# All Questions

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### Can the solar system really fit in a thimble?

Almost every time somebody talks about atoms, at some point they mention something like this: If we remove the spaces between the atoms and atomic components, we can fit the solar system in a ...
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### Why is the sky never green? It can be blue or orange, and green is in between!

I, like everybody I suppose, have read the explanations why the colour of the sky is blue: ... the two most common types of matter present in the atmosphere are gaseous nitrogen and oxygen. ...
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### Does time move slower at the equator?

While answering the question GPS Satellite - Special Relativity it occurred to me that time would run more slowly at the equator than at the North Pole, because the surface of the Earth is moving at ...
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### Is a hard drive heavier when it is full?

Browsing Quora, I saw the following question with contradicting answers. For the highest voted answer: The bits are represented by certain orientations of magnetic fields which shouldn't have ...
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### Trace of a commutator is zero - but what about the commutator of $x$ and $p$?

Operators can be cyclically interchanged inside a trace: $${\rm Tr} (AB)~=~{\rm Tr} (BA).$$ This means the trace of a commutator of any two operators is zero: $${\rm Tr} ([A,B])~=~0.$$ But what about ...
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### Is fire plasma?

Is Fire a Plasma? If not, what is it then? If yes why, don't we teach kids this basic example? UPDATE: I probably meant a regular commonplace fire of the usual temperature. That should simplify ...
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### If photon energies are continuous and atomic energy levels are discrete, how can atoms absorb photons?

If photon energies are continuous and atomic energy levels are discrete, how can atoms absorb photons? The probability of a photon having just the right amount of energy for an atomic transition is ...
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### Wind generators - why so few blades?

Why commercial wind generators usually have just 2-3 blades? Having more blades would allow to increase power OR decrease diameter. Decreased diameter would also reduce stress due to different wind ...
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### How exact is the analogy between statistical mechanics and quantum field theory?

Famously, the path integral of quantum field theory is related to the partition function of statistical mechanics via a Wick rotation and there is therefore a formal analogy between the two. I have a ...
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### What causes ballpoint pens to write intermittently?

After a while, a ball point pen doesn't write very well anymore. It will write for a little distance, then leave a gap, then maybe write in little streaks, then maybe write properly again. It seems ...
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### Why aren't there spherical galaxies? [duplicate]

According to the Wikipedia page on Galaxy Types, there are four main kinds of galaxies: Spirals - as the name implies, these look like huge spinning spirals with curved "arms" branching out ...
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### Does Gödel preclude a workable ToE?

Gödel's incompleteness theorem prevents a universal axiomatic system for math. Is there any reason to believe that it also prevents a theory of everything for physics? Edit: I haven't before seen ...
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### Why is Earth's climate so stable?

Earth wasn't always the only water-world in the solar system. Mars also appear to have started out wet but, as conditions changed, Mars lost its oceans. So, how has Earth managed to avoid a ...
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### Does the sun rotate?

As implied from the question, does the sun rotate? If so, do other stars not including the sun also rotate? Would there be any consequences if the sun and other stars didn't rotate? Me and my friends ...
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### How can a photon have no mass and still travel at the speed of light?

I've read a number of the helpful Q&As on photons that mention the mass/mass-less issue. Do I understand correctly that the idea of mass-less (a rest mass of 0) may be just a convention to make ...
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### Where does the extra force generated by a lever come from?

Given an object being pulled down to earth by a force of 1000 lb: If a machine were to lift it off the ground for me, and I grabbed on to the rope before it was released, there is no way in the world ...
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### Why is it said that without quantum mechanics we would not have modern computers?

I've heard this in many quantum mechanics talks and lectures, nevertheless I don't seem to grasp the idea behind it. What I mean is, at which point is that our modern understanding of quantum ...
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### Why do rocket engines have a throat?

Diagrams of rocket engines like this one, (source) always seem to show a combustion chamber with a throat, followed by a nozzle. Why is there a throat? Wouldn't the thrust be the same if the ...
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### What is the minimum pressure of a medium for which a sound wave can exist?

At what pressure will be particles in a medium be unable to form a sound wave when disturbed? How can this pressure be described mathematically? My guess is that this would correspond to the point at ...
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### Can Jupiter be ignited?

Our solar system itself contains two candidate "Earths" One is Jupiter's moon Europa and another is Saturn's moon Titan. Both of them have the problem of having at low temperature as Sun's heat ...
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### Why has Earth's core not become solid?

The Earth is billions of years old, yet its core has not yet cooled down and become solid. Will this happen in the foreseeable future?
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### What is the difference between “kinematics” and “dynamics”?

I have noticed that authors in the literature sometimes divide characteristics of some phenomenon into "kinematics" and "dynamics". I first encountered this in Jackson's E&M book, where, in ...
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### Is a “third quantization” possible?

Classical mechanics: $t\mapsto \vec x(t)$, the world is described by particle trajectories $\vec x(t)$ or $x^\mu(\lambda)$, i.e. the Hilbert vector is the particle coordinate function $\vec x$ (or ...
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### Why are differential equations for fields in physics of order two?

