Wikipedia:Reference desk/Archives/Science/2013 March 21

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March 21

Timetravel

If the past of a point is in any direction away from it, doesn't that mean that timetravel into the future is limited to one second at a time. Since you can hardly get closer to the point than 0 meters, and the local rate of time is one second at a time. To me it just doesn't mak sense to talk of negative radial displacement from the point. For those who doesn't understand what I'm talking about: one year ago from this moment is one lightyear away in any particular direction, plus any time since that moment. The past travels away from this spacetime coordinate at a rate equal to the speed of light. If you wish to reach the past, you must overtake it by exceeding the speed of light. If you wish to remain in that moment in the past, you must travel in any particular direction at exactly the speed of light. Then you will always remain in the present moment. This follows that you can travel to any coordinate within the past, but you cannot exceed one second into the future, as you cannot travel slower than being stationary. Plasmic Physics (talk) 00:46, 21 March 2013 (UTC)

Either your speaking of time in a way I have never heard, or it's a corrupted version of light cone. Someguy1221 (talk) 00:52, 21 March 2013 (UTC)

Yeah - I can't decipher what you're trying to say - but whatever it is, the answer is "No!" - no backwards time-travel, no matter what speed you or your destination moves or over what distance at whatever speed. Just "No!". SteveBaker (talk) 00:54, 21 March 2013 (UTC)

For one thing "one second" is a arbitrary man-made unit of time, and has no direct correlation with "spacetime", etc. ~:74.60.29.141 (talk) 01:12, 21 March 2013 (UTC)

One second at a time, means one second per second, or zero dilation. However, perhaps I have it back to front: the future is away from the point. Plasmic Physics (talk) 01:15, 21 March 2013 (UTC)

One way to think about it is philosophically: there is only one past, there are an infinite number of possible futures and the present doesn't exist. [That last bit requires some 'splaining] ~:74.60.29.141 (talk) 01:36, 21 March 2013 (UTC)

You can think of time as a physical dimension we are moving through. When we are still with respect to our reference frame in all three spacial axes, we are moving in the direction of time at the speed of light. You travel through space by changing the direction of your velocity through spacetime. A particle traveling at the speed of light through space has zero displacement in time, necessarily. In this model, which reproduces the equation for time dilation just fine, the speed of light is not simply a cosmic speed limit but also a minimum. You are always traveling at the speed of light, and moving through space merely changes the direction of travel. That is the closest explanation of time dilation I have ever seen to what you wrote to start this section (the explanation I just gave came from Brian Greene. In this model, backwards time travel is anything in a forbidden hemisphere of hypothetical velocities (behind you). And speedy forwards time travel requires changing your velocity through spacetime, which is also forbidden. As in most of these models, the restrictions on time travel either do not arise from the model, or are assumed. This is precisely why time travel is still such a topic of discussion amongst physicists - There is no accepted theory of physics that would outright forbid time travel, but there are many good reasons to think it should be impossible. Someguy1221 (talk) 01:41, 21 March 2013 (UTC)

I think you're talking about taking Δτ² = Δt² - Δx² (where τ is proper time and t is coordinate time) and rewriting it as Δt² = Δτ² + Δx², which shows that if you draw a graph of coordinate position versus proper time, the elapsed coordinate time is the ordinary Euclidean length of the worldline. However, in that picture, extending the line downwards takes you backwards in proper time while still going forward in coordinate time. That appears to make even less sense than ordinary time travel; at any rate it's not like jumping into a H.G. Wells style time machine. -- BenRG (talk) 02:12, 21 March 2013 (UTC)

kind of like Event_horizon#Particle_horizon_of_the_observable_universe? Gzuckier (talk) 17:16, 21 March 2013 (UTC)

I don't think so. The particle horizon is a clearly defined concept in cosmology, and isn't related to time travel. -- BenRG (talk) 18:41, 21 March 2013 (UTC)

(edit conflict)

• If you're stationary at a speed u = 0 relative to point A, then you're travelling to A's present according to you.
• If you're travelling at a speed 0 < u < c relative to A, then you're traveling to A's future according to you.
• If you're travelling at a speed u = c relative to A, then you're stationary in A's instant according to you.
• If you're travelling at a speed u > c relative to A, then you're travelling to A's past according to you.

If these are true, then one year since this timespace coordinate according to you, is one radial lightyear away, if you travel in a linear path at a particular speed 0 < u < c relative to that coordinate.

And, then this timespace coordinate according to you, is any radial distance away, if you travel in a linear path at c relative to that coordinate.

And, thene, then one year before this timespace coordinate according to you, is any radial distance away, if you travel in a linear path at a particular speed u > c relative to that coordinate. Plasmic Physics (talk) 01:53, 21 March 2013 (UTC)

The idea that speeds greater than c take you back in time is wrong. The misconception may derive from the fact that the time dilation factor is 1 at v=0 and 0 at v=c, so it looks like it might go negative for v>c. In fact, though, it goes imaginary (the square root of a negative number). Furthermore, since every number has two square roots, you can just as well say the time dilation factor is −1 when v=0. That doesn't make time travel possible.

