Wikipedia:Reference desk/Archives/Science/2014 August 10

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August 10

Blackbody propulsion

Take a sheet with one side silvered and the other black, warm it up, and put it in space with zero external light. It will emit more blackbody radiation on the black side than the silvered side, which produces thrust without any propellant and needing no exotic theory. Am I missing something? Note that I'm not talking about any type of solar sail that uses an external light source. This was also asked on reddit with one poster providing the thrust (2.04μN at 300K) and the rest confusing it with a solar sail. SamuelRiv (talk) 14:02, 10 August 2014 (UTC)

The principle is sound: emitting electromagnetic radiation predominantly in one direction produces a reaction force that acts as propulsion. This is a photon rocket. The earlier responses in the linked reddit discussion make sense. To maintain the radiation at a level above ambient, you will need a source of power. Once it cools to ambient (reached thermal equilibrium, e.g. with the microwave background radiation, the pressure of incident thermal radiation on the reflective side will balance the force from incident absorbed radiation and emitted radiation on the black side. In terms of impulse per unit energy, if the radiation is all in one direction like a in laser, I expect that this is theoretically the most efficient rocket-like propulsion possible. —Quondum 15:02, 10 August 2014 (UTC)

What is the specific impulse of photon rockets with the best available current lasers for the job? Laser propulsion#Photonic Laser Thruster (PLT) has information on thrust but not specific impulse. 104.128.96.117 (talk) 20:32, 10 August 2014 (UTC)

Who needs lasers? It would take forever to justify the additional mass. Since we're not actually building this thing, let's makes some idealizing assumptions:

• Assume a black cavity or surface (as proposed by the OP), heated to emit radiation. The radiation is not collimated as with a laser, but the intensity (and hence momentum) is proportional to cos θ, where θ is the angle from the direction the cavity faces. This gives a momentum of cos² θ in that direction per unit angular area. Integrate over the hemisphere, and we end up with an efficiency due to lack of collimation. I seem to get 1/3; anyone who is interested can verify this.
• Assume heating by annihilating fuel composed of matter and antimatter: total conversion of mass to heat energy.
• Assume non-relativistic speeds to simplify calculations.

The momentum of a collimated photon beam with energy equal to annihilating mass m is energy E = mc², and the momentum is E/c = mc. Applying our efficiency factor for the non-collimated radiation, we get a specific impulse I_sp = (mc/3)/m = c/3. —Quondum 22:12, 10 August 2014 (UTC)

That seems to be in distance/time. Specific impulse is measured in units of time. I love the idea of a heated blackbody thruster though. I wish I had the math skills to follow... Can you get Newtons per Watt? How about using electrical current heating a resistor instead of antimatter annihilation? 104.128.96.117 (talk) 22:38, 10 August 2014 (UTC)

To quote from Specific impulse: If the "amount" of propellant is given in terms of mass (such as in kilograms), then specific impulse has units of velocity. Measuring the quantity of propellant by its weight seems particularly inappropriate in the context of space propulsion, whence the units of velocity that I gave. But if you insist on measuring impulse in terms of the weight of the propellant in terms of Earth gravity, this translates to c/3g ≈ 10⁹ s. —Quondum 00:32, 11 August 2014 (UTC)

I got c/3g = (3*10⁸m/s) / (30m/s²) = 10⁷s. - ¡Ouch! (^{hurt me} / _{more pain}) 06:59, 12 August 2014 (UTC)

Sorry, missed some questions. In terms of thrust per unit power, it is the same as momentum per unit energy, which is mc/(3mc²) = 1/3c ≈ 10⁻⁹ s/m ≈ 10⁻⁹ N/W. This might seem miniscule, until you realize that to compare apples with apples, you need to include the mass–energy of propellant in the "power" in a conventional rocket, and that is worse by many orders of magnitude. Ignoring the mass of your on-board power storage does not make sense. Electrical heating does not particularly make sense – where are you going to store the energy? In some chemical form? Matter–antimatter annihilation seems almost perfect for the job here. One kilogram of propellant would propel a one-kilogram craft to a significant fraction of the speed of light. —Quondum 00:59, 11 August 2014 (UTC)

