All Questions
24
votes
8answers
8k views
Will a hole cut into a metal disk expand or shrink when the disc is heated?
Suppose you take a metal disc and cut a small, circular hole in the center.
When you heat the whole thing, will the hole's diameter increase or decrease? and why?
24
votes
5answers
2k views
With Newton's third law, why are things capable of moving?
I've got a rather humiliating question considering newton's third law
"If an object A exterts a force on object B, then object B exerts an equal but opposite force on object A" -> $F_1=-F_2$
...
24
votes
9answers
3k views
Can Maxwell's equations be derived from Coulomb's Law and Special Relativity?
As an exercise I sat down and derived the magnetic field produced by moving charges for a few contrived situations. I started out with Coulomb's Law and Special Relativity. For example, I derived the ...
24
votes
2answers
1k views
Why does a supernova explode
This is really bugging me. When you look up some educational text about stars life, this is what you find out:
Gravity creates the temperature and pressure to start fusion reactions.
The fusion ...
24
votes
10answers
3k views
What happens to the energy when waves perfectly cancel each other?
What happens to the energy when waves perfectly cancel each other (destructive interference)? It appears that the energy "disappear" but the law of conservation of energy states that it can't be ...
24
votes
3answers
856 views
The Pioneer anomaly finally explained?
Pioneer 10 & 11 are robotic space probes launched by the NASA in the early 1970's. After leaving our solar system, an unusual deceleration of both spacecrafts has been measured to be approximately ...
23
votes
9answers
584 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 ...
23
votes
7answers
907 views
Are these two quantum systems distinguishable?
Suppose Stanford Research Systems starts selling a two-level atom factory. Your grad student pushes a button, and bang, he gets a two level atom. Half the time the atom is produced in the ground ...
23
votes
5answers
2k views
A list of inconveniences between quantum mechanics and relativity?
It is well known that quantum mechanics and (special and/or general) relativity do not fit well. I am wondering whether it is possible to make a list of contradictions or problems between them?
E.g. ...
23
votes
11answers
885 views
Negative probabilities in quantum physics
Negative probabilities are naturally found in the Wigner function (both the original one and its discrete variants), the Klein paradox (where it is an artifact of using a one-particle theory) and the ...
23
votes
3answers
769 views
Why is high temperature superconductivity so hard to solve?
The phenomenon of high temperature superconductivity has been known for decades, particularly layered cuprate superconductors. We know the precise lattice structure of the materials. We know the band ...
23
votes
3answers
2k 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 ...
23
votes
2answers
3k views
Why can Hiroshima be inhabited when Chernobyl cannot?
In Hiroshima, atomic bomb was dropped, but today, there are residents in Hiroshima. However, in Chernobyl, there is no resident living today (or very few). What made the difference?
23
votes
3answers
1k views
When water climbs up a piece of paper, where is the energy coming from?
Take a glass of water and piece of toilet paper. If you keep the paper vertical, and touch the surface of the water with the tip of the paper, you can see the water being absorbed and climbing up the ...
23
votes
5answers
310 views
What are some critiques of Jaynes' approach to statistical mechanics?
Suggested here: What are the justifying foundations of statistical mechanics without appealing to the ergodic hypothesis?
I was wondering about good critiques of Jaynes' approach to statistical ...
23
votes
2answers
2k 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 ...
23
votes
1answer
325 views
Overcharging a black hole
Hubeny's 1998 paper got a lot of people interested in determining whether cosmic censorship can be violated by dropping too much charge onto a black hole. It suggested that you might be able to get a ...
22
votes
10answers
5k views
Discreteness and Determinism in Superstrings?
So Gerard 't Hooft has a brand new paper (thanks to Mitchell Porter for making me aware of it) so this is somewhat of a expansion to the question I posed on this site a month or so ago regarding 't ...
22
votes
6answers
2k views
How do whisky stones keep your drink cold?
From a discussion in the DMZ (security stack exchange's chat room - a place where food and drink are important topics) we began to question the difference between how ice and whisky stones work to ...
22
votes
8answers
2k views
Why don't modern spacecraft use nuclear power?
The Voyager 1 & 2 spacecraft launched in 1977 with Plutonium as their source of electricity. 34 years later they claim these two spacecraft have enough power to last them until at least 2020. That ...
22
votes
3answers
2k 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 ...
22
votes
6answers
606 views
How to write a paper in physics
I really like to do research in physics and like to calculate to see what happen. However, I really find it hard to write a paper, to explain the results I obtained and to put them in order. One of ...
22
votes
2answers
2k views
Why can't we feel the Earth turning?
The Earth turns with a very high velocity, around its own axis and around the Sun.
So why can't we feel that it's turning, but we can still feel earthquake.
22
votes
4answers
2k views
Why is a new moon not the same as a solar eclipse?
Forgive the elementary nature of this question:
Because a new moon occurs when the moon is positioned between the earth and sun, doesn't this also mean that somewhere on the Earth, a solar eclipse ...
22
votes
5answers
4k views
Why does a tire need to be filled with air?
The Mini 4WD's tires aren't full of air, and it can run. Also, the tank doesn't have tires with air. So, the question is: why do real cars on the road need to be filled with air? What is the idea ...
