SETI always makes us ask what human-centered assumptions we are making about extraterrestrial civilizations. When it comes to detecting an actual technology, like the starships we’ve been talking about in the last two posts, we’ve largely been forced to study concepts that fit our understanding of physics. Thus Robert Zubrin talks about how we might detect a magsail, or an antimatter engine, or a fusion-powered spacecraft, but he’s careful to note that the kind of concepts once studied by the Breakthrough Propulsion Physics Project at NASA may be undetectable, since we really don’t know what’s possible and what its signature might be.
I mentioned zero-point energy in a previous post because Zubrin likewise mentions it, an idea that would draw from the energy of the vacuum at the quantum level. Would a craft using such energies — if it’s even possible — leave a detectable signal? I’ve never seen a paper on this, but it’s true that one classic paper has looked at another truly exotic mechanism for interstellar travel, the wormhole. These shortcuts through spacetime make space travel a snap. Because they connect one part of the universe to another, you go in one end and come out the other, emerging into another place and, for all we know, another time.
The fact that we don’t know whether wormholes exist doesn’t mean we can’t think about how to detect one, although the authors of the classic paper on wormhole detection make no assumptions about whether or not any intelligent species would actually be using a wormhole. The paper is “Natural Wormholes as Gravitational Lenses,” and it’s no surprise to find that its authors are not only wormhole specialists like Matt Visser and Michael Morris, but physicists with a science fiction connection like John Cramer, Geoffrey Landis, Gregory Benford and the formidable Robert Forward.
Image: A wormhole presents a shortcut through spacetime. Can one be detected? Credit: Wikimedia Commons.
The analysis assumes that the mouth of a wormhole would accrete mass, which would give the other mouth a net negative mass that would behave in gravitationally unusual ways. Thus the GNACHO (gravitationally negative anomalous compact halo object), which playfully echoes the acronym for massive compact halo objects (MACHOs). Observationally, we can look for a gravitational lensing signature that will enhance background stars by bending light in a fundamentally different way than what a MACHO would do. And because we have MACHO search data available, the authors propose checking them for a GNACHO signature.
In conventional gravitational lensing, when a massive object moves between you and a much more distant object, a greatly magnified and distorted image of the distant object can be seen. Gravitational lensing like this has proven a useful tool for astrophysicists and has also been a means of exoplanet detection. But when a wormhole moves in front of another star, it should de-focus the light and dim it. And as the wormhole continues to move in relation to the background star, it should create a sudden spike of light. The signature, then, is two spikes with a steep lowering of light between them.
The authors think we might find the first solid evidence for the existence of a wormhole in our data by looking for such an event, saying “…the negative gravitational lensing presented here, if observed, would provide distinctive and unambiguous evidence for the existence of a foreground object of negative mass.” And it goes without saying that today’s astronomy, which collects information at a rate far faster than it can be analyzed, might have such evidence tucked away in computer data waiting to be discovered by the right search algorithms.
Would a wormhole be a transportation device? Nobody knows. Assuming we discover a wormhole one day, it would likely be so far away that we wouldn’t be able to get to it to examine its possibilities. But it’s not inconceivable that a sufficiently advanced civilization might be able to create an artificial wormhole, creating a network of spacetime shortcuts for instantaneous travel. Matt Visser has discussed a wormhole whose mouth would be held open by negative energy, ‘…a flat-space wormhole mouth framed by a single continuous loop of exotic cosmic string.’ A primordial wormhole might survive from the early universe. Could one also be created by technology?
Civilizations on the Brink
More conventional means of transport like solar or laser-powered sails present serious problems for detection. In Jerry Pournelle and Larry Niven’s The Mote in God’s Eye, an alien lightsail is detected moving at seven percent of the speed of light, its spectrum the same as the star that it is approaching but blueshifted, which is how analysts have determined it is a sail. The novel’s detection occurs with far more sophisticated observatories than we have in our day, when finding a solar or lightsail in transit would be a tricky thing indeed. A fusion rocket, for example, would emit largely in the X-ray range and could be detectable for several light years, but a lightsail is a highly mutable catch.
I remembered reading something about this in Gregory Matloff’s Deep Space Probes (Springer, 2005) and checked the book to extract this:
If ET prefers non-nuclear travel, he might utilise a laser or maser light sail. If the starship is near enough and the laser/maser is powerful enough, reflections from the sail might be observable as a fast-moving and accelerating monochromatic ‘star.’ However, detection will depend on sail shape and orientation as well as other physical factors.
Therefore, it is not as easy to model the spectral signature of these craft as it is energetic nuclear craft. A starship accelerated using lasers or masers may be easier to detect during deceleration if a magsail is used.
Writing in the comments to yesterday’s post, Centauri Dreams reader James Jason Wentworth recalls Larry Niven’s short story “The Fourth Profession,” which has a lightsail detection something like the one in The Mote in God’s Eye:
“All right. The astronomers were studying a nearby nova, so they caught the intruder a little sooner. It showed a strange spectrum, radically different from a nova and much more constant. It got even stranger. The light was growing brighter at the same time the spectral lines were shifting toward the red.
“It was months before anyone identified the spectrum.
“Then one Jerome Finney finally caught wise. He showed that the spectrum was the light of our own sun, drastically blue-shifted. Some kind of mirror was coming at us, moving at a hell of a clip, but slowing as it came.”
Some sails could be truly gigantic, and we can imagine worldships large enough to require sails the size of a planetary radius, which could be detected when near their home or destination stars, but would be hard to find when in cruise. Matloff goes on to suggest that any search for this kind of ship should look near stars from which an entire civilization might be emigrating. A star like Beta Hydri is a possibility, a nearby (21 light years) solar-type star now expanding from the main sequence. This is the longest shot of all, but finding unusual signatures in visible light near a star leaving the main sequence would at least compel a second look.
The wormhole paper is John Cramer, Robert L. Forward, Gregory Benford et al., “Natural Wormholes as Gravitational Lenses,” Physical Review D (March 15, 1995): pp. 3124-27 (available online). See also Matloff and Pazmino, “Detecting Interstellar Migrations,” in Astronomical and Biochemical Origins and the Search for Life in the Universe, ed. C. B. Cosmovici, S. Bowyer and D. Werthimer, Editrici Compositori, Bologna, Italy (1997), pp. 757-759.
