How can the space between the stars be so full of stuff? So commented a friend who chanced upon this site, reading our discussion of interstellar gas and dust and the troubling fact that moving through it at high speeds bathes a spacecraft in radiation. Not an issue for our current generation of spacecraft, dust and gas rise in significance as we reach velocities that are an appreciable fraction of the speed of light, creating the need for various kinds of shielding. So what exactly is that stuff in outer space?
Break down the interstellar medium and you get almost 90 percent hydrogen, with ten percent or so helium and trace elements like carbon, oxygen, silicon and iron accreted in dust particles. Oleg Semyonov, in his recent Acta Astronautica paper, examines all this, noting that the concentration of interstellar gas varies greatly between 104 cm-3 in galactic clouds to less than 1 cm-3 in the regions between the clouds. Our own Solar System lies in a cavity of low-density gas, with the nearest ‘wall’ located about 170 light years out in the direction of the galactic core.
From Relativistic Dust to FTL
So much for empty space, in regions which, if we produced them in a laboratory here on Earth, we would consider the hardest of vacuums. Writing about anything is always serendipitous, but never more so than when looking at interstellar matters. Had it not been for the Semyonov paper, I wouldn’t have gotten to Stefano Finazzi’s study of warp drive theory as quickly as I did. Both Larry Klaes and Adam Crowl discussed the Finazzi paper in comments to the interstellar dust story. Adam wrote the matter up on his Crowlspace site and Larry pointed to this squib in Technology Review.
Then a note from Kelvin Long pointed to discussion of warp drive instability at the recent interstellar session at the Charterhouse conference in the UK, and suddenly problems at superluminal speed were filling my hard disk. And rightly so, for it turns out that dangerous radiation isn’t simply an external issue, a matter of shielding a spacecraft from matter it encounters while at relativistic speeds. If warp drive is carried through to its logical conclusions, we may well run into something even worse when going superluminal — a devastating radiation that would destroy our fragile human payload.
Alcubierre’s Theory, and a Problem
Let’s look more closely. Warp drive as envisioned by Miguel Alcubierre relied on the concept that although nothing could move faster than the speed of light through spacetime, spacetime itself is not so restricted. We do not, in fact, have any notion of a limit to the ‘stretching’ of spacetime, a fact brought home by inflation theory, which posits an immense expansion of the early universe in a mere flicker of time. Contract the spacetime in front of a vehicle while expanding it behind and the spacecraft itself never exceeds the speed of light even though the ‘warped’ spacetime delivers it to its destination faster than would otherwise be possible.
All of which calls for immense supplies of energy, and negative energy at that, so that recent work has been (more or less unsuccessfully) devoted to understanding how to reduce those requirements to something remotely manageable. Now Finazzi and team have folded quantum mechanics into their consideration of warp drive theory, with the result that warp drive is shown to be untenable for the hapless crew. The inside of the ‘bubble’ housing the spacecraft, in fact, becomes filled with Hawking radiation, emitted by black holes due to these very quantum effects.
The Ultimate Show-Stopper?
Thus we’ve gone from shielding the vehicle from external radiation to coping with a radiation phenomenon arising from the drive itself. And in this context it’s pretty much of a show-stopper. Let me quote Adam Crowl’s treatment of this in Crowlspace, since it’s so much clearer than what I was able to come up with:
…a somewhat more serious difficulty arises because of the horizons that the warp-metric creates. These horizons act just like black-hole Event Horizons and so they produce Hawking radiation. Hawking radiation is normally quite benign as most horizons are pretty large – for example, a collapsed star’s horizon is typically 10 km across. But in a warp-metric there are two horizons – forward and aft, contracting and expanding – and to produce them very thin shells of “negative energy” are needed. And because they’re so very thin (~10-35 metres) the Hawking temperature is very, very high – i.e. a large fraction of the Planck Temperature (1032 K.) Ouch!
