2
votes
2answers
622 views

Penetration of human skin [closed]

I recently found an answer for the "toughness" of steel http://physics.stackexchange.com/a/16821 and was hoping to get a similar answer regarding human flesh. How much pressure is required to ...
2
votes
1answer
335 views

How is it possible to pull out derivatives of a wavefunction?

In an early derivation, the following equation was stated: $$\frac\partial{\partial t}\lvert\psi\rvert^2 = \frac{i\hbar}{2m}\biggl(\psi^*\frac{\partial^2\psi}{\partial x^2} - \frac{\partial^2\psi^*}{\...
2
votes
3answers
102 views

Physical meaning of $\left \langle \psi_{nml} | x | \psi_{n'm'l'} \right \rangle$ for hydrogen atom?

I know how to calculate the matrix element $\left\langle \psi_{nml} | x | \psi_{n'm'l'} \right\rangle$, but what is the physical meaning of it? In general, what does the following mean: $$ \left \...
2
votes
3answers
113 views

Why can the divergence of vector potential be anything?

Purcell in his book was deriving the vector potential $\bf A$ using $\text{curl}\;(\text{curl}\; \mathbf A)= \mu_0 \mathbf J\; .$ After some algebra, he came to this: $$-\frac{\partial^2 A_x}{\...
2
votes
2answers
123 views

Why is the acceleration due to gravity $g$ quoted as if it were constant?

In elementary physics courses the value $g=9.8\:\mathrm{m/s^2}$ is used almost universally. But surely this value must depend to some extent upon one's location on the Earth. Why do we assume a ...
2
votes
1answer
348 views

Is there a relationship between the Cosmological constant and the Hubble constant?

I looked around to see if this precise question was asked before and it appears not to be. So is it just me or has anyone else noticed that, no matter what consistent set of units you use, $$ \...
2
votes
2answers
138 views

Has the Higgs boson always existed?

I am slow and need some help understanding this. So has the Higgs boson always existed? If not, than how did it come to exist? What would have excited it before matter or particles were created? So ...
2
votes
3answers
223 views

Can a photon be absorbed by a proton?

When incident light passes through a hydrogen gas, for example, does it have 50% chance (since it's a 1:1 ratio of protons to electrons) of getting absorbed by the proton? Any chance at all? If no, ...
2
votes
3answers
154 views

What experimental evidence shows that sound velocity is the same for all wavelengths?

I'm studying sound waves with Halliday's book, and after reading the whole chapter, one of the questions suggested was: What is the experimental evidence that allows the assumption that the sound ...
2
votes
2answers
153 views

Why does heat added to a system cause an increase in entropy that is independent of the amount of particles in the system?

Say we have two gas containers of $N_{2}$ at the same temperature of $300 ~\text{K}$, one containing $10^{23}$ particles and the other containing $10^{13}$ particles. If we add a quantity of heat to ...
2
votes
4answers
149 views

Why light moves sideways?

Greatings! I'm trying to understand special relativity and have one question bugging me. In almost every book or video about the subject there is a thought experiment with moving light clock. I hope ...
2
votes
3answers
124 views

Explaning Motion of a Man and Spacecraft Orbiting the Earth [closed]

A man and spacecraft is orbiting around the earth. The man is outside the spacecraft and is not connected to the spacecraft in any way. The distance between the man and the spacecraft remain constant. ...
2
votes
2answers
99 views

Could time have tertiary directions?

This is a late night thought, but it seems interesting enough to ask: Has anyone considered the possibility that time might have more directions that forward and backward? Could time go sideways?
2
votes
1answer
179 views

What is the QFT picture of a static electric field?

Accelerating charge generates electromagnetic waves and loses energy, in QFT terms it emits photons that carry it away. What of a static charge? Moving photons are usually associated with waves, which ...
2
votes
2answers
126 views

Does every Hilbert Space carry a representation of Poincare group?

We know all infinite dimensional Hilbert Spaces are unitarily equivalent. It should follow therefore that if I have an unitary representation of say Lorentz or Poincare group on one infinite ...
2
votes
3answers
303 views

Why are triangles drawn like so when working with gravity on an inclined plane?

This is my first year as a physics student, and I've never learned about vectors past a basic level, so this is confusing me. When we have gravity on an inclined plane, we separate it into two ...
2
votes
1answer
227 views

How can spin be gauge dependent?

