# All Questions

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### If a tree falls in the forest [closed]

The question of whether or not a tree that falls in the forest makes a sound - if there is nothing or no one around to hear it - comes up frequently at my house. So, my question is: is there any way ...
125 views

### Light emitted from galaxies receding faster than $c$

I'm wondering how it is possible that light cannot reach us, even all the way out there where metric expansion is making the distance between our galaxy and their galaxy increase at greater than $c$. ...
67 views

### Passage of time on a faster-than-light (FTL) vessel [closed]

Yes, this is already wild speculation. I'll retract this question if it isn't well received. As I understand, within an event horizon, spacetime gets rotated so that the singularity is in the future, ...
81 views

### Ohm's Law for Accelerated Conductors

My questions are about the Ohm's law for a moving conductor in a stationary magnetic field. As we know this law is stated based on following relation with respect to the fixed frame of reference ...
104 views

### Conservation of momentum 1-D problem: track is not level

This problem is a standard kind of 1-D problem. You have 2 equal carts approaching each other and collide on linear air track. BUT the person setting up the track did NOT do a good job at making the ...
52 views

### Density matrix: error with diagonalization claim and fixing it

On page 174 of Townsend's "A Modern Approach to Quantum Mechanics", 2nd edition, it says the following: "For a mixed state, one for which $p_k$ is the probability that a particle is in the state ...
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### What is the current situation of the Yang-Mills existence problem?

What is the current situation of the Yang-Mills existence and mass gap problem? And who are the physicists and mathematicians working in this nowaday?
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### Can we measure kenetic energy on an absolute scale, or does it have to be measured with reference to something else?

I recently read the question Kinetic energy of an object at rest where it was suggested that the kinetic energy of an object on Earth is measured in the reference frame of Earth. Can we measure ...
72 views

### Defintion of temperature without thermal equilibrium condition

Is temperature only defined in thermal equilibrium? Then how can we explain heat flow by temperature differences?
120 views

### Could one make steel balloons? [duplicate]

Reading this question reminded me of an idea I once had, and I wonder whether it's feasible. Imagine a hollow ball made of something strong, like steel. Imagine there is nothing inside; only vacuum. ...
125 views

### What would be the consequences of time not being “relative” [closed]

So far nobody proved Einstein wrong about Theory of relativity and time but I'm curious: What would be the consequences or problems if someone would prove him wrong when it comes to relativity of ...
25 views

### Max thickness that can be cooled in a certain time

A frozen-food company freezes its spinach by first compressing it into large slabs and then exposing the slab of spinach to a low-temperature cooling medium. The large slab of compressed spinach is ...
55 views

### Will a heavier or lighter tank shell / bullet/ etc. travel further given the same starting energy?

Let's say I have for example two tank shells, both being the same size and having the same shape. Let's say the starting energy is 100kg*m/s^2, and the one shell is 1kg, the other 25kg. 1kg * ...
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### What does it mean to say that “6 tons of dark energy would be found within the radius of Pluto's orbit”?

What does it mean to say that "6 tons of dark energy would be found within the radius of Pluto's orbit"? Does it mean that the dark energy is orbiting the solar system? Or does it mean a flow of ...
89 views

### Damping Constant for a Ball Rolling in a Bowl

How would you find the damping constant of a ball rolling without slipping in half of a sphere? I know how to find the period of the oscillation, but how would I find the damping constant ...
325 views

### Why can we see white objects as white under the sun if the sunlight is yellow?

I get that the sun is producing white light which is scattered threw our atmosphere so that the light of the sun reaching our eyes is yellow. So how come if I look to a piece of white paper under ...
52 views

### Defining photons

I've read every book for my course and all of them describe photons as wave-packets/"bursts" of the EM wave. I just can't appreciate this view of photons. From what I've gathered on photons: Photons ...
206 views

### How can Rosetta spacecraft orbit an object with such a low mass?

Comet 67P Churyumov–Gerasimenko's mass is 11 orders of magnitude lower than Earth's. That means that the comet's gravity force is also weaker than Earth's in the same proportion. Provided that also ...
61 views

### About Right-Hand-Rule and Cross Projuct

In "Physics for Scientists and Engineers with Modern Physics - 8th Edition", p. 842, it gives the total magnetic force on the segment of current-carrying wire of length $L$ in a uniform magnetic ...
116 views

### Statistical physics and momentum conservation

In statistical physics one usually looks at energy as a conserved quantity and e.g. in the canonical ensemble assumes a constant average energy of the ensemble. Now why don't we usually do this for ...
179 views

### Car driving / falling off of a cliff - will it land upright?

In a recent episode of The Walking Dead, a van was pushed off of a bridge and flipped once and landed upright, on all four wheels. Assumption: the ground and cliff are both flat. Will a vehicle ...
110 views

### Is an atom charged after undergoing beta emission?

After beta emission, an atom's mass number remains the same while the number of protons increases by one. As far as I know, the beta particle (electron) is too energetic to be recaptured. If this is ...
94 views

### Collisions of celestial objects

When massive, spherical celestial objects collide, how long does it take for the objects to coalesce into a new, larger, spherical entity under the influence of gravity? Examples of the above:- ...
195 views

### Was the Big Bang actually cold?

As I understand, from watching the Discovery Channel, the total amount of energy in the universe is zero. As such, people like Hawking explain that the universe can be created out of nothing ...
2k views

### Kinetic energy of an object at rest

Why don't we consider that an object on Earth has kinetic energy as it is also moving at the rotating speed of Earth?
259 views

### Does an electron possess potential energy while revolving around a nucleus ?

Does an electron possess potential energy while revolving around a nucleus ? I guess that it wont. Why because when an energy is given it converts into its kinetic energy so that it revolves around ...
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### Are there Planck units for weak or strong “charge”, similar to the electromagnetic Planck charge $\sqrt{4~\pi~\epsilon_0~\hbar~c}~$?