Wow, thank you. I think that is impressive, but ... antimatter? Is there any way to get Newtons per Watt from less exotic sources of heat? 104.128.96.117 (talk) 01:01, 11 August 2014 (UTC)

You need to be more specific about the context. Newtons per watt would usually be used in the context of a power source working with an external mass to act on, such as with a jet or turbo engine. Or from an external power source in space, such as the sun – or a phenomenally powerful laser beam – with a solar sail. (Oh, and the N/W figure is the same for this type of thruster, regardless of the source of heat.) —Quondum 01:17, 11 August 2014 (UTC)

Better idea is this, Black hole starship 108.170.113.22 (talk) 16:19, 11 August 2014 (UTC)

Why is that a good idea? It sounds like a terrible one. The black hole is evaporating to produce that material - it is the "fuel" for the ship. When it's all evaporated away, no more thrust. Furthermore, you have to haul the enormous mass of the black hole around with you. I'm pretty sure that the amount of evaporating material is tiny compared to the mass of the black hole until the last few milliseconds of it's existence - so the amount of acceleration you'd get would be vanishingly small surely? SteveBaker (talk) 19:00, 11 August 2014 (UTC)

It is a terrible idea. It's the most impractical thing I've ever heard of, bar none. 104.128.96.117 (talk) 19:03, 11 August 2014 (UTC)

They already did the calculations. The black hole can last over 500 years, it can be "refueled" by dumping ANY matter into it, and it produces net thrust despite its extreme mass. 108.170.113.22 (talk) 16:49, 12 August 2014 (UTC)

The redditor was off in their calculation, using only the visible spectrum and cm² for power required while using the full spectrum and m^2 for radiation pressure, in addition to not accounting for solid angle, which reduces the pressure by 1/2. The additional power needed to maintain a given additional force, independent of the mass, area, temperature, and albedo (but not geometry) of the craft, is 6×10⁸ Watts per Newton; both force and power scale as the 4th power of temperature.

The actual force produced scales with the surface area of our "sail", but not the mass. At 300K (room temp) we get 1.5μN at a cost of nearly 1000W per m² of our black/silver sail. Note that all spacecraft would require power to maintain temperature, so they try to minimize surface area and maximize albedo, and within the solar system where the sun shines they will absorb more radiation than they emit.

As to whether or not this method is thermodynamically the most efficient propulsion in isolation as Quondum suggested, I'm not so sure. I believe it can be optimized by changing the geometry of the sail, but paradoxically this would reduce efficiency since it would rely on recapturing and re-emitting some radiation. I'll run the calculation later. SamuelRiv (talk) 19:18, 11 August 2014 (UTC)

Deep ocean internet lines

How are deep sea internet lines laid at the deepest part of the oceans. Is it just dropped in or is it lowered down using specialised deep ocean diving machinery? And when these lines get near a coast, does it just go under the sea bed since I'm assuming the cables wouldn't just be on show in shallow water or on a beach. How does it get underground without water also getting int this underground area where the cable goes into? And what happens to the cable when it reaches a deep ocean trench? Does it follow the trench right down and back up again following the seabed terrain exactly or is it bridges across the trench? — Preceding unsigned comment added by Clover345 (talk • contribs) 20:15, 10 August 2014 (UTC)

Atlantic bottom contour in the vicinity of the AFR477 search. Note how the vertical axis is exaggerated by a factor of sixty, so while the terrain is rugged, it is not as crazy so as it first appears. -- ToE 22:37, 11 August 2014 (UTC)

Underwater cables have been around a lot longer than the internet - see Transatlantic telegraph cable, the first of which was laid by ship in 1858. Mikenorton (talk) 20:31, 10 August 2014 (UTC) See also Submarine communications cable, which has more details. Mikenorton (talk) 20:34, 10 August 2014 (UTC)

Unfortunately, our article doesn't really address the questions, other than saying that with 19th century cables, "The portions closest to each shore landing had additional protective armor wires." Our cable landing point article doesn't go into much physical detail, but this KIS-ORCA page says, "At the shore approaches where the cable is landed (the shore end), burial is carried out by divers in the shallow water. If the water is too shallow for the main lay vessel to approach safely, a smaller shallow drafted vessel may be lay used to lay the shore end. The main lay vessel (the cableship) is used to lay the rest of the cable system. In water depths where there are risks due to fishing and other man made activities such as anchoring etc., the cable is generally buried to a target depth of approximately 1 metre." Presumably water does enter the trench that is dug for the subsea portion of the cable, as well as the portion on land dug below the water table, but I don't understand in what way you consider this a problem. The cable is waterproof, after all.

