Author Topic: Theoretical FTL  (Read 40266 times)

Offline colbourne

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Theoretical FTL
« on: 06/23/2008 06:03 AM »
Assumption 1
===========
I am assuming that anti-matter wil be affected by gravity in the reverse to normal matter.

Assumption 2
===========
Therefore if we can create an anti matter object we should be able to accelerate it up to FTL. This in itself should allow FTL communications.

Assumption 3
===========

We can currently contain anti-matter by magnetic means, so we should be able to contain normal matter by a similar means inside our anti-matter spacecraft.

I expect the new CERN accelrator will be able to answer my probably incorrect assumptions.

Offline Eraser

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Re: Theoretical FTL
« Reply #1 on: 06/23/2008 10:08 AM »
No, gravitation equally affects on a matter and an antimatter.

Offline whitewatcher

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Re: Theoretical FTL
« Reply #2 on: 06/23/2008 11:23 AM »
Yep, anti-matter and matter are made of the same thing: energy.

To observe a reverse gravitational effect, you would need something like anti-antimatter (composed of anti-energy).
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Offline William Barton

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Re: Theoretical FTL
« Reply #3 on: 06/23/2008 12:16 PM »
I forget who, but somebody one suggested the force of negative gravity be referred to as "levity."

It's probably not a good idea to try to prognosticate the enabling technologies of soft SF (unless you're a high-end theoretical cosmologist or something). FTL, teleportation, time-travel, etc. do for SF what magic wands and incantations do for fantasy. One of their hallmarks is, they enable secondary technologies that allow us to bypass the secondary (practical) limitations imposed by physics. For example, if you have teleportation, you instantly have fuelless rockets. You sink a transmitter in Jupiter's atmosphere, a receiver at the back end of your spaceship, and la voila! The ignored magic trick is the energy density required for something like teleportation to work. They are all effectively perpetual motion machines, and if you had the command of physics necessary to make them work, you wouldn't need them.

The issue with trying to get past the contraints imposed by physics as we know it is, first you have to get past the contraints imposed by practical engineering. Somebody comes up with a theory that allows FTL, and Step 1 turns out to be, "Accummulate 400 vigintillion tonnes of neutronium and shape it into a rotating torus 4cm in diameter..."

A brilliant example of the borderland of achievable technology was Arthur C. Clarke's black-hole rocket engine in "Imperial Earth."

Offline cpcjr

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Re: Theoretical FTL
« Reply #4 on: 06/23/2008 02:12 PM »
Assumption 1
===========
I am assuming that anti-matter wil be affected by gravity in the reverse to normal matter.

This assumption is wrong anti-matter is affect by gravity the same way ordinary matter is. 

Quote
Assumption 2
===========
Therefore if we can create an anti matter object we should be able to accelerate it up to FTL. This in itself should allow FTL communications.

Wrong even if assumption 1 were correct, which it is not.
1. The Gravity of the sun would only accelerate it to at most 617.5 Km/s
2. Anti-mater would still be limited by relativity to the speed light.

Quote
Assumption 3
===========

We can currently contain anti-matter by magnetic means, so we should be able to contain normal matter by a similar means inside our anti-matter spacecraft.

This does work but getting enough anti-matter for even the most basic anti-matter rocket would be exreamly expensive: over a $100,000,000.00 / ounce.

Offline scienceguy

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Re: Theoretical FTL
« Reply #5 on: 06/23/2008 08:13 PM »
According to this paper

A `warp drive' with more reasonable total energy requirements
Chris Van Den Broeck
Class. Quantum Grav. 16 No 12 (December 1999) 3973-3979

Even if antimatter responded to gravity in an opposite way to matter, you would still need -10^30 kg of it for a warp drive.
e^(pi)i = -1

Offline Martin FL

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Re: Theoretical FTL
« Reply #6 on: 06/23/2008 09:34 PM »
Here's a curveball, but this area of science is all theory, what is there are gaps in the facts and FTL is possible?

