## Search found 513 matches

Fri Jun 13, 2014 2:33 pm UTC
Forum: Science
Topic: Alcubierre drive (Warp Drive)
Replies: 77
Views: 17582

### Re: Alcubierre drive (Warp Drive)

If the only time mass-energy is not conserved is when there is a corresponding change in space-time (like in the aforementioned redshift example), could we not say that some combined space-time-mass-energy quantity is what is always conserved? Basically, you're trying to take a differential conserv...
Thu Jun 12, 2014 3:32 pm UTC
Forum: Science
Topic: Alcubierre drive (Warp Drive)
Replies: 77
Views: 17582

### Re: Alcubierre drive (Warp Drive)

The uncertainty principle doesn't break energy conservation, and GR just tells you that what you REALLY meant by energy conservation was dT=0. And yes of course you meant that. And given that people with some understanding of SR but not GR already accept that the conservation is of mass + energy, I...
Wed Jun 11, 2014 5:02 pm UTC
Forum: Science
Topic: Alcubierre drive (Warp Drive)
Replies: 77
Views: 17582

### Re: Alcubierre drive (Warp Drive)

PM 2Ring wrote:Ok. How about we rule out time travel trajectories that break energy conservation?

GR already breaks energy conservation, even without closed time-like trajectories. At least, to the extent that you can define energy conservation without having a "now".
Tue Jun 10, 2014 11:03 pm UTC
Forum: Mathematics
Topic: Vectors without components
Replies: 19
Views: 4530

### Re: Vectors without components

The easiest to see, though the proof is a bit hairy, is L 1 [0,1]. The candidate for the inner product is what we use on L 2 , but you aren't guaranteed convergence of the requisite integrals in L 1 . Demonstrating that the only thing you need to consider is the candidate is tricky. Just to check t...
Tue Jun 10, 2014 1:55 am UTC
Forum: Mathematics
Topic: Vectors without components
Replies: 19
Views: 4530

### Re: Vectors without components

The fact that "a person with a hat on" means a different thing from "a person" shouldn't suggest to you that there are people who can't wear hats. We also don't have a specific word for "a person with a hat on". Any person could wear a hat, so we don't have a need for ...
Mon Jun 09, 2014 7:08 pm UTC
Forum: Mathematics
Topic: Vectors without components
Replies: 19
Views: 4530

### Re: Vectors without components

Given a vector and a basis, coordinates for the vector in that basis can be calculated by taking the dot product of the vector with each of the basis vectors. Assuming the existence of a dot product, ie that the space is a HIlbert space (or more acurately still, that it can be extended into a Hilbe...
Mon Jun 09, 2014 2:34 pm UTC
Forum: Mathematics
Topic: Vectors without components
Replies: 19
Views: 4530

### Re: Vectors without components

It's been a while since I took linear algebra, so I may be wrong, but I believe there's a theorem that every vector space has a basis. Given a vector and a basis, coordinates for the vector in that basis can be calculated by taking the dot product of the vector with each of the basis vectors. Thus, ...
Tue Jun 03, 2014 8:14 pm UTC
Forum: Science
Topic: How efficient is ATP?
Replies: 10
Views: 2530

### Re: How efficient is ATP?

My simple understanding of these molecular motors is that the extension is fixed per cycle, so even if the resistance is less than 1.4 pN, you'll get the same displacement. So efficiency may suffer away from the optimal force. (But I could be completely wrong about this point.) I am... not sure... ...
Tue Jun 03, 2014 5:49 pm UTC
Forum: Science
Topic: How efficient is ATP?
Replies: 10
Views: 2530

### Re: How efficient is ATP?

A typical double headed myosin motor uses a single ATP to displace 10nm with 1.4pN. ATP contains 7.3 kcal/mol (about -30.6 kJ/mol). Someone else will have to do the math. Uh, calling gmal? You just have to mutliply the force by the displacment to get work: 8.4 kJ/mol . This suggests an efficiency o...
Tue May 27, 2014 8:43 pm UTC
Forum: Science
Topic: Solar Road Ways
Replies: 82
Views: 13258

### Re: Solar Road Ways

it's also more uncomfortable to drive on, because it is not viscuous and thus not as smooth as asphalt. Asphalt's viscosity leads to washboarding, notably at traffic lights. I wouldn't rate that an advantage. That said, concrete does need expansion joints, unlike asphalt, so score one for blacktop.
Tue May 27, 2014 8:21 pm UTC
Forum: Science
Topic: Solar Road Ways
Replies: 82
Views: 13258

