Wikipedia:Reference desk/Archives/Science/2008 February 19

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February 19

About time dilation and planet habitation

I've been wondering about the effects of time dilation on planets orbiting at different velocities around a star. My question is this: if Planet A is orbiting around its star at say twice the velocity of Planet B, does time pass slower on Planet A in relation to B? Also, if you're sitting stationary in a solar system, does time pass slower on an orbiting planet in relation to you? There's probably some weird gravitational dilation that I'm not taking into account here, but I'm really aching to find out the answer to this, so any help will be greatly appreciated. --Closedmouth (talk) 04:10, 19 February 2008 (UTC)

Yes to both, but not significantly. See the Lorentz factor for the underlying math. — Lomn 05:08, 19 February 2008 (UTC)

The question doesn't really provide sufficient information to say for sure that time passes more slowly on the surface of planet A. Besides the orbital speeds of the planets, you also need to take into account the rotational speeds of the planets as of where the observers are, as well as differences in gravitational time dilation between the two planets. MrRedact (talk) 07:32, 19 February 2008 (UTC)

Orbiting objects are always accelerating. Accelerating things are not really in the realm of special relativity. General relativity has Gravitational time dilation, which indeed says that time in lower gravitational potential (closer to the star) runs slower. --131.215.220.112 (talk) 12:09, 19 February 2008 (UTC)

The time dilation effect would most likely be insignificant. All objects that are traveling at different speeds experience the phenomenon, but it goes unnoticed. A good example is a traveler on a jet airliner. They experience a minute and insignificant time dilation with respect to observes on the ground. Wisdom89 _{(T / ^C)} 18:37, 19 February 2008 (UTC)

The above statement that "accelerating things are not really in the realm of special relativity" is overly general. This is a common misconception about special relativity vs. general relativity.

Special relativity works just fine for dealing with rapidly moving, accelerating objects in the absence of gravity, such as a charged particle in the presence of an electromagnetic field, or a spaceship that's not near an astronomical body, that's accelerating due to thrust from its engines. For example, see Hyperbolic motion (relativity), which deals with an accelerating object, but only uses special relativity.

It's only when dealing with a curved space-time, i.e., when dealing with gravity, that you need to use general relativity. This particular problem does indeed require general relativity, but that's because gravity is involved, not just because acceleration is involved. MrRedact (talk) 00:22, 20 February 2008 (UTC)

Thanks, guys. Most interesting :) --Closedmouth (talk) 12:58, 20 February 2008 (UTC)

Aircraft carrier design

Current US aircraft carrier design features two runways, one for launches and one for landings. Would it be possible to put the launch deck below the landing deck to dramatically reduce the length of a carrier ? One potential problem with this would be ventilation, as the jet engines on the planes burn large quantities of oxygen and put out large quantities of exhaust. Perhaps this could be dealt with by having open sides and ends on the launch deck (with steel columns supporting the landing deck, of course), massive exhaust fans, maximizing the use of catapults, and only going to full jet engine thrust at the final moment of launch (I visualize the launch sequence being fully automated, with no pilot interaction until the plane is airborne). Besides the smaller size, other advantages of such a setup might be the ability to launch in harsher weather conditions and less reduction of launch capability due to bombs dropped on the deck from above. Does this approach seem feasible ? StuRat (talk) 09:30, 19 February 2008 (UTC)

My first thoughts are:

• Airplanes and concrete runways tend to be heavy
• Runways tend not to work so well with steel supports sticking out of them

:D\=< (talk) 11:42, 19 February 2008 (UTC)

The additional launch decks ("second flying-off decks") are actually mentioned in the aircraft carrier article, in the aircraft carrier#Hurricane bow section. The article makes it sound like a bad idea, but provides no explanation. I would think that the main problem with such a configuration would be that the planes are taking off too close to the water. An increased threat of flooding would also be a concern. Besides, the carrier size is not crucially dependent on the takeoff strip size. It may well be (I am not sure) that the area and volume requirements on hangars and depots are more important for carrier design; in that case, placing launch deck below the landing deck is counterproductive. Cheers, --Dr Dima (talk) 11:41, 19 February 2008 (UTC)

Takeoff height and hangar volume are easily solved -- build the second deck higher. I expect, though, that Froth has hit on the problem -- how do you support the upper deck without interfering with flight ops? Topside, aircraft wings can overhang as needed, which an interior flight deck wouldn't permit. On that alone, you're losing a great deal of usable area.