Possible, no chance, I should go back to reading about shuttles? ;)
« Last Edit: 06/23/2008 09:34 PM by Martin FL »

Offline scienceguy

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Re: Theoretical FTL
« Reply #7 on: 06/23/2008 11:34 PM »
Miguel Alcubierrre (1994) published a paper that showed that a warp drive is at least mathematically possible, although it would require huge amounts of negative energy. Pfenning and Ford (1997) showed that a warp bubble wall as proposed by Alcubierre would have to be impossibly thin in order to work. Low (1999) showed that a warp in spacetime could travel no faster than the speed of light and that such a warp would require exotic matter (negative energy). Natario (2002) had a little more positive result: he showed that warp drives could be possible in that they wouldn’t need to compress spacetime ahead of themselves and stretch it behind in order to move. Lobo and Visser (2004) most recently published on this and they showed that in order for a warp drive to work, a couple of things need to happen:
1.   The spaceship can’t travel faster than light
2.   The amount of negative energy must be a significant fraction of the mass of the ship.
As I understand it, a spaceship using a warp drive can’t travel faster than light because gravity only travels at the speed of light, and such a ship would be relying on a negative gravitational force generated by the negative energy it is carrying along in its warp of spacetime.

References

The warp drive: hyper-fast travel within general relativity
Miguel Alcubierre
Class. Quantum Grav. 11 No 5 (May 1994) L73-L77

Fundamental limitations on 'warp drive' spacetimes
Francisco S N Lobo and Matt Visser
Class. Quantum Grav. 21 No 24 (21 December 2004) 5871-5892

Speed limits in general relativity
Robert J Low
Class. Quantum Grav. 16 No 2 (February 1999) 543-549

Warp drive with zero expansion
J Natario
Class. Quantum Grav. 19 No 6 (21 March 2002) 1157-1165

The unphysical nature of `warp drive'
M J Pfenning and L H Ford
Class. Quantum Grav. 14 No 7 (July 1997) 1743-1751
e^(pi)i = -1

Online hop

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Re: Theoretical FTL
« Reply #8 on: 06/24/2008 12:29 AM »
No, gravitation equally affects on a matter and an antimatter.
AFAIK, this is current strong consensus of people in the field, but hasn't been verified experimentally yet.

edit:
Not that FTL or antigravity would necessarily follow even if some difference was detected.
« Last Edit: 06/24/2008 12:30 AM by hop »

Offline khallow

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Re: Theoretical FTL
« Reply #9 on: 06/24/2008 04:14 AM »
My take is that if FTL was possible, we'd probably have seen it by now in particle accelerator experiments and supernova observations. High energy events cover a lot of theoretical possibilities. If there were FTL possibilities, one would need to explain why those possibilities aren't been seen in the trillions of recorded collisions by particle accelerators and why we don't see anything precede the neutrino (and sometimes gamma ray) burst from a supernova.

A technology that might be feasible is the wormhole. Mathematically, it's a "handle" or hole in space-time, that provides an alternate path to a destination that isn't equivalent to the usual way of going between two points. In particular, at no time is anything traveling faster than the speed of light. This changes the topology of space which may or may not be possible.

Optimistically, this new path is considerably shorter than the usual one. For example, Alpha Centauri is 4+ light years away from Earth. A wormhole might provide an alternate path that is say 20 AU long instead. That might be useful merely for communication (under six hours round trip communication time) or even for travel if the hole can be made wide enough (and the environment inside the wormhole is survivable for a vehicle).

As I understand it, the two ends of the wormhole would be created next to one another. Each end would go to an appropriate destination. I have no idea how you'd move it around, keep it from pinching shut, or change its length.
Karl Hallowell

Offline TyMoore

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Re: Theoretical FTL
« Reply #10 on: 06/24/2008 03:43 PM »
Miguel Alcubierre's "Warp Metric" relies on the bizarre behavior that a negative energy mass and a positive energy mass will have for each other: the positive mass attracts the negative mass, but the negative mass repels the positive mass. Or more correctly, the positive mass has the usual gravity field, but the negative mass has a gravity field with a negative sense (as felt by the positive mass.) So a system composed of two equal magnitude but opposite sensed masses (total energy of system: zero) will accelerate in the direction of the positive mass.

Miguel Alcubierre took the idea to the extreme by positing large masses: neutronium density or more. Further he used a nifty little gravitational trick: the gravity field inside a spherically symmetric shell of mass is zero--in general relativity terms, the spacetime inside a spherical shell is approximately flat. So putting the two ideas together you get a spherical shell with the forward end composed of positive energy matter, the aft half is composed of negative energy matter, and the 'vessel' or transport is at the center of the shell in the flat spacetime 'island' in the middle. Increase the density of the shell until it comes close to the density of neutronium, and voila you have massive acceleration that the occupants inside won't feel (they're in free fall.)