Other than that, nothing that you wrote above makes any sense to me. More distant things in the universe look older because we see them indirectly, via light that is right here on Earth, in our telescopes, and has been in transit for a long time. This is true even in a Newtonian world if light doesn't go infinitely fast. It has nothing to do with time travel. -- BenRG (talk) 02:12, 21 March 2013 (UTC)

I'm sure the assumption that v>c => backwards time travel comes from the idea that for an object traveling at superluminal velocity, there is a reference frame in which causality seems to be going backwards (a gun sucking a bullet out of the air, for instance). IIRC my college physics. Someguy1221 (talk) 03:10, 21 March 2013 (UTC)

I'm pretty sure both misconceptions exist. Among people who haven't taken college physics there seem to be a lot who think that your wristwatch will start running backwards when you exceed c, with no Lorentz transforms or two-way signaling protocols involved. That has got to be a simple extrapolation of "time stops at the speed of light" (together with the misconception that the slowdown only affects wristwatches, not your thought processes). Among people who have taken college physics there are a lot who think that you can send signals into the past with tachyons, though the truth is that the standard model of causality simply breaks down if there are tachyons, and can't be said to make any prediction at all. -- BenRG (talk) 18:41, 21 March 2013 (UTC)

I took a cosmology paper, and the lecturer never discussed faster than light travel. Plasmic Physics (talk) 21:51, 21 March 2013 (UTC)

Does that imply that there is an imaginary dimension of time? Otherwise what are thte implications of imaginary dilation factors. Plasmic Physics (talk) 02:26, 21 March 2013 (UTC)

There is nothing to indicate that imaginary time dilation factors are anything more than a mathematical artifact. Someguy1221 (talk) 03:10, 21 March 2013 (UTC)

Travelling faster than the speed of light does not mean you are going back in time. It just means you are going somewhere else, and the light from the place you came from will reach you some time after you arrive. If you consider that to be the definition of time travel, then fine. This would then mean that you are simultaneously travelling forward in time, as well as backward, in relation to the object you are flying towards and the object you are coming from, respectively. (Explanation: if you travel 10,000 times the speed of light towards an object you can see from Earth, and that object is 1,000 light years away, you will not get to that object's location 9,000 years before it arrived there. In fact, it won't be there any more, having moved on 1,000 years ago - this will seem to you like you have gone forward in time, and a thousand years later, you will get light from the present-day Earth.) This is my understanding of it, anyway. KägeTorä - (影虎) (TALK) 11:07, 21 March 2013 (UTC)

See here for how you can exploit faster than light travel to travel back in time. Count Iblis (talk) 12:48, 21 March 2013 (UTC)

Age of the universe

Many of our current articles claim that the universe is around 13.73 billion years old. Three years ago, there was a claim (reported in National Geographic) that this should be 13.75, in December last year this was updated to 13.77, and now the Planck space telescope's observations indicate that the figure should be 13.81 or 13.82 billion. How reliable was the 13.73 billion figure, and was it based on other evidence that casts doubt on the Planck space telescope data? Dbfirs 12:36, 21 March 2013 (UTC)

Our article on Age of the universe lists the evidence for the estimated age and lists some of the assumptions made. --Guy Macon (talk) 14:10, 21 March 2013 (UTC)

The current preferred estimate from the WMAP team is 13.772±0.059 Gyr at 68% confidence, which (doubling the error bars) means 13.65 – 13.89 Gyr at 95% confidence. So all of these small changes to the central point are well within the uncertainty that's existed all along. A while back (when the error bars were larger) I changed the Wikipedia articles to say "13.5 to 14 billion light years" in an attempt to discourage bad reporting, but it looks like it got changed back. -- BenRG (talk) 16:35, 21 March 2013 (UTC)

Thanks. I expect there will be new papers published soon, but, as Ben points out, the new estimate is less than one standard deviation above the 13.77 figure quoted (and even closer to the new combined figure), so I agree that we should continue to report that figure for now. I've changed a few of the old 13.73 figures in other articles because that figure is clearly many years out of date. There are lots more to be changed if anyone wants a boring task. Dbfirs 22:45, 21 March 2013 (UTC)

Resolved

C/2013_A1 collision effects

I know the chances of C/2013_A1 hitting Mars is well below 1% now, but I will ask my question anyway. Everything I have found online about the impact is blah-blah-blah crater size. Is there any good speculation on the long-term effects? Specifically, that seems like a whole lot of water. Would it change the atmospheric composition in the long run? Or the pressure? Estimates for its size vary wildly, but we're still talking cubic miles of ice. That seems like a lot. — Preceding unsigned comment added by Tdjewell (talk • contribs) 12:39, 21 March 2013 (UTC)

I haven't seen any references about what you're asking. However, consider that according to Atmosphere of Mars, the Martian atmosphere has a mass of about 25 teratonnes. A cubic kilometer of ice would have a mass of 1 teratonne. If that all made it into the atmosphere, then yes, it would be a huge amount of water - it would raise the concentration of water to about 4% by mass. However, I don't know how much would be ejected, buried and frozen, or lost over time. A proper analysis probably hasn't been done because it is considered so unlikely, and there is a lot to take into consideration. I did check, and our Terraforming of Mars article doesn't mention anything about directing a comet into Mars to change conditions - it seemed like the kind of thing someone may have considered. 38.111.64.107 (talk) 15:49, 21 March 2013 (UTC)

I detect the hand of the Brennan-monster in this affair.... --Trovatore (talk) 17:23, 21 March 2013 (UTC)

One gigatonne. - ¡Ouch! (^{hurt me} / _{more pain}) 10:08, 25 March 2013 (UTC)

Thanks for the correction - I know I looked it over twice since I was surprised that it would make that much of a contribution. A cubic kilometer is huge, but a planet's atmosphere is on a whole different scale. That means a cubic kilometer only contributes .004% which sounds a lot more reasonable to me. Still, it is hard to say what effect that would have on the planet in the long run. 38.111.64.107 (talk) 15:57, 26 March 2013 (UTC)

new space program

I want to set up my own space program and launch rockets and such, obviously it would take a lot of work and money and finding the right people to actually get anywhere interesting, such as the moon, but would it be possible to send a first test rocket at least a little way into space, and what would I need to sort out, to start doing so?

