4
votes
1answer
637 views

Physical significane and context in which Dirac introduced the Dirac Delta function

I'd like to know the exact context in which Paul Dirac introduced the Dirac delta function. What was the physical significance of the Dirac delta function when he first used it in Physics ?
11
votes
5answers
8k views

Stephen Hawking says universe can create itself from nothing, but how exactly?

Stephen Hawking says in his latest book The Grand Design that, Because there is a law such as gravity, the universe can and will create itself from nothing. Is it not circular logic? I mean, how ...
5
votes
0answers
100 views

What can quantum adiabatic computation provably accomplish?

Let's say I have a quantum adiabatic computer in a black-box that works perfectly, doesn't suffer from decoherence/noise problems, etc. Are there any proven bounds for an adiabatic algorithm that ...
6
votes
5answers
384 views

Inertia in an empty universe

I was reading a recent article on Mach's Principle. In it, the author talks about inertia in an empty universe. I'll quote some lines from the article: Imagine a single body in an otherwise empty ...
2
votes
2answers
322 views

Question concerning Isospin symmetry

I'm currently taking an introductory course to particle physics and I'm now trying to understand the concept of isospin. However I do have some trouble. So let's write the up- and down Quark as a ...
3
votes
1answer
311 views

Lorentz invariance of a frequency- and wavelength- dependent dielectric tensor

Suppose we have a material described by a dielectric tensor $\bar{\epsilon}$. In frequency domain, this tensor depends on the wave frequency $\omega$ and the wave vector $\vec{k}$. Clearly not all ...
1
vote
0answers
143 views

Question on energies obtained via WKB approximation

Suppose we are given an ODE problem $$ y''(x)+V(x)f(x)=E_{n} y(x) $$ with boundary conditions $ y(0)=y(\infty)=0$. Here $V(x)$ is a potential function. Then is it always true that (for $n ...
3
votes
2answers
621 views

Does superluminal travel imply travelling back in time?

Is this statement true? You can not travel back in time. If I travel fast enough the clock will start to go backwards, but that does not mean I am traveling back in time. It would only mean that ...
6
votes
3answers
1k views

Does the “Energy Catalyzer” by Andrea Rossi et al. generate energy by converting Nickel to Copper?

From Wikipedia: The Energy Catalyzer is an apparatus built by [...] Andrea Rossi, [and] Sergio Focardi. The 2009 patent application claims "a method and apparatus for carrying out nickel and ...
0
votes
0answers
56 views

Virtual particles contributing to universal expansion. [duplicate]

Possible Duplicate: Virtual particle production in space-time. Does virtual particle production contribute to observed universal expansion?
1
vote
1answer
175 views

Virtual particle production in space-time.

Are virtual particles created within existing space-time, or do they create new space-time for them to occupy?
4
votes
4answers
151 views

What objects/states of objects with absolute magnitude do we know of?

For measuring distances the knowledge of absolute magnitude or luminosity is often crucial, especially for very big distances. Unfortunately we can't measure the diameter of far distant objects and ...
6
votes
2answers
1k views

A Fermi-question: Are there more stars than grains of sand?

This claim is pretty popular in the web and obviously cannot be answered by counting grains or stars. Thats what physicists call a Fermi-question. It deals with problems/questions where we dont have ...
3
votes
1answer
91 views

When will Enceladus run out of water?

So its geyser eruptions are the primary source of water in Saturn's atmosphere (from this thread). And they even manage to contaminate Titan's atmosphere with it. But has the outgassing rate ...
13
votes
7answers
2k views

The philosophy behind the mathematics of quantum mechanics

My field of study is computer science, and I recently had some readings on quantum physics and computation. This is surely a basic question for the physics researcher, but the answer helps me a lot ...
5
votes
4answers
3k views

Trampoline physics - what is the right time to crouch and to jump to maximize your height?

Please bear in mind that I'm neither a physics guy nor particularly a math guy. So I will probably need a bit more hand-holding than just a complex equation. For a game, I'm trying to calculate a ...
2
votes
2answers
379 views

“Observed” angular velocity

I have a homework question based on the following diagram: I need to find the angular velocity of the object as seen by an observer at the origin of the frame. The question says that the observed ...
13
votes
2answers
413 views

The entropic cost of tying knots in polymers

Imagine I take a polymer like polyethylene, of length $L$ with some number of Kuhn lengths $N$, and I tie into into a trefoil knot. What is the difference in entropy between this knotted polymer and ...
5
votes
1answer
3k views

Why are lasers inefficient?

