# All Questions

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### Newton's third law.. please explain here

According to Newton's third law, the force acting on object is equal and opposite to the other object that is applying the force. If I push a table with 5N force then table is doing the same. But the ...
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### How far has a 13.7 billion year old photon travelled

I've read that the size of the observable Universe is thought to be around ~46 billion light years, and that the light we see from the most distant galaxies were emitted ~13.7 billion years ago as a ...
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### What does the exponential decay constant depend on?

We know the law of radioactivity: $$N=N_0e^{-\lambda t}$$ where $\lambda$ is the exponential decay constant. My question is: This constant depends of what?
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### How long is a second in zero gravity?

General relatively predicts that gravity creates time dilation and a relatively slower passage of time for the observer experiencing larger acceleration. For an observer experiencing zero acceleration,...
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### Why the photon can't produce electron and positron in space or in vacuum?

$$\frac{hc}{\lambda} = K_e + K_p + 2m_e c^2$$ could be the energy conservation equation for a photon of wavelength $\lambda$ decaying into a electron and positron with kinetic energies $K_e$ and $K_p$ ...
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### What is the physical meaning of $a_{\vec{p}} \! \mid \! 0 \rangle$

$a^\dagger_{\vec{p}} \! \mid \! 0 \rangle = \mid \! p \rangle$ is interpreted as a creation of a particle with momentum $p$ from the vacuum. $a_{\vec{p}} \! \mid \! p \rangle = \mid \! 0 \rangle$ is ...
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### Calculating the Riemann tensor for a 3-Sphere

I have worked out all the connection symbols for the 3-sphere using calculus of variations, cf. this Phys.SE post. So to find the Riemann tensor I am trying to find all the nonzero components of: \...
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### What “exactly” happens when a ray is incident at the critical angle?

The question arose when my physics teacher taught me geometrical optics and told that the phenomenon of both refraction and $reflection$ occur by change in medium. First of all, I'm not sure about the ...
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### Is a superposition of (anti)symmetric states (anti)symmetric?

Let's say we have the following wavefunction of two identical particles, $A$ and $B$: $$\frac{1}{2}[(\chi(A)\psi(B)\pm\psi(A)\chi(B))+(\phi(A)\eta(B)\pm\eta(A)\phi(B))]$$ Is this properly (anti)...
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### Experiment demonstrating interference patterns of neutrons

The question is about experiments (and references) that demonstrate interference patterns of neutral (chargless) particles (fermions), especially neutrons. Like double-slit experiments for neutrons (...
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### Why is the Schwarzschild radius the radius of an event horizon?

I've been searching the web and many references without much success. My question is how do we know that, in the Schwarzschild black hole solution, the surface with coordinate $r=2M$ (in the geometric ...
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### Will my car consume more if I plug in my laptop?

I drive a Volvo v40 with a 110hp diesel engine. It usually use 4.6L/100km. If I plug my laptop which is 85W, will this change the fuel need? If so, by how much?
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Though the mathematical concepts underlying quadratic air drag are quite straightforward (a single variable differential, just like the linear drag equation), my text book (and online text books) ...
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### Why does earth cool? [duplicate]

This question is boggling me for some time. We know that heat can be transferred from matter to matter and heat is nothing more than tiny atoms vibration intensity (correct me if I'm wrong). But space ...
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### If wavefunction is just a probability function, how does an electron interfere with itself

I have read lots of quantum mechanics books. The chapters that are talking about De Broglie, lots of them name the chapter as "Wave-particle duality" and says: "Electrons are both waves and ...
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### Thermal Conductivity of Metals

My textbook says on conduction: These 'free' electrons can rapidly carry energy from the hotter to the cooler regions of the metal, so metals are generally good conductors of heat. A metal rod at ...
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### Two definitions of Riemann curvature tensor

I am relatively used to the coordinate free expression of the Riemann tensor: $$R(X, Y)Z=\nabla_X\nabla_Y Z - \nabla_Y\nabla_X Z - \nabla_{[X, Y]} Z,$$ where $\nabla$ is the Levi-Civita connection ...
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### Why are the 'color-neutral' gluons confined?