The system does not appear to violate conservation of energy as long as the spherical shell has a net-zero energy (sum of the positive energy mass and negative energy mass is zero;) the vessel inside does not possess any more or less kinetic energy than what it started out with since the 'spacelike' FTL movement is similar in some regards to expansion of the early universe.

This is what I got out of Mr. Alcubierre's paper (without being able to do the General Relativistic Tensor mathematics involved in the transforms!)

Offline gospacex

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Re: Theoretical FTL
« Reply #11 on: 06/24/2008 04:21 PM »
Further he used a nifty little gravitational trick: the gravity field inside a spherically symmetric shell of mass is zero--in general relativity terms, the spacetime inside a spherical shell is approximately flat. So putting the two ideas together you get a spherical shell with the forward end composed of positive energy matter, the aft half is composed of negative energy matter,

Well, spherical shell's interior has zero gravity field _only if_ the density and thickness of the shell is the same eveywhere. The above description seems to violate that.

Offline colbourne

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Re: Theoretical FTL
« Reply #12 on: 06/26/2008 02:18 AM »
If anti-matter did respond to gravity differently to normal matter one of the results might have been after the big bang all the anti-matter would have quickly accelerated away at faster than light speed which may explain why there appears to be an absence/shortage of anti-matter in the universe.

This space craft seems expensive but as with everything you get what you pay for. I would say it is a bargain if it really could be built !!!

Initially once we get some results proving how gravity and anti-matter are linked , it might be a useful study for the SETI people.

If my assumptions were correct could we build a communications device that would work ?

Online hop

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Re: Theoretical FTL
« Reply #13 on: 06/26/2008 02:56 AM »
If anti-matter did respond to gravity differently to normal matter one of the results might have been after the big bang all the anti-matter would have quickly accelerated away at faster than light speed which may explain why there appears to be an absence/shortage of anti-matter in the universe.
No, as previously pointed out, even if antimatter doesn't respond as we expect to gravity (which itself would be a huge surprise) that doesn't imply FTL.

Offline Suzy

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Re: Theoretical FTL
« Reply #14 on: 06/26/2008 06:11 AM »
As I understand it (not being a physicist!), you can't go Faster-than-light at all, but there are ways around this (in science fiction, at least!). One as mentioned is to use a wormhole. Two points in space are brought together and a hole is poked through spacetime, and the starship just pops through without traversing any distance in real space. (If anyone saw the movie Event Horizon there is a scene where Sam Neil explains this with a piece of paper and a pencil). A wormhole is a hole, not a tunnel (as incorrectly depicted in some sci-fi films).

A similar concept is folding space - I am not sure if it is the same as a wormhole - where a starship pulls or warps space toward it until it reaches the place it wants to be (without actually moving itself physically).

My take is that if FTL was possible, we'd probably have seen it by now in particle accelerator experiments and supernova observations. High energy events cover a lot of theoretical possibilities. If there were FTL possibilities, one would need to explain why those possibilities aren't been seen in the trillions of recorded collisions by particle accelerators and why we don't see anything precede the neutrino (and sometimes gamma ray) burst from a supernova.

A technology that might be feasible is the wormhole. Mathematically, it's a "handle" or hole in space-time, that provides an alternate path to a destination that isn't equivalent to the usual way of going between two points. In particular, at no time is anything traveling faster than the speed of light. This changes the topology of space which may or may not be possible.

Optimistically, this new path is considerably shorter than the usual one. For example, Alpha Centauri is 4+ light years away from Earth. A wormhole might provide an alternate path that is say 20 AU long instead. That might be useful merely for communication (under six hours round trip communication time) or even for travel if the hole can be made wide enough (and the environment inside the wormhole is survivable for a vehicle).

As I understand it, the two ends of the wormhole would be created next to one another. Each end would go to an appropriate destination. I have no idea how you'd move it around, keep it from pinching shut, or change its length.

« Last Edit: 06/26/2008 06:13 AM by Suzy »

Offline Lampyridae

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Re: Theoretical FTL
« Reply #15 on: 06/26/2008 06:37 AM »
I forget who, but somebody one suggested the force of negative gravity be referred to as "levity."

It's probably not a good idea to try to prognosticate the enabling technologies of soft SF (unless you're a high-end theoretical cosmologist or something). FTL, teleportation, time-travel, etc. do for SF what magic wands and incantations do for fantasy. One of their hallmarks is, they enable secondary technologies that allow us to bypass the secondary (practical) limitations imposed by physics. For example, if you have teleportation, you instantly have fuelless rockets. You sink a transmitter in Jupiter's atmosphere, a receiver at the back end of your spaceship, and la voila! The ignored magic trick is the energy density required for something like teleportation to work. They are all effectively perpetual motion machines, and if you had the command of physics necessary to make them work, you wouldn't need them.

