All Questions
34,044
questions
27
votes
10
answers
20k
views
Why I think tension should be twice the force in a tug of war
I'm going to provide my argument for why I think the tension in a rope should be twice the force exerted on either side of it.
First, let's consider a different example.
Say, there is a person named <...
- 1,335
26
votes
1
answer
3k
views
The choice of measurement basis on one half of an entangled state affects the other half. Can this be used to communicate faster than light?
It is often stated, particularly in popular physics articles and videos about quantum entanglement, that if one measures a particle A that is entangled with some other particle B, then this ...
- 133k
25
votes
2
answers
7k
views
How does the Hubble parameter change with the age of the universe?
How does the Hubble parameter change with the age of the universe?
This question was posted recently, and I had almost finished writing an answer when the question was deleted. Since it's a shame to ...
- 355k
9
votes
3
answers
3k
views
Why are thermodynamic potentials minimised?
Why is it that, at equilibrium, certain potentials are minimised?
That is, for a system at constant temperature and pressure, the Gibbs free energy is minimised, and for fixed volume and temperature, ...
- 1,933
105
votes
5
answers
107k
views
How and why do accelerating charges radiate electromagnetic radiation?
Let's consider it case by case:
Case 1: Charged particle is at rest. It has an electric field around it. No problem. That is its property.
Case 2: Charged particle started moving (it's accelerating)....
- 7,255
42
votes
5
answers
10k
views
Is information entropy the same as thermodynamic entropy?
Context
In one of his most popular books Guards! Guards!, Terry Pratchett makes an entropy joke:
Knowledge equals Power, which equals Energy, which equals Mass
Pratchett is a fantasy comedian and ...
- 539
32
votes
14
answers
83k
views
Recommendations for good Newtonian mechanics and kinematics books
What are some good books for learning the concepts of Kinematics, Newton laws, 2D Motion of Object etc.?
31
votes
4
answers
11k
views
Speed of light in a gravitational field?
How do I solve the speed of light in gravitational field?
Should I just add gravitational acceleration in speed of light?
$$c'=c_0+g(r)t~?$$
- 936
24
votes
6
answers
11k
views
Conservation law of energy and Big Bang?
Did the law of conservation of energy apply to the earliest moments of the Big Bang? If so, what theoretical physics supports this?
I hear that Einstein's theory of relativity disputes the law of ...
- 241
102
votes
13
answers
63k
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:
...
74
votes
7
answers
31k
views
Is spacetime discrete or continuous?
Is the spacetime continuous or discrete?
Or better, is the 4-dimensional spacetime of general-relativity discrete or continuous? What if we consider additional dimensions like string theory ...
- 1,267
46
votes
8
answers
12k
views
If I am travelling on a car at around 60 km/h, and I shine a light, does that mean that the light is travelling faster than the speed of light?
The title says it all.
If I was on a bus at 60 km/h, and I started walking on the bus at a steady pace of 5 km/h, then I'd technically be moving at 65 km/h, right?
So my son posed me an interesting ...
- 1,435
43
votes
6
answers
8k
views
Why are so many forces explainable using inverse squares when space is three dimensional?
It seems paradoxical that the strength of so many phenomena (Newtonian gravity, Coulomb force) are calculable by the inverse square of distance.
However, since volume is determined by three ...
- 544
26
votes
6
answers
25k
views
Why can't photons have a mass?
Why can't photons have a mass? Could you explain this to me in a short and mathematical way?
- 1,219
20
votes
1
answer
6k
views
Why path integral approach may suffer from operator ordering problem?
In Assa Auerbach's book (Ref. 1), he gave an argument saying that in the normal process of path integral, we lose information about ordering of operators by ignoring the discontinuous path.
What did ...
- 553
19
votes
3
answers
4k
views
If it was possible to dig a hole that went from one side of the Earth to the other...
...And you jumped in.
What would happen when you got to the middle of the Earth? Would you gradually slow down, until you got to the middle and once you were in middle would every direction feel like ...
- 387
18
votes
1
answer
9k
views
Why/How is this Wick's theorem?
Let $\phi$ be a scalar field and then I see the following expression (1) for the square of the normal ordered version of $\phi^2(x)$.
\begin{align}
T(:\phi^2(x)::\phi^2(0):) &= 2 \langle 0|T(\...
