## Search found 950 matches

- Sat Jul 16, 2011 12:52 pm UTC
- Forum: Science
- Topic: Scattering off vaccum particles
- Replies:
**13** - Views:
**2863**

### Re: Scattering off vaccum particles

Photon -> Photon + electron + positron cannot conserve energy and momentum at the same time. The right side has a physical center of mass system, the left side does not. You can express the same thing with 4-vectors to calculate it. Pair production always needs some particle with mass (usually a nuc...

- Sat Jul 16, 2011 12:04 pm UTC
- Forum: Science
- Topic: nature of elementary particles...
- Replies:
**66** - Views:
**7658**

### Re: nature of elementary particles...

Feynman diagrams are great stuff. But when you try to calculate anything interesting, the maths just gets too complicated...too many loops! :? So how do you compute things then? On the lattice? Nice for some QCD approximations, but I doubt your computer would reach the precision to compete with the...

- Sat Jul 16, 2011 11:18 am UTC
- Forum: Science
- Topic: Elements of Physics?
- Replies:
**5** - Views:
**1398**

### Re: Elements of Physics?

Maybe that and the category tree here and here can help you. But there is no single tree where each concept got a unique position and is not related to anything other than the parts given by the tree. To use your example: waves can appear as sound (moving particles) or as electromagnetic waves (elec...

- Sat Jul 16, 2011 10:47 am UTC
- Forum: Mathematics
- Topic: A set in the complex plane edit: ?s about complex variables
- Replies:
**26** - Views:
**3350**

### Re: A set in the complex plane edit: ?s about complex variab

It does not matter whether you use a<=t<=b or 0<=t<=1 as parameter for your curves.

Note that the theorem tells you something about the curves itself, not about some parameterizations of them.

Note that the theorem tells you something about the curves itself, not about some parameterizations of them.

- Sat Jul 16, 2011 10:28 am UTC
- Forum: Mathematics
- Topic: Chaos pendulum
- Replies:
**19** - Views:
**4089**

### Re: Chaos pendulum

The last 4 equations in the part "Lagrangian" are the best thing you can get there. If you want to calculate the position at time t, you have to calculate it numerically step by step, and your calculations will get larger and larger errors over time. There is no way to prevent this. That i...

- Thu Jun 30, 2011 5:49 pm UTC
- Forum: Science
- Topic: The CERN LHC Blackhole debacle!
- Replies:
**18** - Views:
**3083**

### Re: The CERN LHC Blackhole debacle!

You don't need the mass of the moon for a (relativly) stable black hole. If you want one which needs 3 years for its decay, it needs a mass of ~10^8 kg - far beyond the energy in the LHC, but less than the largest ships on earth. For 1000 years, it needs 10 times the mass, and for 3 billion years ju...

- Wed Jun 29, 2011 9:13 pm UTC
- Forum: Science
- Topic: Why do we measure half-life instead of just life?
- Replies:
**62** - Views:
**13551**

### Re: Why do we measure half-life instead of just life?

But fundamentally, decay calculations are statistical; so the accuracy of prediction is reduced when the sample size is small. So for a particular material, maybe we can't reach 95% confidence if we have fewer than 100 atoms to consider. In a population of 1000, then, any estimate shorter than the ...

- Sat Jun 25, 2011 8:16 pm UTC
- Forum: Mathematics
- Topic: Intersecting circles [of doom] 3.0 (Now with more sanity!)!
- Replies:
**8** - Views:
**1479**

### Re: Intersecting circles [of doom] 2.0!

Strange thing. With Ra, d and theta, it is possible to calculate M and an equation for the line of L1. So far, it is no easy. But then we get two undefined parts: - the length of L1 - the angle phi There is just one additional constraint which comes from the angle beta together with Rc. But that is ...

- Thu Jun 23, 2011 2:55 pm UTC
- Forum: Mathematics
- Topic: How practical is a B.S. in Math?
- Replies:
**20** - Views:
**7571**

### Re: How practical is a B.S. in Math?

e^(ipi)+1=0 wrote:So what can someone do if they got a B.S. in mathematics?

Getting a master in mathematics.

It will help a lot if you look for a useful mathematics-related job (and there are many).

- Mon Jun 13, 2011 12:27 pm UTC
- Forum: Mathematics
- Topic: Expected value of the product of a sample?
- Replies:
**4** - Views:
**1072**

### Re: Expected value of the product of a sample?

More details: Let P=(x_1,x_2,x_3, ... ,x_N) with x 1 != x 2 be a set of N positive real numbers. You take randomly n values out of that (where each value has the same chance to get picked) and calculate the product of all of them. Let X be the expectation value of that product. Now I will sh...