Ouch indeed. Add to this the other problem noted by the Finazzi team, that the bubble of spacetime we are manipulating seems itself to be unstable, too much so for our spacecraft, sitting in its zone of flat spacetime, to take advantage of the warp drive effect. Possible use of warp drive design at subluminal speeds does not seem to be ruled out, but is a definite come-down when compared to the ease with which we hoped warp drive might span the stars.
Is this the end of Alcubierre-style warp drive theory? Surely not, but it’s now up to the next round of investigators to look hard at the Finazzi results to see whether there are ways around the quantum challenge. The paper is Finazzi et al., “Semiclassical instability of dynamical warp drives,” available online.
Comments on this entry are closed.
For warp-drive or wormholes to be made real, and realistic, possibilities we need some means of creating negative energy over a larger volume than a ~Planck Scale Casimir ‘Gap’. Bob Forward imagined “negative mass” a couple of decades ago – in his mind it seemed a physically reasonable prospect. I’m stretching a long-bow to suggest it, but perhaps the large-scale voids seen between galactic clusters indicate regions of negative mass/energy. Or even the Dark Energy might provide a new physics that will make warp-drive and stable wormholes a reasonable prospect. The Cosmological Constant is typically seen to be expanding space-time just like one side of a warp-bubble.
And then there’s all the possibilities opened up by higher dimensionalities…
Quoting Paul Gilster from the main article:
“Our own Solar System lies in a cavity of low-density gas, with the
nearest ‘wall’ located about 170 light years out in the direction of the
Before it was revealed that a Bussard ramjet starship would be a
much more efficient braking system than a propulsion drive, I used
to wonder if the lack of natural fuel in the interstellar realm around
our star system was one reason why we weren’t getting visitors
who might have used a Bussard style propulsion system for their
But now maybe it is because there isn’t enough material for them
to break with is why they aren’t obviously popping by for a visit.
And will this be a problem for us when we start visiting our
Regardless of whether any kind of a warp drive ever becomes
feasible, isn’t it amazing that we are alive to be able to discuss
in serious terms a real possible FTL drive?!
If this helps any, someone recently wrote a paper about the
possibility, “at least in principle, to construct a traversable wormhole
that is stable to linearized radial perturbations by specifying relatively
simple conditions on the shape and redshift functions.”
Let’s assume negative energy&mass exist in the Great Voids. The major problem is that we can’t get there. Those regions are even further than M31.
Assuming we had a way to harness the energy requirements today to warp or bend space as described in theoretical models, I don’t think we could actually build this technology. At least not in the foreseeable future. FTL travel is pretty much still conjecture, unfortunately.
Sometimes, I wonder what the bigger problem for FTL travel is – energy requirements or know-how.
I hope the new generation of scientists will make interstellar propulsion a priority of the human race, and not just a thought experiment.
I’ve got a strong suspicion that in this universe, FTL is theoretically possible but unusable for any practical purposes.
One thing I often wonder about Alcubierre warp drive: Once you’ve “stretched out” the space behind you to travel FTL, does it stay “stretched”? Does a warp drive automatically leave a spacetime shortcut in its wake, like a Krasnikov tube? Or are you, as some descriptions have it, simply creating new spacetime behind you, so that your wake is indistinguishable from ordinary spacetime?
If the former were true, might it provide a solution to the problems detailed in the Finazzi paper? Maybe you could send a robot ship ahead to create the warp passage, something robust enough to withstand the Hawking radiation, and then follow behind in manned ships. Although that wouldn’t solve the instability problem.
Re: the instability, even if a warp bubble does collapse very quickly, how far might you travel in that fraction of a second? If it were a great enough distance, might it be possible to devise a drive that would advance you in a series of very brief warp hops one after the other?
We must not feel bad if warp drive proves untenable for the crew so far. As Paul said, this is not necessarilly a show stopper for warp drive.
Essentially unlimited gamma factors still might present them selves as alternatives in the event that FTL travel proves untenable.
We have lots of ambient energy is space ranging from starlight, CMBR, gravity waves, baryonic matter, and cold dark matter as well as hot dark matter and dark energy. I am confident that these alone could be harnessed to bring us up to arbitratilly high gamma factors.