Kitutal (talk) 16:09, 21 March 2013 (UTC)

Among other things, you'll have to obtain clearance from your national airspace authority (the Federal Aviation Administration for the US, the European Aviation Safety Agency for the EU, etc. This page includes a brief overview of the FAA's role in small rocket launches. You may also be interested in the Civilian Space eXploration Team. — Lomn 16:20, 21 March 2013 (UTC)

(ec) It would take a group of dedicated amateurs, tens of thousands of dollars and flight clearance from your national government. The GoFast rocket reached 100 miles in 2004.[1] 75.41.109.190 (talk) 16:21, 21 March 2013 (UTC)

Tens of thousands? No, try billions. SpaceX has had 1 billion dollars to work with over 10 years, and they haven't gotten anywhere near the Moon. --140.180.249.152 (talk) 18:48, 21 March 2013 (UTC)

No thousands is correct. Follow the links Lomn and I provided. The OP only asked about getting to space - not all the way to the moon. Rmhermen (talk) 20:57, 21 March 2013 (UTC)

• Since you'd basically be trying to replicate SpaceX, you might find that article informative. Looie496 (talk) 16:54, 21 March 2013 (UTC)

I suppose it will be cheaper than previous programs, since a lot of what has been done was research, which could be re-used, but you'll still need a huge amount of money. Besides that, you'll need the approval of some government, any government. Uganda may be a good choice, since they have already a (rather pathetic) space program and they are on the equator. Being a quite corrupt country should be no problem for you, since you already have billions to throw away, don't you? If not, I don't see any way of gathering the fund for a project without any economical use like reaching the moon, although a private satellite launching company could be a viable commercial enterprise. See List of private spaceflight companies too. OsmanRF34 (talk) 20:08, 21 March 2013 (UTC)

The most frequently-claimed economical use in reaching the moon is to mine Helium-3 - which is thought to be common there, while it's extremely rare here on Earth. Helium-3 is the dream-fuel for fusion reactors - and if even relatively small amounts could be mined on the moon and shipped back to earth, it would be a phenomenal source of revenue. There are other possible economic gains...solar energy plants at the poles where you get 24/7 sunlight and no atmospheric attenuation. Telescopes on the 'dark side' where no human-created light or radio signals ever reach. Mining ice from deep craters, which your polar solar power site splits into hydrogen and oxygen - which you can sell as rocket fuel at your Lagrange-point refuelling station. How about tourism? I'm sure there are other things. SteveBaker (talk) 20:18, 21 March 2013 (UTC)

This doesn't look quite realistic right now, although it's nice to just imagine how things could be in the far future. But from a present day perspective fusion reactors won't necessarily work some day, electricity for a population of 0, no sure source of water, you can send a probe, instead of creating a moon station, tickets much more expensive than low-cost airlines, indeed, as expensive as an airplane ... OsmanRF34 (talk)

Rather than reinventing the wheel and trying to send a rocket into space, perhaps a better goal is something like Inspiration Mars, which aims to use commercial space technology to send humans to Mars by 2018, or Mars One, which aims to set up a Mars colony by 2023. Both of these projects build upon the proven technology of companies like SpaceX and Paragon Space Corporation to do what nobody has ever done before. --140.180.249.152 (talk) 03:46, 22 March 2013 (UTC)

Calanus

Is calanus a shellfish and is it a scavenger? — Preceding unsigned comment added by 208.38.232.40 (talk) 16:56, 21 March 2013 (UTC)

Calanus are copepods, closely related to shrimp, so I suppose you could consider them shellfish, but they are very small, only a fraction of an inch long. I don't think creatures that small are ordinarily thought of as shellfish. They feed on plankton both living and non-living, so they are partly scavengers and partly predators. Looie496 (talk) 18:26, 21 March 2013 (UTC)

Laser light polarization in laser cutting

I have a rather nifty laser cutter - and I thought I understood all about it...until I read about a gizmo that turns the linearly polarized light from the laser into circular polarization - supposedly to improve it's ability to cut stuff.

The way the laser cutter works is that the light from the CO2 laser is bounced off of three mirrors, then through a focussing mirror and into the material you want to cut. (There is a good diagram HERE). For clarity, the mirrors are numbered 1, 2 and 3 with the number 1 mirror being the first in the path of the laser, then two, then three. So the beam is turned through three right-angles by this mirrors.

The #1 mirror is stationary. When the machine cuts in the "Y" direction, the #2 and #3 mirror are moving together towards or away from the laser. When it cuts in the "X" direction, the #2 mirror is stationary and #3 moves to the left or right along the X axis. The focussing lens is fixed with respect to the #3 mirror.

The claim here is that with the linearly polarized light from the laser tube, that you get a noticable change in the amount of energy you get onto the target depending on whether the system is cutting in the X or Y direction.

I find that rather hard to believe - but there are a paper here that tries to explain it - and at least one manufacturer (eg here) have tricks involving two laser sources that claim to avoid the problem.

I'm having a hard time understanding that paper - can someone explain in more layman's terms why the MOTION of the mirrors can affect the amount of laser energy delivered to the target if the beam is linearly rather than circularly polarized? It seems entirely counter-intuitive to me.

