98,434 reputation
7128268
bio website motls.blogspot.com
location Czech Republic
age 40
visits member for 3 years, 6 months
seen 1 hour ago

Hi, I am a string theorist and a publicist.


1h
revised Is it possible to design a large permanent magnet that creates a multi-Tesla field?
added 291 characters in body
1h
answered How to define the thermodynamic temperature
1h
answered supersymmetry vs multiverse
1h
comment Is it possible to design a large permanent magnet that creates a multi-Tesla field?
Perhaps it is, "multi" is "greater than one". For example, an average multicultural country contains 1.4 cultures (or 1.4 cultural ethnic groups).
2h
comment If virtual particles have negative mass why do they contribute positive mass to atoms?
Dear @BrandonEnright ,they are off-shell in general but they are surely allowed to have negative energy, too.
2h
comment What is the difference between negative acceleration and decreasing acceleration?
One of them says that the second derivative of position is negative, the other says that the third derivative is negative. These two things are different because $2\neq 3$ or, equivalently, $1+1\neq 3$.
2h
answered Long string model of black hole
3h
answered Is it possible to design a large permanent magnet that creates a multi-Tesla field?
4h
comment Would magnetic flux be necessary for analogous systems?
@Waffle'sCrazyPeanut, sorry, they don't disagree, you used the $c$ notation while I call it $m\gamma$.
4h
comment Showing a fourth rank tensor in $\epsilon$'s reduces to one in the metric $g$
It's just a normalization factor. Don't get carried away.
4h
comment Can an underdensity in space act as a negative matter density powering a warp drive?
Sorry, what you write is just completely wrong. I fully understand you but it is wrong, @lurscher. AdS spacetimes have a perfectly well-defined conserved total energy. Without that, I assure you that AdS/CFT wouldn't work. dS don't admit any nontrivial conserved total energy because it doesn't have globally timelike Killing vectors or an asymptotic region in spatial infinity, for that matter. None of these things matters for the question whether the kind of negative energy needed to get a warp drive in the real world is possible. It is not possible.
9h
awarded  Nice Answer
13h
comment Can an underdensity in space act as a negative matter density powering a warp drive?
You may "call" just the cosmological constant "negative relative energy" but that's just misleading terminology. It's still non-negative genuine energy density, and will therefore lead to no inconsistencies such as warp drives. It's a matter of conventions how the C.C. is included to the stress-energy tensor, and the energy conditions must be rewritten accordingly, but whatever it is, there is some behavior for the vacuum in any C.C., it doesn't contain any warp drives, and the addition of any matter makes the warp drives "even further" from reality.
13h
comment Showing a fourth rank tensor in $\epsilon$'s reduces to one in the metric $g$
That's perfectly fine. An error is your conclusion that all the constants vanish. The three equations are clearly not independent, are they? Subtract the last one from the first one, for example.
13h
comment Would magnetic flux be necessary for analogous systems?
Of course that it's just a dictionary that shows the mathematical isomorphism between differential equations that appear on both sides. Some of your entries disagree with Feynman - why didn't you copy his table accurately?
13h
awarded  Nice Answer
14h
comment Showing a fourth rank tensor in $\epsilon$'s reduces to one in the metric $g$
You made a mistake.
14h
comment Lifting frictional force and relative values for kinetic vs static friction
There is really no contradiction here. Cold welding also involves matching of crests and troughs. See the picture powerguru.org/wordpress/wp-content/uploads/2012/07/… at powerguru.org/reliability-of-pressfit-connections
14h
comment Showing a fourth rank tensor in $\epsilon$'s reduces to one in the metric $g$
Let me say that these tests are only simple in the frame where $g$ is diagonal, with the usual diagonal entries, so for a general coordinate system, you may either run the more general calculation, or you may argue that the objects behave properly under the transformations. In a general system, the most natural fully antisymmetric tensor really has scalar components proportional to $\sqrt{|\det g|}$
14h
comment Showing a fourth rank tensor in $\epsilon$'s reduces to one in the metric $g$
Yes, $a_1/a_2=-2$ to cancel the 1111 and 2222 terms. It also cancels the 1122 and 2211 terms, of course. You may determine the value of $a$ uniquely out of some nonzero components. Of course, if you rescale $a,a_1,a_2$ by the same factor, it will still be OK - it's still just multiplying the equation by a number.