When one takes into account that the zero point fields may have a latent energy density 120 orders of magnitude greater than that of the average 3-D volumetric mass density of our current universe, it seem like miracles could be accomplished by learning how to do reactioary propulsion powered by ZPF energy extraction or perhaps even using the ZPF as a field effect propulsion mechanism.
Then there is always the possibility that the ZPF properties could be manipulated and so alter the electromagnetic properties of space and time perhaps leading to novel ways to actually increase the speed of light, perhaps by many orders of magnitude along the path of the ship.
One of my favorites to contemplate is the notion of somehow setting up multiple connectivities between the space occupied by a space craft and the space around a star or galaxy located at a cosmic distance through space time in ordinary 4-D Einstienian Space time. The idea is that the ship would travel to the transition point in such a multiply connected set of space time regions and then suddenly enter the cosmically distant region of space time.
I can think of other potential ways around the warp drive conundrum but posting them in this set of comments would take up to much room.
The real point is take heart. We are going to the stars. Ad Astra Incrementis!
If the people can’t survive the trip, how about sending someTHING that would be useful to follow-on travelers?
1) Save time and expense by sending unmanned probes to examine target systems beforehand. It would probably be faster to have them come back, download data, and move to the next target rather than wait for a normal-space signal to crawl back.
2) Those same probes can self-replicate and set up a beamed energy or particle stream station to decelerate incoming manned spacecraft.
3) If you have the tech to create a warp bubble, why not a traversable wormhole? Drag the far end to the target system at FTL and drop through at your leisure…
Hi Chris Bennett
The Warp metric returns to flat-space outside the warp itself, so it’s not a permanent thing. And the Hawking radiation removes the stress-energy forming the warp – at such extreme temperatures the warp decay lasts only a couple of Planck-times. But that’s for a ship sized warp. The instability propagates at lightspeed and a warp 300,000 km across might last a second – though at a ridiculously large cost in energy. Conceivably a 300 metre warp might be regenerated at a million times per second, but the energy input to recreate it each time would be beyond belief.
However, as the authors point out, the temperature is related to the narrowness of a warp-metric, and that’s from using the Casimir effect to create the extreme negative energies required. If – a very big ‘if’ – the volume can be increased to something more reasonable then the temperature will go way, way down. But how do we make negative energy like that???
What about warping the shockwave in order to achieve hypersonic speed without breaking the Sound Barrier? The same way we might learn how to warp space itself for FTL.
the warp drive is a very attractive idea, since it opens up a much larger amount of the universe to exploration and transport. of course, it will remain theoretical for quite some time. but if we could even send a probe through a space bubble or wormhole, that would be highly valuable.
of course, we dont NEED a warp drive to achieve interstellar travel, although its the best possibility we know of. simply traveling at .3 c or .5 c could put several stars within our reach. however, intergalactic travel is an entirely different question, with the closest galaxy, the andromeda galaxy, being 2.5 million light years away. to get there, wed most likely need to master wormholes or some other space-time shortcut.
overall, there are many difficulties and challenges in interstellar travel, which, even with our advanced technology and concepts, we still struggle with. it is difficult… but the key is that it is not impossible.
In the event that FTL inertial travel through space leads to backward time travel, wormhole travel leads to backward or forward space time travel, etc., the notion of gradually accelerating to light speed wherein relativistic time dilation takes effect still seems philosophically useful to me.
Many backward and forward time travel schemes hold that actually traveling backward in time, or traveling forward in time and then backward to the present or past would actually dump oneself into a parallel history akin to the parallel histories of the Many World’s Interpretation of Quantum Theory. Although such histories might at first appear real, perhaps they are somehow not quite as real as the here and now history that we live in and progress toward the future in a special relativistic and non-space time warp based general relativistic way.
If we were to travel into the future to see ourselves in the future and then come back to the present, would we still not percieve the future meeting of our future selves as just another distinct person. We in theory would not percieve our selves as our future selves but would seem to be talking to a complete stranger.