SteveBaker (talk) 20:41, 21 March 2013 (UTC)

Not my field at all, but seeing as nobody else has answered, after a quick scan of your citations, I think you can visualise it this way: Visualise an XY table drawing an image with a medium weight pencil. If the pencil was sharpened in a normal sharpener that gives a circular point, the lines will be equal in width and density regardless of whether plotting with X fixed and Y moving, or vice versa. Now, visualise it with a pencil sharpened the way you were taught in high-schoool woodwork class, i.e., a chisel point. Let's say the long axis of the chisel point is aligned in the X direction. You can see that with Y fixed and the pencil moving in the X direction, the line drawn will be narrow and full density. When plotting with X fixed and Y moving, the line will be wider and less dense.

You can now see that it is NOT the motion of the mirrors that is the key to understanding it. The key is the direction of beam movement over the workpiece with respect to the polarisation, the polarisation being unchanged by the mirror movement (I mean the beam does not rotate as the mirrors move - the polarisation is fixed for any given mirror configuration).

It appears similarly that a linearly polarised laser cuts metal wider if the polarisation is aligned with the direction of beam movement. If the cut is wider, the energy absorbed per unit width must be less. By making the polarisation circular, the effect is midway between the aligned direction and the orthogonal direction, reagrdless of direction of travel.

Wickwack 121.221.25.39 (talk) 00:46, 22 March 2013 (UTC)

That just sounds totally bizarre to me. The laser beam is not literally fatter in the direction of its polarization, and thin in the direction orthogonal to it. Someguy1221 (talk) 01:25, 22 March 2013 (UTC)

Nobody said it was, though in practice it could be another (minor) factor. But the two references the OP cited indicate that polarisation vis a vis direction of travel does matter (due to another reason). The essence of the OP's query is that he could not visualise how movement of mirrors affects cutting. In fact it doesn't - Steve has misunderstood what's happening. I provided an analogy to help him understand that movement of mirrors is a red herring. Perhaps you could read more carefully what the OP said, what I said, and what the references said. Wickwack 124.182.55.66 (talk) 02:33, 22 March 2013 (UTC)

You said it was, in your pencil analogy. Someguy1221 (talk) 07:30, 22 March 2013 (UTC)

Well, that is the danger of using analogy I suppose. Some people extrapolate beyond what is intended in a foolish way, just to show they haven't got the point. How would you have explained to the OP that the movement of mirrors is not the key as he thought it was? The point is not about pencils, the point is that the mirrors do not rotate the beam just as the pencil is not rotated - this results in different conditions at the workpiece as the cutting/marking direction changes - something the analogy makes obvious. Wickwack 121.221.211.67 (talk) 14:55, 22 March 2013 (UTC)

I understand from your analogy how a non-circular cross-section beam might produce different cut widths when cutting vertically instead of horizontally...but as far as I know, the beam is more or less circular in cross-section - and focussed down to a small circular point. So, OK, maybe this is an analogy - but how does linear polarization versus cicular have anything to do with the beam width? SteveBaker (talk) 15:49, 22 March 2013 (UTC)

You are like Someguy1221 - you have focussed on cross section, whereas my analogy was to help the OP undertsand that the orientation of the beam wrt the workpiece does not change but does change wrt cutting direction. The pencil with a chisel cut is "polarised" and the laser beam is polarised, but not in the same way. I thought that was so obvious as to not require qualification - but it seems for some people that it is a confusion. Wickwack 58.170.154.119 (talk) 01:09, 23 March 2013 (UTC)

If I'm making the same misconceptions as SteveBaker, I feel pretty good about myself. Someguy1221 (talk) 01:38, 23 March 2013 (UTC)

It would seem that from Steve's comments below, he later understood what I was getting at, and devised a test that could prove or disprove it. So you feeling good is not for now justified. Wickwack — Preceding unsigned comment added by 58.170.154.119 (talk) 01:56, 23 March 2013 (UTC)

I'm sorry your analogy sucked. Someguy1221 (talk) 04:47, 23 March 2013 (UTC)

I think this is related to the principle behind Brewster's angle, i.e., when light hits a surface at a glancing angle, the rate of absorption is higher when the polarization direction is aligned with the surface normal. It's not obvious what polarization would be best for laser cutting, but it makes sense that it might matter. According to the paper, for the simplest transverse beam mode (TEM₀₀), circular polarization (C) is preferable to linear polarization either parallel (P) or perpendicular (S) to the cut, for some reason related to the fact that it's absorbed equally in the forward and sideways directions. -- BenRG (talk) 04:08, 22 March 2013 (UTC)

That certainly sounds reasonable to me, though I'd have thought you'd want the energy of the beam to be absorbed preferentially in the direction of the cut rather than the two sides so getting the polarization right might lead to not only a quicker cut but a thinner one. Dmcq (talk) 13:49, 22 March 2013 (UTC)

Yes, but that would require that the polarisation be continually adjusted to match the direction of cut as the direction of cut changes. The advantage of circular polaristion is that you get a consistent cut finish without the complexity that rapid polarisation change would require. Wickwack 121.221.211.67 (talk) 14:44, 22 March 2013 (UTC)

Sure, the plane of polarization may change at each of the three mirrors, but these silicon-backed, gold-coated mirrors are claimed to be 98% reflective, so clearly not much energy is lost due to the linear polarization. Indeed, if they were much less reflective than they are then the energy that they absorb would have them glowing red hot within seconds. (And that's exactly what happens if you don't keep the mirrors scrupulously clean!). The circular beam geometry is still circular as it enters that final lens - and I know that it is because if I put a piece of tape in place of the focussing lens, I get a neat, almost perfectly circular, 1/4" hole chopped out of it.

