79
votes
7answers
13k views

How can a black hole produce sound?

I was reading this article from NASA -- it's NASA -- and literally found myself perplexed. The article describes the discovery that black holes emit a "note" that has physical ramifications on the ...
79
votes
5answers
19k views

What does it mean for two objects to “touch”?

If you've ever been annoyingly poked by a geek, you might be familiar with the semi-nerdy obnoxious response of "I'm not actually touching you! The electrons in the atoms of my skin are just ...
79
votes
5answers
9k views

Gauge symmetry is not a symmetry?

I have read before in one of Seiberg's articles something like, that gauge symmetry is not a symmetry but a redundancy in our description, by introducing fake degrees of freedom to facilitate ...
79
votes
2answers
4k views

Why does my wooden door disperse light into a rainbow color spectrum?

On a clear morning, light comes through this window (viewed from the inside) It then hits the door on the opposite side (so viewing from the outside the door straight on the other side) There is ...
78
votes
6answers
9k views

Does a gun exert enough gravity on the bullet it fired to stop it?

My question is set in the following situation: You have a completely empty universe without boundaries. In this universe is a single gun which holds one bullet. The gun fires the bullet and the ...
78
votes
9answers
79k views

If photons have no mass, how can they have momentum?

As an explanation of why a large gravitational field (such as a black hole) can bend light, I have heard that light has momentum. This is given as a solution to the problem of only massive objects ...
78
votes
14answers
8k views

Superluminal neutrinos

I was quite surprised to read this all over the news today: Elusive, nearly massive subatomic particles called neutrinos appear to travel just faster than light, a team of physicists in Europe ...
76
votes
18answers
11k views

Why do most formulas in physics have integer and rational exponents?

I mean, why is $F=ma$? Why not $m^{0.123}$, $a^{1.43}$ or some random non-integers or irrational? I hope you understand that my question isn't limited just to force, energy, velocity, etc.; it also ...
75
votes
5answers
6k views

What was the major discovery on gravitational waves made March 17th, 2014, in the BICEP2 experiment?

The Harvard-Smithsonian Centre for Astrophysics held a press conference today to announce a major discovery relating to gravitational waves. What was their announcement, and what are the implications? ...
75
votes
8answers
8k views

Why does space expansion not expand matter?

I have looked at other questions on this site (e.g. "why does space expansion affect matter") but can't find the answer I am looking for. So here is my question: One often hears talk of space ...
74
votes
12answers
23k views

Why are radiators always placed under windows?

I don't know if anyone else has noticed this but in most buildings and most rooms, radiators are predominantly placed under a window. Now, in my eyes, that is the worst place to put them; hot air ...
73
votes
6answers
15k views

Am I attracting Pluto?

My question is simple: as the title says, am I exerting a force over for example Pluto, although it is a very small force, it is there, right? Or maybe let's go further: Am I exerting a gravity force ...
73
votes
2answers
16k views

Reading the Feynman lectures in 2012

The Feynman lectures are universally admired, it seems, but also a half-century old. Taking them as a source for self-study, what compensation for their age, if any, should today's reader undertake? ...
72
votes
9answers
12k views

Can we theoretically balance a perfectly symmetrical pencil on its one-atom tip?

I was asked by an undergrad student about this question. I think if we were to take away air molecules around the pencil and cool it to absolute zero, that pencil would theoretically balance. Am I ...
72
votes
4answers
13k views

Can tin foil hats block anything?

This is joked about all the time, but... Can tin foil hats actually block anything? If they can, what frequencies? Is there any research into tin or aluminum foil and radio blocking or amplifying ...
72
votes
3answers
4k views

Where is the flaw in this machine that decreases the entropy of a closed system?

I was thinking about a completely unrelated problem (Quantum Field Theory Peskin & Schroeder kind of unrelated!) when the diagram below sprang into my mind for no apparent reason. After some ...
72
votes
5answers
8k views

Does someone falling into a black hole see the end of the universe?

This question was prompted by Can matter really fall through an event horizon?. Notoriously, if you calculate the Schwarzschild coordinate time for anything, matter or light, to reach the event ...
71
votes
8answers
21k views

Why does hot water clean better than cold water?

I had a left over coffee cup this morning, and I tried to wash it out. I realized I always instinctively use hot water to clean things, as it seems to work better. A Google search showed that other ...
70
votes
7answers
6k views

Does juggling balls reduce the total weight of the juggler and balls?

A friend offered me a brain teaser to which the solution involves a $195$ pound man juggling two $3$-pound balls to traverse a bridge having a maximum capacity of only $200$ pounds. He explained that ...
70
votes
10answers
11k views

Is time continuous or discrete?

While working on physical system simulation software, I noticed that I had implemented discrete time, which means that there was an update mechanism advancing the simulation for a fixed amount of time ...
70
votes
6answers
4k views

Thought experiment - would you notice if you fell into a black hole?

I've heard many scientists, when giving interviews and the like, state that if one were falling into a black hole massive enough that the tidal forces at the event horizon weren't too extreme, that ...
69
votes
17answers
7k 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 ...
69
votes
4answers
4k views

The Role of Rigor

The purpose of this question is to ask about the role of mathematical rigor in physics. In order to formulate a question that can be answered, and not just discussed, I divided this large issue into ...
69
votes
10answers
8k views

How can I stand on the ground? EM or/and Pauli?

There is this famous example about the order difference between gravitational force and EM force. All the gravitational force of Earth is just countered by the electromagnetic force between the ...
68
votes
7answers
9k views

Why does public mains power use 50-60 Hz and 100-240 V?