Additionally, the angled-deck design isn't simply one of "two runways". It's perhaps more important as "runway plus storage". The angle allows for safe execution of landings, with the allowance for an aborted landing attempt where the plane returns to full power, with other aircraft stowed forward on the usual launch area. There they can be refuelled, rearmed, and otherwise quickly maintained without introducing the additional bottleneck of the aircraft elevator. — Lomn 14:48, 19 February 2008 (UTC)

American vs British style aircraft carrier

Are, I wonder, flight decks the way of the future in any form? The current crop of aircraft carriers under construction now are probably the last that will feature full complement of human-operated aircraft. The next generation will surely have drone fighters (and probably fighter-bombers and attack/bomber aircraft). It's not clear how this will affect shipboard operations (and it'll probably be another generation before they fully figure that out). It's quite possible that launch systems for drone fighters might move to a rocket-cartridge system (like that used for naval TLAMs now, leaving the flight deck only for recovery and (for those navies that operate them) for other aircraft like tankers and ELINT aircraft. For a carrier that only hosts small agile hardy fighters (which can take the high forces associated with rocket-launches and "giant butterfly net" landings) it's quite possible the flightdeck will shrink or maybe even disappear altogether. -- Finlay McWalter | Talk 15:13, 19 February 2008 (UTC)

It all depends on what type of plane you want on the carrier. The British carriers are already much smaller (see figure to right for a comparison) since they use the Sea Harrier. David D. (Talk) 15:27, 19 February 2008 (UTC)

This is kind of a misleading statement. You seem to be insinuating that the British carrier is better. It isn't. It's a lot worse in fact. Bigger aircraft carriers, hold more aircraft, and thus can project more power. 64.236.121.129 (talk) 19:33, 19 February 2008 (UTC)

The converted cruisers: Glorious, Courageous, and Furious all had small, auxiliary flight decks below the main deck over the forecastle which could launch Fairey Flycatcher fighters. They were too small to operate the aircraft of the '30s and '40's and were converted to gun decks during later refits.—eric 16:38, 19 February 2008 (UTC)

Thanks for all the info so far. As for the issue of the steel columns interfering with flight ops, in a fully automated system the pillars could be only slightly farther apart than the widest wingspan, since the pilot doesn't need to navigate between them. If this didn't provide sufficient support for the upper flight deck (recovery deck), then arches could be used between the columns (this might require increasing the height a bit, though). As for reducing usable area, aircraft wings could still extend out over the edge of the carrier, between the columns. It would also be helpful if a system were used where the planes could be moved sideways to quickly get them onto the runway from a storage slot on the side. StuRat (talk) 18:34, 19 February 2008 (UTC)

The "usable area" point I wanted to make was one of launching. Note, for instance, that a Nimitz carrier's two forward catapults are set far enough to the sides that the wings overhang. Putting the catapults on a lower deck would likely force launch ops to a single catapult (I concur that otherwise you can trim the clearance margins). — Lomn 19:33, 19 February 2008 (UTC)

On an other note, see past dreams of grandeur with Project Habakkuk. 200.127.59.151 (talk) 22:38, 19 February 2008 (UTC)

...which inspired the 'mili-habbakuk' as a unit for measuring impracticability.—eric 23:04, 19 February 2008 (UTC)

The British design already has separate launch/recovery. Because they are using the Sea Harrier they typically (or at least optionally) recover planes by vertical landing. Also carriers are rarely launching and recovering at the same time. The problem is not the need to do both at once, but the equipment for each - the catapaults and arrestor hooks. If memory serves the Harrier-based design needs neither. DJ Clayworth (talk) 16:11, 21 February 2008 (UTC)

They may rarely do both at once, but the ability to do so during combat is still critical. If the "first wave" is returning while the "second wave" is launching, any time delay could be disastrous for the returning planes, which may be low on fuel, and the departing planes, as this may give the enemy time to prepare their defenses. StuRat (talk) 19:06, 22 February 2008 (UTC)

Presumably if the cost saving is great enough you could make up for this by having two smaller carriers, with the extra flexibility that you could deploy them separately for a "single strike" operation rather than for a sustained attack. -- Q Chris (talk) 12:00, 25 February 2008 (UTC)

The Hall Voltage in Ionic Fluids

I've derived a formula to find the Hall voltage of a flowing ionic fluid:
$V_{H}={\frac {-IB/d}{ne}}.$