Deriving the spin (density) of an electromagnetic wave, I obtained the formula $$\mathbf S = \mathbf E \times \mathbf A$$ But under a gauge transformation $\mathbf A+\nabla f$ this function seems to ...
2
votes
2answers
408 views

Maximum limit of charging a capacitor

We say that we can charge a capacitor in proportion to the potential difference we apply across its plates and the maximum potential difference depends on the dielectric strength of the medium. Now ...
2
votes
2answers
225 views

Can all of physics be described by simple math? [closed]

Recently I was browsing through A Dynamical Theory of electromagnetic field by Maxwell and wondered because the paper did not seem to include any vector calculus or any vectors. I thought of the ...
2
votes
4answers
713 views

Stone dropped from a moving train

This may look like a stupid question, but it is really getting to me. Imagine a train moving with an acceleration $a$, and a person drops a stone from the window. To an observer on the ground, the ...
2
votes
1answer
93 views

Why does Moon always poses the same face towards Earth?

If both Earth and Moon are rotating as well as revolving around some focus, shouldn't they have drifted out of phase with each other long ago? So, why do we always see the same side always?
2
votes
2answers
161 views

Is there any $SU(\infty)$ gauge theory in quantum field theory?

The groups $U(N)$ and $SU(N)$ are the most important Lie groups in quantum field theory. The most popular are the $U(1),SU(2),SU(3)$ groups (these gauge groups form the Standard model). But is there ...
2
votes
1answer
127 views

How can the Hubble telescope see the light from galaxies that are millions of light years distant?

The Hubble Space Telescope is in the news every now and then. How can it photograph galaxies that are millions of light years from Earth? We can understand that light comes from galaxy and Hubble "...
2
votes
2answers
119 views

Why does the Dirac equation require matrices to be rotationally invariant?

Why does the Dirac equation derivation require matrices? Starting from $$i\hbar \frac{\partial \psi}{\partial t} = \left(\frac{\hbar c}{i}\alpha^k\partial _k + \beta m_0 c^2 \right) \psi =H \psi.$$ ...
2
votes
3answers
302 views

How can I calculate whether a shelf is strong enough to support a falling cat?

I'm building some shelves for my cat to climb on. The highest position the cat will be is 2 meters above the next shelf. The cat has a mass of 6.5kg. The shelf has a maximum load of 20kg. Using ...
2
votes
3answers
319 views

How do I know what variable to use for the chain rule?

In my textbook the tangential acceleration is given like this: $$a_t=\frac{dv}{dt}=r\frac{dw}{dt}$$ $$a_t=rα$$ I understand that the chain rule is applied here like this: $$a_t=\frac{dv}{dt}=\frac{...
2
votes
3answers
364 views

Solving the two body problem numerically

I'm trying to solve the two body problem numerically, setting up $G$, $m1$ and $m2$ to be equal to 1. then I located each mass on positions -5 and 5 respectively along the $x$ axis and gave them both ...
2
votes
3answers
259 views

Why does Hamiltonian follow the property $H^*_{ij} = H_{ji} $?

I was reading Feynman's Lectures III's Hamiltonian Matrix. There I found this property of Hamiltonian Matrix: The Hamiltonian has one property that can be deduced right away, namely, that $$H^*_{...
2
votes
2answers
1k views

error propagation with an integral

My question here is about how to determine the error of an integral given individual uncertainties in two parameters defining the function being integrated. I used a curve fitting function to ...
2
votes
3answers
269 views

Would a person gain or lose weight after expelling a flatus?

Considering the chemical composition, mass, and pressure range of typical human flatulence, would a person gain or lose weight after passing gas?
2
votes
2answers
127 views

What is high energy physics?

Is high energy physics the same as particle physics? Does research in high-energy physics include things like quantum gravity, string theory and quantum field theory? Is unifying the four ...
2
votes
1answer
115 views

Coulomb's law with an $r^3$, not $r^2$, in the denominator [duplicate]

I am reading an older physics book that my professor gave me. It is going over Coulomb's law and Gauss' theorem. However, the book gives both equations with an $r^3$, not $r^2$, in the denominator. ...
2
votes
4answers
212 views

Particle energy in general relativity?