- Mon Jun 13, 2011 11:40 am UTC
- Forum: Mathematics
- Topic: This geometry seems fishy...
- Replies:
**12** - Views:
**2096**

### Re: This geometry seems fishy...

Proportions should not matter. 4 "N"-shapes on the outside 4 reduced "N"-shapes on the inside. 4 squares on the top 4 squares on the bottom 8 rectangles on the inclines -- 24 faces If the N's and the reduced N's consists of 5 "faces" each (see greengiant), the number in...

- Mon Jun 13, 2011 11:18 am UTC
- Forum: Mathematics
- Topic: Question on probability (19 independent lights)
- Replies:
**7** - Views:
**1120**

### Re: Question on probability (19 independent lights)

You can generate tables for n (or for 0 to n) lights or use existing tables and just add (subtract) these values to answer all question like that, but except from "use the computer" there is no real way to speed it up. 4.75 is the right answer for question 1. Expectation values can be numb...

- Mon Jun 13, 2011 11:10 am UTC
- Forum: Mathematics
- Topic: Is there an intuitive appeal for multiplication of integers?
- Replies:
**22** - Views:
**3641**

### Re: Is there an intuitive appeal for multiplication of integ

If you can convice them that (a+b)*c = a*c + b*c, you can try:

0 * (-6) = 0

(5 - 5) * (-6) = -30 + ? = 0

But I think that tables like at greengiant's post are even better to show that. Any other values than the right ones would really break the pattern.

0 * (-6) = 0

(5 - 5) * (-6) = -30 + ? = 0

But I think that tables like at greengiant's post are even better to show that. Any other values than the right ones would really break the pattern.

- Mon Jun 13, 2011 10:34 am UTC
- Forum: Science
- Topic: An interesting question: Perfect Mirror Ball
- Replies:
**56** - Views:
**12646**

### Re: An interesting question: Perfect Mirror Ball

There are clearly cases where we can have two equal energy state areas, and a wall that is a ramp on one side and a sharp drop on the other. Sufficiently energetic objects can clearly cross the ramp from one side, but rebound off the sharp drop on the other. The question is simply can we create a m...

- Sat Jun 04, 2011 11:11 am UTC
- Forum: Mathematics
- Topic: Socks in a Drawer (OCD Rant)
- Replies:
**5** - Views:
**1579**

### Re: Socks in a Drawer (OCD Rant)

Assuming you know the number of left and right socks for each color (and you should, if you want to calculate those numbers!) and it is possible to divide all socks of each color in proper pairs, you can use undifferentiated socks earlier. I just look at one color and assume that S^l > S^r to reduce...

- Sat Jun 04, 2011 10:22 am UTC
- Forum: Science
- Topic: Physics and Assumptions
- Replies:
**11** - Views:
**2118**

### Re: Physics and Assumptions

A theory can be falsified. The theory "there is no gravitational force and all objects are floating around" can be falsified without any problems. Newton's theory is wrong, too, which has been shown in many tests of General Relativity (GR). However, it is a good approximation for slow velo...

- Sat Jun 04, 2011 10:11 am UTC
- Forum: Science
- Topic: Spearing a fish
- Replies:
**3** - Views:
**1075**

### Re: Spearing a fish

To calculate that, as I don't like a "proof by sketch": http://i54.tinypic.com/10xu2ax.jpg n is the refractive index, the other one is WLOG set to 1 as only the ratio is interesting. The first ray hits the surface at A=(sin(alpha),0) and the outgoing ray has an angle of alpha' with sin(alp...

- Sat Jun 04, 2011 7:48 am UTC
- Forum: Science
- Topic: Systems of propulsion on motorways
- Replies:
**36** - Views:
**3954**

### Re: Systems of propulsion on motorways

Getting all the benefits of centralized transport systems with all the benefits of cars for last mile seems like an impossible task. It has been the holy grail for public transport for a long time now. So you are looking for PRT . That, combined with an efficient railway system for longer distances...

- Sun May 29, 2011 7:29 pm UTC
- Forum: Science
- Topic: Orbital Paths
- Replies:
**4** - Views:
**1745**

### Re: Orbital Paths

As long as just one object is relevant, you can use an ellipse (or hyperbola) to calculate the path. But putting together more of them will not work, unless your ship is really fast compared to the relevant velocities in the system. You always get in regions where the gravitational pull of at least ...