- 4,619
18
votes
2
answers
10k
views
Gravitational field intensity inside a hollow sphere
It is quite easy to derive the gravitational field intensity at a point within a hollow sphere. However, the result is quite surprising. The field intensity at any point within a hollow sphere is zero....
- 1,572
88
votes
8
answers
9k
views
Why is the application of probability in Quantum Mechanics fundamentally different from application of probability in other areas?
Why is the application of probability in Quantum Mechanics (QM) fundamentally different from its application in other areas? QM applies probability according to the same probability axioms as in other ...
- 5,142
71
votes
6
answers
53k
views
When is the Hamiltonian of a system not equal to its total energy?
I thought the Hamiltonian was always equal to the total energy of a system but have read that this isn't always true. Is there an example of this and does the Hamiltonian have a physical ...
- 1,780
61
votes
2
answers
89k
views
Difference between $\Delta$, $d$ and $\delta$
I have read the thread regarding 'the difference between the operators $\delta$ and $d$', but it does not answer my question.
I am confused about the notation for change in Physics. In Mathematics, $\...
- 899
54
votes
9
answers
47k
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 ...
- 565
36
votes
14
answers
134k
views
Why does the (relativistic) mass of an object increase when its speed approaches that of light?
I'm reading Nano: The Essentials by T. Pradeep and I came upon this statement in the section explaining the basics of scanning electron microscopy.
However, the equation breaks down when the ...
- 1,483
25
votes
3
answers
6k
views
Grassmann paradox weirdness
I'm running into an annoying problem I am unable to resolve, although a friend has given me some guidance as to how the resolution might come about. Hopefully someone on here knows the answer.
It is ...
- 6,301
14
votes
4
answers
19k
views
Magnetic field due to a single moving charge
The Biot-Savart law can only be used in the case of magnetostatics (constant current) so how do we calculate the magnetic field of a single charge moving at constant velocity at a distance r. I tried ...
- 261
9
votes
2
answers
2k
views
Is propagation of uncertainties linear?
I'm in doubt with one thing: let's imagine that we have $n+1$ quantities, $n$ of them being directly measured, and the other one being related to the first $n$ by a function $f : \mathbb{R}^n \to \...
- 35.9k
2
votes
8
answers
2k
views
Visualising gas temperature and gas pressure
Gas pressure is created when gas molecules collide with the wall of the container creating a force.
Gas temperature is a measure of how fast the molecules are moving / vibrating.
However, they both ...
- 317
81
votes
11
answers
121k
views
What determines color -- wavelength or frequency?
What determines the color of light -- is it the wavelength of the light or the frequency?
(i.e. If you put light through a medium other than air, in order to keep its color the same, which one would ...
- 4,151
57
votes
4
answers
6k
views
Hilbert space of harmonic oscillator: Countable vs uncountable?
Hm, this just occurred to me while answering another question:
If I write the Hamiltonian for a harmonic oscillator as
$$H = \frac{p^2}{2m} + \frac{1}{2} m \omega^2 x^2$$
then wouldn't one set of ...
- 14.8k
54
votes
4
answers
31k
views
Why do electrons occupy the space around nuclei, and not collide with them? [duplicate]
We all learn in grade school that electrons are negatively-charged particles that inhabit the space around the nucleus of an atom, that protons are positively-charged and are embedded within the ...
- 3,439
35
votes
5
answers
4k
views
Intuition for multiple temporal dimensions
It’s easy, relatively speaking, to develop an intuition for higher spatial dimensions, usually by induction on familiar lower-dimensional spaces. But I’m having difficulty envisioning a universe with ...
- 477
29
votes
7
answers
29k
views
Is the universe finite or infinite?
I thought the universe was finite, but then I read this:
How can something finite become infinite?
And they seem to assume it is infinite. So which is it?
- 1,387
25
votes
14
answers
8k
views
Does the collapse of the wave function happen immediately everywhere?
It is usually taught that when we measure some measurable value the wave function collapses immediately everywhere. This idea sounds like a simplification of some more complicated mechanism.
Are ...
