explanation
for this is, that conserved quantities are related to geometrical symmetries
via Noethers theorem [1], and on a curved background these symmetries may or
may not be there. ( A rather handwaving explanation would be, that there is no
straightforward way to define the energy density of a gravitational field.)

In fact it is possible to build a perpetuum mobile by just filling some region
of space with dark energy. This region of space will then expand at constant
energy density, therefore creating energy.

[1] <https://en.wikipedia.org/wiki/Noether%27s_theorem>

~~~
maxerickson
"In fact it is possible" is a pretty strong way to talk about moving something
that might not exist.

~~~
yk
Agreed, the wording is rather strong. But at least it is shorter than:
"According to current understanding of GR and the contents of the universe it
should be possible."

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keyle
Can someone explain what happens to _time_ while you travel this way?

Is your family at home going to be older when you come back from one of those
trips?

~~~
jonnathanson
Simple answer: probably not. You wouldn't experience time dilation in this
scenario, because the ship itself isn't actually moving faster than light
speed. In fact, it's not moving very quickly at all. It's bending space around
itself, while remaining within a flat and undistorted region of space inside
the bubble. With respect to objects inside the bubble (its own frame of
reference), the ship is not moving faster than light. (A beam of light inside
the warp bubble will still travel faster than the ship, for instance).

Basically, the ship is able to traverse distances faster than light can, but
it isn't actually _moving_ faster than light.

~~~
EthanHeilman
>You wouldn't experience time dilation in this scenario, because the ship
itself isn't actually moving faster than light speed.

Time dilation occurs at speeds far less than the speed of light. For instance
GPS satellites and jet planes experience measurable time dilation.

I am very curious about the time effects of such a drive. Does the ability to
warp space using such a device necessarily include the ability to warp time?
Is it possible, within a frame of reference, to locally reverse the arrow of
time and thereby reverse entropy within that pocket?

>A beam of light inside the warp bubble will still travel faster than the
ship, for instance.

But that beam of light seen from outside the bubble will appear to travel
faster than the speed of light.

~~~
jonnathanson
_"Time dilation occurs at speeds far less than the speed of light. For
instance GPS satellites and jet planes experience measurable time dilation."_

Sure, but I think (?) the original question was asking about time dilation in
the massive sense: i.e., you go on a round trip to some distant point in space
and return, and it's only been a few days for you, but a few hundred or
thousand years have passed on Earth. That sort of time dilation.

That sort won't occur with an Alcubierre ship. It's my understanding that the
ship isn't actually moving _at all_ inside the bubble -- or, if it is, it's
moving extremely slowly.

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powertower
I don't see how this will work considering the contraction of space-time right
in front of the spaceship within this warp-bubble, will also in-turn cause a
natural dilution of space-time right in front of the warp-bubble.

Even if that was not true, how will the warp-drive propagate the warp-bubble
_field_ itself faster than C to grab onto space-time in a timely manner?

It would seem to me this would cause the space-ship to feel that it's
traveling faster than C, but to an outside observer it would only be traveling
at C or less.

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brudgers
I am not a physicist - My understanding is that it is information that is
limited to the speed of light. So the technology for sending people to a
nearby star tends to be analogous to the to that between Clarke's hominids and
the obelisk rather than that between Captain Kirk and Star Fleet.

Warp drive will allow humans to pollenize, not colonize, other planets.

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Tipzntrix
If we can't move the spaceship fast enough, we'll move the space around it! No
wonder all the sci-fi movies show the area around warp speed ships distorting.
They must have been onto something. I don't want to know how a human could
ever survive that though.

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shin_lao
The article doesn't mention exotic matter. Did they find a way to build an
engine without exotic matter?

~~~
panacea
No. See: <http://news.ycombinator.com/item?id=4534359>

