20
votes
3answers
2k views

Prove that negative absolute temperatures are actually hotter than positive absolute temperatures

Could someone provide me with a mathematical proof of why, a system with an absolute negative Kelvin temperature (such that of a spin system) is hotter than any system with a positive temperature (in ...
6
votes
3answers
3k views

Please recommend a good book about physics for young child (elementary school aged) [closed]

I'm looking for a book that would be appropriate for an advanced elementary school aged kids (say, 6-11 YO) describing the basics of physics (or sciences in general) in entertaining way. The ...
-2
votes
3answers
202 views

New theories and publications [closed]

When someone develops a new theory on physics, which is barely on schetch (so there are no measurements, nor simulations) with just a mathematical and conceptual description, in which scientific ...
0
votes
1answer
230 views

Simplifying some math for an ant-on-rubber-band problem

OK, I've been doing this problem for fun (it's a great problem, BTW!): http://www.physics.harvard.edu/academics/undergrad/probweek/prob76.pdf Here is the solution: ...
1
vote
1answer
182 views

How to explain relativistic mass with 2 moving systems, but not 3?

All the visual explanations I know work in some kind of "If you are moving relative to something A, while inside A something is moving, the stuff in A has to move slower due time dilation and ...
3
votes
4answers
2k views

Why water inside a vessel placed in a body of boiling water does not boil?

Imagine a plastic sac filled with water placed in a pan of boiling water. I wonder why the water inside the sac does not boil. The temperature of the water inside the sac should be the same as that of ...
1
vote
2answers
1k views

Infinite Atwood's machine

I tried doing the following problem just for fun (please see the following link): (I'm not allowed to post the entire problem here since I'm a new user.) Here are my equations: ...
7
votes
0answers
244 views

Instantons and Borel Resummation

As explained in Weinberg's The Quantum Theory of Fields, Volume 2, Chapter 20.7 Renormalons, instantons are a known source of poles in the Borel transform of the perturbative series. These poles are ...
6
votes
1answer
203 views

Why does separating plastic from a cough drop create flashes?

I was tearing open individually-wrapped cough drops in the dark, and noticed that when I pulled apart two adhesive-joined pieces of silvery plastic film, small flashes of light were produced. It was ...
22
votes
5answers
2k views

Does a charged particle accelerating in a gravitational field radiate?

A charged particle undergoing an acceleration radiates photons. Let's consider a charge in a freely falling frame of reference. In such a frame, the local gravitational field is necessarily zero, ...
5
votes
1answer
25 views

Is there any site/place which gives access to astronomical signals acquired from space?

I am an engineer and I'd like to know if there are any places which provide access to any kind of astronomical signals acquired from space using radio telescopes.
1
vote
1answer
164 views

Educational game [closed]

I am in the process of conceptualizing an iOS or computer game (a modification of Tetris) that would be suitable for motivated high school students and university undergraduates. It is meant to be an ...
4
votes
1answer
671 views

Addition of a neutral electrolyte to water— how can it increase conductivity?

Sparked off by Is sea water more conductive than pure water because "electrical current is transported by the ions in solution"? This question really belongs on chemistry.SE, which is ...
2
votes
1answer
290 views

A missing factor of 2 in the standard Hartree-Fock mean field?

Let's start from a very simple argument: If $A$ and $B$ are some operators, then I can write their product as $$AB = (A-\langle A\rangle)(B - \langle B \rangle) + \langle A \rangle B + A \langle B ...
5
votes
3answers
231 views

Analyticity and Causality in Relativity

A few weeks ago at a conference a speaker I was listening to made a comment to the effect that a function (let's say scalar) cannot be analytic because otherwise it would violate causality. He didn't ...
0
votes
1answer
387 views

Energy versus free-energy diagram

Energy versus free energy diagram. I haven't been able to find an adequate definition of these two terms in relation to each other. Could someone point me in the right direction, please? From Borrell ...
6
votes
4answers
635 views

The Dual Cloud Chamber Paradox

2012-04-07 Addendum: The Dual Cloud Chamber Paradox Two 10m diameter spheres $A$ and $B$ of very cold, thin gas have average atomic separations of 1nm. Their atoms are neutral, but ionize easily and ...
4
votes
1answer
68 views

What is the weekly average of coronal mass ejections the sun sends out?

