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Grad student: jlackm.wordpress.com


Oct
9
awarded  Autobiographer
Aug
30
comment How was the formula for kinetic energy found, and who found it?
@user17574 There are much much simpler proofs, all of these conserved quantities, momentum, energy, angular momentum could be proved rigorously just from Newton's laws. Also considering the Lagrangian involves the kinetic energy, I don't think this helps.
Jul
18
comment The Hole Argument
Well I'm glad you see what I mean. So then are the two spacetimes distinguishable? I mean, can we tell which one we "live" in? I think part of my confusion arises from the fact that the coordinates don't seem to be defined before finding for the metric.
Jul
18
comment The Hole Argument
But they only mention the coordinate transformation thing later, on page two they seem to be literally considering different metrics on the same manifold. They even write $G(r)$ and $G'(r)$, and since they are using the same letter I can only assume that $r$ represents the same point on the manifold, resulting in two different metrics. Which part of this isn't right?
Jul
18
comment The Hole Argument
I don't understand that, why do you call them the same thing if they are two different things? You seem to have two different metrics on the same manifold, hence should be able to use the same points.
Jul
18
comment The Hole Argument
So take two points $p,p'$ on $M$. Using $g, g'$ you measure two different distances between them, but since they are isometric there is no observable difference in the physics?
Jul
18
revised The Hole Argument
added 53 characters in body
Jul
18
revised The Hole Argument
added 76 characters in body
Jul
18
revised The Hole Argument
added 799 characters in body
Jul
18
comment The Hole Argument
@ACuriousMind I didn't just change the coordinate system, I changed the metric also, ie. if $p$ is a point on the manifold, then we have $g'(p)\ne g(p)$.
Jul
18
comment The Hole Argument
But if $p$ is a point on the manifold, then we have $g'(p)\ne g(p)$.
Jul
18
revised The Hole Argument
added 799 characters in body
Jul
18
asked The Hole Argument
Jul
2
awarded  Curious
Jun
25
comment How do we know that $F = ma$, not $F = k \cdot ma$
@JánLalinský I think the notion may be problematic if you don't define it...how do you measure it without knowing what to measure? In any case, I was in fact talking about inertial mass, and I think it does get at what the second law is about. Inertia is a bodies resistance to movement...this makes that statement precise.
Jun
24
comment How do we know that $F = ma$, not $F = k \cdot ma$
@JánLalinský Well you haven't proposed an alternate definition of mass...would you prefer it if I said inertial mass?
Mar
18
answered How do we know that $F = ma$, not $F = k \cdot ma$
Mar
7
accepted Momentum of particle in a box
Nov
10
answered Motivation for form of Lagrangian
Oct
28
comment Does GR imply a fundamental difference between gravitational and non-gravitational acceleration?
I think you have it wrong. An accelerometer in free fall would measure $0$ acceleration. The statement that you can't tell the difference between being in free fall and at rest in a vacuum is (approximately) true. You can clearly tell the difference between free fall and upward acceleration in an elevator. In one if you drop a ball it will fall beneath you, in the other it won't. And if you mean downward acceleration of the elevator, then that is exactly free fall so the statement is vacuous.