Eniac
“As it turns out, and you apparently agree, our common goal of 0.2c can be achieved either by
1) A 50% effective fusion rocket and a 1:1000 mass ratio, about 100x100x100 m in size fully fueled, or any non-cubic variations of that.
2) A unspecified atomic rocket fueled from flying buckets accelerated and filled using an arrangement of many neutral particle guns of unheard-of beam intensity which at least according to your previous descriptions are about Earth sized. If you have thought of smaller ones, do tell how long they are now.
I am tiring of my efforts to let you see the absurdity of your design.”
I agree, with several addendums:
The “unspecified atomic rocket” can be a fission rocket, within our present technological means;
I will comment shortly about the required precision of the accelerator; and later with regards to its size;
The accelerator I presented is far closer to our current technological level than a 50% efficient fusion rocket; if it is “absurd”, then the rocket is even more so.
“One light minute is 2*10^10 meters. Assuming your buckets are no more than a meter across, you’d have to have an aiming accuracy of 5*10^-11. Groping for an analogy, I think this corresponds roughly to hitting a quarter on the moon, shooting from Earth.”
About the container:
I took one as being 100 g heavy and carrying 10 kg worth of fuel. At this weight, 10 tonnes worth of containers can carry 1000 tonnes worth of fuel, enough to accelerate a 100 tonnes ship to a delta v of 0,2c.
I can make a container be 500 g heavy and carry 10 kg worth of fuel; 50 tonnes worth of containers is more than acceptable for carrying 1000 tonnes worth of fuel.
About precision:
Let us say that the magnetic/electrostatic sail of the container has a radius of 10 meters.
Let us make two accelerator stations 1 light-minute distant. The fuel particles cross that distance, at 0,1c, in 10 minutes.
The sideways velocity of the fuel particles must be at most 10 meters per 600 seconds = 1,66*10^-2 m/s.
This needed precision is FAR BELOW your stated figure of 5*10^-11, Eniac.
And the needed aiming accuracy can be made even less precise:
We arrange the accelerator stations in two curved rows; in each row, the accelerator stations are 1 light-second (300000 km) apart.
The fuel particles will cross this distance in 10 seconds at 0,1c. For a magnetic/electrostatic sail of the container with the radius of 10 m, the maximum sideways velocity of the fuel atoms must be below or equal to 1 meter/second.
At this sideways velocity, I no longer need to turn the atoms exiting the accelerator into dust. I only need to neutralize the atoms and focus them with a few very small lenses.
The container will, of course, follow a circular path in a magnetic field; but 2 large segments of this circular path will have no accelerator stations placed on them.
“I am curious about what this mysterious arrangement might be. Are you keeping it secret?”
Not at all:
You take a conducting plate, bend it into a cylinder, without closing the cylinder (viewed in cross-section, this conducting plate will stretch 1/2 of the cylinder).
For the remaining 1/2, you take another conducting plate and bend it – in cross-section, it will describe 1/2 of a larger cylinder.
Both these conducting plates are electrically positively charged. The one describing 1/2 of a larger cylinder will be charged to a somewhat higher field strength then the other. The purpose is – at specific radius from the center of the atom beam, the positive electrostatic fields generated by the 2 plates must have the same value.
These positive electrostatic fields must be 2*the strength of the electrostatic field generated by the atom beam. The electrostatic field of the atom beam will interact with them – the strength of the electrostatic fields generated by the atom beam and the plates will become equal.
Another approach:
You take an insulator in the form of a cylinder, with breakdown voltage above the electrostatic field generated by the atom beam.
You apply a voltage higher than the breakdown voltage to the insulator, charging him uniformly, positively to the level of the electrostatic field generated by the atom beam.
“Just to be specific: “magical lenses” is backed up by the law of conservation of radiance, “buckets blown up by particle beams” is backed up by a calculation of the power impinging on such buckets, and the “expanding cloud of plasma” is backed up in several instances by realistic calculations of the energy that would be released in designs that you have put forward.”
About the lenses – I backed them up by showing the laws their operations are based upon – Faraday’s law of induction; Lenz’s law (and I do intend to derive/post the equations of motion for the fuel particles when time permits).
Also – conservation of radiance does not prevent the lenses from functioning.
About the containers – I answered your objection by indicating that once could send few particles to the container at the beginning of the acceleration (when the difference in velocity between the container and the particles is large AKA the kinetic energy of the particles is large). The number of particles sent will increase as their kinetic energy relative to the container decreases (the transferred momentum increasing).
About Coulombic repulsion – I dispersed the atom beam until the electrostatic field of the atom beam became manageable.
This configuration will create a transverse field, from the lower potential plate to the higher potential one. It will not focus the beam, but bend it. Right into the wall, resulting in a rapidly expanding cloud of plasma.
This will result in zero field, just as in the case of the conductive cylinder, because the insulator is charged uniformly. The beam will diverge, hit the wall, and yield a rapidly expanding cloud of plasma.
Judging from these proposals, you do not have a proper grasp of Gauss’s law or Maxwell’s equations. I suggest “The Feynman Lectures on Physics: Volume 2, Electromagnetism and Matter” (http://en.wikipedia.org/wiki/The_Feynman_Lectures_on_Physics#Volume_2._Mainly_electromagnetism_and_matter)
I am looking forward to this excitedly. I really hope you get around to it. I also hope it comes out better than your above field configurations.
I beg to differ. You are proposing to increase beam radiance (to a degree that would make a laser blush) with lenses. The law says radiance is, at best, conserved. Something has to give.
“This configuration will create a transverse field, from the lower potential plate to the higher potential one. It will not focus the beam, but bend it. Right into the wall, resulting in a rapidly expanding cloud of plasma.”
Not if the higher potential plate is further away from the atom beam than the lower potential plate.
Why that? So that the conducting plates do not touch each other, making a Faraday cage.
Of course, the distance/strength of electric field of the conducting plates must be calculated so as to ensure that in the atom beam pipe (the atoms should stay inside a calculated cylinder with the radius a little smaller then the radius of the cylinder described by the closer conducting plate), the electric potential from both conducting plates is the same.