Are there Planck units for "charge" of weak or strong interaction, similar to the Planck unit of electromagnetic charge: $\sqrt{4~\pi~\epsilon_0~\hbar~c}$ ? Are there perhaps direct substitutes, ...
43 views

### What makes a system linear?

Lately I have been interested in Image Processing, and I started by following this course: https://class.coursera.org/digital-001, which is quite awesome in my opinion. But in weeks 2, Linear ...
120 views

### Thermal conductivity in vacuum

It is my assumption that material objects preserve/gain heat via: 1) material contact (air or other touching material/object) and/or 2) absorbing radiation / electromagnetic waves So, imagine that ...
104 views

### Relative acceleration: frame of references

There is a question I've been struggling with, and I ask for your help to understand it fully. Q: An inclined plane, fixed to the inside of an elevvator, makes a 32 degrees angle with the floor. A ...
47 views

### Dimensional analysis of sums [closed]

Quick question: What is the dimension of the following fraction ($\sum_{i=1}^{n}a_i b_i )/ \sum_{i}^{n}a_i$ where $a_1,a_2,...,a_n$ are in kg and $b_1,b_2,...,b_n$ are in Newton?
83 views

### Photoelectric effect intensity

I understand the PE effect quite well but I'm failing to understand one thing. Intensity is the amount of energy per second incident to a given area. So can you can increase the intensity by either ...
64 views

### Definition of mass gap [duplicate]

Why do we say that a system with mass gap has correlation function which decays exponentially and one without a mass gap has a slower power law decay?
19 views

### Does a photo-excited semiconductor produce a constant voltage output equal to the band gap?

Does the voltage produced by a photo-excited semiconductor always equal the band gap of that semiconductor, or does the voltage vary over a range similar to the photon energies in the emission spectra ...
73 views

### Electric Potential-can anyone give the concept behind this?

Three points A, B and C lie in a uniform electric field (E) of 5 x 103 NC-1 as shown in the figure. Find the potential difference between A and C. I think there is some trick? How can the pd ...
60 views

### Gravitational Redshift and Length Contraction

Gravitational redshift is based on the time-time component of the metric (e.g., http://en.wikipedia.org/wiki/Redshift). Why does length contraction not contribute to redshift?
255 views

### How many hours will there be in a day 5,000,000,000 years from now?

It is known that the moon is moving away from Earth 2cm a year, and in doing so makes the days longer. I want to know how many hours will have one day, when our planet is near its end.
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### Electrical Generator [duplicate]

I have seen the generator that produces electricity from the earth's magnetic field demonstrated I have a video if it working It was taken approximately 15 years ago in California
881 views

### Does the heat equation violate causality?

I've ran across the idea that, besides simply writing partial differential equations in covariant form, they need to be hyperbolic with all characteristic speeds less than the speed of light. A ...
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### How do we know if distant stars we see by their light are real objects? [closed]

Is there a way to be sure if they are not just light, but real objects?
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### If an common metal is induced with a static dipole moment, won't the the positive side be weaker?

To my knowledge, common metals with metallic bonding is formed through free electrons within its lattice structure. If say we are to shift the free roaming electrons to one side and therefore minutely ...
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### Muscular energy transformations with regards to gravity

Maybe this has more to do with biology than I know, but I have been wondering just what kind of energy muscles use and convert when lifting a weight off the ground or lowering a weight back to the ...
2k views

### What do physicists mean by “information”?

On the question why certain velocities (i.e. phase velocity) can be greater than the speed of light, people will say something like: since no matter or "information" is transferred, therefore the ...
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### Symmetry breaking and band gaps?

Can the discontinuity in the E-K dispersion relation of a periodic lattice (at the boundary of a Brillouin zone) be understood as a consequence of breaking continuous translation symmetry into ...
117 views

### Is it possible to prove that units can be manipulated algebraically?

With expressions such as $$4\ \mathrm{\frac{m}{s}} \times 2\ \mathrm{kg} = 8\ \mathrm{\frac{m}{s}} \times 1\ \mathrm{kg}$$ We can justify that a $2\ \mathrm{kg}$ mass moving at $4\ \mathrm{m/s}$ has ...
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### Does the conductivity of a wire in a vacuum decrease over time?

Does the conductivity of a wire in a vacuum decrease over time, say over the period of years or decades? In other words: Does current degrade a wire, making it less conductive? If so, by how much, and ...
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### Maximum velocity on Unbanked roads and why should friction be more than centripetal force? [closed]

Why is maximum velocity on unbanked circular road given by $\sqrt{urg}$ where $u$ is coefficient of friction, $r$ is radius of circular track and $g$ is acceleration due to gravity? Why should ...
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### Induced EMF of a fixed wire?

I've studied many examples where there is a "moving" conductor passing a magnetic field or in a magnetic field, or a moving magnetic passing a solenoid or conductor, and the induced $\epsilon$ is ...
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### Movie Interstellar - Followup Question to Escape Velocity

Continuing the discussion on this thread: Movie Interstellar - Question about Escape Velocity The movie Interstellar shows people on a water planet where time is dilated so much that 1 hour is equal ...
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### Can you heat something in AL foil using IR?

Is it possible to heat up, let's say, a blob of mud, golf ball size, covered in AL foil, with IR? Also, how efficient would that be? Is there an IR wavelength that would get absorbed far better than ...

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