What is the average number of coronal mass ejections that the sun sends out each week, including those that are not directed towards Earth?
6
votes
2answers
1k views

“Dear radioactive ladies and gentlemen” - Letter by Wolfgang Pauli

In 1930, Wolfgang Pauli wrote a letter to Lise Meitner for a convention in Tübingen, considering the problem of beta decay. Does anybody know, where to find the original letter online ?
1
vote
1answer
10k views

Is sea water more conductive than pure water because “electrical current is transported by the ions in solution”?

Apparently, electrical charge is transported by the ions dissolved in water, is this true?
7
votes
3answers
1k views

What makes a supermassive black hole move through space?

In this example of a Neutrino jet, it seems that the supermassive black hole moves around the space and meets up with a interstellar object sometimes. Source of the above image and a larger ...
5
votes
1answer
2k views

What would happen to Earth if Saturn were to exit the Solar System?

Suppose Saturn simply vanished. How would that affect the rest of the Solar System, specifically Earth?
9
votes
1answer
763 views

Identification of the state of particle types with representations of Poincare group

In the second chapter of the first volume of his books on QFT, Weinberg writes in the last paragraph of page 63: In general, it may be possible by using suitable linear combinations of the ...
-1
votes
2answers
214 views

Entropy change in a cycle with two isochoric and two adiabatic processes [closed]

Prove that the change of Entropy in a cycle with two isochoric and two adiabatic processes is 0. How can I prove that? Thanks!
2
votes
3answers
612 views

Simple home pressure experiment: how to understand the dependence on the glass opening?

Suppose you put a drinking glass under water such that it is completely filled with water. Then hold it upside down and slowly raise the drinking glass out of the water. The water in the drinking ...
3
votes
1answer
1k views

How does a stronger magnet affect the MRI image quality?

In which ways is a stronger magnet better for magnetic resonance imaging? I read that: The field strength of the magnet will influence the quality of the MR image regarding chemical shift ...
-2
votes
1answer
396 views

Which areas of physics are most demanding of good intuition or physical insight? [closed]

I'm a beginning graduate student, and I have 2 years before I have to pick a specialization. I would like to know which areas of physics call for the most intuition or physical insight. If this sounds ...
1
vote
5answers
227 views

Normal distribution of x, xdot [closed]

I have some real measurements from a process and I happened to look at the mutual distribution of (x(t), xdot(t)). I found that they seem to follow 2d normal distribution around (mu, 0). See image, ...
9
votes
4answers
1k views

Electrons - What is Waving?

If an electron is a wave, what is waving? So many answers on the internet say "the probability that a particle will be at a particular location"... so... the electron is a physical manifestation of ...
5
votes
2answers
9k views

Convective and Diffusive terms in Navier Stokes Equations

My question has 2 parts: I just followed the derivation of Navier Stokes (for Control Volume CFD analysis) and was able to understand most parts. However, the book I use (by Versteeg) does not ...
2
votes
4answers
2k views

Explanation about black color, and hence color

I'm bit confused about 'black' as a color. As per my knowledge, it is not given in visible color spectra like other colors for example red, violet etc. Also I'm confused with definition of color--does ...
1
vote
3answers
469 views

What is the reason behind specific values for charge of electrons, protons?

Why do things like protons and neutrons have specific values. Also speed of light is a speed in which even if you go towards it, the speed does not vary. But why does light have to travel at speed?
1
vote
2answers
236 views

What is the relation between extra dimensions and unification of theories?

One of the most used methods in unification of theories is the use of higher dimensions. How does it actually work? If these dimensions are extremely small curled up, how does it affect the universe. ...
1
vote
4answers
179 views

Is a macroscopic pair where I observe one of them quantum entanglement?