• So if even the worst case for polarization only loses us 2% of energy per each of the three mirrors - then no improvement due to polarization will change that by much.
• Since the final beam is circular (at least to the precision that I can visually estimate it), there can't be any gross geometric reason for this effect.
• The laser hits the material at 90 degrees to the surface.

Are we saying that in a brief *zap* with no motion of the mirrors - that even though the beam is circular, the polarization results in an elliptical hole being formed? The "pencil analogy" (well, the "Calligraphy nibbed pen analogy" might work better!) seems to require that to be a true statement.

The hole that my laser cutter makes is a few hundredths of a millimeter across - so I'm going to need a microscope to know that for sure!

I do have a "kerf test" pattern that I can probably use to measure the thickness of the hole that's produced - I habitually measure a Y-direction cut when I'm testing to be sure that the laser is properly focussed - I guess I could easily rotate the test pattern by 90 degrees to verify that.

SteveBaker (talk) 15:49, 22 March 2013 (UTC)

Steve - great to see you reading all this! Many times we respond to a queston, but never hear from the OP again, so we don't know whether we helped, or it was just a lot of blather to the OP, or whathever. As I said right at the start, laser cutting is NOT a field I am expert in, but I could see right away that which mirros move have nothing to do with why circular polarisation may be beneficial, so I explained that. I'm not qualified to say whether a spot zap will produce an eliptical hole, though it seems likely. The tape test could be misleading, as it is at different intensity per unit area, and (I presume) a different material. A test using a rotated kerf test pattern seems a good idea. Not the least because if you the user can't tell the difference, all this theory matters not - you need not waste money on the circular polarisation gizmo. I never thought the mirrors had any significant loss, for the reason you gave (they'd get hot), but even if they did have significant loss, as the beam is virtually of constant width between the mirrors (or should be), moving the mirrors will not change the total loss. Wickwack 58.170.154.119 (talk) 01:52, 23 March 2013 (UTC)

Wickwack, I see nothing in the paper to suggest that linear polarization is preferable to circular even when you only cut in one direction. It seems to say exactly the opposite. Steve, the mirrors have nothing to do with it. I was talking about absorption by the material that's being cut. As the paper says, the cut is usually much deeper than the diameter of the beam, so most of the beam energy is absorbed by the walls of a deep pit in the material, at a small glancing angle. -- BenRG (talk) 18:04, 22 March 2013 (UTC)

I just had a read of that paper and it explains why what I was saying about just using p polarized light didn't work as well as might be hoped. The cut rapidly became V shaped in the direction of the cut and so the beam wasn't being absorbed properly. The circularly polarized light gave a U type cut instead and was about as good as the p-polarized light and better than the s-polarized and since it was easier it is a good general solution, but the radially polarized light was even better even though it is a more complicated business to produce it. At least that's my reading of it. Dmcq (talk) 23:31, 22 March 2013 (UTC)

So P-polarised (aligned with cut direction) does work better that S-polarised (orthogonal to cut direction), contary to what BenRG says? That was my reading of it as well. Wickwack 58.170.154.119 (talk) 02:06, 23 March 2013 (UTC)

Most of the time, the cut is *much* deeper than the diameter of the beam. I mostly cut materials between 3 and 6mm thick - but my machine can delicately cut paper and thin cloth - and up to maybe 15mm wood. The unfocussed laser beam is about the diameter of a pencil - but it's focussed down to a few hundredths of a millimeter. It's a 100 watt laser - the amount of energy it's putting out is no more than a 100 watt lightbulb. The reason it slices through wood and plastic like it wasn't there is because you're taking all of that energy output and concentrating it into a microscopic dot.

The business of the shape of the bottom of the cut is interesting...but the actual practical situation is more complicated. We have a gizmo called "Air Assist" which is a high power air jet aimed into the slot as the laser cuts. This helps the laser to cut thick materials by pushing the intensely hot air surrounding the laser beam down into the slot instead of allowing it to rise up out of the slot. That hot air pre-heats the material before the laser hits it.

With the air assist turned on, I'd expect a very different leading edge to the cut than without it. SteveBaker (talk) 03:21, 24 March 2013 (UTC)

Depending on the material you are cutting, Air Assist might be causing combustion, somewhat like oxy-acetaline cutting steel. If you attempt to cut steel with a standard oxy-acetaline welding torch, what you do is locally melt the steel - resulting in an ugly blobby cut without good penetration. Oxy-acelatine cutting torches have an extra oxygen feed - when you turn on the extra oxygen (after first locally heating the start of the cut) it rapidly burns the workpiece, resulting in much deeper penetration and a much better cut finish.

Did you try rotating your kerf test pattern 90 degrees to see what the effect was?

Wickwack 60.230.231.106 (talk) 03:53, 24 March 2013 (UTC)

Isn't the air assist just to keep the area clear of smoke and actually to stop the hot air affecting anything else, so only the point where the laser is pointed is affected and you have a nice clean cut. Dmcq (talk) 18:53, 24 March 2013 (UTC)

Neaspora in cattle

Can a bull whose mother is positive for neaspora pass on the disease to his offspring — Preceding unsigned comment added by 92.41.39.54 (talk) 20:55, 21 March 2013 (UTC)

According to our article (Neospora), it can be passed from cow to calf, but not from bull to calf. Tevildo (talk) 21:21, 21 March 2013 (UTC)

How's this animal called?