Why are lasers inefficient? Is it because of the heat lost during lasing? Why couldn't there be thermocouples or turbines in parts of the cooling circuits to extract something out of that heat?
6
votes
3answers
1k views

Book suggestions for foundation of Newtonian Mechanics

I'm not looking for books which deal with the mathematical foundations of Newtonian mechanics. What I'm looking for are modern books which deal with conceptual foundations of Newtonian mechanics - by ...
4
votes
3answers
1k views

What is the specific meaning of “Fourier frequency” (as opposed to simply “frequency”)?

I've noticed that many journal articles (in optics) use the phrase "Fourier frequency" to describe, well, the frequency of something. Google scholar search for "Fourier frequency". Example: ...
3
votes
1answer
163 views

What exactly is the definition of motion and its relation to Mach's conjecture?

The notion of "movement" seems to be well understood in physics. In fact, I don't recall any physics text-book defining motion. Special relativity theory says that there is no absolute frame of ...
3
votes
3answers
139 views

Are there convective movements similar to those in clouds everywhere in the atmosphere?

I heard there were powerful convective movements in clouds which were responsible for increasing the size of water droplets or ice crystals. My question is: do the same movements appear outside of ...
2
votes
2answers
191 views

Is there a reason for photodynamic therapy to not cause cancer if mobile phones might?

I don't want to open a debate about whether cell phones can cause cancer, I read the thread: Could cell-phone radiation cause cancer? For the sake of this question let's assume there's a chance for ...
25
votes
1answer
1k views

Is gecko-like friction Coulombic? What is the highest known Coulombic $\mu_s$ for any combination of surfaces?

Materials with large coefficients of static friction would be cool and useful. Rubber on rough surfaces typically has $\mu_s\sim1-2$. When people talk about examples with very high friction, often ...
4
votes
2answers
391 views

How to calculate the highest theoretical artificial hill?

The biggest peak in the world is Mount Everest. Imagine someone starting to make an artificial hill (like pyramide) from soil (earth). So, when starting with an 200x200 Km base area, with 45degree ...
4
votes
3answers
118 views

What would a rotating black-hole look like to a “geo-stationary” observer orbiting the black hole

A rotating black hole is believed to contain a ring singularity rather than a point. However, if an astronaut is orbiting the black hole at exactly the same angular velocity as the blackhole (in ...
2
votes
1answer
253 views

Next crescent moon?

How do I find the date of next crescent moon on wolframalpha or any other site which tells the appearance date time of next crescent on a specific location? I am searching for it to look for 1st of ...
5
votes
2answers
343 views

Proof that Statistical Mechanics is a model of Themodynamics

The laws of thermodynamics are essentially four axioms of a mathematical theory. The expectation values of a statistical ensemble are supposed to satisfy the axioms of thermodynamics (under the ...
4
votes
2answers
180 views

How is the shape of the universe measured by scientists?

I would like to learn how scientists go about measuring the large-scale curvature of the universe to determine if the universe is closed 'i.e. spherical', flat, or open 'i.e. saddle shaped'. My ...
3
votes
1answer
315 views

metamaterials for gravitational waves

Is there such a thing as refractive index of a material for gravitational waves ? How would the index of a material depend on density - Say density of interstellar cloud vs planet vs star vs neutron ...
1
vote
0answers
197 views

decay with initial velocity:

decay with initial velocity: All static particle decay to particles with momentum distribution in the same shape like that's implied by structure function and calculated by MG. but the decay of W ...
5
votes
4answers
396 views

What free software is there for observing the sky (sky map software)

I used until now only stellarium.org and I'm curious if there is any other software that is better than stellarium. By better, I mean: doesn't have high system ...
3
votes
2answers
284 views

What devices could be taken round dwellings to measure the thermal conductivities of buildings in situ?

