6:57 AM
> I still haven't read the original Conan
I assume that it is a bit more subtle
The original books were by Robert E. Howard, and his Conan is just a thug whose approach to any problem is to hit it with something hard and heavy.
I have to concede there is a certain appeal to this philosophy :-)
But dozens of authors have picked up the Conan stories since, and their Conan tends to be more sophisticated. I find the original stories get a bit wearing - there are only so many variants of hitting things with large, heavy objects.

3 hours later…
10:21 AM
If you've got a metric like $ds^2=adx^2+bdy^2$ and you need to calculate the volume of the manifold, you'd use $\sqrt{|g|}dx\wedge dy=\sqrt{|ab|}dx\wedge dy$, which you would just integrate as $\int_V \sqrt{|ab|}dxdy$ right? I.e. the wedge product is "hidden" in the integral and you just calculate the volume like this?

Well the definition of the integral on a manifold is $$\int_U dx^1 \wedge \ldots \wedge dx^n = \int_{\phi(U)} dx^1 \ldots dx^n$$
there is a correspondance between the integral on a form and the integral on the Lebesgue measure

with the metric determinant thingy in there too right?

it's not "hidden"
sure yes

ah yeah i meant that in a very loose sense :P
ty

2 hours later…
12:27 PM
There's apparently something called a "Taub-Bolt space"
Physicists are having too much fun naming stuff

2 hours later…
2:38 PM
Where does the energy in uranium come from? When unstable uranium splits, where does the energy come from and what form is it in? A guess I have is as the uranium splits, the two parts gain potential energy as they get further apart, but I'm not sure how it works exactly

3:05 PM
Does anyone know how to share pdfs on telegram?

Hi All..
What if person moves with the speed of light. There frame is inertial because it is moving with constant velocity by this way it can also move more than speed of light. How it perceive light?

3:21 PM
@JingleBells it's from the nuclear binding energy - a curious fact is that the curve of nuclear binding energy, i.e. the binding energy per nucleon, is not constant across elements. So when you split an atom to get two atoms that have lower binding energy per nucleon, you get the difference in binding energy as "usable" energy.
@123 there is no frame moving with the speed of light, see e.g. physics.stackexchange.com/q/54162/50583
A frame must be reachable by Lorentz transformation from the rest frame of a massive observer, but Lorentz boosts only exist for speeds smaller than $c$

Hello @ACuriousMind . We know there is no absolute rest or motion. What if person B moving more than speed of light his frame is inertial with respect to person A. Is that possible. If not why..
@ACuriousMind it means no inertial exist which can attain speed of light with respect to another inertial frame?

@123 nothing can move with "more than speed of light"

What i think this is the problem lorentz transformation or light which can not reach the inertial frame which is moving more than speed of light. What do you say. Because there is no absolute rest/motion. Who is going to decide whether frame is moving with speed of light or not.
@ACuriousMind Let say we observe all frame those are inertial. Frame A at rest , Frame B moving close to speed of light with respect to frame A, But Frame C can be move more than speed of light with respect to A but less than speed of light with respect to frame B. There is no absolute rest/motion. Who is going to decide whether frame moving more than speed of light or not. This is the problem with light speed not with frame.

@123 the point is that all observers will always see massless things moving with the speed of light, and all massive things moving with less than the speed of light
you can't observe a "frame", you can only observe objects
and the mind-boggling fact of relativity is precisely that no matter how fast and long anything accelerates, it will never be seen by anyone to be travelling at or faster than the speed of light
@123 you're using classical velocity-addition, that just doesn't work. If two objects move with velocities $c/2$ and $-c/2$, they won't see each other moving at $c$, but at less than $c$. See relativistic velocity addition for the correct way of "adding" velocities relativistically.

3:41 PM
@ACuriousMind Thanks. The confusion is that if there is no absolute motion/rest. How we decide with what velocity our inertial frame is moving? Until there must be some absolute frame. Let say light has inertial frame.
In my opinion this is the restriction of light which allow only calculation with respect to light. Which is the only way we receive information from the outside world. This is not the restriction of nature.

I'm afraid I don't understand what you're saying
there is no absolute velocity "with which our frame is moving"
there are only velocities relative to other frames/objects with which it is moving
And these velocities are always smaller than the speed of light.

I am trying say. Who is going to decide we can not move more than the speed of light.

no one is "deciding" that - it's a fact of nature that we cannot observe any objects moving with velocities faster than the speed of light relative to any observer

It means it is a matter of observation/calculation. not frame?

I don't know what that means, either

3:50 PM
What if i am in inertial frame which is moving c/2. And i decide to move from this frame with speed c/1.5. Is that possible?

2 hours later…
5:37 PM
@123 It's an easy trap to fall into, it seems reasonable to ask what happens if you're moving at $c/2$ relative to me in some direction and you see an object travelling at $c/2$ relative to you in the same direction, what will I see. The answer is that velocities don't just add between frames in special relativity.

5:48 PM
Uh, if you have a representation of the Clifford algebra in even dimensions and you "get" a representation in $d+1$ dimensions by taking the $\gamma^5$ matrix as your additional gamma matrix, apparently you can also choose $-\gamma^5$ and get an inequivalent representation. How are they related? I'm just wondering if they're something like conjugate representations or something

@Charlie they're not generally conjugate since there are Majoranas in some odd dimensions, but for the representation to be real, it needs to be self-conjugate

Is there a general term for how they are related? Or are they just two inequivalent representations in odd dimensions

I don't think there's any special relation between them

Okay that's fine, just wondered ty

6:41 PM
I was just trying to understand is it possible to move inertial frame more than the speed of light with respect to other inertial frame. Because there is no absolute rest/motion.
Because in inertial frame no one decide whether we are moving or not until we relate our motion with some other frame. We have all laws of physics valid in that frame which is moving more than speed of light with respect to other inertial frame.
In inertial frame we are dependent on the information we receive by light. If some inertial frame moving more than speed of light toward other frame what happened. Is that possible or not?

7:12 PM
no

@ACuriousMind Gotchya, thx
@ACuriousMind in what form does the "usable" energy get released?

@JingleBells kinetic energy of the fission products, and hence overall theoremal energy

So different atoms have different nuclear binding energy, and let's say when uranium becomes unstable (due to a hitting neutron), it splits into two smaller atoms that have less combined nuclear binding energy than the original uranium, and since energy in the system has to be conserved, the two new atoms have a bit more kinetic energy than the original uranium? Can we say that in the process nuclear binding energy has been converted to kinetic/thermal energy?

yes

Alright, thanks!

2 hours later…
9:13 PM
0

Is it possible to "swim" in a vacuum, taking advantage of relativity? (Is this a joke or hoax?) is a question I posted just now. Everything seems fine now, but while on this page which is an old question of mine: Why does moderately distant lightning sound the way it does: relatively quiet high p...