### Re: Solar Road Ways

Hexagons and weather aside, I wonder if materials science can come up with a feasible replacement for asphalt. Requiring less maintenance for cracked roads and potholes would be a huge benefit. You're describing concrete. It's more expensive to install than asphalt, but it lasts much longer. Amorti...
Tue May 27, 2014 6:34 pm UTC
Forum: Science
Topic: Solar Road Ways
Replies: 82
Views: 13258

### Re: Solar Road Ways

I think the heat going out of the road into the air would be sufficient to melt the snow as it falls. So the question is how much power would be consumed in order to continually maintain the temperature of the roadway, rather than melting a particular quantity of snow. That would depend on how much...
Tue May 27, 2014 5:48 pm UTC
Forum: Science
Topic: Solar Road Ways
Replies: 82
Views: 13258

### Re: Solar Road Ways

Electrical heating elements can be pretty high-efficiency. So what's the power consumption from a square meter of asphalt to a square meter of air at, say, 10 degrees C up to -10 degrees C? Let's run some ridiculously optimistic numbers. Let's consider 1 inch snow water equivalent over a lane-mile ...
Tue May 27, 2014 4:50 pm UTC
Forum: Science
Topic: Solar Road Ways
Replies: 82
Views: 13258

### Re: Solar Road Ways

I think that's where the benefit from this is. Doesn't even really need to be this tiles form factor. Just forego the solar panel and just pump electricity from the grid into them. Keep the roads at slightly above freezing all winter. We'd save tens of millions of dollars on plows every year, tens ...
Wed May 21, 2014 3:29 pm UTC
Forum: Mathematics
Topic: Bad math
Replies: 88
Views: 16527

### Re: Bad math

I mean, imagine a prime number Q 1 , which is otherwise uninteresting (not a power of two minus one or anything like that) But there is no such number, since all positive integers are interesting. Proof: Suppose there are uninteresting positive integers. Then there is a smallest uninteresting posit...
Wed May 21, 2014 3:24 pm UTC
Forum: Science
Topic: Star Gazing Question
Replies: 7
Views: 1839

### Re: Star Gazing Question

Whizbang wrote:Camera plus projector? You can buy gaming projectors (they are not HD, but if all you want is spots of light, they'll work) for \$60. Add in a cheap camera, some wires and electricity, and there you go.

I suspect that, with a cheap camera, the spots of light you see will be shot noise, not stars.
Wed May 14, 2014 9:31 pm UTC
Forum: Science
Topic: Henrietta Lacks
Replies: 7
Views: 2550

### Re: Henrietta Lacks

There's a book, The Immortal Life of Henrietta Lacks, that describes the HeLa cells and the ethical issues surrounding them. I haven't read it, but it got good reviews when it was released.
Mon Apr 28, 2014 9:31 pm UTC
Forum: Science
Topic: Energy requirements to launch to LEO (from vacuum zeppelin)
Replies: 46
Views: 35837

### Re: Energy requirements to launch to LEO (from vacuum zeppel

Reaching 300 km altitude requires 2.94 MJ/kg, equivalent to 2425 m/s. Launch at high altitude isn't going to cut more than a tenth or so of this out. Reaching 8000 m/s requires 32 MJ/kg. Launch at high altitude doesn't change anything here. For an approximation of gravity drag, thrusting vertically...
Thu Apr 24, 2014 4:59 am UTC
Forum: Science
Topic: Relativistic rod passing over a gap.
Replies: 19
Views: 3122

### Re: Relativistic rod passing over a gap.

Except I don't think that's the kind of rigidity meant. After all, thickness doesn't affect speed of sound, but it does affect flexibility, and it would seem that flexibility is what must have a limit in this case. Sure, you need to know the thickness to work out the configuration in a static situa...
Thu Apr 24, 2014 2:59 am UTC
Forum: Science
Topic: Relativistic rod passing over a gap.
Replies: 19
Views: 3122

### Re: Relativistic rod passing over a gap.

teusry wrote:Well, thats what I'm questioning now. This paradox seems to imply that there is a theoretical limit on rigidity of any object.

