# All Questions

2k views

### What determines which frames are inertial frames?

I understand that you can (in principle) measure whether "free particles" (no forces) experience accelerations in order to tell whether a frame is inertial. But fundamentally, what determines which ...
704 views

### How could spacetime become discretised at the Planck scale?

I didn't have much luck getting a response to this question before so I have tried to reword and expand it a little: In early 2010 I attended this inaugural lecture by string theorist- Prof. ...
8k views

### Why glass is transparent?

Once I asked this question from my teacher and he replied "because it passes light", "and why it passes light" I asked and he said "because it is transparent". Same question again, Why glass is ...
709 views

### Why space expansion affects matter?

If space itself is expanding, then why would it have any effect on matter (separates distant galaxies)? Space is "nothing", and if "nothing" becomes bigger "nothing" it's still a "nothing" that ...
1k views

### The origin of the value of speed of light

Meaning, why is it the exact number that it is? Why not 2x10^8 /mps instead of 3? Does it have something to do with the mass, size or behavior of a photon? To be clear, I'm not asking "how we ...
4k views

### Why can you see virtual images?

In optics it is widely mentioned real images are projectable onto screens whereas virtual ones can only be seen by a person. Isn't that contradictory? I mean in order to see the virtual image it has ...
4k views

### What equation describes the wavefunction of a single photon?

The Schrödinger equation describes the quantum mechanics of a single massive non-relativistic particle. The Dirac equation governs a single massive relativistic spin-½ particle. The photon is a ...
2k views

### Books for Condensed Matter Physics

What are some good condensed matter physics books that can fill the gap between Ashcroft & Mermin and research papers? Suggestions for any specialized topics (such as superconductivity, CFT, ...
4k views

### What does it mean to say “Gravity is the weakest of the forces”?

I can understand that on small scales (within an atom/molecule), the other forces are much stronger, but on larger scales, it seems that gravity is a far stronger force; e.g. planets are held to the ...
2k views

### Experimental evidence of a fourth spatial dimension?

As human beings, we observe the world in which we live in three dimensions. However, it is certainly theoretically possible that more dimensions exist. Is there any direct or indirect evidence ...
1k views

### Does $E = mc^2$ apply to photons?

Photons are massless, but if $m = 0$ and $E = mc^2$ then $E = 0c^2 = 0$. This would say that photons have no energy, which is not true. However, given the formula $E = ℎf$, a photon does have energy ...
787 views

### Why do physicists believe that particles are pointlike?

String theory gives physicists reason to believe that particles are 1-dimensional strings because the theory has a purpose - unifying gravity with the gauge theories. So why is it that it's popular ...
631 views

### Would it be possible to develop special relativity without knowing about light?

My understanding of special relativity is that it is fundamentally based on the constancy of the speed of electromagnetic radiation - that this speed is a physical law (or derivable from physical laws ...
2k views

### Are tidal power plants slowing down Earth's rotation?

Are tidal power plants slowing down Earth's rotation to the speed of the orbiting moon? (1 rotation per 28 cca days) Are they vice versa increasing the speed of moon orbiting by generating some ...
1k views