Picture: http://imgur.com/Nc8Izej --109.173.37.164 (talk) 20:59, 21 March 2013 (UTC)

A Tapir (Tapirus terrestris, the Brazilian tapir, in this particular example). Tevildo (talk) 21:23, 21 March 2013 (UTC)

Thanks! --109.173.37.164 (talk) 21:52, 21 March 2013 (UTC)

Why faces matter?

Why do we humans care so much about faces? An ugly woman could be perfectly able to generate offspring, so why bother about her wide nose, big ears, or whatever trace that could make a face ugly? Do faces reflect the functioning of internal organs? OsmanRF34 (talk) 21:04, 21 March 2013 (UTC)

Sex appeal, maybe? I, for instance, would be much more turned on by a woman with a hot face than with an ugly face, if the rest of their bodies were (hypothetically) equal or almost equal in appearance. Futurist110 (talk) 22:04, 21 March 2013 (UTC)

I'd say it's far more about free choice than generation of offspring. Personally I don't care if someone is ugly or beautiful; it depends on their intelligence and such. Again, that's fairly subjective as well. This sort of thing happens with animals all the time. If you're a male peacock, it doesn't matter if you're the most fertile if the female doesn't like your plumage. RMoD (talk) 22:16, 21 March 2013 (UTC)

Futurist110: your answer is kind of tautological

RMoD: it leaves often why people freely care about faces. OsmanRF34 (talk) 22:41, 21 March 2013 (UTC)

"The Evolutionary Psychology of Facial Beauty" talks about the various theories: "Theorists have proposed that face preferences may be adaptations for mate choice because attractive traits signal important aspects of mate quality, such as health. Others have argued that they may simply be by-products of the way brains process information." Personally, I believe it's driven by the inexorable evolutionary pressure to get a Wikipedia article, the pinnacle of success. See WP:HOTTIE. Clarityfiend (talk) 22:19, 21 March 2013 (UTC)

nice link. OsmanRF34 (talk) 22:41, 21 March 2013 (UTC)

Also note that "beauty is in the eye of the beholder". I, for one, place a high value on facial beauty, but I know that my idea of beauty is quite different to that of other people (or at least to the one portrayed by the media). All those "supermodels" don't hold any appeal to me. 86.136.42.134 (talk) 22:45, 21 March 2013 (UTC)

is there any evidence that symmetry and proportion in face and body is reflective of a deeper symmetry and proportion of organs, bones, and even a proper balance of chemicals and chromosomes?68.36.148.100 (talk) 01:36, 22 March 2013 (UTC)

The Roman encyclopaedist Aulus Cornelius Celsus.

Greeks and Romans definition of beauty was a straight nose that was in line with the forehead. I read in Africa the male prefer the real weighty women. Japanese women ought to have small feet, some african tribe preferres very long necks etc. etc. Its all specific cultural tradition obviously. In the end it seems simply the feature or shape all or most of your local competitors are after, that you adapt. --Kharon (talk) 05:59, 22 March 2013 (UTC)

Scientifically Determining One's Relatives

Is it scientifically possible (excluding paternity and maternity tests) to determine how closely related someone is to someone else? For instance, would it be scientifically possible (without looking at records and documentation) to determine that my first cousin is more closely related to me than my third cousin or fourth cousin? I apologize if this is a stupid question, but I am genuinely curious about this. Futurist110 (talk) 22:03, 21 March 2013 (UTC)

The only stupid question is the one you don't ask. So, what information do you allow? Are we allowed eyewitness testimony? If there is only one doctor that delivers all the babies, he could establish maternity. Are you allowed to compare whether people look alike? --Guy Macon (talk) 22:20, 21 March 2013 (UTC)

Nope, eyewitness testinomy is not allowed. Neither is comparing if people look alike. Only scientific (especially genetic) testing is allowed for the purposes of this question. Futurist110 (talk) 00:42, 22 March 2013 (UTC)

It's not that hard, actually. You share 50% of your genes with each of your siblings, 25% of your genes with each first cousin, 12.5% with each second cousin, and so on. For every x generations back your nearest common relative is you'll share 1/2^x of your genes with them. For removed cousins, replace the x with (x+y)/2 where x and y are the number of generations back for each of you to that nearest common relative. --Jayron32 22:25, 21 March 2013 (UTC)

I believe Jayron's excellent answer becomes more correct with liberal insertions of "on average". That is, siblings on average have 50% of their alleles in common (let's also remember that, strictly speaking, genes are not alleles: the gene is the locus, the allele is what code is there). A given pair of siblings may share more or less than 50%, due to some randomness in recombination, meiosis, and other reproductive processes. SemanticMantis (talk) 23:01, 21 March 2013 (UTC)

Pardon my ignorance on this, but it there any scientific way to test for the percentage of common genes between two people? Futurist110 (talk) 00:42, 22 March 2013 (UTC)

Yes, you completely sequence their entire genomes, and then you can define a degree of similarity. Someguy1221 (talk) 01:06, 22 March 2013 (UTC)

For a shortcut, since chromosomes normally stay more or less intact, you can just check the number of chromosomes in common. This is a lot less work than comparing every base pair (you do need to compare some base pairs, to determine if the chromosomes are common or not). One of the chromosomes may not even require genetic testing. Excluding abnormal sex chromosomes, if one of the two people being compared is male, and the other is female, then you know they don't share the chromosome which is X in the female and Y in the male. StuRat (talk) 02:10, 22 March 2013 (UTC)