“This will result in zero field, just as in the case of the conductive cylinder, because the insulator is charged uniformly. The beam will diverge, hit the wall, and yield a rapidly expanding cloud of plasma.”
An insulator reacts differently from a conductor to charge, Eniac:
With an insulator, the charge does not go to the outer part of the material, but stays inside it.
You would only get a zero field inside if the insulator was shaped as an hollow sphere.
In this case, the insulator is shaped as a cylinder.
“I beg to differ. You are proposing to increase beam radiance (to a degree that would make a laser blush) with lenses. The law says radiance is, at best, conserved. Something has to give.”
Does the law of conservation of radiance considers and incorporates the effects of EMF/Lenz’s law on atoms/particles?
If not, it does not prevent my lenses from functioning.
And – an accuracy of 1 m/s “makes a laser blush”? Really?
Well, yes, really. It would be a proud laser whose beam diverged only 1 meter over a 300,000 km distance. And you must have made a mistake somewhere with your 1 m/s number, because a light minute is a lot longer than 300,000 km. 60 times longer, to be exact. A beam at 0.1 c with +/- 1 m/s transverse velocity would spread to a diameter of 1,200 m over that distance.
Here you go again. Your concrete proposals do not work out, so you resort to the “so as to” approach, without specifying what “so” might be. You could save yourself a lot of trouble if you understood the general laws governing fields. Gauss’s or Maxwell’s, in this case. You would then not have to keep chasing after the absurd.
For more on the blushing laser, see this quote from here:
http://www.rp-photonics.com/beam_divergence.html
You can see that even a perfect laser will have a divergence of 0.34 mrad, which corresponds to a “sideways velocity” of 0.00034 c = 10 km/s. So we are talking a blush factor of 4 orders of magnitude, plus an additional 3 orders for your apparent miscalculation of the actual accuracy needed to focus a beam on a bucket from a light minute away.
“And you must have made a mistake somewhere with your 1 m/s number, because a light minute is a lot longer than 300,000 km. 60 times longer, to be exact.”
There’s no mistake.
From my last post:
‘We arrange the accelerator stations in two curved rows; in each row, the accelerator stations are 1 light-second (300000 km) apart.
The fuel particles will cross this distance in 10 seconds at 0,1c. For a magnetic/electrostatic sail of the container with the radius of 10 m, the maximum sideways velocity of the fuel atoms must be below or equal to 1 meter/second.
[…]
The container will, of course, follow a circular path in a magnetic field; but 2 large segments of this circular path will have no accelerator stations placed on them.’
If, when exiting the accelerators&lenses, the fuel atoms/particles have a sideways velocity of 1 m/s (which is, relatively speaking – with regards to fine tunning -, quite large AKA achievable), these fuel atoms/lenses will diverge FAR less than light.
Eniac, you previously made the argument that light is better than particle beams for transporting momentum, due to dispersion issues; you just debunked that argument.
Ah, it seems I skipped one iteration in your design, I could swear it was a light minute, before. So, you can take the 3 orders of magnitude, but I will keep the 4. It does not really change anything.
Debunk it? Me? I did no such thing. Far from it. I stand by my argument. The degree of collimation you expect from your particle beams (which, unlike laser light, are not coherent) is absurd.
The buckets have somehow now became 10 m across, not a small feat for a 100 g container that can hold 10 kg of material. Perhaps you could comment on the new shape. Are we talking giant dinner plates, now?
This is another whopper I have not yet commented on.
I think you should elaborate on the amount of charge you put on the buckets, the radius of curvature, the Field strength necessary to achieve it at 0.1 c, and the nature of the magnets that will create this field while admitting a 10 m sized bucket (or dinner plate, or whatever the new shape is). You might also want to roughly estimate the number of these giant magnets needed to close the circle, as well as their mass and cost. Numbers, if possible, not waving hands and squishy words like “quite large”, “achievable”, “FAR less”, etc.
If you do this, you might convince yourself that this is not the way to go, a feat I seem incapable of.
“The buckets have somehow now became 10 m across, not a small feat for a 100 g container that can hold 10 kg of material. Perhaps you could comment on the new shape. Are we talking giant dinner plates, now?”
In the same post in which I made the accelerator stations 300000 km distant, I made the containers 500 grams heavy – resulting in 50 tonnes worth of containers to carry 1000 tonnes worth of fuel (per 10 years).
“Debunk it? Me? I did no such thing. Far from it. I stand by my argument. The degree of collimation you expect from your particle beams (which, unlike laser light, are not coherent) is absurd.”
I expect my particles to have a sideways velocity of, at most, 1 m/s – that is a macroscopic value (by a quite a margin), achievable (probably, if we use atoms, as opposed to dust).
At this macroscopic value, the degree of collimation of my particle beam is 4 orders of magnitude better than that of a laser.
About the Lorentz force turning:
The 500 g container will have an electrostatic sail of ~450 g. The sail will be composed of a 10 micrometer wires, radially expanding from the center.
The linear mass density of the wires is ~10^-7 kg/m.
A beta radiation emitter can apply voltage to the wires of up to 10^6 V. Capacitance is ~10^-12 f/m. Charge is voltage*capacitance.
q/m is ~10 coulomb/kg.
Jupiter’s magnetic field of 10^-3 tesla.
Gyration frequency is B*(q/m).
Gyration frequency is ~10^-2 per second. One complete turn around Jupiter in 100 seconds, 3000000 km.
Aong these 3000000 km, the 10000 accelerator stations will be places – one each 300 km (not 300000 km). The container will go from one accelerator station to the next in 0,01 seconds. The accuracy of the fuel atoms (we don’t need to turn them into dust any longer) is 10/0,01 m/s = 1 KM/S.
Eniac, that’s FAR better than we need.
And, if we don’t want to use Jupiter, we could just take 10 tesla magnets, formed as rings with a radius of 10 m. The magnetic fields they produce, together, must be 300 km in length.
We arrange these magnets around a circular path.
