187
votes
Accepted
Why does nature favour the Laplacian?
Nature appears to be rotationally symmetric, favoring no particular direction. The Laplacian is the only translationally-invariant second-order differential operator obeying this property. Your "...
- 98.6k
178
votes
How can time dilation be symmetric?
The answer to this is that our twins, $A$ and $B$, are not measuring the same thing on their clocks. Since they are not measuring the same thing there is no paradox in the fact that each twin thinks ...
116
votes
Accepted
Why isn't the Euler-Lagrange equation trivial?
Ah, what a tricky mistake you've made there. The problem is that you've simply confused some notions in multivariable calculus. Don't feel bad though-- this is generally very poorly explained. Both ...
- 11.3k
90
votes
Accepted
What is really curved, spacetime, or simply the coordinate lines?
Congratulations! You stumbled upon an important question of differential geometry:
How can I know whether the curvature is caused by my choice of coordinates or the space I live in?
As has been ...
- 1,070
68
votes
Accepted
Partial derivative notation in thermodynamics
That's because in thermodynamics we sometimes use the same letter to represent different functions. For example, one can write the volume of a system as $V=f_1(P,T)$ (a function of the pressure and ...
- 1,484
62
votes
Accepted
Rotate an object about the time axis
This a great question, and leads to some interesting ideas.
Firstly, the notion of a "rotation axis" is restricted to three dimensions. In more than three dimensions the rotation axis ...
- 48.1k
52
votes
Accepted
Why is this vector field curl-free?
The vector $\hat \varphi$ is not defined at the origin, because the coordinate transformation $$(x,y) \mapsto (r,\varphi) = \left(\sqrt{x^2 + y^2}, \arctan(y/x)\right)$$
is singular there. Hence your ...
- 14.4k
44
votes
Accepted
What are holonomic and non-holonomic constraints?
If you have a mechanical system with $N$ particles, you'd technically need $n = 3N$ coordinates to describe it completely.
But often it is possible to express one coordinate in terms of others: for ...
- 4,749
43
votes
Does it make sense to take an infinitesimal volume of shape other than a cube?
Infinitesimal volume elements do not have to be cubes.
Some familiar examples come from typical solids of revolution problems from calculus 1/2. Typically one discusses using either the "disk/...
- 55k
40
votes
Given two vectors (with no common point), is a dot product between them possible?
Vector by definition only has magnitude and direction. Origin is not part of the vector. In particular, you can unambiguously define a vector between two points $A$ and $B$ by finding the distance ...
- 27.8k
40
votes
Accepted
Does Newtonian mechanics work in polar coordinates?
Your teacher is incorrect. $\vec F = m \vec a$ is valid in any inertial (non-accelerating) coordinate system. You must account for the fact that the unit vectors for position in some coordinate ...
- 9,283
34
votes
In the theory of special relativity speed is relative so who decides which observer’s time moves slower?
A core idea of special relativity is there is no right frame of reference. It doesn't matter which of the two observers you use as your point of reference, the math will work out either way. Yes, they'...
- 2,666
32
votes
Accepted
Does spacetime position not form a four-vector?
You are correct.
Position is a vector when you are working in a vector space, since, well, it is a vector space. Even then, if you use a nonlinear coordinate system, the coordinates of a point ...
- 10.3k
31
votes
Accepted
Why is clock synchronisation such a big deal in physics?
All of special relativity is based on the assumption that any observer can set up a coordinate system and then label spacetime events with their coordinates in that system. Then we can use the Lorentz ...
- 345k
31
votes
Accepted
Why/When can we separate spacetime into space and time?
The notion that a spacetime can be decomposed into a spacelike and timelike part is generally called a spacelike foliation, where your spacetime manifold can be decomposed into purely spatial ...
- 15.2k
30
votes
Is Fermat's principle only an approximation?
In general relativity, it's not entirely clear what "least time" means, since you have to ask "whose time are you talking about"? Are you talking about the time as measured by the emitter? The ...
- 44.7k
30
votes
Does light really travel more slowly near a massive body?
The simple way to show that the speed derived from the Schwarzschild coordinates has no absolute meaning is to derive an expression for the speed measured by a different observer and show that they ...
- 345k
30
votes
Why does nature favour the Laplacian?
This is a question that hunted me for years, so I'll share with you my view about the Laplace equation, which is the most elemental equation you can write with the laplacian.
If you force the ...
- 436
30
votes
Accepted
Massless Kerr black hole
It's simply flat space in Boyer-Lindquist coordinates. By writing
$\begin{cases}
x=\sqrt{r^2+a^2}\sin\theta\cos\phi\\
y=\sqrt{r^2+a^2}\sin\theta\sin\phi\\
z=r\cos\theta
\end{cases}$
you'll get good ol'...
- 5,277
29
votes
What is the cause of the constancy of the speed of light in vacuum?
The invariance of the speed of light follows from the principle of relativity. This says there is no experiment that can distinguish between inertial reference frames: physical laws are the same in ...
- 1,143
27
votes
Derive vector gradient in spherical coordinates from first principles
You asked for a proof from "first principles". So let's do it. I'll highlight the most common sources of errors and I'll show an alternative proof later that doesn't require any knowledge of tensor ...
- 1,824
27
votes
Accepted
Is it strange that there are two directions which are perpendicular to both field and current, yet the Lorentz force only points along one of them?
The universe is not preferentially selecting one direction over another. The fact that it appears that this is happening is an artifact of how we represent the magnetic field.
It is well-known that ...
- 5,938
27
votes
Accepted
Why can vector components not be resolved by Laws of Vector Addition?
Indeed, any vector can be resolved in terms of two components (in $n$-dimensional space in terms of $n$ components). For this being possible the components should be linearly independent, i.e. in your ...
- 54.9k
26
votes
Accepted
On mathematical level, what exactly is time in Newtonian mechanics?
Here is one way to address your question.
"Time is defined so that motion looks simple." - Misner, Thorne, and Wheeler in Gravitation, p.23.
Continue through to p. 26 where they say
"...
- 10.7k
25
votes
Does Newtonian mechanics work in polar coordinates?
Your teacher is definitely incorrect. In fact, the whole point $\vec{F}=m\vec{a}$ is written as a vector equation is to emphasize that the equation does not depend on the coordinate system you choose ...
- 441
24
votes
Why is this vector field curl-free?
There already are very good answers so I would just like to give some physical intuition why this vector field is curl-free even though it has non zero circulation.
We can make an analogy of the curl ...
- 17k
24
votes
Lagrange's equation is form invariant under EVERY coordinate transformation. Hamilton's equations are not under EVERY phase space transformation. Why?
You should really think about the variables we use as being like coordinates on some manifold, the configuration space (roughly the same as the phase space, I won't be careful about the distinction). ...
- 4,799
24
votes
What exactly would it take to show (hypothetically) that the speed of light (in vacuum) is not constant?
You are correct and in good modern treatments, people are careful to say that it's only meaningful to say that dimensionless quantities change with time. The "grown up version" of looking at ...
- 42.9k
23
votes
Accepted
Why do we need coordinate-free descriptions?
That's a very good question. While it may seem "natural" that the world is ordered like a vector space (it is the order that we are accustomed to!), it's indeed a completely unnatural requirement for ...
- 16.2k
Only top scored, non community-wiki answers of a minimum length are eligible