I'm quite sure it's been explained before, most people will probably have zero chromosomes in common with their parents or anyone else except identical twins or clones (ignoring random mutations) due to chromosomal crossover. While our article doesn't discuss it, I believe sources have been presented before showing that most chromosomes have at least one during successful meiosis and if they weren't, it's easy to find sources saying that for C. elegans [2] and humans [3]. As our Pseudoautosomal region article mentions, we shouldn't even exclude sex chromosomes since it appears to be needed for X-Y chromosomes as well. Nil Einne (talk) 13:36, 22 March 2013 (UTC)

Yes, but isn't the total volume of genetic change resulting from the one or two chromosome crossovers quite small, such that it could be ignored when trying to do a quick comparison of "relatedness" ? StuRat (talk) 15:57, 22 March 2013 (UTC)

Companies like 23andMe can work up detailed genetic profiles and will identify relationships. Their "relative finder" tool will flag siblings, parents, and cousins. For more distant relationships they say things like 3rd to 6th cousin, or 4th to "distant" cousin, etc., which give estimated ranges for relatedness. Dragons flight (talk) 01:22, 22 March 2013 (UTC)

One issue that will come up in any genetic comparison, though, is whether the genetic differences are "significant". That is, there are many differences in DNA which don't actually do anything important, while one base pair different, somewhere else, can make a huge difference. So, just looking at the total volume of genetic differences isn't very useful in telling how different two people are. (It's a bit like comparing the contents of two houses to determine the similarity in wealth of the owners, and saying "there's only 1 difference found, so they must have a similar wealth", even though that one difference is the presence of a 1000 carat diamond in one of the two houses.) StuRat (talk) 02:13, 22 March 2013 (UTC)

True (sort of), but not relevant to the OP's question. A better analogy is this: Two houses should not be regarded as being built to different designs just because one has more furniture, or painted in different colours. Wickwack 124.182.55.66 (talk) 02:42, 22 March 2013 (UTC)

The OP mentions "excluding paternity and maternity tests", but allows genetic testing, without perhaps realizing that most Parental testing (especially such that would be recognised in court) IS genetic. Vespine (talk) 02:51, 22 March 2013 (UTC)

Consider: A man and his wife have sex several times a week on a regular basis. One day, the wife has an affair with her husband's identical twin brother. 9 months later she gives birth. I don't think there is any test of any kind that could tell you who the father of that baby is.

It's all a matter of the degree of certainty you require and biological similarity of the potential fathers and mothers:

• If they are genetically kinda similar (same blood groups, for example), then figuring it out is harder, if there are identical twins involved, it's probably impossible.
• If the potential candidates they are sufficiently genetically different then it's much easier. Suppose the mother and one potential father have dark skin and the other potential father is caucasian - then the skin color of the child is almost certainly a dead giveaway as to who the father was (not 100% certain - but pretty damned close). I'm sure that having certain genetically-linked diseases pop up in parent and child would also make the result a near-certainty.
• In more average situations, you might be able to study the family tree of one possible parent versus another and look at the hair and eye color, the propensity for certain genetic diseases and thereby come up with a fairly certain idea of who was the parent...but you'd never be 100% sure.

The degree of certainty (it's never going to be 100%) depends on the degree of genetic diversity in the candidate parents and the amount of information you have about how those genes are expressed in ways that you can measure within the constraints of the rules you're arbitrarily applying. If we can do a complete gene-sequence of everyone involved - then we'll be very certain. If we're only allowed to talk to them on the phone, we'll be very uncertain. If they are identical twins, we won't know - if they are wildly opposite genetically, then we can probably tell at a glance.

We can't give you a definite yes or no answer on this one. SteveBaker (talk) 17:01, 22 March 2013 (UTC)

There are genetic tests that can tell identical twins apart: basically, you do a full sequencing of the genome and look for mutations that took place after the egg divided into two embryos. There won't be very many of them, which is why you need to look at the entire DNA sequence rather than the limited areas that a typical DNA fingerprint uses. --Carnildo (talk) 22:32, 22 March 2013 (UTC)

Can a chimpanzee give birth to a human baby?

Suppose we implant a fertilized egg in the womb of a chimpanzee. Will this lead to a healthy baby after 9 months? Would the baby be born after 9 months or would it have to be removed using a C-section? Count Iblis (talk) 23:23, 21 March 2013 (UTC)

Almost certainly not by natural birth. The human pelvis is adapted to be able to give birth to something with a huge head compared to other primates. --Guy Macon (talk) 00:01, 22 March 2013 (UTC)

And if you put apart the anatomical issue (where a tremendous number of babies died through all primate species) and then put apart any tissue rejection issues, which would seem to be the most major issue, the hormonal triggers in homosapien pregnancy differ in some subtle, and not so subtle ways from other primates. Even Rh factor matters in human births. It's an incredibly sensitive process, and it fails about as much as it works. That all said, the better question is, what are the impediments to an interpsecies birth. I think that question's a lot more answerable, and maybe has been done? I'd like to hear the expanded explanation of this. Shadowjams (talk) 01:16, 22 March 2013 (UTC)

I would also be interested to hear about the feasibility of implanting a Wooly Mammoth embryo in an elephant. (Insert Jurassic Park joke here) --Guy Macon (talk) 02:44, 22 March 2013 (UTC)

Or maybe in a ferret, where after a while you'll be singing, "Pop! Goes the weasel." ←Baseball Bugs ^{What's up, Doc?} carrots→ 04:37, 22 March 2013 (UTC)