Assume that a particle is moving with four-velocity $u^\mu$ through a spacetime with metric $g_{\mu\nu}$ ($-+++$ signature). Let us also assume that there exists a time-like vector field $t^\mu=(1,0,...
2
votes
2answers
334 views

If the ionosphere reflects radio waves then why use satellites? [closed]

Please explain the main physical advantages of using satellites for communication as I'm sure there are many.
2
votes
2answers
297 views

Integration by parts to derive $d\langle x \rangle / dt$

I am reading "Introduction to Quantum Mechanics" by David Griffiths and I am having trouble understanding part of a derivation of $\frac{d\langle x\rangle }{dt}$ in section 1.5 - Momentum - of the ...
2
votes
1answer
282 views

How to visualize a Schrödinger cat state?

I recently read about Schrödinger cat states, which are basically a superposition of two coherent states $|\alpha\rangle$ with opposite phases, that is, $$ |\mathrm{cat}\rangle = |\alpha\rangle \pm |{-...
2
votes
3answers
79 views

How and when are the relativistic corrections applied to GPS satellites?

It is known that there is a need to correct the onboard clocks to reduce the time difference from 38μs to 50ns. Where is relativity playing its role here? Why cant the clocks be simply synchronised ...
2
votes
4answers
145 views

Photon Emission/Absorbsion from the Photons Perspective [duplicate]

First some assumptions. 1) Photons travel at the speed of light. 2) From the photon's reference spacetime is contracted to 0 length in the direction of photon travel. 3) From the photon's reference ...
2
votes
1answer
175 views

Why is the electromagnetic four-potential $A_{\mu}$ not an observable?

Why within classical field-theory the electromagnetic four-potential (usually $A_{\mu}$) not an observable? In classical mechanics we don't have problems with energy measurements and in quantum ...
2
votes
2answers
340 views

How can we feel the effects of a Black Hole if all the mass is gone?

This question may help me learn more about the subtleties involved between the notions of gravity, in the Newtonian sense and those of curved spacetime, in the General Relativity sense. I will take ...
2
votes
3answers
251 views

Why doesn't a global frame of reference exist for GR?

I only have at best a layperson's familiarity with GR, so forgive me if I am asking a basic question, but I have heard that in GR, we cannot have a global frame of reference, that is a frame of ...
2
votes
2answers
241 views

Different signatures

I was working out the christoffel symbols, once where the metric that I am using has (+---) signature and another time where it has (-+++) signature because two books had different signatures and I ...
2
votes
1answer
403 views

Is there a significant possibility of the LHC missing “exotic” particles or events?

In his popsci book, "Particle at the end of the universe", Sean Carroll says that the LHC, due to it's sheer information gathering capability, necessarily needs to completely discard most of the data ...
2
votes
2answers
541 views

Is Marshmallow solid or liquid?

A marshmallow is a sugar candy that, in its modern form, typically consists of sugar, whipped to a spongy consistency, molded into small cylindrical pieces, and coated with corn starch. Link :...
2
votes
2answers
247 views

Relativity question about 4-velocity

Given a 4-velocity $u^0$, how do you find $u_0$? Do you use $u_{\alpha}u^{\alpha} = -1$?
2
votes
1answer
324 views

What are Einstein constraint equations?

Yesterday, I met Einstein constraint equations in a thesis? I failed to understand them. Do they have physical meaning? And what do they "constrain"?
2
votes
2answers
62 views

Do the outer gas planets radiate their mass?

We know that the sun experiences angular momentum loss, and radiates a portion of it's mass (though helicity is conserved). Can we say the same about massive Jupiter, or even Saturn, Uranus or Neptune?...
2
votes
1answer
78 views

Poincare non-invariance in real world and field theory

This may be a very blunt question but I wonder why we always use Poincare invariant Lagrangians in field theory. After all, the entire world around us is by no means homogeneous, isotropic and so on. ...
2
votes
5answers
1k views

If the electrostatic potential is zero, why doesn't the electric field have to be zero?

I thought the relation between the electrostatic field $\vec E$ and the electrostatic potential $V$ is as follows: $$\vec E = - \nabla V$$ Thus, when $V$ is zero, $\vec E$ is also zero.
2
votes
1answer
192 views

How much heating Earth inner core provide to the surface?

Compare to the energy that the Earth surface receives from the sun, how much power comes from the inner melted core ? How important is this contribution to the surface temperature ?

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