- Sun May 29, 2011 1:01 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 0903: "Extended Mind"
- Replies:
**666** - Views:
**177792**

### Re: 0903: "Extended Mind"

Seriously Wikipedia could be edited within two months to give an 55% to 95% change on articles leading to philosophy?! Just modify the philosophy-loop to exclude philosophy, and the fraction will drop to less than 55%. "Quantity is a [[Mathematics|mathematical]] concept and a kind of property ...

- Fri May 27, 2011 10:19 pm UTC
- Forum: Science
- Topic: A statistics question about propagation of random errors
- Replies:
**10** - Views:
**2466**

### Re: A statistics question about propagation of random errors

Maybe you did the same mistake 10 times. Some systematic errors will be highly correlated through all experiments, so you cannot estimate them from your distribution of the P-values, but have to determine them separately. No matter how many experiments you perform, they will not go away. The standar...

- Fri May 27, 2011 10:07 pm UTC
- Forum: Science
- Topic: No such thing as velocity?
- Replies:
**6** - Views:
**1582**

### Re: No such thing as velocity?

'Particles' don't have actual positions, I get that And the same thing is true for velocity. In fact, it is possible to transform the wave function in space to a wave function in velocity (or momentum, which is better for calculations). There, a velocity (momentum) is a point of the wave function a...

- Thu May 26, 2011 10:04 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 0903: "Extended Mind"
- Replies:
**666** - Views:
**177792**

### Re: #903: Extended MInd

We should totally make an application which scrapes all the first links in all wikipedia articles and finds all the loops and then determines what percentage of articles are directed to each loop. For science. And so we know how correct/incorrect Randall is. That, in fact, is the only way to get so...

- Wed Apr 06, 2011 9:29 am UTC
- Forum: Science
- Topic: How come there are no beneficial diseases?
- Replies:
**69** - Views:
**12480**

### Re: How come there are no beneficial diseases?

We just don't call it a disease if it does not harm us. But look how many different species we have (and use) in our bowel.

- Mon Apr 04, 2011 2:00 pm UTC
- Forum: Science
- Topic: Microgravity
- Replies:
**5** - Views:
**987**

### Re: Microgravity

During 2g-sections, it is easy: The pilots can control the angle of the airplane to get the direction of the effective force where they want to (with some technical restrictions, of course). Contributions from a rotation of the plane are small. During 0g-sections, the direction of force is not defin...

- Mon Apr 04, 2011 12:52 pm UTC
- Forum: Science
- Topic: a CAPACITOR PROBLEM
- Replies:
**3** - Views:
**946**

### Re: a CAPACITOR PROBLEM

At the battery, you have a potential difference of 20 V. Depending on the direction of the measured 10V and the orientation of the battery, you have to add or subtract it, as the sum of potential differences at all three elements is 10V. So U capacitor1 -U battery + U capacitor2 =10V which is the sa...

- Mon Apr 04, 2011 12:42 pm UTC
- Forum: Science
- Topic: Most efficient way to approach a red traffic light.
- Replies:
**16** - Views:
**2907**

### Re: Most efficient way to approach a red traffic light.

Ok, with realistic models for the fuel consumption, things get really difficult. I will go back to the original rules: No acceleration as long as the traffic lights are red, try to maximize the minimal speed (=v(t L )) If t L is known, the problem is solved: Brake to d/t L and keep that velocity. Th...

- Mon Sep 27, 2010 12:43 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 0798: "Adjectives"
- Replies:
**97** - Views:
**25701**

### Re: "Adjectives" Discussion (#798)

At the moment:

peristeronic -> 8010

"fucking peristeronic" -> 4

"peristeronic as shit" -> 5

Blue -7,4

Red -7,6

Let's see what google got tomorrow .

peristeronic -> 8010

"fucking peristeronic" -> 4

"peristeronic as shit" -> 5

Blue -7,4

Red -7,6

Let's see what google got tomorrow .

- Tue Mar 03, 2009 11:09 pm UTC
- Forum: Science
- Topic: Stern-Gerlach Experiment
- Replies:
**11** - Views:
**1893**

### Re: Stern-Gerlach Experiment

The spin does not have a dipole length like normal dipoles. But it has a dipole moment µ (parallel or antiparallel to the magnetic field), and a force µ * dB/dz.

- Thu Jan 29, 2009 2:42 pm UTC
- Forum: Science
- Topic: Rapidly rotating wire
- Replies:
**52** - Views:
**3814**

### Re: Rapidly rotating wire

Lets reduce the problem to two wires, one in rest (relative to the observer), one moving in vertical direction: ---___ t=0 ------- t=1 ---¯¯¯ t=2 For an observer in the right wire, the average velocity in the right wire is v. For us, the average velocity in the left wire is v, too, and the average v...