Ah, now we are talking. The Lorentz Turner around Jupiter. That one is a LOT better than all your previous ideas combined. As it happens, I did a related calculation just a few days ago. I got a different result, requiring 1,000 C/kg for 0.1c, not 10. Not sure what the discrepancy is. In any case, that is a very interesting idea, especially with the following simplifications:
1) Use solar sails to do the accelerating, or maybe even electromagnetic disturbances from the moons. Surfing, if you will, on the moving magnetic fields around Jupiter. No accelerator devices needed. Also a possibility: Use a solar powered electron gun to create both the charge and propulsion.
2) Don’t bother with nuclear rockets and buckets of fuel, simply accelerate your probe that way to begin with. You can slow down the same way at the target system, if you pick one with a suitable highly magnetic gas giant.
Apart from Jupiter, the solar magnetic field could also be used for Lorentz surfing. A forced solar orbit requires 10-100 times larger charge than around Jupiter, but would make much more solar energy available for propulsion and charging. It would also help with the radiation problem which is said to be quite brutal around Jupiter.
Much better. However, it would still require the molecular beam to be collimated about the same as a laser. Not feasible, I think. Also, perhaps not needed, see above.
A magnet with 10 Tesla in a 10 m opening would be a truly magnificent beast, but not entirely out of the question, I suppose. 30,000 of them would probably be difficult to procure, on any sort of budget. Luckily, we have Jupiter.
I think I know where at least part of the discrepancy between our Lorentz force calculations might come from. You say a turn around Jupiter is 3,000,000 km, but Wikipedia gives a value for the radius of 70,000 km, which would make the circumference about 500,000 km.
Ah, I think you also left out a 2*Pi from your gyration frequency calculation. That would go most of the way towards reconciling our results, with a somewhat smaller value for the magnetic field I used (5*10^-4) perhaps accounting for the rest.
I also did a calculation similar to yours about the charge/mass ratio. I obtained 10,000 C/kg at 1 MV on wire that is 1 micrometer thick. For 10 micrometers I would have gotten 100 C/kg, an order of magnitude better than your result. Not sure what the discrepancy is here. In any case, it appears that with really thin wire we would theoretically be able to sustain a 0.1 c Lorentz orbit around Jupiter, and perhaps even the sun.
If the wire doesn’t break from the electrostatic self-interaction, that is. There will be a minimal thickness for this not to happen which needs to be determined for candidate materials. Carbon fiber will likely come out on top.
About the Lorentz turning:
I took the wire to have a 10 micrometer diameter; its mass density to be ~10^-7 kg/m.
I took the voltage applied to be 10^6 V; the capacitance to be ~10^-12 f/m (this is the suspect for the difference in our calculations; I couldn’t find anywhere the capacitance of a 10 micrometer wire in the interstellar medium; I only approximated based on what I could find).
I took Jupiter’s magnetic field to be of 10^-3 tesla.
I took the gyration frequency to be =B*(q/m).
The result of these premises: Gyration frequency is ~10^-2 per second.
Consequently, one complete turn around Jupiter is in 100 seconds, 3000000 km.
“”The accuracy of the fuel atoms (we don’t need to turn them into dust any longer) is 10/0,01 m/s = 1 KM/S.”
Much better. However, it would still require the molecular beam to be collimated about the same as a laser. Not feasible, I think. Also, perhaps not needed, see above.”
Eniac, 1 m/s sideways velocity should be achievable; 1 km/s even more so.
You disagree because this means particle beams can surpass lasers with regards to collimation. I don’t view this as a show-stopper; so what if they surpass lasers? Lasers have no mystical secure first place on this issue.
“”And, if we don’t want to use Jupiter, we could just take 10 tesla magnets, formed as rings with a radius of 10 m. The magnetic fields they produce, together, must be 300 km in length.”
A magnet with 10 Tesla in a 10 m opening would be a truly magnificent beast, but not entirely out of the question, I suppose.”
The issue is: at what magnetic field strength is the magnet mass/field strength most favorable.
For example, I could use 5 tesla magnets (formed as 10 m radius circles). And the total length of the magnetic field produced is 600 km:
Notice, that’s the added length of the magnetic fields produced (which have an average of 5 tesla), NOT the added width of the magnets.
About acceleration/deceleration:
By using the surprisingly potent Lorentz force, we can have our 100 tonnes ship rotating around Jupiter. This could also be around the Sun.
Or traveling near artificially built high-power magnets – these cannot be circles with a 10 m radius any longer) rather, they should be stations, their positions describing a circle far larger than 3000000 km, (as I already said, we have enough space).
The problem of acceleration/deceleration remains:
Solar sails are FAR too slow-acting (you would wait decades in order to accelerate; the same for deceleration). A fission rocket with a top speed of 0,05c would be, per total, faster.
Lasers, at such short ranges, are a possibility (little diffraction), but are FAR more energy intensive than particle beams for the same acceleration. More importantly, they CANNOT BE USED FOR DECELERATION.
I remain of the opinion that particle beams should be used.
Why?
Acceleration:
THE SHIP can be accelerated by the kinetic energy of particles. When the ship has reached 0,1c, we can consider the acceleration problem closed.
Deceleration:
Afterwards, fuel (thorium atoms or lithium deuteride) will be sent to the ship. IN THE SHIP, most fuel will be ‘packaged’ inside containers (no more fine-tunning work with the containers). Some fuel will be kept inside the ship.
Full containers (containing 10/100/1000 kg of fuel each) will be released from the ship; they will also have an electric charge sufficient to circle at 0,1c, along with the ship.
When the ship is targeted toward alpha centauri, it will cancel its charge and fly at 0,1c. The fuel containers – which will be used for deceleration – will be sent shortly after it, by canceling their charge.
The ship will use the fuel on board for maneuvering and for the initial deceleration.
Afterwards, it will wait for the containers – now faster – to catch up with it, in order to decelerate further.
AN EFFICIENT INTERSTELLAR SHIP – FINAL SYSTEM:
Three components are needed:
1. Lorentz force turning: the ship at o,1c – or even 0.2c to 0,5c – must rotate on a fixed trajectory around Jupiter, the Sun or using the interstellar magnetic field.