In some sense, living our sub-C special and general relativistic world line seems to indicate to me that there may be more than one temporal dimension and that the sub-C special and general relativistic time we experience as the continuity of our dailly conscious lives in the here and now may have primacy over any alternate histories of general relativistically produced backward or forward than backward time travel.
Since traveling ever closer to C but less than C slows us down relative to our Earth based observers and relatives, the continuity of the relative simultaneity seems to remain intact.
There seems to be a higher meaning to time, especially to the subjectively experienced form of time in each one of our personal here and nows that trancends general relativistic abrupt space time warp based , or wormhole travel ,into the past or into the future and then back into the present or past.
Perhaps there are multiple levels of time and the primary dimension of time is that which each and everyone of us experiences as we live out our world lines within the context of the continuity in our sense of personal identity.
Well, there is another problem:
The old aphorism goes “Causality, Relativity, FTL travel: chose any two.”
You cannot have all three.
You had to drag out that stinking corpse… causality. I wonder if it’s a meaningful concept the way it’s formulated in relativity since simultaneity is already out the window.
What’s the problem with causality in relativity? Causality and simultaneity both exist in special relativity and the theory is consistent. The definition of simultaneity in relativity is, however, different from that in Newtonian dynamics.
Instead of trying to find the ultimate fuel that might get us the trillions of watts of power for warp speed, why don’t you, guys, not start another mathematical research in the area of electromagntism? It’s the cousin of gravity anyway. When you find a way to manipulate gravity with electromagnetic forces, you won’t need any magic superenergy source. Simple Polywell Fusor will be enough as power device on board.
I have just finished reading The Age of Entanglement:When Quantum Physics Was Reborn by Louisa Gilder. In this remarkable book, she recounts a story about Charles Townes, the inventor of the laser. Dr. Townes was almost discouraged by von Neumann and Bohr, two of the greatest minds of the 20th century. They assured Dr. Townes the laser was impossible, forbidden by the uncertainty principle.
Fortunately Dr. Townes perserved. Perhaps this problem with warp drive is real; perhaps not.
A thought just occurred.
Warp drives and wormholes don’t seem feasible at the moment due to energy requirements and lack of understanding. However, it has been said that everything in nature has a unique frequency. If this is true, so should distant space-time points in the fabric of the universe. Couldn’t we somehow exploit this by creating a matching resonance pattern with a distant space-time point which in effect results in a “jump” from one point in space to the other because of an imbalance? It’s highly speculative, but at least it can be experimentally falsified.
Oh wait, I actually remember someone proposed this in the 1980’s. Alan C. Holt of the NASA Johnson Space Center.
There is news!
To bigdan201: Andromeda is merely the nearest large spiral galaxy. There are at least 22 dwarf elliptical and irregular galaxies closer than it is.
To James Essig: According to Greenberger and Svozil 2005, quantum physics would constrain any time traveller to their original timeline. Since you’re part of a quantum system that’s already been measured, its wavefunction resolved into a certain history, then if you go into the past, you’re still part of that measured system and are thus constrained to perceive that set of outcomes. So there’s no way to time-travel to any quantum timeline other than the one you started from.
This is all quite fascinating, and an illustration that one model of such a system may need additional elaboration before it is more theoretically viable for human conveyance. I have yet to see any thing making the process *impossible*, merely “inconvenient”. Which is intriguing at least.
I am also reminded, after studies of the history of science with one of Kuhn’s proteges, that every time we think we have things all figured out, we are wrong. It will be equally fascinating to see what new understandings come along in the next few decades that incorporate or add deeper comprehension to our present outlook. Or overturn it altogether.
@christopher bennett, you are correct, i shouldntve forgot about all the smaller nearby galaxies. i believe the closest, the canis major dwarf galaxy is only about 25,000 ly away from us.
still, even that is a daunting distance for sublight travel. i personally have doubts about FTL travel through spacetime. it would be nice, but relativity, causality, and science in general are against it. im more optimistic about warping spacetime itself to skip across vast distances… but of course i cant guess what we might discover in the future.