### Vortex in liquid collects particles in center

At xmas, I had a cup of tea with some debris at the bottom from the leaves. With less than an inch of tea left, I'd shake the cup to get a little vortex going, then stop shaking and watch it spin. ...
The standard treatment of the one-dimensional quantum simple harmonic oscillator (SHO) using the raising and lowering operators arrives at the countable basis of eigenstates $\{\vert n \rangle\}_{n = ... 3answers 3k views ### A fly in an accelerating car A fly is flying around in a car, the fly never touches any surface in the car only fly’s around in the air inside the car. The car accelerates. does the fly slam in to the rear window. or does the fly ... 1answer 238 views ### Does non-mass-energy generate a gravitational field? At a very basic level I know that gravity isn't generated by mass but rather the stress-energy tensor and when I wave my hands a lot it seems like that implies that energy in$E^2 = (pc)^2 + (mc^2)^2$... 1answer 401 views ### What are the average matter, antimatter, and binding energy composition of protons and neutrons? For a free baryon at rest at room temperature, how much of its ~1Gev (rest) mass can (on average) be considered as matter, as antimatter, and as binding energy? For a baryon in a nucleus, I assume ... 1answer 277 views ### What are common methods for calculating the time dependency of elliptical orbit? After playing a game called "Kerbal Space Program" I got interested in orbital mechanics and started messing with simplified calculations to determine$\Delta v$requirements. In which I compared two ... 4answers 1k views ### A Basic Question about Gravity, Inertia or Momentum or something along those lines Why is it that if I'm sitting on a seat on a bus or train and its moving quite fast, I am able to throw something in the air and easily catch it? Why is it that I haven't moved 'past' the thing during ... 3answers 762 views ### Why would Antimatter behave differently via Gravity? Confinement of antihydrogen might help provide a future answer. http://arxiv.org/abs/1104.4982 4answers 3k views ### Does matter with negative mass exist? Or does it exist mathematically? Is it really inconsistent with a common-sense, mathematics or known physical laws? As far as I understand, if it exists, it must be far away from the "positive" ... 14answers 2k views ### Learning physics online? I'm thinking of following some kind of education in physics online. I have a master degree in Computer Science and have reasonable good knowledge in physics. I would like a program of 1-2 years and ... 13answers 8k views ### Home experiments to derive the speed of light? Are there any experiments I can do to derive the speed of light with only common household tools? 4answers 5k views ### Why do same/opposite electric charges repel/attract each other, respectively? I know plus pushes another plus away, but why, really, do they do that? On the other hand, molecules of the same type are attracted to each other. I find that weird. I do know some stuff about four ... 2answers 21k views ### On this infinite grid of resistors, what's the equivalent resistance? I searched and couldn't find it on the site, so here it is (quoted to the letter): On this infinite grid of ideal one-ohm resistors, what's the equivalent resistance between the two marked nodes? ... 8answers 2k views ### Why$\displaystyle i\hbar\frac{\partial}{\partial t}$can not be considered as the Hamiltonian operator? In the time dependent Schrodinger equation$\displaystyle, H\Psi = i\hbar\frac{\partial}{\partial t}\Psi$, the Hamiltonian operator is given by$\displaystyle H = -\frac{\hbar^2}{2m}\nabla^2+V$... 5answers 3k views ### Does Coulomb's Law, with Gauss's Law, imply the existence of only three spatial dimensions? Coulomb's Law states that the fall-off of the strength of the electrostatic force is inversely proportional to the distance squared of the charges. Gauss's law implies that a the total flux through a ... 10answers 9k views ### How did Newton discover his second law? I've always assumed/been told that Newton's 2nd law is an empirical law — it must be discovered by experiment. If this is the case, what experiments did Newton do to discover this? Is it related to ... 4answers 2k views ### Have we figured out how to analyze turbulent fluids? I was surprised to read that we don't know how to analyze turbulent fluids. On page 3-9 of The Feynman Lectures on Physics (Volume One), Feynman writes: Finally, there is a physical problem that ... 3answers 1k views ### Are elementary particles actually more elementary than quasiparticles? Quarks and leptons are considered elementary particles, while phonons, holes, and solitons are quasiparticles. In light of emergent phenomena, such as fractionally charged particles in fractional ... 7answers 3k views ### Why do people still talk about bohmian mechanics/hidden variables [closed] I was reading the Feynman lectures in physics and after thinking about it for a while it seems particularly unreasonable to talk about hidden variables. Let us say that the electron has some internal ... 1answer 962 views ### Why does gravity need to be quantised? The electroweak and strong forces seem to be completely different types of forces to gravity. The latter is geometric while the former are not (as far as I'm aware!). So why should they all be ... 9answers 3k views ### Book about classical mechanics I am looking for a book about "advanced" classical mechanics. By advanced I mean a book considering directly Lagrangian and Hamiltonian formulation, and also providing a firm basis in the geometrical ... 6answers 3k views ### Why is there no absolute maximum temperature? If temperature makes particles vibrate faster, and movement is limited by the speed of light, then temperature must be limited as well I would assume. Why there is no limits? 5answers 936 views ### What is$\Delta t$in the time-energy uncertainty principle? In non-relativistic QM, the$\Delta E$in the time-energy uncertainty principle is the limiting standard deviation of the set of energy measurements of$n$identically prepared systems as$n$goes to ... 6answers 1k views ### What is a tensor? I have a pretty good knowledge of physics but couldn't understand what a tensor is. I just couldn't understand it, and the wiki page is very hard to understand as well. Can someone refer me to a good ... 3answers 910 views ### Introduction to string theory I am in the last year of MSc. and would like to read string theory. I have the Zwiebach Book, but along with it what other advanced book can be followed, which can be a complimentary to Zwiebach. I ... 5answers 3k views ### What if the universe is rotating as a whole? Suppose in the milliseconds after the big bang the cosmic egg had aquired some large angular momentum. As it expanded, keeping the momentum constant (not external forces) the rate of rotation would ... 5answers 1k views ### How are forces “mediated”? I hope this is the right word to use. To me, these forces seem kind of fanciful (except for General Relativity and Gravity, which have a geometric interpretation). For example, how do two charged ... 5answers 2k views ### Deterministic quantum mechanics I came across a very recent paper by Gerard 't Hooft The abstract says: It is often claimed that the collapse of the wave function and Born's rule to interpret the square of the norm as a ... 4answers 966 views ### What is the physical meaning of a “complete” Hilbert space in QM? What does the word "complete" means from the physical point of view? I do not understand what it physically means to say that a Hilbert space is a complete vector space. 10answers 1k views ### Why can't the outcome of a QM measurement be calculated a-priori? Quantum Mechanics is very successful in determining the overall statistical distribution of many measurements of the same process. On the other hand, it is completely clueless in determining the ... 6answers 1k views ### Why isn't dark matter just matter? There's more gravitational force in our galaxy (and others) than can be explained by counting stars. So why not lots of dark planetery systems (ie without stars) ? Why must we assume some undiscovered ... 1answer 2k views ### What is the definition of how to count degrees of freedom? This question resulted, rather as by-product, the discussion on how to count degrees of freedom (DOF). I extend that question here: Are necessary1 derivatives such as velocities counted as ... 2answers 698 views ### Symmetrical twin paradox Take the following gedankenexperiment in which two astronauts meet each other again and again in a perfectly symmetrical setting - a hyperspherical (3-manifold) universe in which the 3 dimensions are ... 4answers 715 views ### Is the world$C^\infty$? While it is quite common to use piecewise constant functions to describe reality, e.g. the optical properties of a layered system, or the Fermi–Dirac statistics at (the impossible to reach exactly) ... 5answers 805 views ### What are some useful ways to imagine the concept of spin as it relates to subatomic particles? The answers in this question: What is spin as it relates to subatomic particles? do not address some particular questions regarding the concept of spin: How are some useful ways to imagine a ... 6answers 1k views ### Why is there a$\frac 1 2$in$\frac 1 2 mv^2\$?
For elastic collisions of n particles, we know that momentum in the three orthogonal directions are independently conserved:$$\frac{d}{dt}\sum\limits_i^n m_iv_{ij} =0,\quad j=1,2,3$$ From this, it ...