2. The ship must generate a positive electrostatic field (generated by one grid; by two grids, in a capacitor-like arrangement) capable of stopping positively charged atoms at a relative velocity of 0,1c – or 0,2c to 0,5c.
3. Containers that can carry 1-100 kg of fuel. They will be collected by the ship and used one at a time.
FOR 0,1C
Acceleration:
A 600 tonnes ship (100 tonnes payload + 500 tonnes fuel) flies in a circle due to the Lorentz force. Along this circle are placed STATIONARY containers (containing fuel atoms; the containers will charge the fuel atoms inside – with energy received from an external source – and release them when the ship approaches).
The ship will collect the atoms from a container, use them, then collect the atoms from the next container, etc.
From the POV of the ship: the electrostatic field of the ship will decelerate these fuel atoms from 0,1c to 0 when they are collected by the ship; then it will accelerate the fuel atoms from 0 to 0,1c when they are ejected by the fission engines of the ship.
No momentum is gained by the ship from this operation.
While in the ship, the fuel undergoes fission – and this accelerates the ship. With a fuel effective exhaust velocity of 0,02c, 3000 tonnes of fuel must be so used in order to bring the 600 tonnes ship to 0,1c.
Deceleration:
The ship is 600 tonnes heavy – 100 tonnes payload and 500 tonnes fuel. The 500 tonnes fuel are placed in containers.
Shortly before it reached 0,1c, the payload will release the 500 tonnes worth of fuel containers on a trajectory to alpha centauri (or another star).
After it reached 0,1c, the 100 tonnes payload will stop circling on the Lorentz force trajectory and set course for alpha centauri (or another star).
En route, the payload will overtake the containers.
When deceleration is to begin, the payload (100 tonnes, among which 1 tonne is fuel) will decelerate by using the 1 tonne of fuel to a velocity smaller than the velocity of the containers.
The containers will reach the payload one at a time and will be used by the payload 1 at a time.
Each container will charge the atoms it contains (with energy received from the payload) and release them toward the payload. The fuel will undergo fission, being used by the ship to decelerate – 500 tonnes will bring the ship from 0,1c to 0.
CONCLUSION:
For a mere 3500 tonnes of fuel, I can give a 100 tonnes ship a delta v of 0,2c (top speed of 0,1c).
Obviously, this method can be used in order to reach a delta v greater than 0,1c (perhaps significantly greater).
Its limits – the capacity of the electrostatic field generated by the ship.
You must have missed my posts on these:
This should be B*(q/m)/(2*Pi)
I get 500,000 km, not 3,000,000.
The equatorial field is more like 5*10^-4.
These three together explain the difference of two orders of magnitude, unfortunately not in our favor.
To catch protons at 0.1 c, you would need to charge your catcher to 100 MV. The power needed to keep up that charge may well be prohibitive. For heavy nuclei with 2-3 elementary charges it would be 10 GV. And there are fine lines between losing the fuel into space, catching it gently, or damaging the craft by violent impact.
It would be much nicer if we could use some means other than a nuclear rocket driven by fuel caught from space. Using natural electric or magnetic fields around Jupiter or the sun for acceleration as well as turning would be ideal. Solar power in a solar orbit could also be considered, either in the form of a sail, or with a solar powered electron beam. We have a few years to get up to speed, so I am not convinced that solar power is “FAR too slow-acting”. You could get pretty close to the sun to get more power plus a stronger field.
The great advantage of self-sufficient methods like that is that they can be used for both departure and arrival, as both fields and light will be found at the target as well as the source. At the target there is also the possibility of braking passively through the plasma pressure on the charged craft.
As I said, in order for my system to work, I need:
1.Lorentz turning (around the Sun, preferably) – and, in this thread, we established this is possible at 0,1c.
2.Electrostatic fields that can stop fuel atoms with a relative velocity of 0,1c (or more.
With regards to this, the paper D. Whitmore – “Relativistic Spaceflight and the Catalytic Nuclear Ramjet”* contains the necessary calculations – showing that this is possible at relative velocities greater than 0,1c (by quite a margin).
3.Containers that carry the fuel along a trajectory and charge/release it when opportune.
All three requirements are met.
As such, my system is feasible. As for performances:
For 3500 tonnes of fuel, I can give a 100 tonnes ship a delta v of 0,2c (top speed of 0,1c). Acceleration/deceleration time: a few months.
For 35000 tonnes of fuel, I can give a 1000 tonnes (YES – 1000 tonnes) ship a delta v of 0,2c (top speed of 0,1c). Acceleration/deceleration time: a few months.
*link: http://www.google.ro/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CBwQFjAA&url=http%3A%2F%2Fwww.askmar.com%2FRobert%2520Bussard%2FCatalytic%2520Nuclear%2520Ramjet.pdf&ei=WyueUJaBI8fGtAbV3YHQCQ&usg=AFQjCNGiTduxHP49xSi6MeKmr1lsYVDRDg
Eniac
Your ships – using sunlight/etc – reminds me of sail ships.
They are smaller, slower (for a 100 tonnes ship you would need ~a decade to accelerate to 0,1c; 1000 tonnes ships need too much time for acceleration to 0,1c to be efficient).
But they are…poetic, living off the land, able to soar alone among the stars.
Perhaps they will come first. An interstellar age of sail and exploration.
My ships – using fuel – remind me of steam ships.
They are faster, far larger, amenable to tighter control. With 35000 tonnes 1000 tonnes ships can accelerate to 0,1c in a few months; a trivial amount of fuel and lack of megaengineering, by comparison to any other interstellar strategies humanity, until now, came up with.
But they need an industrial infrastructure for fuel (although, considering the small amount, the crew should be able to synthesize it themselves). They are the tools of commerce and colonization, traveling the roads of empire, of expansion among the stars.
An interstellar age of steam. Spreading humanity among the stars.
PS
About Lorentz force turning – what value did you used for capacitance (the terms for the charge/mass ratio, in general), Eniac?
I would not use such strong words. We covered a few obvious obstacles, but the next limitation we identified is tensile strain on charged wires, which we have nothing on. Plus, field emission, which would be another one that needs to be addressed.