22
votes
3answers
1k views
Scattering of light by light: experimental status
Scattering of light by light does not occur in the solutions of Maxwell's equations (since they are linear and EM waves obey superposition), but it is a prediction of QED (the most significant Feynman ...
22
votes
4answers
488 views
How do the โhot Jupiterโ planets get so close to their host star?
Many of the extrasolar planets to date are Jovian sized planets that orbit very very close to their parent star. Traditional planetary formation models say that it is extremely unlikely (if not ...
22
votes
7answers
383 views
An entropy of the Wigner function
Is there an entropy that one can use for the Wigner quasi-probability distribution?
(In the sense of a phase-space probability distribution, not - just von Neumann entropy.)
One cannot simply use ...
22
votes
6answers
2k views
Formalizing Quantum Field Theory
I'm wondering about current efforts to provide mathematical foundations and more solid definition for quantum field theories. I am aware of such efforts in the context of the simpler topological or ...
22
votes
4answers
991 views
What is information?
We're all familiar with basic tenants such as "information cannot be transmitted faster than light" and ideas such as information conservation in scenarios like Hawking radiation (and in general, ...
22
votes
3answers
877 views
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 ...
22
votes
8answers
2k views
Why doesn't the bike fall if going with a high speed?
Why does the bike fall when its speed is very low or close to zero and is balanced when going with a high speed?
22
votes
6answers
2k views
Why does space expansion not expand matter?
REFORMULATED:
I have looked at the other questions (ie "why does space expansion affect matter") but can't find the answer I am looking for.
My question:
There is always mention of space expanding ...
22
votes
8answers
1k views
What is the Earth truly rotating about/revolving around?
Earth rotates on its axis and revolves around the sun, the sun revolves around the galaxy, the galaxy is also moving. So Earth's net rotation as observed from a fixed inertial frame consists of all ...
22
votes
1answer
197 views
Mermin-Wagner theorem in the presence of hard-core interactions
It seems quite common in the theoretical physics literature to see applications of the "Mermin-Wagner theorem" (see wikipedia or scholarpedia for some limited background) to systems with hard-core ...
22
votes
3answers
1k 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 ...
22
votes
0answers
431 views
On the Coulomb branch of N=2 supersymmetric gauge theory
The chiral ring of the Coulomb branch of a 4d N=2 supersymmetric gauge theory is given by the Casimirs of the vector multiplet scalars, and they don't have non-trivial relations; the Casimirs are ...
21
votes
12answers
2k views
Is it possible for information to be transmitted faster than light by using a rigid pole?
Is it possible for information (like 1 and 0s) to be transmitted faster than light?
For instance, take a rigid pole of several AU in length. Now say you have a person on each end, and one of them ...
21
votes
7answers
2k views
What is the difference between a white object and a mirror?
I was taught that something which reflects all the colors of light is white. The function of a mirror is the same, it also reflects all light. What's the difference?
Update:
But what if the white ...
21
votes
9answers
2k 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, ...
21
votes
9answers
4k 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 ...
21
votes
5answers
1k views
Is a proton collision (collisions like in the LHC) visible to the human eye?
I was curious if a proton collision is visible to the human eye.
(This might sound like a really basic question and forgive me if it is. I am very inexperienced in Physics and just wanted an answer ...
21
votes
7answers
3k views
Why does holding something up cost energy while no work is being done?
I read the definition of work as
$$W ~=~ \vec{F} \cdot \vec{d}$$
$$\text{ Work = (Force) $\cdot$ (Distance)}.$$
If a book is there on the table, no work is done as no distance is covered. If I ...
21
votes
13answers
2k views
Suggested reading for renormalization (not only in QFT)
What papers/books/reviews can you suggest to learn what renormalization "really" is? Standard QFT textbooks are usually computation-heavy and provide little physical insight in this respect - after my ...
21
votes
3answers
1k views
Why do we need 12 atoms to store 1 bit of data?
Recent research at IBM has found a way to store 1 bit of data in 12 atoms.
While that is a big accomplishment compared to what we have today, it does seem like a waste to a non-physics eye like me.
...
21
votes
5answers
2k views
Hamilton's Principle
Hamilton's principle states that a dynamic system always follows a path such that its action integral is stationary (that is, maximum or minimum).
Why should the action integral be stationary? On ...
21
votes
4answers
2k views
What is the usefulness of the Wigner-Eckart theorem?
I am doing some self-study in between undergrad and grad school and I came across the beastly Wigner-Eckart theorem in Sakurai's Modern Quantum Mechanics. I was wondering if someone could tell me why ...
21
votes
2answers
431 views
Why does laser light not affect glowing materials?
I have this childrens rubber ball which glows in the dark after it's exposed to light. I "charge" it with a flash light then play with my dogs at night. I thought to try a very intense green laser, ...
21
votes
1answer
1k views
Why does this object periodically turn itself?
See this video about 30 sec in. http://www.youtube.com/watch?v=dL6Pt1O_gSE
Is this a real effect? Why does it seem to turn periodically?
Can it be explained by classical mechanics alone?
Is there a ...
21
votes
1answer
728 views
Energy in bolognese reduction - lid on or off?
Generally, to let my bolognese thicken, I leave the lid off in order to "let water vapor escape." I am however distracted from enjoying the taste because I'm having doubts that my physical reasoning ...