Current = charge passing through area cross section per second $I=vAne$
Where the ions in solution have charge -1, are traveling at velocity v through area A, and there are n ions per unit volume.
Substituting, $V_{H}={\frac {-vAneB}{ned}}$
$V_{H}=-vwB$
where w is the width of container.
Is this correct - the voltage depends only on the velocity, width of container and field strength? Why doesn't concentration of ionic solution make any difference?
In practice, does it make any difference with different ionic solutions / concentrations (due to grouping of ions/mobility), if so how will concentration affect voltage?
Also, I need to setup a strong magnetic field - does it matter if the field is not uniform and how can I arrange electromagnets to give a uniform field over about 0.2m²?
Does the choice of electrodes matter? Thanks --90.241.222.228 (talk) 10:12, 19 February 2008 (UTC)

I'm assuming this is a followup to a previous question about the Hall effect. Your question, "Why doesn't concentration of ionic solution make any difference?" is a good one. Remember this voltage was derived on the basis of an equilibrium between Lorentz force (from the magnetic field) and electric force (from the charge buildup). The concentration of ions will affect how fast you approach the equilibrium, but once the equilibrium is there, the net sideways force on any new moving charge is zero, regardless of how many other moving charges are around it.

I am not any sort of expert on Hall sensors, but I would guess that small variations in the magnetic field would be balanced by small variations in the charge buildup, and the resulting equilibrium would be "near" the equilibrium of a perfectly uniform field, making your answer "near" the real values. None the less, a nice uniform field can only help you. The classical way to get a fairly uniform magnetic field is to use a Helmholtz pair. JohnAspinall (talk) 16:05, 19 February 2008 (UTC)

Thanks. Is there any way of predicting how changing concentration will affect how fast equilibrium is reached? Can you suggest any other area to investigate other than changing concentration / ions in solution / speed / field strength? —Preceding unsigned comment added by 90.241.222.228 (talk) 21:08, 19 February 2008 (UTC)

You've already mentioned mobility. Mobility will determine how fast the ions "drift", to build up the charge separation. The drift velocity from the Lorentz force will work the same way as for the electrical force. I will guess that mobility will be independent of concentration, for low concentrations at least, so if you wanted to change the mobility, you might have to change the ionic species. Don't forget that (since the fluid is electrically neutral, on average) there may be a positive and a negative ion moving (in opposite drift directions) and they will (in general) have different mobilities. I would suggest a little searching on how mobility is measured. JohnAspinall (talk) 15:26, 20 February 2008 (UTC)

Supermagnet

A strange question just popped into my head. What if someone, with incredible technology or magic (basically the same thing) developed a monopole electromagnet with incredible strength? I had a similar discussion with a friend a while ago and she said that a strong enough magnet could tear electrons away from all the atoms in its range. Would this make such a silly device applicable as a superweapon? What horrifying effects would such a device have on matter, if any? 206.252.74.48 (talk) 17:17, 19 February 2008 (UTC)

I'm sure I've read a science fiction story about a scientist that invented a monopole magnet and submitted a paper about it, and then suddenly found himself being shadowed by agents from at least one government. I'm sorry that I don't remember the title or author. APL (talk) 17:35, 19 February 2008 (UTC)

At a given point in space, the magnetic field produced by a magnetic monopole would no different from the field produced by a dipole or any other distribution of "magnetic charge". The difference is in the global geometry of the field, not the quality of the field itself. So the effects of an incredibly strong monopole magnet on materials would be the same as those of an incredibly strong (but otherwise ordinary) dipole magnet. —Keenan Pepper 18:09, 19 February 2008 (UTC)

That is a good point, so just ignore the monopole part with this question. I want to know what the effects would be. 206.252.74.48 (talk) 18:28, 19 February 2008 (UTC)

Except that a monopole field would be felt over much longer distances. — Laura Scudder ☎ 20:13, 19 February 2008 (UTC)

Right, that's the only relevant difference in this context: a monopole field falls off as the inverse square of the distance rather than the inverse cube. —Keenan Pepper 04:25, 20 February 2008 (UTC)

So, I think the question boils down to "What are the effects of very strong magnetic fields on matter?". In that case, the answer is "nobody knows, exactly", but we can say a few things. For example, the field wouldn't "tear electrons away" from atoms, because magnetic fields do no work, and tearing an electron away from an atom requires work. However, as Magnetar#Magnetic field says, a very strong magnetic field (10 gigatesla) would rip your body apart because of the diamagnetism of water, and stretch atomic orbitals into thin needles.