Quite right. Specifically, the speeds of of sound in the material must be less than c.
Wed Apr 23, 2014 10:03 pm UTC
Forum: Science
Topic: Energy requirements to launch to LEO (from vacuum zeppelin)
Replies: 46
Views: 35837

### Re: Energy requirements to launch to LEO (from vacuum zeppel

Gravity drag is significant, but I believe the climb to altitude itself takes more energy. Since orbits tend to be at 200 km or 300 km, that results in a contribution to the Delta v budget of nearly 2 km/s. That explains the large part of how we go from 8 km/s orbital velocity to 10 km/s Delta v bu...
Mon Apr 21, 2014 7:59 pm UTC
Forum: Science
Topic: Global Thermodynamics
Replies: 20
Views: 2895

### Re: Global Thermodynamics

watch a pot of water at 0C and at 100C. the latter will be a lot more turbulent. The reason the water at 100C will be more turbulent is that it's being heated from below. This makes the water on the bottom of the pot be less dense than the water above it. This configuration is subject to the famous...
Mon Apr 21, 2014 5:00 pm UTC
Forum: Science
Topic: Global Thermodynamics
Replies: 20
Views: 2895

### Re: Global Thermodynamics

watch a pot of water at 0C and at 100C. the latter will be a lot more turbulent. The reason the water at 100C will be more turbulent is that it's being heated from below. This makes the water on the bottom of the pot be less dense than the water above it. This configuration is subject to the famous...
Thu Apr 17, 2014 2:52 pm UTC
Forum: Science
Topic: Formal demonstration that classical paths won't interfere
Replies: 12
Views: 2610

### Re: Formal demonstration that classical paths won't interfer

Only when something gets special enough to invoke quantum behavior do you end up with something that appears at only one of the slits at a time, but also causes interference patterns. Careful -- if you measure carefully enough to determine which of the slits the electron goes through, you'll destro...
Wed Apr 16, 2014 4:15 am UTC
Forum: Science
Topic: Formal demonstration that classical paths won't interfere
Replies: 12
Views: 2610

### Re: Formal demonstration that classical paths won't diffract

I didn't think I was that confusing. The solutions to Newton's equations for particles must be continuously differentiable functions from R to R 3 , where the domain corresponds to time and the range to position. An electron's position, pre 1900's, was presumed to satisfy Newton's equations. I call...
Tue Apr 15, 2014 9:18 pm UTC
Forum: Science
Topic: 1338 changed my life! (veganism)
Replies: 153
Views: 21086

### Re: 1338 changed my life! (veganism)

there's an easy solution to the mistreatment of animals, just buy from butchers that treat their animals well. It's not so crazy to consider the killing of an healthy animal "mistreatment". And if this is your position, I don't see any logical conclusion other than vegetarianism. (Well, I...
Tue Apr 15, 2014 8:54 pm UTC
Forum: Science
Topic: Formal demonstration that classical paths won't interfere
Replies: 12
Views: 2610

### Re: Formal demonstration that classical paths won't diffract

Quantum Mechanics is frequently motivated by observing that classical path's can't produce diffraction patterns. I'm not quite sure what you mean here. Classical waves certainly diffract, be they light, sound, water, etc. Is the question, why don't we describe everything in classical mechanics as w...
Thu Apr 10, 2014 2:32 am UTC
Forum: Science
Topic: quarkcosh1's miscellaneous science posts
Replies: 33
Views: 3486

### Re: quarkcosh1's miscellaneous science posts

Nor have I. But I'm a ℏ = 0 kinda guy. Cannot tell if this is a mistake or actual classical physics. Well, I would claim to never make mistakes, but that probably won't fly in this thread. This isn't one, though; I'm a soft condensed matter person, happily living in the land of continuum mechanics....
Thu Apr 10, 2014 1:53 am UTC
Forum: Science
Topic: quarkcosh1's miscellaneous science posts
Replies: 33
Views: 3486

### Re: quarkcosh1's miscellaneous science posts

why are people claiming the gravitational coupling constant αG is the same as G? i k ow nothing about the subject but what i read on wimipedia, but tere it is explained to be dimensionless. I have never seen any physicist ever actually refer to this thing. Nor have I. But I'm a ℏ = 0 kinda guy, so ...
Thu Apr 10, 2014 12:42 am UTC
Forum: Science
Topic: quarkcosh1's miscellaneous science posts
Replies: 33
Views: 3486