Ultimately, this question is unanswerable as no one has tried. Good luck floating this one past an ethics committee. Should have tried it while the Soviet Union was still around. Someguy1221 (talk) 10:27, 22 March 2013 (UTC)

They did! See Humanzee. Although this was the creation of a hybrid, not the implantation of a human embryo. (I seem to recall a plan to implant human embryos into cows, as well - I'll see if we have an article on it). Tevildo (talk) 10:38, 22 March 2013 (UTC)

They didn't! Read the article... --Jayron32 20:02, 22 March 2013 (UTC)

I didn't say they _succeeded_, and I note the large number of "citation needed" tags in the article (which are on material which wasn't there the last time I read it). But the attempt was seriously considered, at least. Tevildo (talk) 20:41, 22 March 2013 (UTC)

A followup question. Can a human female (a woman) give birth to a chimpanzee baby if a fertilized Chimpanzee egg is implanted in the woman's womb? The head of chimpanzee baby is much smaller, so I don't think anatomy would be an issue. --PlanetEditor (talk) 11:14, 22 March 2013 (UTC)

Thanks for all the answers so far. If this sort of thing can be made to work, women would no longer need to get pregnant, so it would have huge economic benefits. Count Iblis (talk) 12:29, 22 March 2013 (UTC)

someone has to post it [4] Gzuckier (talk) 14:36, 22 March 2013 (UTC)

OK - that's just a dumb comment! You aren't thinking this through.

Firstly: You're making the grave mistake of assuming that women don't want to get pregnant in order to have children...which is certainly not universally true. Many women enjoy their pregnancies...many more want to have the experience at least once, even though they know it can be tough on their bodies. Also, handing over a baby that the mother didn't carry for all those months causes all kinds of bonding issues - it would prevent lactation, so the baby would have to be bottle-fed (which is known to have all sorts of adverse consequences).

Secondly: There is the demographic issue - there are 12.7 babies born for every 1,000 people in the US every year - so in a population of 300 million, you'll be needing around four million female chimps in good health and child-bearing age at all times. That means you'll need to breed those animals, feed and house them somehow throughout their lives. Suppose one female chimpanzee could survive a dozen pregnancies - she could serve as surrogate womb for only between 4 and 6 human females - depending on the birth rate, etc. Ultimately, the total male, female, adult and baby chimpanzee population would have to be perhaps a tenth to a quarter of the human population size to make this a sustainable effort. That is not a small economic cost! Over a the life of a human female, she would (in effect) have to pay for perhaps a quarter of the lifetime cost of breeding, owning, housing and rearing a chimpanzee in reasonably hygenic and humane conditions - and her only benefit is saving the time she'd not be working due to pregnancy. That's got to be millions of dollars over the life of the chimp. Most women these days are able to work and produce economic gain until within weeks of the birth. My g/f's daughter worked until 3 days before she gave birth. All of the economic "loss" is in rearing the child for the first months until she can return to work. So almost none of the economic loss due to her pregnancy would be saved by surrogacy. So under your scheme, the earnings lost over perhaps a month or two total in a woman's life - maybe $10,000 - would have to pay for her share of a quarter of the cost of housing a chimpanzee for 30 or 40 years!

Thirdly: The cost of the specialised medical intervention required to transplant eggs into the chimp, the cost of the (required) cesarean birth, the cost of anti-rejection drugs...that alone would dwarf the economic loss of a few weeks of the natural mother's work.

I don't think there is an economic gain here at all! It's a gigantic loss. Biology and ethics aside - I don't think such a scheme would be met with the wild economic enthusiasm you imagine.

SteveBaker (talk) 16:40, 22 March 2013 (UTC)

Steve, I agree with your conclusion, that it's unlikely to come out in the green economically, but did you really mean to say that it would cost several million dollars to maintain a chimp for life? Is the chimp eating at Chez Panisse every night, or what? --Trovatore (talk) 20:53, 22 March 2013 (UTC)

Actually, it wouldn't prevent lactation at all. Also, the fact that surrogates exist and are often engaged at significant cost to the genetic parents, there certainly could be a market for this type of thing, even if it is hideously expensive. 202.155.85.18 (talk) 03:42, 23 March 2013 (UTC)

The uterine environment matters enough that people talk about it affecting sexual orientation, obesity and so forth depending on what happens to a human mother. The effects of a chimp uterine environment are hard to predict, but I doubt they would be negligible. Wnt (talk) 20:42, 22 March 2013 (UTC)

• Intergeneric hybrids are known. It seems rather silly to assume there is something especially different between chimps and humans metabolically that would make chimps standing surrogates to humans difficult, other than the size of the human infant and its skull. μηδείς (talk) 21:25, 22 March 2013 (UTC)

In biology you never know until you do the experiment ... that's what makes it so much fun! The reverse experiment would be easier, of course - all we need is a puckish technician at a pro-life in vitro fertilization clinic who has a friend at the zoo... Wnt (talk) 14:48, 23 March 2013 (UTC)

But where would you find a woman willing to go along with this scheme? 24.23.196.85 (talk) 19:15, 23 March 2013 (UTC)

That's what makes him puckish. :) Wnt (talk) 23:20, 23 March 2013 (UTC)

About the costs of using Chimps, there may be other options, like artificial wombs or biological versions of that. You can e.g. imagine growing a womb using a woman's stem cells in the lab... Count Iblis (talk) 15:12, 23 March 2013 (UTC)