- 1,686
21
votes
2
answers
3k
views
Vector spaces for the irreducible representations of the Lorentz Group
EDIT: The vector space for the $(\frac{1}{2},0)$ Representation is $\mathbb{C}^2$ as mentioned by Qmechanic in the comments to his answer below! The vector spaces for the other representations remain ...
- 1,862
11
votes
6
answers
15k
views
Do photons have acceleration?
Photons travel at the fastest speed in our universe, the speed of light.
Do photons have acceleration?
- 1,027
119
votes
7
answers
27k
views
Why is the $S_{z} =0$ state forbidden for photons?
If photons are spin-1 bosons, then doesn't quantum mechanics imply that the allowed values for the z-component of spin (in units of $\hbar$) are -1, 0, and 1?
Why then in practice do we only use the $...
- 1,776
73
votes
12
answers
24k
views
What is a tensor?
I have a pretty good knowledge of physics, but couldn't deeply understand what a tensor is and why it is so fundamental.
- 3,316
69
votes
5
answers
24k
views
Why is the harmonic oscillator so important?
I've been wondering what makes the harmonic oscillator such an important model. What I came up with:
It is a (relatively) simple system, making it a perfect example for physics students to learn ...
- 2,805
66
votes
10
answers
21k
views
Why doesn't a bike/bicycle fall if going with a high speed?
Why does a bike/bicycle fall when its speed is very low or close to zero and is balanced when going with a high speed?
- 777
50
votes
5
answers
4k
views
Is the world $C^\infty$?
While it is quite common to use piecewise constant functions to describe reality, e.g. the optical properties of a layered system, or the Fermi–Dirac statistics at (the impossible to reach exactly) $T=...
- 6,768
48
votes
9
answers
29k
views
Why is the gravitational force always attractive?
Why is the gravitational force always attractive? Is there another way to explain this without the curvature of space time?
PS: If the simple answer to this question is that mass makes space-time ...
- 1,762
39
votes
2
answers
13k
views
How does the Higgs mechanism work?
I'm not a particle physicist, but I did manage to get through the Feynman lectures without getting too lost.
Is there a way to explain how the Higgs field works, in a way that people like me might ...
- 17.1k
36
votes
8
answers
10k
views
What determines which frames are inertial frames?
I understand that you can (in principle) measure whether "free particles" (no forces) experience accelerations in order to tell whether a frame is inertial. But fundamentally, what determines which ...
Joss L
30
votes
5
answers
6k
views
If the universe is expanding, what is it expanding into? [closed]
If the universe is expanding, what is it expanding into?
When the big bang happened where did it occur?
When the big bang happened how did it occur?
Where did the energy come from? Energy can not be ...
- 441
27
votes
7
answers
22k
views
Why are objects at rest in motion through spacetime at the speed of light? [closed]
I read that an object at rest has such a stupendous amount of energy, $E=mc^2$ because it's effectively in motion through space-time at the speed of light and it's traveling through the time dimension ...
- 4,595
26
votes
2
answers
9k
views
Proof that the one-dimensional simple harmonic oscillator is non-degenerate?
The standard treatment of the one-dimensional quantum simple harmonic oscillator (SHO) using the raising and lowering operators arrives at the countable basis of eigenstates $\{\vert n \rangle\}_{n = ...
- 397
22
votes
2
answers
7k
views
How do I derive the Lorentz contraction from the invariant interval?
While reviewing some basic special relativity, I stumbled upon this problem:
From the definition of the proper time:
$$c^2d\tau^2=c^2dt^2-dx^2$$
I was able to derive the time dilation formula by using ...
- 16.3k
7
votes
2
answers
1k
views
Example in motivation for Lagrangian formalism
I started reading Quantum Field Theory for the Gifted Amateur by Lancaster & Blundell, and I have a conceptual question regarding their motivation of the Lagrangian formalism. They start by ...
- 203
115
votes
6
answers
114k
views
Why is glass transparent?
Once I asked this question from my teacher and he replied "Because it passes light.". "And why does it pass light?" I asked and he said, "Because it is transparent.".
The same question again, Why ...
- 5,317
76
votes
10
answers
8k
views
Is 3+1 spacetime as privileged as is claimed?
I've often heard the argument that having 3 spatial dimensions is very special. Such arguments are invariably based on certain assumptions that do not appear to be justifiable at all, at least to me. ...
- 2,155