### Re: quarkcosh1's miscellaneous science posts

why are people claiming the gravitational coupling constant αG is the same as G? i k ow nothing about the subject but what i read on wimipedia, but tere it is explained to be dimensionless. My mistake, sorry. But in my defense, the OP didn't contain the word "coupling" anywhere (the URL b...
Wed Apr 09, 2014 10:23 pm UTC
Forum: Science
Topic: quarkcosh1's miscellaneous science posts
Replies: 33
Views: 3486

### Re: quarkcosh1's miscellaneous science posts

Except its not quackery. People here really seem to underestimate the amount of physics reading I have done and while I don't claim to understand it all there are parts of it I do understand enough to have an informed opinion on or to be creative with those parts. I don't know how to put this gentl...
Wed Apr 09, 2014 9:13 pm UTC
Forum: Science
Topic: Are physics in the particles, or reality itself?
Replies: 60
Views: 10966

### Re: Are physics in the particles, or reality itself?

thoughtfully wrote:A gentler introduction to Quantum Field Theory might be Feynman's QED.

Certainly. But I wasn't trying to be gentle.
Wed Apr 09, 2014 8:35 pm UTC
Forum: Science
Topic: Are physics in the particles, or reality itself?
Replies: 60
Views: 10966

### Re: Are physics in the particles, or reality itself?

quarkcosh1 wrote:I think physicists are the ones who don't understand them since everything I have read about them is so vague.

In that case, I think you will find Peskin & Schroeder pleasingly specific.
Wed Apr 09, 2014 3:24 pm UTC
Forum: Science
Topic: quarkcosh1's miscellaneous science posts
Replies: 33
Views: 3486

### Re: Are there any theories of gravity based on factorials?

Everyone knows that G = 1. Your "approximation" is terrible.

(And off to the coincidences thread we go!)
Wed Apr 09, 2014 3:13 pm UTC
Forum: Science
Topic: Are physics in the particles, or reality itself?
Replies: 60
Views: 10966

### Re: Are physics in the particles, or reality itself?

quarkcosh1 wrote:Is it too hard for experimenters to count virtual particles or something?

Yes, by definition.

Virtual particles are those that live inside interactions and cannot be directly detected. We infer their existence from their impact on those interactions.
Tue Apr 08, 2014 2:17 am UTC
Forum: Mathematics
Topic: how do i find F(g(x)) given f(x), g(x), or compute it?
Replies: 5
Views: 3106

### Re: how do i find F(g(x)) given f(x), g(x), or compute it?

I have the function [ + inf E(v) = | tanh(arctanh(M)v) * e^[-(v-1)^2/(2S^2)]/(sqrt(2*pi)S) dv ] -inf No, you don't. The thing on the right-hand side depends on M and S . But v is a dummy variable, being integrated over, so this thing is not a function of v . What you really have is E ( M , S &#...
Sat Mar 22, 2014 8:43 pm UTC
Forum: Science
Topic: Question about Planck Units
Replies: 13
Views: 3214

### Re: Question about Planck Units

We can say that the Planck length presents a practical limit to our measurements: A photon with a wavelength less than the Planck length would collapse into a black hole. How does that jibe with relativity? We don't know -- we need a quantum theory of relativity to understand what happens here. To ...
Wed Mar 19, 2014 5:27 pm UTC
Forum: Mathematics
Topic: Axiomatic mathematics has no foundation
Replies: 158
Views: 32852

### Re: Axiomatic mathematics has no foundation

From the director's extended cut of Pachyderm . I can prove elephants don't exist. Fine. Go ahead. As everyone knows, elephants fly by flapping their ears. This is completely implausible. An elephant's ears have minimal structural support. There's no way it could properly maintain the correct cross-...
Tue Mar 18, 2014 9:33 pm UTC
Forum: Mathematics
Topic: Axiomatic mathematics has no foundation
Replies: 158
Views: 32852

### Re: Axiomatic mathematics has no foundation

The XKCD Players are proud to present Pachyderm being an Allegorie in one itty-bitty little act Dramatis Personae Dit A. Ert , being the only sane man Chorus , being Geek A bare stage, conveying no meaning. Elephants do not exist! Sure they do, for we have seen them. Elephants do not exist! I have ...
Tue Mar 18, 2014 8:30 pm UTC
Forum: Science
Topic: Question about Planck Units
Replies: 13
Views: 3214

### Re: Question about Planck Units

But of course Max Plank chose this length (in 1899) many years before Schwarzschild showed Black Holes as a result in GR (1916), and decades before their significance in this context was realized (1960s? later?). It's not like the length was chosen because it was the smallest meaningful size. True....

Go to advanced search