# All Questions

46k 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" -> $F_1=-F_2$ ...
6k views

### Is it possible to “see” atoms?

As per my knowledge, atoms are small beyond our imaginations. But there is an image on Wikipedia that shows silicon atoms observed at the surface of silicon carbide crystals. The image: How can we ...
9k views

### Why is water clear?

Water appears transparent to visible light, yet most other objects are opaque. Why is that? Is there an explanation why water appears transparent? Is water transparent at all wavelengths, or are ...
4k views

### What is information?

We're all familiar with basic tenets such as "information cannot be transmitted faster than light" and ideas such as information conservation in scenarios like Hawking radiation (and in general, ...
11k 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 ...
16k views

### Why does a candle blow out when we blow on it? Our breath is 16% oxygen and only 4% CO2

Don't say that a layer of carbon dioxide covers the flame, because our breath has more oxygen than carbon dioxide. Also, our breath does not cool the flame as it is itself warm. So what is happening ...
5k views

### Is there something similar to Noether's theorem for discrete symmetries?

Noether's theorem states that, for every continuous symmetry of an action, there exists a conserved quantity, e.g. energy conservation for time invariance, charge conservation for $U(1)$. Is there any ...
3k views

### Do all black holes have a singularity?

If a large star goes supernova, but not enough mass collapses to form a black hole, it often forms a neutron star. My understanding is that this is the densest object that can exist because of the ...
3k views

### What happened to David John Candlin?

This is an ultra-soft question about relatively recent history. While reading some of Mandelstam's papers, I noticed that he cites David John Candlin consistenly whenever he does anything with ...
7k views

### Why doesn't light affect a compass?

In our daily life a lot of photons of visible light, infrared and radio etc move around us. We know that light is an electromagnetic radiation. So why doesn't that electromagnetic radiation affect a ...
5k views

### Why do the big nuts always remain at top? The Brazil-nut Effect

Most of the time I mixed different nuts in a bowl, I observed that the big Brazil nuts were always at top while the small peanuts settled near the base. No matter how you take it, if the big nuts are ...
6k views

### Why isn't length contraction permanent even though time dilation is?

It's my understanding that when something is going near the speed of light in reference to an observer, time dilation occurs and time goes slower for that fast-moving object. However, when that ...
6k views

### Why does a cup with 100 g water float when placed on another cup with 50 g of water?

Imagine we have cup A with 50 g of water and cup B (smaller in width than A) with 100 g of water. Now put cup B into cup A. If the width of both cups are of comparable size then the cup with ...
8k views

### Dimensionless Constants in Physics

Forgive me if this topic is too much in the realm of philosophy. John Baez has an interesting perspective on the relative importance of dimensionless constants, which he calls fundamental like alpha, ...
2k views

### Why does our voice sound different on inhaling helium?

This question (and answer) is an attempt to clear the air on what appears to be a very simple issue, with conflicting or unclear explanations on the internet. Arguments, negations, etc are invited. ...
4k views

### Why do we think there are only three generations of fundamental particles?

In the standard model of particle physics, there are three generations of quarks (up/down, strange/charm, and top/bottom), along with three generations of leptons (electron, muon, and tau). All of ...
3k views

### Neutrinos vs. Photons: Who wins the race across the galaxy?

Inspired by the wording of this answer, a thought occurred to me. If a photon and a neutrino were to race along a significant stretch of our actual galaxy, which would win the race? Now, neutrinos ...
1k views

### What is the upper-limit on intrinsic heating due to dark matter?

Cold dark matter is thought to fill our galactic neighborhood with a density $\rho$ of about 0.3 GeV/cm${}^3$ and with a velocity $v$ of roughly 200 to 300 km/s. (The velocity dispersion is much ...
20k views

### Why don't electric fish shock themselves?

Fish like electric eels and torpedoes have specially designed nerve cells that allow them to discharge hundreds of volts of electricity. Now, while pure water is usually nonconductive, the dissolved ...
13k views

### Why does dust stick to rotating fan propeller?

Why does dust stick to rotating fan propeller? Intuitively, most people (including I) think of the dust will not stick to rotating fan propellers. EDIT 1: Thank you for the great explanations. I am ...
8k views

### Can kicking a falling phone save it from shattering?

So I saw this tip but I don't think this is true, it would be that your leg or shoe is more flexible than a hard floor so the momentum change would be slower right?
6k views

### How can Magnets be used to pick up pieces of metal when the force from a magnetic field does no work?

I learned that the force from magnetic fields does no work. However I was wondering how magnets can be used to pick up pieces of metal like small paperclips and stuff. I also was wondering how magnets ...
82k views

### Why do grapes in a microwave oven produce plasma?

Some of you may know this experience (Grape + Microwave oven = Plasma video link): take a grape that you almost split in two parts, letting just a tiny piece of skin making a link between each ...
8k views

