33
votes
4answers
2k views

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 ...
33
votes
9answers
20k views

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 ...
33
votes
13answers
3k views

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 ...
33
votes
7answers
5k views

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 ...
33
votes
4answers
5k views

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, ...
33
votes
5answers
4k views

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 ...
33
votes
3answers
3k views

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 ...
33
votes
7answers
4k views

“Falling upward” - how far you have to be from Earth to start falling to the Moon?

Talking about gravity with my 9 y/o she asked when do we start "falling upward" to the Moon. What is the distance at which the Moon's gravitational attraction is higher than that of the Earth and thus ...
33
votes
4answers
2k views

What is needed to claim the discovery of the Higgs boson?

As I understand the Higg's boson can be discovered by the LHC because the collisions are done at an energy that is high enough to produce it and because the luminosity will be high enough also. But ...
33
votes
9answers
9k views

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 ...
33
votes
11answers
13k 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 ...
33
votes
9answers
3k views

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 ...
33
votes
12answers
4k views

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 ...
33
votes
3answers
794 views

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 ...
33
votes
4answers
1k views

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 ...
33
votes
4answers
1k views

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 ...
33
votes
4answers
10k views

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 ...
32
votes
7answers
7k views

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 ...
32
votes
3answers
3k views

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 ...
32
votes
3answers
2k views

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?
32
votes
10answers
1k views

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 ...
32
votes
4answers
3k views

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 ...
32
votes
5answers
3k views

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?
32
votes
6answers
3k views

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 ...
32
votes
11answers
42k views

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 ...
32
votes
4answers
4k views

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 ...
32
votes
3answers
6k views

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 ...
32
votes
8answers
2k views

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 ...
32
votes
5answers
2k views

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 ...
32
votes
6answers
2k views

Path integral vs. measure on infinite dimensional space

Coming from a mathematical background, I'm trying to get a handle on the path integral formulation of quantum mechanics. According to Feynman, if you want to figure out the probability amplitude for ...
32
votes
6answers
3k views

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 ...
32
votes
10answers
3k views

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 ...
32
votes
1answer
383 views

$(\mu,P,T)$ 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 $(\mu, P, T)$ ...
32
votes
2answers
6k views

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 ...
32
votes
10answers
3k views

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): ...
32
votes
3answers
4k views

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 ...
32
votes
5answers
4k views

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 ...
32
votes
0answers
1k views

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 ...
31
votes
9answers
7k views

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.
31
votes
24answers
2k views

Your favorite Physics/Astrophysics blogs? [closed]

What are the Physics/Astrophysics blogs you regularly read? I'm looking to beef up my RSS feeds to catch up with during my long commutes. I'd like to discover lesser-known gems (e.g. not well known ...
31
votes
8answers
44k views

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?
31
votes
4answers
4k views

Why does water rise through a hole in ice

I've just been watching a doco where some guys cut a hole in a frozen lake, the ice is 3 feet thick, I was surprised that the water rose to the level of the ice. My friend assures me it's because ...
31
votes
6answers
5k views

Why can't you hear music well over a telephone line?

Why can't you hear music well well over a telephone line? I was asked this question in an interview for a university study placement and I unfortunately had no idea. I was given the hint that the ...
31
votes
5answers
3k views

What is the most efficient information storage?

What is the most efficient way to store data that is currently hypothesized? Is it theoretically possible to store more than one bit per atom? Until Floris' comment, I hadn't considered that ...
31
votes
6answers
3k views

Why does calculus of variations work?

How does it make sense to vary the position and the velocity independently? Edit: Velocity is the derivative of position, so how can you treat them as independent variables? Doesn't every physics ...
31
votes
10answers
3k views

Why are the laws of thermodynamics “supreme among the laws of Nature”?

Eddington wrote The law that entropy always increases holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in ...
31
votes
9answers
4k views

Rigor in quantum field theory

Quantum field theory is a broad subject and has the reputation of using methods which are mathematically desiring. For example working with and subtracting infinities or the use of path integrals, ...
31
votes
8answers
10k views

Would it matter if the Earth rotated clockwise?

In the Futurama episode "That Darn Katz!" they save the world by rotating the Earth backwards saying it shouldn't matter (which direction Earth rotates). If Earth rotated clockwise and remained in ...
31
votes
4answers
6k views

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. ...
31
votes
7answers
3k views

Homemade salad dressing separates into layers after it sits for a while. Why doesn't this violate the 2nd law of thermodynamics?

The oil, vinegar and other liquids in homemade salad dressing separate into layers after sitting for a while, making the mixture become more organized as time evolves. Why doesn't this violate the ...

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