The issue with trying to get past the contraints imposed by physics as we know it is, first you have to get past the contraints imposed by practical engineering. Somebody comes up with a theory that allows FTL, and Step 1 turns out to be, "Accummulate 400 vigintillion tonnes of neutronium and shape it into a rotating torus 4cm in diameter..."

A brilliant example of the borderland of achievable technology was Arthur C. Clarke's black-hole rocket engine in "Imperial Earth."

A classic thought experiment exists on the teleportation idea. Drill a hole through the Earth (yes, yes, I know...), evacuate it and drop an object down it. It will travel down, pop up in Australia or wherever the exit is, allowing you to catch it. Zero energy is required (apart from the catch) so in effect you have teleportation for nothing (after drilling the hole through Earth...).

It's a way of using potential energy to move something, and not actually pay the gazillion dollar energy bill. So it may therefore be possible to use potential energies such as casimir effect (which theoretically can only be tapped once, like dropping a ball is for gravitational potential) and using it for whatever purpose. I don't believe we will manipulate gazillion tonnes of neutronium anytime soon but the ability to manipulate if not harvest quantum effects might give you would give you options into these FTL options.

As for FTL itself, nothing prevents you travelling faster than light (in special relativity), it just prevents you from accelerating past the lightspeed barrier. Decelerating from FTL is also bad; you re-enter normal space as a blinding cascade of Cherenkov radiation... but then again wasn't it Stephen Hawking who made that bet about "information never being able to leave a black hole?"

A lot of basic assumptions have been spectacularly demolished over recent years, and have been replaced by "erm, we don't know." Case in point, the inflationary universe. So our standard physics models may be due for updates soon. It's the nature of science.
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Offline scienceguy

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Re: Theoretical FTL
« Reply #16 on: 06/27/2008 07:48 PM »
Let's say antimatter responds to gravity in an opposite manner to matter. Would it be cheaper to harvest it from Jupiter's radiation belts or dedicate a single particle accelerator to make it?
e^(pi)i = -1

Offline khallow

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Re: Theoretical FTL
« Reply #17 on: 06/29/2008 07:48 AM »
Probably neither. It'd probably be easier to build a large scale solar powered plant in close orbit around the Sun (say a few million kilometers out or less) and farm the solar wind.
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Offline Suzy

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Re: Theoretical FTL
« Reply #18 on: 07/08/2008 08:28 AM »
...A brilliant example of the borderland of achievable technology was Arthur C. Clarke's black-hole rocket engine in "Imperial Earth."

An explanation of the Asymptotic Drive (from here):

The main character takes a trip from the Saturnian moon of Titan to Earth in a vessel powered by the “Asymptotic Drive” which is basically a small mass black hole (in the book it was one with “one or two thousand tons mass” – something which would be proton-sized or thereabouts) suspended in a very powerful magnetic field. The way it works is like this: The black hole “eats” matter as we all know, but it can only consume matter at a set rate depending on its mass and the size of the Event Horizon which surrounds the singularity. Now, (as Clarke visualizes) you dump a few grams per second of plain hydrogen onto the black hole which attempts to consume all of the hydrogen but can’t so the remaining hydrogen compresses against itself and the event horizon to the point that it gets hot.

Very hot.

Hot enough to fuse.

Now the singularity is suspended via powerful magnetic fields at the end of a tube open at one end to space and the superheated hydrogen jets out of the open end of the tube. This superhot gas is plasma which means it is electrically active and also means it can be shaped, focussed and directed by magnetic fields. The result is one has a fusion powered rocket which creates thrust with a nozzle velocity far in excess of anything a chemically powered rocket could produce, and do it for weeks on end. In the book, the billion-mile trip from Titan to Earth took 20 days with 10 days accelerating and 10 days decelerating.


You're still stuck going through normal space, though.

In the Event Horizon movie a "black hole drive" was used to somehow fold space so the ship could jump instantly from one point in space to another. Any idea of the details of how that could work?

Offline Eerie

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Re: Theoretical FTL
« Reply #19 on: 07/08/2008 01:18 PM »
Suzy, you could just use a fusion rocket, without messing with a HEAVY black hole.

And antimatter rocket would be better anyway.

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