I'm actually going to the open house of the National High Magnetic Field Laboratory in Tallahassee tomorrow, so I'll ask this question and post any interesting replies here. —Keenan Pepper 15:57, 20 February 2008 (UTC)

Actually, I believe the only reason a magnetic field can do no work is because there are no monopoles. — Laura Scudder ☎ 22:03, 20 February 2008 (UTC)

True: if monopoles exist, then a magnetic field can do work on monopoles. But it still can't do work on normal matter (such as the electrons being discussed here) that lacks magnetic monopole charge. --mglg_(talk) 23:21, 20 February 2008 (UTC)

You're forgetting that once we accept monopoles, we have to consider magnetic current loops. They should do work on electric monopoles. — Laura Scudder ☎ 03:35, 21 February 2008 (UTC)

Yes, a monopole current loop can create an electric field, which of course can do work on electric charges. But that fact has nothing to do with the question of whether a magnetic field can do work on electric charges (which it can't). For the magnetic-field question it is irrelevant how the magnetic field was generated, so the existence or non-existence of monopoles doesn't change anything. --mglg_(talk) 16:59, 21 February 2008 (UTC)

So, when an electric current creates a magnetic field, the current is actually doing the work, but if a magnetic current created an electric field, the electric field is doing the work? — Laura Scudder ☎ 00:48, 22 February 2008 (UTC)

(This off-topic sub-thread is getting out of hand long, but here goes:) Of course the work ultimately originates in whatever made the monopoles move in the first place. But the question wasn't about the ultimate source of the energy, but about whether the magnetic field can do (or convey, if you want) work on electric charges. You said above that you "believe the only reason a magnetic field can do no work is because there are no monopoles" (my emphasis), so you clearly do agree that a field can be considered to do work. In the monopole current loop case it is not the magnetic field that is doing any work, but the electric field. Using your analogy, if a normal electromagnet is used to lift a piece of iron, you would not say that it was the electric field of the electrons in the coil that was doing the work, would you? It is the magnetic field that does the work. That said magnetic field is generated by moving electric charges is beside the point. (Over and out.) --mglg_(talk) 04:34, 22 February 2008 (UTC)

When my foot falls asleep

I know that when my feet "fall asleep", it's because there's not enough circulation to it. One question, though: when that happens, what causes the "tingly" feeling, as if I'm being poked by little pins or having little electrical shocks? Nyttend (talk) 21:29, 19 February 2008 (UTC)

It's not just that you're cutting off the circulation to the cells and tissue of the limb, but also you're physically preventing nerve impulses from entering the CNS and vice versa. This causes the neurons/nerves to fire erratically after the pressure is released. Wisdom89 _{(T / ^C)} 21:35, 19 February 2008 (UTC)

See Paresthesia. —Steve Summit (talk) 14:16, 20 February 2008 (UTC)

MATERIALS/CHARATERISTICS (Homework?)

LOOK,GUYS sorry I used the f word. When,I last posted this question but that`s a testament to How I much I need it answered. Just,tell Me The Measurements and Charateristics of a balloon going across a room. —Preceding unsigned comment added by Yeats30 (talk • contribs) 23:56, 19 February 2008 (UTC)

Homework much?Zrs 12 (talk) 00:21, 20 February 2008 (UTC)

Perhaps you could rephrase your question? Something like:

I deeply regret posting previous homework questions under false pretence and undertaking plagiarism in using these responses to dishonestly complete my homework. I promise not to do it again. Furthermore, I will not get upset with you, the Wikipedia Reference Desk volunteers, if you are not willing to aid me in continuing the aforementioned. Nevertheless, I request your assistance in understanding the concept of how to measure and characterize a balloon crossing a room. Sincerely, The "Physics Magazine" Guy

Honestly, we'd like to help you. We just don't want to get yelled at for not being willing to help you cheat. (EhJJ)^TALK 01:07, 20 February 2008 (UTC)

I like the way you phrased that EhJJ. Furthermore, I agree. Zrs 12 (talk) 01:18, 20 February 2008 (UTC)

I don't care if this might be a homework problem, I'm willing to help you anyway. The most important measurements and characteristics of a balloon going across a room are as follows:

1) Since the balloon is (usually) a prolate spheroid, it's important to consider the balloon's various radii of curvature, which will generally be on the order of 4 diopters.