I think this paper is quite far from showing feasibility. The way I read it, he says a voltage of 9 GV is necessary, and that there is sufficient energy, but not a single word about how to create and maintain the charge. Note that the negatively charged grid he describes is subject to field emission of electrons, presumably at much lower voltages than this. It is also subject to erosion by impacts from the protons it is supposed to stop.
I fail to see where you get these numbers from. At the very least, the time it takes to accelerate should not change with the ship’s mass, if the collector surface is scaled accordingly.
Yes, that and, in addition, a scheme to supply coal to them from land using catapults.
The formula for the electric field of a line charge is simple, and although the potential diverges, it does so only weakly. With an assumption on the Debye length (~7 m is what I used) as the outer boundary for the potential, you can calculate the voltage as a function of linear charge density. The dependence on the Debye length is logarithmic (i.e. weak).
See http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elecyl.html
I have not been able to figure out how to calculate the strain on a charged wire. I think you have to take the volume distribution into account, as a line charge appears to have infinite strain on it.
Eniac
“We covered a few obvious obstacles, but the next limitation we identified is tensile strain on charged wires, which we have nothing on. Plus, field emission, which would be another one that needs to be addressed.”
About the Lorentz force:
Tensile strength – 10 micrometer wires should be sturdy enough.
BTW, you still haven’t told me what values you used for your q/m ratio.
Field emission becomes a problem beyond 10^9 V/m.
At o,1c, lorentz turning can be achieved with fields in the megavolts range.
Of course, we may want to exceed 0,1c – at which point, in order to prevent field emission, the charged wires could be coated in insulating material.
“The way I read it, he says a voltage of 9 GV is necessary, and that there is sufficient energy, but not a single word about how to create and maintain the charge. Note that the negatively charged grid he describes is subject to field emission of electrons, presumably at much lower voltages than this. It is also subject to erosion by impacts from the protons it is supposed to stop.”
About erosion – the grid will stop fuel atoms targeted precisely to specific areas (by their containers). As such, erosion will be prevented.
About field emission – coat the wires in insulating material, increasing the barrier strength, decreasing the probability of tunneling for the electrons.
“a scheme to supply coal to them from land using catapults.”
Which I have provided.
“I fail to see where you get these numbers from. At the very least, the time it takes to accelerate should not change with the ship’s mass, if the collector surface is scaled accordingly. ”
At 100 tonnes, in order to bring the ship to 0,1c (and the corresponding kinetic energy) by using solar light, how large must be the sails?
Enormous – if your ships rotates around the Sun via Lorentz force – meaning, it can’t get too close.
Increasing the already enormous sails 10fold – for a 1000 tonnes ship – can prove extremely difficult (and expensive with regard to mass).
In my system, on the other hand, I don’t have to increase the size of the electrostatic field grids at all to transition from a 100 tonnes ship to a 1000 tonnes one.
Oh, really? What gives you the confidence? And how sturdy is sturdy enough, anyway? Will pretty sturdy do? Extra sturdy would probably be better, and enormously sturdy would surely save the day…
You are confusing field and potential. The problem with the fine wires is that on a scale of micrometers, megavolts can give rise to fields of 10^12 V/m. In fact, field emission happens at just a few volts on atomically fine tips. Luckily, field emission of positive charges is 3-4 orders of magnitude less likely, so we might just get away with a positively charged thin wire.
I marvel at your ability to draw quantitative conclusions (100 is ok, but 1000 is not) from such accurate assumptions as “enormous” and “extremely difficult”. The fact is, with solar power, acceleration depends on the power to weight ratio, not total weight.
The closer the better for Lorentz turning. Dipole fields generally decrease with he third power of distance. The limit is when it gets too hot.
“The closer the better for Lorentz turning.”
The radius of Lorentz turning depends on the magnetic field and on the electrostatic charge of the ship.
With q/m of 100 c/kg and a magnetic field of 6nT, the orbit of a ship around the sun is quite a ways off from the Sun.
I marvel at your ability to draw quantitative conclusions (100 is ok, but 1000 is not) from such accurate assumptions as “enormous” and “extremely difficult”. The fact is, with solar power, acceleration depends on the power to weight ratio, not total weight.
“The closer the better for Lorentz turning.”
The radius of Lorentz turning depends on the magnetic field and on the electrostatic charge of the ship.
With q/m of 100 c/kg and a magnetic field of 6nT, the orbit of a ship around the sun is quite a ways off from the Sun.
“I marvel at your ability to draw quantitative conclusions (100 is ok, but 1000 is not) from such accurate assumptions as “enormous” and “extremely difficult”. The fact is, with solar power, acceleration depends on the power to weight ratio, not total weight.”
About this, Eniac – you told me a few times to come with the calculations.
You have proposed your sunlight-driven ship, but came with no calculations to see the power required to accelerate 100 tonnes to 0,1c – and how large the sail must be to capture this much power from light.
I predict the size of this sail will be ENORMOUS.
“Oh, really? What gives you the confidence? And how sturdy is sturdy enough, anyway? Will pretty sturdy do? Extra sturdy would probably be better, and enormously sturdy would surely save the day”
The energy of a capacitor is 1/2 C V^2 or 1/2 Q^2/C. As before, C = e*l, where e = 2 pi eps0 / ln (a2/a1). e is about 10^-12.
The strain on the wire will be the derivative of the energy by length, i.e. F = (1/e) Q^2/L^2 = 1/e q^2, where q is the linear charge density in C/m.
This strain must be, at most, equal to the yield strength of the wire which is given by the specific strength S, density rho and cross section a as Fy = S*rho*a. The maximum specific strength of known materials goes up to about 6*10^6 m^2/s^2.
When the numbers are run, a 10 micrometer wire should be sturdy enough.
“The problem with the fine wires is that on a scale of micrometers, megavolts can give rise to fields of 10^12 V/m. In fact, field emission happens at just a few volts on atomically fine tips. Luckily, field emission of positive charges is 3-4 orders of magnitude less likely, so we might just get away with a positively charged thin wire.”
Field emission is due to quantum tunneling of electrons/protons.
The probability of electrons/protons tunneling decreases as the barrier strength increases.
As said – coating the wires with insulator will increase the strength of the barrier and reduce/eliminate field emission.
