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2d
comment Induced metric on a null hypersurface
It's been a long time since I've looked at this stuff, and I"m thinking of it to give you a better answer, but you should look at Sean Hayward's articles on this stuff.
2d
comment Induced metric on a null hypersurface
however, I'm now trying to think through ane example where the statement is false. Note that when particles enter the black hole, the black hole horizon becomes a spacelike, not a null, surface, so this null geometry is no longer valid.
2d
comment Induced metric on a null hypersurface
It's not true in general. Generally, the condition that the horizon is isolated, rather than dynamic, is equivalent to the null tangent vector being a killing field of the induced metric, which gives you the condition you're looking for.
Feb
1
answered Does time symmetry still holds when a particle drops into a black hole?
Jan
27
reviewed Approve Heisenberg Uncertainty Principle scientific proof
Jan
5
awarded  Nice Answer
Jan
5
comment What is gravitational energy in general relativity?
@ThomasElliot: it's true about gravitational waves, but definig where the wave is and what they are carrying energy from is a bit subtle.
Dec
28
comment About the complex nature of the wave function?
@ruskin23: I should have said <i>unphysical</i> time and space dependencies in the wave function. When you solve the Schrödinger equation, and you get an answer that depends on the spatial variables, it's giving you real information about where the particle is. It's different when you multiply by a phase -- that's not giving you information about where the particle is, it's giving you information about how that particle might interfere with other particles.
Dec
10
comment If there were infinite many stars
Or, alternately, the stars are receding so quickly that their light has been redshifted to below the visible spectrum.
Dec
4
awarded  Popular Question
Dec
3
revised Is there any evidence of black holes existing?
edited body
Dec
3
comment Is there any evidence of black holes existing?
The only truly missing piece is gravitational waves, and we have indirect evidence of that, and hopefully LIGO or LISA (hey, LISA Pathfinder went into the sky yesterday!) will directly detect those.
Dec
3
answered Is there any evidence of black holes existing?
Nov
25
comment Why is cold fusion considered bogus?
@Joshua: fine. But Tokamaks aren't economically viable, but <i>they can and have been built</i>.
Nov
12
awarded  Yearling
Nov
10
comment How does the second law of thermodynamics forbid the possibility of perpetual machine of the second kind?
I understand, I was just asking for clarity.
Nov
10
comment How does the second law of thermodynamics forbid the possibility of perpetual machine of the second kind?
Which statement of the second law are you using? One of the statements is "no process can cause heat to go from cold to hot spontaneously", and this is pretty obviously violated by a machine like what you describe.
Oct
25
awarded  Good Answer
Oct
22
comment Are mass and angular momentum related at a quantum level?
Are you talking about the spin angular momentum, or the orbital angular momentum in an atom?
Oct
20
comment Can you take the integral of $ d^2x\over dt^2$?
@UysofSpades: why is integrating twice weird? You had to differentiate twice to get the thing you're integrating.