2) The balloon's modulus of elasticity, assuming a rubber balloon, would be in the range of 0.01 to 0.1 GPa, if there were only a small strain. However, due to the balloon’s relatively large stress, the nonlinearity of the balloon material's stress-strain curve must be taken into consideration. Therefore, you’ll need to use the strain tensor for large deformations, which is $\varepsilon _{ij}={1 \over 2}({g_{ij}-g_{ij}^{(0)}})$

3) Since the balloon is in motion across the room, it will undergo a relativistic length contraction. The amount of this effect is given by the formula $L'={\frac {L}{\gamma }}=L\,{\sqrt {1-v^{2}/c^{2}}}$

4) It's important to take into consideration the balloon's de Broglie wavelength, so that interference effects can be computed as the balloon travels down various hallways and through windows and doorways. The balloon's de Broglie wavelength is given by $\lambda ={\frac {h}{p}}={\frac {h}{\gamma mv}}={\frac {h}{mv}}{\sqrt {1-{\frac {v^{2}}{c^{2}}}}}$

I hope this was helpful. MrRedact (talk) 01:32, 20 February 2008 (UTC)

Oh! I thought of another very important characteristic of a balloon going across a room: the balloon’s relative static permittivity. I don’t know how such an important measurement could have slipped my mind; you’d have a heck of a hard time trying to figure out the balloon's capacitance without it! According to our article, the relative static permittivity of rubber is about 7. MrRedact (talk) 01:59, 20 February 2008 (UTC)

Would time dialation not be an important factor as well? Zrs 12 (talk) 02:18, 20 February 2008 (UTC)

Don't forget isotopic analysis. Any helium 3 that may happen to be in the balloon will contribute more to buoyancy than your regular helium 4 will. --Anon, 02:50 UTC, February 20/08.

Not true...the He-3 nuclei weigh less than He-4, but that's because there are fewer particles in those nuclei. They're lighter but also smaller and you can thus fit more into a given balloon volume. On average the gas still weighs the same per volume. That's a simple result of the ideal gas law: constant density of a gas regardless of what gas it is. DMacks (talk) 03:00, 20 February 2008 (UTC)

Are they really smaller? Yes, the nuclei are slightly smaller, but the nucleus is orders of magnitude smaller than the atom anyway. AlmostReadytoFly (talk) 08:47, 20 February 2008 (UTC)

...Besides which, the ideal gas law assumes pointlike atoms.AlmostReadytoFly (talk) 09:13, 20 February 2008 (UTC)

I agree, DMacks is wrong. The ideal gas law gives constant molar volume of a gas, not density. Helium-3 would indeed contribute more to the buoyancy. —Keenan Pepper 15:44, 20 February 2008 (UTC)

Sorry, forgot we were keeping the answers "literally correct" not "completely obvious bullshit":). Helium is small enough that lots of quantum effects and/or other weirdness happens. He-3 liquid density is about half that of He-4. DMacks (talk) 18:02, 20 February 2008 (UTC)

Also, there may by oxygen, nitrogen, etc. Furthermore, there will constantly be helium passing through the balloon into the atmosphere due to the pressure inside; there may even be some quantum tunneling (although probably not much).Zrs 12 (talk) 02:54, 20 February 2008 (UTC)

You guys are neglecting impulse and delta-v. The balloon is operating as a rocket, but in an atmosphere. IF he balloon were released in a vaccuum, we could neglect many of the effects mentioned above. However, assuming a vaccum will have important consequences for the delta-v. -Arch dude (talk) 03:58, 20 February 2008 (UTC)

Fuel efficiency of a Hot Air Balloon (and other questions)

If one would travel with a Hot Air Balloon from Texas to Florida, how would the fuel efficiency compare to a typical passenger airliner or personal jet? How safe is Ballooning compared to other forms of air travel? Malamockq (talk) 23:59, 19 February 2008 (UTC)

Hot air balloons are only safe when used under some rather specific conditions, like slow, steady winds and relatively flat ground (or water) which is clear of trees, buildings, and other obstructions. When used by a trained balloonist in airspace clear of aircraft, under those conditions, balloons can be fairly safe for short distances. Note, however, that balloons only go where the prevailing winds blow them, which makes them quite useless for general transportation. As for fuel economy, it's probably better than aircraft when measured on a per hour per passenger basis, but worse when measured per mile per passenger. StuRat (talk) 07:49, 20 February 2008 (UTC)