I think what you are missing is that the magnetic field depends on the distance, too. Closer in, the magnetic field is stronger, which more than compensates for the higher acceleration required.
Solar sails have been done over many times. I do not need to provide calculations, you can just look it up.
Sure it will be enormous. Like the array of wires that generates the Lorentz force, which is also ENORMOUS. The point is, the mass of the sail/collector is always the same fraction of the mass of the ship.
I think the whole assembly will need to be very long and snakelike, in order to keep the forward cross-section low so it will not be slowed down too much by the plasma which it is repelling due to the large positive charge.
I think this will be of no help whatsoever.
Insulators have no more or less of a work function than conductors. The surface field will be reduced, but only by the same amount as it is when you simply make the wire thicker. And, of course, heavier, which you cannot afford.
Another way to look at this is: There is no better insulator than vacuum, so what good would adding a different insulator do?
We shall see when we run them. Have you? Your approach seems reasonable, so why did you stop short of actually running the numbers?
From our earlier calculations, I recall 10 micrometer is too thick to get the Jupiter turn up to 0.1 c, much less the solar turn. Remember we reconciled our respective calculations on that with 3 orders of magnitude in our disfavor? After that, I think you will find 10 microns to be too thick to achieve the necessary charge/mass ratio.
Yes, with about the same amount of feasibility (and laugh factor).
I looked at my scribblings again. According to them, in the near surface fields of Jupiter, you need a charge/mass ratio of 1000 C/kg to get to 0.1 c. Plugging this into your equation of 1/e q^2 = S*rho*a, and expressing your q [C/m] in the charge mass ratio q/m [C/kg] as q = (q/m)*a*rho, I get S = (1/e) a rho (q/m)^2. For a 10 micrometer wire I get 4*10^11 m^2/s^2, which is 5 orders of magnitude higher than the maximum specific strength of known materials.
I guess we lose on this one. It was too good to be true, anyway. Sigh. The only tweak we can really do is make the wire thinner. That, however, sends us headlong into the field emission trap. Sigh again.
Conclusion: Lorentz turning may work for, at best, a few hundred km/s around Jupiter. Not for fast interstellar travel, alas.
Well, according to the above simple (and brutal) equation, a single-wall nanotube would actually achieve our goal, because of its really small cross-section, mostly, and also its high strength. With positive charge, maybe we do not have to worry about field emission. What would have to be emitted is carbon ions, and presumably they would not tunnel nearly as easily as electrons. What would be required is a stability analysis of the carbon nanotube structure with every 100,000th electron removed. It seems plausible that such a “CNT ion” would be stable. Of course, it is not easy to put together what amounts to an ENORMOUS spider web of single nanotube filaments, but it is not outright impossible. Thus, there remains a ray of hope….
“the near surface fields of Jupiter, you need a charge/mass ratio of 1000 C/kg to get to 0.1 c”
Jupiter again…
The Sun is a much better candidate (near its surface, the magnetic field is 10^-4 T – and it’s a LOT bigger than Jupiter).
Even the interstellar medium has a magnetic field of 1nT. With a 10 micrometer wire, we get a mass of 10^-7 kg/m and a q/mu of 7 c/kg. A complete circle would be completed in 5 years (and this is the worst case scenario, when the Sun doesn’t allow for better figures).
“field emission”
Do we even know the magnitude of this problem? At 9GV, given the most favorable geometry of the structure, how large, exactly, are the losses from field emission?
About the electrostatic grid – do read D. Whitmore paper – “Relativistic Spaceflight and the Catalytic Nuclear Ramjet” again.
The field strength necessary is only 9*10^5 V/m for a proton impacting at 0,5 c.
Even the necessity of doing work to maintain the charge is mentioned.
So – beta radiation emitters are sufficient in creating and maintaining this charge.
At 9*10^5 V/m, field emission is manageable .
My version of the ship (and the far more controlled method of receiving fuel) mitigates the erosion problem – and opens the door for more convenient grid design geometries.
And I can afford to have my ship at any distance from the sun while accelerating. Lorentz turning would be useful, but it’s not necessary in order to accelerate the ship.
Also note – the top velocity for my ship – if using thorium as fuel – is ~0,22 c – delta v of 0,44 c.
And also – thorium fission, deuterium fusion, etc (reactions used to power my “steam” ship) are FAR more energetic than the CNO cycle proposed in the D. Whitmore paper – “Relativistic Spaceflight and the Catalytic Nuclear Ramjet”.
Even your ship could work
For acceleration:
The sail (composed of one or more sub-sails) is separated from the ship, fixed on a tight orbit near the Sun, beaming the energy via laser/etc to the ship which rotates by Lorentz force as close as possible to the Sun.
The ship is accelerated to 0,1 c and departs, the sail remaining.
For deceleration:
The ship carries a folded sail.
When reaching the target system, the ship will deploy and stop a sub-sail (of relatively small mass). This sub-sail will stop the other sub-sails. All sub-sails will, then, stop the ship from 0,1 c to 0.
It’s a bit too hot, there, though. I am pretty sure Jupiter is the very best the solar system has to offer in terms of magnetic field without melt-down. There is the radiation issue, though…
As I said, they show nothing about feasibility, except that there is enough power.
Sorry, but no. The 9*10^5 V/m is in the space between the grids. The field right at the surface of the wire is MUCH stronger. At 9GV and 10 microns it should be on the order of 10^15 V/m. Field emission will discharge the grid instantaneously. And we have already shown that 10 um wire at this voltage will be ripped to shreds, anyway.
The only way around field emission is to use only positive charge, something that Whitmore apparently just did not think about.
I do not know where you get this idea. It seems you are still confused about the difference between voltage and field strength. Beta electrons are good for 100 kV, there might be a few up to 1 MV. 9 GV? Forget it. And even in the 100kV range, we need to consider the current required, and how much beta emitter we will need to keep the voltage up against the inevitable discharge from the surrounding plasma. If the amount is too large, again no dice.