What makes the two 'color-neutral' gluons $(r\bar r−b\bar b)/\sqrt2$ and $(r\bar r+b\bar b −2g\bar g )/\sqrt6$ different from the pure $r\bar r +b\bar b +g\bar g$ ? Why don't they result in long ...
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### Why is using Newton's First Law failing me?

A factory worker pushes a 29.7kg crate a distance of 5.0m along a level floor at constant velocity by pushing downward at an angle of 32$^\circ$ below the horizontal. The coefficient of kinetic ...
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### What is the relative speed of two near-light speed particles headed towards each other?

I understand that nothing can move faster than light due to time dilation. I want to build upon my understanding of Einstein's theory of Special Relativity, so I came up with this hypothetical problem ...
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### How to pronounce $\textrm{eV}\!/c^2$

It seems that $\textrm{eV}\!/c^2$ (and its multiples) is commonly used as the unit of mass in particle physics. For example, David Griffiths uses it quite naturally in Introduction to Elementary ...
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### Does Fermi-Dirac Statistics explain anti-particles?

I wondered whether the Fermi-Dirac Statistics describes the anti-fermion particles. Does it include the anti-particles?
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### Dark matter and tides

As I understand, "dark matter" is what they call a theoretical substance which is only known by it's influence upon velocity curve of the galaxy. If indeed the gravity of "dark matter" is so strong ...
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### Can Flow of time become still?

According to theory of time dilation, flow of time slows down significantly at the speed of light.Is there any conditions practically or theoretically when flow of time is reduced to zero means it ...
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### Why gravity decreases as we go down into the earth? [duplicate]

We all know that gravity decreases as the distance b/w the two increases. Hence $$g = G \frac{Mm}{r^2}$$ hence acceleration due to gravity is inversely proportional to the distance b/w the 2 ...
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### Twin Paradox, calculating spacetime intervals from both perspectives

I've very recently started to try to understand special relativity. I'm want to get a decent understanding of the twin paradox. I'll post what I've done so far and highlight what's gone wrong for me. ...
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### Oil drop experiment and quantization of charge

How to systematically show that the resulting charges in oil drop experiment are integers multiplied by $e$ in other word how to extract $e$ from the data?
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### Angular speed of the planets

Do all the planets in our solar system have the same angular speed? Physics teacher says yes, my research is not crystal clear. I want to make sure I have the right information for future reference.
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### Why is the blue color of sky darker in high altitude regions?

I read that as the air density is lower at high altitudes, the scattering is less, hence the darker blue but then other colors are scattering lesser than blue, why don't we see a darker shade of the ...
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### Is it possible to split baryons and extract useable energy out of it?

Since baryons (e.g. protons, neutrons) are composite particles it should be possible to split them apart. If so, is it then possible to extract useable energy out of the splitting of baryons in ...
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### Why are there dust particles on TV screens?

My professor gave us the following reason: The screen is positively charged. When dust particles fly near it, the positive charges in the screen induce a charge in the dust particle, pulling the ...
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### How can I calculate the pressure against the sides of a container? [closed]

While answering this question on DIY.SE, I wanted to calculate the amount of pressure (in psi) that would be exerted on the walls of the container. However, I don't know much about how to calculate ...
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### Expansion of the universe and strain

From cosmological models that involve expansion of the universe, can we not say that there are ever increasing tidal forces felt by solid bodies? If so, the material in solid bodies like metal blocks,...
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### Is there an EMF in a conductor moving at constant speed across the uniform magnetic field [duplicate]

If a conductor - a long rod - moves at constant speed across the "lines" of a uniform magnetic field, is there an EMF within this conductor? Or, if a conducting rod rotates at uniform rate, pivoted ...