Hi Christopher L. Bennett;
The paper about Greenberger and Svozil 2005 is indeed interesting.
However, I am still not convinced that one that travels back into time must necessarily stay in his own original world line.
It may be possible that our universe contains an infinite number of domains or at least an infinity scrapper number of domains wherein each domain on average has its unique laws of physics, force constants, other constants, perhaps other numbers and types of fundamental particles and the like. If the number of domains is high enough, perhaps there in an infinite scrapper or even an infinite number of domains with thermodynamic histories identical to our domain. The past throughout each such identical domain could be identical to that or ours.
Any act of time travel that would enable me to kill my own father before I was concieved might force nature in some deep way to transfer me over to an alternate domain in which my presence was already previously absent from the start, since in that domain I would have not been concieved by my father since I would have killed him before my conception. In that domain I would be percieved as an interlopper who somehow showed up on the scene while relative to my original domain I would simply show up missing at the time I left our domain.
How nature would act to preserve truth or the integrety of information by transfering me over to an alternate domain in which I was not concieved but for which I showed up one day as an interlopper is amyones guess. If we can assume the inviolatability of the conservation of electric charge or mass energy, perhaps we can assume that nature works hard to avoid self contradiction and distruction of quantum thermodynamic truth.
I had to mention one more thing while we are on the subject of time travel and warp drive.
Perhaps the mechanism that causes a time traveler to sucessfully change a world line including his involves some sort of degeneracy effect akin to the Pauli Exclusion Principle. Perhaps such a degeneracy effect from the two almost entirely identical world histories causes them to necessarily seperate and become distinct whereas in the very moment that world line altering act was being performed, the histories would be one and the same up to that point whereupon at the instant the world line altering act was completed, an act of decoherence would take place thus seperating the now distinct histories. Degeneracy effects would then seperate them as they would still be close to identicallity to the point that a Pauli Exclusion Principle would then operate.
well here it is a good discussion about warp drive and wormholes and me,lol, late to the party! what is this like the 26th reply or something!? sorry.but i just reviewed all the great things you guys had to say above.the trouble i agree with most of this stuff is that we lack the knowledge to make it all come true.not to mention the lack of the ability to generate the huge amounts of energy needed!can anybody do it though? i know i read above…maybe not.all the above was good to read and only serves once again to remind me how lucky i am to be a part of such a group as this! however,having said that,space is BIG space is OLD – who can say…nobody?! respectfully to all your friend george
June 30, 2009
Device Makes Radio Waves Travel Faster Than Light
Written by Nancy Atkinson
Polarization Synchrotron. Credit: Singleton, et al., via Current.com
A scientist has created a gadget that can make radio waves travel faster than light. Einstein predicted that particles and information can’t travel faster than the speed of light, but phenomena like radio waves are a different story, said John Singleton, who works at the Los Alamos National Laboratory.
The polarization synchrotron combines the waves with a rapidly spinning magnetic field, and the result could explain why pulsars — which are super-dense spinning stars that are a subclass of neutron stars — emit such powerful signals, a phenomenon that has baffled many scientists.
Singleton said the polarization synchrotron basically abuses radio waves so severely that they finally give in and travel faster than light. This may be what happens in pulsars, as well.
“Pulsars are rapidly rotating neutron stars that emit radio waves in pulses, but what we don’t know is why these pulses are so bright or why they travel such long distances,” Singleton said. “What we think is these are transmitting the same way our machine does.”
The device consists of a 2 meter-long gently curving arc of alumina (a dielectric material), with a series of electrodes fitted at regular intervals along its length. Applying a sinusoidal voltage across each electrode and displacing the phase of the voltage very slightly from one electrode to the next generates a sinusoidally-varying polarization pattern that moves along the device. By carefully adjusting the frequency of the voltage and the phase displacement the researchers say they can make the wave travel at greater than the speed of light. However no physical quantity of charge travels faster than light speed.