Is there a physical reason behind the frequency and voltage in the mains electricity? I do not want to know why exactly a certain value was chosen; I am rather interested to know why that range/order ...
68
votes
4answers
8k views

Where does the extra kinetic energy come from in a gravitational slingshot?

I read in this answer in this site that the KE a free-falling ball acquires is not originated by the attracting body but that energy was actually stored in the ball when it had been lifted to the ...
68
votes
10answers
12k views

What is a field, really?

There was a reason why I constantly failed physics at school and university, and that reason was, apart from the fact I was immensely lazy, that I mentally refused to "believe" more advanced stuff ...
68
votes
12answers
22k views

Best books for mathematical background?

What are the best textbooks to read for the mathematical background you need for modern physics, such as, string theory? Some subjects off the top of my head that probably need covering: ...
68
votes
5answers
7k views

Is it possible to start fire using moonlight?

You can start fire by focusing the sunlight using the magnifying glass. I searched the web whether you can do the same using moonlight. And found this and this - the first two in Google search ...
68
votes
10answers
7k views

Is Angular Momentum truly fundamental?

This may seem like a slightly trite question, but it is one that has long intrigued me. Since I formally learned classical (Newtonian) mechanics, it has often struck me that angular momentum (and ...
67
votes
6answers
12k views

Which way does the scale tip?

I found the problem described in the attached picture on the internet. In the comment sections there were two opposing solutions. So it made me wonder which of those would be the actual solution. So ...
67
votes
5answers
10k views

Why wet is dark?

When something gets wet, it usually appears darker. This can be observed with soil, sand, cloth, paper, concrete, bricks ... What is the reason for this? How does water soaking into the material ...
66
votes
8answers
26k views

What's inside a proton?

What constitutes protons? When I see pictures, I can't understand. Protons are made of quarks, but some say that they are made of 99% empty space. Also, in this illustration from Wikipedia, what's ...
66
votes
5answers
8k views

How literally should you take “The Higgs boson gives other particles mass”?

A standard phrase in popular discussions of the Higgs boson is that "it gives particles mass". To what extent is this a reasonable, pop-science, level of description of the Higgs boson and it's ...
66
votes
10answers
29k views

Why don't electrons crash into the nuclei they “orbit”?

I'm having trouble understanding the simple "planetary" model of the atom that I'm being taught in my basic chemistry course. In particular, I can't see how a negatively charged electron can stay ...
66
votes
8answers
17k 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 ...
65
votes
6answers
16k views

Why are most metals gray/silver?

Why do most metals (iron, tin, aluminum, lead, zinc, tungsten, nickel, etc.) appear silver or gray in color? (What atomic characteristics determine the color?) What makes copper and gold have ...
64
votes
3answers
8k views

Why doesn't the motion of a car affect the frequency of radio stations?

When we go in a car and tune to an FM radio station, why doesn't our motion disturb the frequency? Like the Doppler effect?
64
votes
2answers
7k views

In the earth's crust, why is there far more uranium than gold?

In parts per million in the Earth's crust Uranium is around 1.8ppm and Gold 0.003ppm. Given that it takes far more energy to create Uranium than Gold, why is this?
64
votes
10answers
10k views

What makes running so much less energy-efficient than bicycling?

Most people can ride 10 km on their bike. However, running 10 km is a lot harder to do. Why? According to the law of conservation of energy, bicycling should be more intensive because you have to ...
64
votes
7answers
7k views

Number theory in Physics [closed]

As a Graduate Mathematics student, my interest lies in Number theory. I am curious to know if Number theory has any connections or applications to physics. I have never even heard of any applications ...
64
votes
5answers
6k views

What challenges needed to be overcome to create (blue) LEDs?

In light of today's announcement of the 2014 Nobel laureates, and because of a discussion among colleagues about the physical significance of these devices, let me ask: What is the physical ...
63
votes
7answers
11k views

When I boil a kettle, what stops all the water from turning (exploding!) in to steam in one go once it reaches 100°C?

While making a cup of tea in the office kitchen, a colleague asked me this question and neither of us could answer with any certainty. We're guessing it has something to do with the pressure of the ...
63
votes
10answers
7k views

Quantum Entanglement - What's the big deal?

Bearing in mind I am a layman - with no background in physics - please could someone explain what the "big deal" is with quantum entanglement? I used to think I understood it - that 2 particles, say ...
63
votes
3answers
6k views

Why do we need to “create our own” Higgs boson in order to see one?

I understand that the LHC found the Higgs boson by pumping so much energy into a tiny space (via near light speed proton-proton collisions) that a Higgs boson appeared momentarily, then instantly ...
63
votes
2answers
7k views

Why do chimneys have these spiral “wings”?

While walking around I noticed something very peculiar. Many chimneys had spiral "wings", while others didn't. I came up with two possibilities: The wind circles around the chimney upwards which ...
63
votes
6answers
4k views

What is known about the topological structure of spacetime?

General relativity says that spacetime is a Lorentzian 4-manifold $M$ whose metric satisfies Einstein's field equations. I have two questions: What topological restrictions do Einstein's equations ...
63
votes
2answers
42k 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? ...
63
votes
1answer
2k views

Does the 4/3 problem of classical electromagnetism remain in quantum mechanics?

In Volume II Chapter 28 of the Feymann Lectures on Physics, Feynman discusses the infamous 4/3 problem of classical electromagnetism. Suppose you have a charged particle of radius $a$ and charge $q$ ...
62
votes
5answers
12k views

Why is it cold on the sea floor if pressure heats things?

I was reading this and it says that Microsoft put a server farm at the bottom of the ocean because it's cooler there. Particularly it seems to imply that it get's colder as you go deeper, "Since ocean ...

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