There is another problem with the interplanetary magnetic field: It does not point in the right direction for Lorentz turning. Apparently, the field lines are mostly open, and run more or less along the orbital plane (or more precisely, the “interplanetary current sheet” or “heliospheric current sheet”), at a 45 degree angle from the radial direction. Not suitable for a forced orbit, unfortunately. See: http://pluto.space.swri.edu/image/glossary/IMF.html
As you say, the galactic field should be suitable for Lorentz turning on a grand scale, but there is no source of energy along those trajectories, so this does not really help us, here.
I am afraid it will have to be Jupiter, and single nanotubes. Nothing else will work. There is not a lot of solar energy there, but maybe we can surf the Alfven waves instead….
“9 GV? Forget it”
Why so fixated on 9 GV, Eniac?
As said, 9*10^5 V/m is quite sufficient.
“The 9*10^5 V/m is in the space between the grids”
AND AT THE GRIDS. As you know, the electric field generated by an infinite plate does not decrease with distance.
An infinite plate or close to a finite one – that’s why the grids are 10 km in diameter.
As said, at 9*10^5 V/m, beta emitters are sufficient to charge the grids
And for 10 micrometer wires, field emission is a minor problem.
As for using only positively charged grids near the ship – I already mentioned the idea (in my initial post on this system).
I went with capacitor-like grids only because the relevant calculations were already done.
About Lorentz turning:
“It’s a bit too hot, there, though. I am pretty sure Jupiter is the very best the solar system has to offer in terms of magnetic field without melt-down.”
At a q/m of 10, you will be far away from the Sun that you’ll wish you were closer.
Indeed, Lorentz force turning farther away from the magnetic field is trivially easy (decrease the electric charge). Getting closer is tricky – and the Sun, with his magnetic field/size/available energy, beats Jupiter by a larger margin.
“There is another problem with the interplanetary magnetic field: It does not point in the right direction for Lorentz turning. Apparently, the field lines are mostly open, and run more or less along the orbital plane (or more precisely, the “interplanetary current sheet” or “heliospheric current sheet”), at a 45 degree angle from the radial direction. Not suitable for a forced orbit, unfortunately. See: http://pluto.space.swri.edu/image/glossary/IMF.html”
The solution: accelerate to 0,1 c while on a plane at a 45 degree angle from the radial direction, then, while en route, use the intergalactic magnetic field to turn the ship in the direction you want.
“As you say, the galactic field should be suitable for Lorentz turning on a grand scale, but there is no source of energy along those trajectories, so this does not really help us, here.”
My ship doesn’t need external energy sources; it uses fuel seeded along a trajectory to accelerate.
Again you confuse potential and field. I believe the 9 GV are from the paper you cited, and they square with your field number when you assume the grids to be oppositely charged and 20 km apart, which I think is approximately the configuration described in the paper.
As you (hopefully) know, a grid is not a plate. When you get a little closer, a grid looks more similar to a fine wire, and the field decreases with distance linearly. For a 10 micron wire at 9 GV, the surface field is around 10^15 V/m, whether you like it or not.
Again you confuse potential and field. The beta electrons must be able to escape into infinity, and they can do so only when they have enough energy to escape the potential, measured in Volts (9 GV here). The field, measured in Volts/meter (Here: 9*10^5 V/m between the grids, ~10^15 V/m at the wire surface) is irrelevant. Typical beta energies are around 1 MeV, so megavolts is all you get. Gigavolts, forget about it.
Note that if you want to stop a proton coming in at 0,1 c, you need 100 MV right there. With heavy ions, even much more. Beta emitters are not an option. Whether you like it or not.
Depends on the field, which depends on the voltage and wire radius as approximately U/r. For a negatively charged 10 micron wire, it should start to become a problem around 1-10 kV.
You are wrong. At q/m of 10, you’d have to be skimming the sun’s surface. As you go farther away, you have to increasethe charge. You disappoint me, apparently you have not been reading with much attention. If the sun’s were a dipole field, it would decrease with r^-3. Thus, the closer the better. No doubt about it. As it is, there is no usable field to begin with, except maybe inside the corona, but there the field is extremely bumpy, and the nanotube mesh would evaporate in nanoseconds from the heat.
Before I found out the field is all mangled up and pointing in the wrong direction, I did the calculation at 6 nT and 1 AU. the values in our neighborhood, as if it were oriented the ideal way. The q/m needs to be 300 times larger for this situation than for near the surface of Jupiter. 300,000 C/kg instead of 1,000. Closer will be better, but not that much better.
You do not understand. “45 degrees from radial” will send you into the sun or into interstellar space before you can pick up enough speed. There is no closed Lorentz orbit in which you can accelerate with the field as it is.
You had a few lucid moments there, earlier, but your latest posts again demonstrate the profound lack of understanding of the physics involved I have lamented before. You should spend some time trying to understand the issues I have raised before brushing them off. The issues themselves will be much less forgiving than I am….
Which is why, as I have said, there is no need for Lorentz turning in this situation. You can pick up the fuel on a straight path just as well.
About the 9GV you’re fixated on:
Do read the paper again: http://www.google.ro/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CBwQFjAA&url=http%3A%2F%2Fwww.askmar.com%2FRobert%2520Bussard%2FCatalytic%2520Nuclear%2520Ramjet.pdf&ei=WyueUJaBI8fGtAbV3YHQCQ&usg=AFQjCNGiTduxHP49xSi6MeKmr1lsYVDRDg
9*10^5 V/m (NOT V – as in, there’s NO confusion on my part between potential and field) is FROM THE PAPER.
9 GV is not (regardless of what you believe) – as I’ve been trying to tell you for the last ~5 posts.
The front and aft electric grids are specifically – and repeatedly – named capacitors in said paper.
BTW, your ‘get a little closer’ is irrelevant. You see, a continuous plate is also porous when one gets close enough. The good thing is that we don’t have to get “closer”. You see, a lot of wires put near each other behave far closer to a plate than a single wire – or do you actually think their combined electric charge will decrease linearly with distance, as seen from beyond microscopic distance?
The distance between the 2 grids forming each capacitor is 10 km; and a grid has a 10 km diameter – also from said paper.
“You disappoint me, apparently you have not been reading with much attention.”
Well, you disappoint me too, Eniac – also due to your lacking attention. Actually READ the paper I linked to.