What is the reason for the observation that across the board fields in physics are generally governed by second order (partial) differential equations? If someone on the street would flat out ask ...
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### How can anything ever fall into a black hole as seen from an outside observer?

The event horizon of a black hole is where gravity is such that not even light can escape. This is also the point I understand that according to Einstein time dilation will be infinite for a ...
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### What is the use of a Universal-NOT gate?

The universal-NOT gate in quantum computing is an operation which maps every point on the Bloch sphere to its antipodal point (see Buzek et al, Phys. Rev. A 60, R2626–R2629). In general, a single ...
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### Does quantum mechanics violate the equivalence principle?

I have a question about equivalence principle in quantum mechanics. Consider a Schroedinger equation under gravitional field \left[ - \frac{1}{2m_I} \nabla^2 + m_g \Phi_{\mathrm{grav}} \right]\psi ...
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### Why can't we see gases?

I am not sure what causes gas molecules to be invisible.This question may look silly but I really want to know the story behind it.
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### Why doesn't the Moon fall upon Earth?

Why doesn't the Moon, or for that matter anything rotating another larger body, ever fall into the larger body?
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### Why does the speed of light have no uncertainty?

I could understand that the definition of a second wouldn't have an uncertainty when related to the transition of the Cs atom, so it doesn't have an error because it's an absolute reference and we ...
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### How close can you get to lava before burning?

As the title asks: How close can you get to lava before burning? I know that it depends on an number of factors; speed of lava flow, wind direction/strength, type(?) of lava flow (related to speed, ...
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### What are washers for? [closed]

When you attach a bolt to something using a nut, it is clear what the roles of the nut and bold are. The more you tighten the bolt the more secure your fastening. However, you are often also told ...
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### What is the fallacy in this infinite motion machine?

I realize this isn't possible, but I can't see why not, especially if you change the model a little bit so that the balls simply travel through a tube of water on the way up, rather than exactly ...
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### Examples of number theory showing up in physics

My question is very simple: Are there any interesting examples of number theory showing up unexpectedly in physics? This probably sounds like rather strange question, or rather like one of the ...
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### How can earthquakes shift the earth's axis?

One often comes across news articles that claim that an earthquake shifted the earth's axis. http://news.google.com/?q=earthquake%20shifted%20OR%20shifts%20earth%27s%20axis If you ignore the ...
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### How is it possible to accelerate a neutron?

It is possible to accelerate a charged particle in an electric field, how is it possible to accelerate a neutron? How can we control its velocity?
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### Why is information indestructible?

I really can't understand what Leonard Susskind means when he says that information is indestructible. Is that information that is lost, through the increase of entropy really recoverable? He ...
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### Are atoms unique?

Do atoms have any uniquely identifying characteristic besides their history? For example, if we had detailed information about a specific carbon atom from one of Planck's fingerprints, and could ...
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### Why is the sky not purple?

I realise the question of why this sky is blue is considered reasonably often here, one way or another. You can take that knowledge as given. What I'm wondering is, given that the spectrum of ...
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### Why does everything spin?

The origin of spin is some what a puzzle to me, everything spin from galaxies to planets to weather to electrons. Where has all the angular momentum come from? Why is it so natural? I was also ...
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### If there were fundamental forces weaker than gravity, would we know about it?

We know that gravity is a very weak force compared to electromagnetic forces and the nuclear forces. We know about the other forces because they're necessary to explain atoms, and we can detect ...
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### What is spontaneous symmetry breaking in QUANTUM systems?

Most descriptions of spontaneous symmetry breaking, even for spontaneous symmetry breaking in quantum systems, actually only give a classical picture. According to the classical picture, spontaneous ...
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### $(\mu,P,T)$ pseudo-ensemble: why is it not a proper thermodynamic ensemble?

While teaching statistical mechanics, and describing the common thermodynamic ensembles (microcanonical, canonical, grand canonical), I usually give a line on why there can be no $(\mu, P, T)$ ...
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### Before a once-warm lake starts to freeze, must its temperature be 4°C throughout at some point?

This is a problem I just started puzzling over, and I felt this would be a good forum to check my reasoning. So here are the relevant observations followed by my question: Water achieves its maximum ...
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### Mechanics around a rail tank wagon

Some time ago I came across a problem which might be of interest to the physics.se, I think. The problem sounds like a homework problem, but I think it is not trivial (i am still thinking about it): ...
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### A No-Nonsense Introduction to Quantum Field Theory

I found Sean Carroll's "A No Nonsense Introduction to General Relativity" (about page here. pdf here), a 24-page overview of the topic, very helpful for beginning study. It all got me over the hump ...
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### Why is it hard to solve the Ising-model in 3D?

The Ising model is a well-known and well-studied model of magnetism. Ising solved the model in one dimension in 1925. In 1944, Onsager obtained the exact free energy of the two-dimensional (2D) model ...
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### On the Coulomb branch of N=2 supersymmetric gauge theory

The chiral ring of the Coulomb branch of a 4D $\mathcal N=2$ supersymmetric gauge theory is given by the Casimirs of the vector multiplet scalars, and they don't have non-trivial relations; the ...