### What technology can result from such expensive experiment as undertaken in CERN?

I wonder what technology can be obtained from such very expensive experiments/institutes as e.g. undertaken in CERN? I understand that e.g. the discovery of the Higgs Boson confirms our understanding ...
5k views

### Quantum mechanics and everyday nature

Is there a phenomenon visible to the naked eye that requires quantum mechanics to be satisfactorily explained? I am looking for a sort of quantic Newtonian apple.
5k views

### Why quantum mechanics?

Imagine you're teaching a first course on quantum mechanics in which your students are well-versed in classical mechanics, but have never seen any quantum before. How would you motivate the subject ...
7k views

### What enables protons to give new properties to an atom every time one is added?

How does adding one more particle to the nucleus of an atom give that atom new properties? I can see how it changes it's mass, that's obvious... But how does it give that new atom different properties ...
26k views

### How exactly does gravity work?

The electromagnetic force and strong and weak forces require particles like photons and gluons. But in case of gravity there is no such particle found. Every mass bearing object creates a ...
4k views

### Why is the Earth so fat?

I made a naive calculation of the height of Earth's equatorial bulge and found that it should be about 10km. The true height is about 20km. My question is: why is there this discrepancy? The ...
8k views

### Was the 2013 meteor over Russia stronger than an atomic bomb?

Reports of the Russian meteor event (2013) say that it released more energy than 20 atomic bombs of the size dropped on Hiroshima, Japan: Scientists estimated the meteor unleashed a force 20 times ...
7k views

### What can the D-Wave quantum computer do?

The media are reporting the commercially sold 128-bit quantum computer from D-Wave http://news.google.com/news?ned=us&hl=us&q=d-wave+quantum&cf=all&scoring=n which of course ...
7k views

### Cyclist's electrical tingling under power lines

It's been happening to me for years. I finally decided to ask users who are better with "practical physics" when I was told that my experience – that I am going to describe momentarily – prove that I ...
4k 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 ...
852 views

I am confused by most discussions of analog Hawking radiation in fluids (see, for example, the recent experimental result of Weinfurtner et al. Phys. Rev. Lett. 106, 021302 (2011), ...
6k views

### Why do electric sparks appear blue/purple?

Electric sparks tend to appear blue or purple or white in color. Why?
3k views

### Is the uncertainty principle a property of elementary particles or a result of our measurement tools?

In many physics divulgation books I've read, this seems to be a commonly accepted point of view (I'm making this quote up, as I don't remember the exact words, but this should give you an idea): ...
10k views

### If I'm floating in space and I turn on a flashlight, will I accelerate?

Photons have no mass but they can push things, as evidenced by laser propulsion. Can photons push the source which is emitting them? If yes, will a more intense flashlight accelerate me more? Does ...
11k views

### 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 ...
4k views

### 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 ...
41k views

### Why do power lines buzz?

When near high tension power lines, particularly after a good rain, the lines themselves emit a buzzing noise. A similar noise can be heard coming out of the electric meters attached to my apartment. ...
3k views

### Photons from stars--how do they fill in such large angular distances?

It would seem that far-away stars are at such a distance that I should be able to take a step to the side and not have the star's photons hit my eye. How do stars release so many photons to fill in ...
6k views

### Why is the prospective new kilogram standard a sphere?

I can understand the choice of material, silicon 28, but why is it a sphere rather than (say) a cube? Article here I would have thought that a sphere would have been the hardest shape to machine ...
6k views

### Why are gold mirrors yellow?

Why are golden mirrors yellow? Do they add a yellow component to the spectrum or absorb non-yellow components? If they absorb, then why are they used in telescopes being imperfect? If they add a ...
3k views

### Explaining UV radiation to a 6 year old

My (just completed) PhD involved a considerable amount of research involved with the detection of solar UV radiation. This generated quite a bit of interest, especially when I was conducting my ...
6k views

### Why should the Planck constant be a constant throughout all space?

Our value for the Planck constant $h$ can be found on experiments on Earth, but how do we know that the Planck constant doesn't change throughout space, for instance it depends weakly upon the ...
5k views

### Why Do Sausages Always Split Lengthwise?

Sausages universally split parallel to the length of the sausage. Why is that?
5k views

### What physics paper would a high school student be able to read?

I'm looking for a physics paper which a typical high school student who is new to physics would be able to read and grasp the general idea of the purpose, setup and results, if not the details. To be ...
18k views

### What practical issues remain for the adoption of Thorium reactors?

From what I've read on thorium reactors, there's enormous benefit to them. Their fuel is abundant enough to power human civilization for centuries, their fission products are relatively short-lived, ...
Why do we have an elementary charge $e$ in physics but no elementary mass? Is an elementary mass ruled out by experiment or is an elementary mass forbidden by some theoretical reason?