The same KIS-ORCA page also says, "The cable is laid to conform to the contours of the seabed to avoid cable lying in suspension." -- ToE 22:03, 11 August 2014 (UTC)

File:AF447Cross-Section.jpg appears to be mislabeled. The "260" km on the horizontal axis should be "250" (probably just a typo during construction). This observation is based both on the consistent spacing between the marks, the others being labeled in 50 km increments, and the calculated width of the graph of 258.44 km, based on the coordinates given at the top. -- ToE 22:52, 11 August 2014 (UTC)

Working from the above image only, not the original data, it appears that the steepest areas on this graph are only sloped 30° from the horizontal. Our oceanic trench article states, "Transects across trenches yield asymmetric profiles, with relatively gentle (~5°) outer (seaward) slope and a steeper (~10–16°) inner (landward) slope." The point being that subsea contours are typically not as steep as many people's impressions, so spanning trenches would require tremendous tensions and would not reduce the cable run lengths by very much. It is not as if a 100 m span would save descending a 1 km trench. -- ToE 14:44, 13 August 2014 (UTC)

Wall plugs that look like a nail/screw hybrid

In Romania, before the fall of the communism, instead of using the plastic wall plugs invented by Fischer, we were using some nails with screwing thread on one end. They were locked into the wall using a pistol that pushed them into their place. My question is if they have a distinct name or they are simply named "wall plugs". Who invented them and when? I am also curious if they were used in the West or maybe only in the communist countries? Thanks. —  Ark25  (talk) 23:47, 10 August 2014 (UTC)

I think just nail guns are used for that purpose here, although I don't understand what was screwed into the ends of those. StuRat (talk) 00:32, 11 August 2014 (UTC)

I haven't seen them in the UK either, and, like Stu, I'm puzzled about how they were used once the pistol had put them in place. Dbfirs 09:08, 11 August 2014 (UTC)

In England there's a similar product called a "threaded nail", which serves much the same purpose - the spiral means the nail rotates as it's hammered in. They're not common, and usually have to be bought from specialist building shops rather than general hardware stores. I'd guess that their relative rarity in Western Europe compared to Romania is due to differing construction techniques - wood isn't a common building material here, and a wall in Britain, France, Germany etc is usually plaster over brick, meaning the additional strength of the plastic plug is necessary to hold a nail or screw in place without cracking the plaster. 89.242.88.157 (talk) 09:26, 11 August 2014 (UTC)

The threaded nail you are describing is quite different than this image. On a spiral-shank nail, the (almost-)full length is the "nail", and is itself threaded (usually with a fairly coarse thread)--see nice pictures at [1]. Instead, this image looks like two separate zones along the length: a completely smooth nail then a fairly fine machine thread. It's like a hanger bolt or anchor bolt, but with a nail-like end rather than a wood-screw or bent part. The straight part would go into the wall (either driven with a hammer or placed in the plaster/mortar/etc as the wall was being constructed). The threaded end (the "head of the nail") would be left protruding for latter attachment using nuts similar other threaded fasteners. DMacks (talk) 16:55, 11 August 2014 (UTC)

Thanks for the answers. Those nails were used in walls made of concrete, not sure if they were used in walls of bricks covered with plaster. To the best that I can remember, the concrete walls were not covered in plaster, so there were no concerns for not cracking the plaster. If anyone would have used them in wooden walls, they would have used hammers instead of pistols, I guess. You could hang something on that nail - like for example a painting, and then use the thread in order to lock it with a nut. The nails were improperly named "Bolţ" (in Romanian, "Bolţ" means "Gudgeon pin") - which comes from the German word "Bolzen" - my bad, I should have said that from the very beginning. Now, nobody uses those nails anymore in Romania, of course. Fischer wall-plugs are so much better and easier to use. —  Ark25  (talk) 01:17, 14 August 2014 (UTC)