Of what little I know/understand about quantum entanglement can somebody confirm if the following experiment is a good analogy to quantum entanglement of pair of particles? PS: please don't laugh as ...
2
votes
1answer
105 views

Why do we need the concept of dark energy to explain the expansion of the universe?

Why can't the expansion of the universe be thought as the Big Bang itself still in progress? Why do we need to introduce dark energy? The Big Bang was powerful, and that explosion itself could still ...
1
vote
2answers
956 views

Noise amplitude increases as sample rate increase

I am testing the material properties of some very low stiffness materials. I'm using a force probe connected to software, sensing at about a hundredth of a gram of force. Now, what's interesting is ...
-1
votes
1answer
613 views

Ping pong ball levitation control system question, modeling as a damper? [closed]

I am building a control system to levitate a ping pong ball at a given height in a clear plastic tube using an arduino and implementing PID control. I have to come up with a transfer function... I am ...
1
vote
2answers
1k views

Irreducible representation in physics

I am confused about something. Group theory books written for physicists say that any reducible representation can be decomposed in terms of irreducible representations (so correct me if I am wrong, ...
25
votes
5answers
9k views

Why do electron and proton have the same but opposite electric charge?

What is the explanation between equality of proton and electron charges (up to a sign)? This is connected to the gauge invariance and renormalization of charge is connected to the renormalization of ...
2
votes
1answer
42 views

Existing photometry data of the open star cluster Cepheus OB3?

I am doing research with my teacher on photometry of the open star cluster Cepheus OB3, and we are trying to find research papers so we can target stars in the photos we took. Are there any existing ...
1
vote
1answer
380 views

Is there orbital angular momentum for all particles?

Light as an electromagnetic wave can be polarized in different ways, e.g. linear or circular. As far as I understand it currently this can be compared to the spin direction of a propagation electron ...
-2
votes
1answer
456 views

A quanta of time

A question of Quantum Time: Does a minimum interval of time cause wave-like behavior? If we think about the uncertainty principle, could it derive from a quanta of time? Does plank’s constant somehow ...
5
votes
5answers
871 views

Computing Planetary Positions

Shortly, I will be beginning my third year at University in Computer Science, I am a software developer and I will be required to work on a final year project. My idea for my final year project is ...
3
votes
1answer
112 views

Charge of a field under the action of a group

What does it mean for a field (say, $\phi$) to have a charge (say, $Q$) under the action of a group (say, $U(1)$)?
1
vote
0answers
199 views

How to derive a multi-reflection echo from a given single-reflection “trace”

Consider a one-dimensional semi-bounded "ray" (or thin "cylinder") of stratum (or just any homogeneous substance) which has several "features" along it's length (say, thin films). Consider a wave that ...
7
votes
1answer
2k views

Why does electrical current start to flow?

What happens microscopically when an electrical current starts to flow? I'd like to understand microscopically what happens in detail when electrons start moving (quasi-classically). Electrons can ...
12
votes
7answers
2k views

What is the proof that the universal constants ($G$, $\hbar$, $\ldots$) are really constant in time and space?

Cavendish measured the gravitation constant $G$, but actually he measured that constant on the Earth. What’s the proof that the value of the gravitation constant if measured on Neptune would remain ...
2
votes
3answers
708 views

Repulsive gravity [duplicate]

IANAP, so feel free to berate me for thinking apocryphal thoughts! Just as magnetism has two charges, in which particles of like-charge repulse and particles of dissimilar charge attract, might ...
6
votes
2answers
2k views

How can I determine if my telescope mirror is parabolic?

This week I purchased the “Skywatcher Explorer 130P Newtonian Reflector Telescope”, specifically as it has the parabolic mirror. I do not have the outer packaging for sundry reasons. Therefore, after ...
0
votes
0answers
85 views

Springs yet again, this time with a picture. Infinite displacement, makes no sense [duplicate]

Possible Duplicate: How could this damped oscillator ever go to infinity? Or negative infinity for that matter? Consider this ! Where I purposely drew the right arrow bigger than the left ...

15 30 50 per page