Your lateral thinking and knowledge of lab kit could help us solve a tricky measuring problem in building-physics. One of the problems we get in modelling the heat demands of buildings, is getting an ...
4
votes
1answer
1k views

Finding the position of a planet between two other planets of known mass and distance

Here is the question: A planet with mass $m$ and a second with mass $M$ are separated by a distance $d$. A third planet with mass $m_3$ happens to be midway between $M$ and $m$. Where could ...
3
votes
2answers
986 views

Could a human run horizontally inside a Wall of Death?

A popular circus stunt is for a motorcycle rider to ride inside a bowl shaped depression called a "Wall of Death." The rider goes higher and higher up the wall until they are actually horizontal. I ...
6
votes
2answers
314 views

Physical Interpretation of a Scalar Quantity Related to Currents/Conservation Laws

Let $Q_{ab} = (\psi_{;a})(\psi_{;b}) - (1/2)g_{ab}|\nabla \psi|^2$ be the energy-momentum tensor of the wave equation in some space time. I will use semicolons to refer to covariant differentiation ...
7
votes
2answers
2k views

how can you keep a gyroscope turning?

I'm trying to understand how do gyroscopes and cmgs used for navigating keep on spinning even though there's friction. Any engine/pushing mechanism I can think of will restrict the gyro's rotor to a ...
8
votes
3answers
21k views

Hollow Tube Stronger than Solid bar of same Outside Diameter (O.D.)?

I was listening to some co-workers talking about problems meeting stiffness requirements. Someone said that even with a solid metal rod (instead of the existing tube) we could not meet stiffness ...
1
vote
3answers
48 views

The orbit of 2010 TK7

Having just heard about the asteroid 2010 TK7 in Trojan asteroid seen in Earth's orbit by Wise telescope, I want to know more about its orbit. The BBC article says it moves above and below the ...
5
votes
1answer
288 views

What does scalar phi represent in spacetime?

Trying to understand one-forms and vectors via Schutz's A First Course In General Relativity. His example uses a spacetime diagram, a scalar field phi, a curve (worldline) parametrized using proper ...
3
votes
2answers
425 views

Force exerted by rotating bar

Say we have a bar immersed in liquid, and that the bar in being rotated at a given constant angular speed. What force is exerted on the liquid by the faces of the bar moving through the liquid (so the ...
7
votes
1answer
402 views

The role of metric in the Wave Equation

The wave equation is often written in the form $$(\partial^2_t-\Delta)u=0,$$ involving the Laplace-Beltrami operator $\Delta$. However, the Laplace-Beltrami operator $\Delta$ is defined only in the ...
7
votes
2answers
815 views

Sunflowers and radioactivity

I read yesterday that sunflowers were used and to cleanup radioactivity at Chernobyl and the Atomic Bomb sites in Japan and may be used as part of a campaign to clean up the Fukushima area. But my ...
7
votes
1answer
336 views

Charged particle close to a charged black hole - what happens?

Let's assume the Reissner–Nordström metric (charged black hole, non-rotating), for simplicity. The black hole is charged with a powerful electric charge. There's a particle nearby, of non-zero mass, ...
6
votes
1answer
490 views

Boundary Conditions Invariant Under Conformal Transformations in Electrostatics?

in two dimensional electrostatics it is assumed that the whole physical system is translationally invariant in one direction. Here, the two-dimensional Laplace equation $$\Delta \phi(x,y) = ...
2
votes
1answer
416 views

Using heat energy to increase temperature

It's been a long time since I studied thermodynamics in college years ago, but I was just wondering this: I know that a large bathtub of 10 C water has more heat energy than a small cup of 20 C ...
4
votes
2answers
1k views

Radial Schrodinger equation with inverse power law potential

Recently I read a paper about solving radial Schrodinger equation with inverse power law potential. Consider the radial Schrodinger equation(simply set $\mu=\hbar=1$): ...
9
votes
3answers
591 views

How do we explain accelerated motion in Newtonian physics and in modern physics?

Maybe my question will seem stupid, but I am not a physicist so I have some problems understanding a classic Newtonian experiment: in the bucket experiment, why does he have to introduce the absolute ...
1
vote
1answer
34 views

Could dark energy be linked to dark flow?

I couldn't find any other questions regarding dark flow on Wikipedia, so I'm curious. As far as I can tell, dark energy's existence seems to be hard to explain. Since there may be a couple of ideas ...

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