And beyond explaining what has been a bit of a mystery to the astronomical community, Singleton’s discovery could have wide-ranging technological impacts in areas such as medicine and communications, he said.
“Because nobody’s really thought about things that travel faster than light before, this is a wide-open technological field,” Singleton said.
Full article here:
My understand of the Warp Drive theory is that it would not result in “time travel” because the vehicle itself would not be undergoing any acceleration. The whole point is that spacetime itself would be moving. Like if you put a toy ship on a plate and moved the plate. The toy hasn’t moved itself, rather its enviroment has moved.
The key toward interstellar possibilities lies not with propulsion, since that only increases the limiting factors associated with matter and gravity and time, but rather with an electromagnetic dissociation from our physical space/time realm.
Much like a photon of light is able to disassociate itself from the inherent forces of mass and gravity; an interstellar ship would have to be able to mimic this feat in order to have success with interstellar voyages. Light is able to do this by having no mass and therefore is not affected by mass, gravity, or even time but only by the inherent electromagnetism in this duality type existence.
The acceleration of the universe is the result of a disparity in the levels of electromagnetism that exists within various regions of space. The outer edges moving more and more rapidly from the center. The electromagnetism which is a derivative of the force of the space/time momentum at a certain point would be greater towards the outer edges of the universe since that area is more closely tied to the initial and greater force created by the Big Bang event.
It is because light is a derivative of this that nothing can exceed the speed of light in a certain region but it is also the same reason why the acceleration of space exceeds the speed of light. Our location in space/time leaves us with our own unified electromagnetic speed derivative which appears as a constant to us since it is directly tied to the initial force created by the Big Bang.
Separating our relativistic mass from all the limitations and inherent dangers of the material space/time created by the relativistic energy-momentum of the Big Bang into the derivative quantum realm in which light travels in massless freedom will have to be performed by some trick of physics we must learn to perform.
Hawking radiation as seen by an infalling observer
Authors: Eric Greenwood, Dejan Stojkovic
(Submitted on 3 Jun 2008 (v1), last revised 8 Sep 2009 (this version, v2))
Abstract: We investigate an important question of Hawking-like radiation as seen by an infalling observer during gravitational collapse. Using the functional Schrodinger formalism we are able to probe the time dependent regime which is out of the reach of the standard approximations like the Bogolyubov method.
We calculate the occupation number of particles whose frequencies are measured in the proper time of an infalling observer in two crucially different space-time foliations: Schwarzschild and Eddington-Finkelstein.
We demonstrate that the distribution in Schwarzschild reference frame is not quite thermal, though it becomes thermal once the horizon is crossed. We approximately fit the temperature and find that the local temperature increases as the horizon is approached, and diverges exactly at the horizon. In Eddington-Finkelstein reference frame the temperature at the horizon is finite, since the observer in that frame is not accelerated.
These results are in agreement with what is generically expected in the absence of backreaction. We also discuss some subtleties related to the physical interpretation of the infinite local temperature in Schwarzschild reference frame.
Comments: Added analysis of the Hawking-like radiation as seen by an observer in Eddington-Finkelstein coordinates. Accepted for publication in JHEP
Subjects: General Relativity and Quantum Cosmology (gr-qc); Astrophysics (astro-ph); High Energy Physics – Theory (hep-th)
Cite as: arXiv:0806.0628v2 [gr-qc]
From: Eric Greenwood [view email]
[v1] Tue, 3 Jun 2008 20:03:19 GMT (310kb)
[v2] Tue, 8 Sep 2009 21:18:07 GMT (1831kb)
You had to drag out that stinking corpse… causality. I wonder if it’s a meaningful concept the way it’s formulated in relativity since simultaneity is already out the window.
Light is able to do this by having no mass and therefore is not affected by mass, gravity, or even time but only by the inherent electromagnetism in this duality type existence.
I’ve got a strong suspicion that in this universe, FTL is theoretically possible but unusable for any practical purposes.
“I’ve got a strong suspicion that in this universe, FTL is theoretically possible but unusable for any practical purposes.”
Now that would be annoying!