genug
Joined 11 years ago
Karma
27
Submissions
14
Member since
Nov 17
Recent submissions
Comment · 11 years ago
Comment · 11 years ago
Systems like the bullet cluster (https://en.wikipedia.org/wiki/Bullet_Cluster) are taken as evidence that most of the matter in large-scale, gravitationally bound astrophysical systems does not behave like luminous matter.
Comment · 11 years ago
It's like the old saying: offense is taken, not given.
Comment · 11 years ago
Seems like an innocuous question. Why is it being downvoted?
Comment · 11 years ago
> Who cares?
Everyone trying to pressure these people into having children, apparently.
Comment · 11 years ago
There's really no reason to, you know, for the transcription to include every filler word and little, you know, rewording. In fact, it makes the, I think it makes it really hard to read.
Comment · 11 years ago
Oxford University banned a magazine from being distributed in a stall at a freshman orientation fair, which is not nearly as sweeping as the headline suggests.
Comment · 11 years ago
I don't think there is any connection. But since the submission was titled "covariant and contravariant" without any kind of context, it seemed appropriate that we could all share our own personal ideas about what these words were supposed to mean.
Comment · 11 years ago
A contravariant object transforms like a vector.
A covariant object transforms like the gradient of a scalar.
Comment · 11 years ago
> the current sub-standard understanding of Best Practice infrastructure
> Best Practice has been established. It's ridiculous to try and reinvent the wheel.
> It's simple if Paris wants it to be.
If the author thinks that the understanding of "Best Practice" is substandard, maybe he should post a link to what he means, instead of leaving the term undefined and then acting smug about it.
Googling "Best Practice bicycle" returns a number of documents from different cities. It's unclear which of these (if any) correspond to the author's idea of "best practice" for cycling infrastructure.
I did find a post by the same author (http://www.copenhagenize.com/2013/04/the-copenhagenize-bicyc...) which advertises itself as a "planning guide", but is actually just a poster made by someone who was apparently more interested in graphic design than in actually conveying information.
Comment · 11 years ago
> Or how about this: a break-through that goes beyond classic antibiotics. Something that hasn't been thought of yet.
So what should the BBC headline read, then? "We're running out of antibiotics, but surely some clever person will think of something!"?
Comment · 11 years ago
We have no experimental evidence that the calculations for gravitational Hawking radiation (or their physical interpretation) are valid. The notion that dumb holes exhibit all the same properties as black holes is conjectural.
Comment · 11 years ago
120 years ago, people thought that experimental results on wave propagation in continuous media could be used to make statements about wave propagation in electrodynamics. That's how we ended up with ether theories, which were eventually falsified with interferometric experiments.
To my mind, we should take this as a lesson that the notion of analogue systems (e.g., using results from fluid dynamics to make statements about gravitation) is bogus. The ether episode seems to indicate that if we want to make statements about gravitational Hawking radiation, we need to do tests on a system where the gravitational dynamics are important (e.g., a black hole), not an unrelated system that just happens to be governed by an identical set of equations in some regime.
Another example of people incorrectly conflating properties of an analogue system with the real system of interest is the Nature monopole paper: http://www.nature.com/nature/journal/v505/n7485/full/nature1... The actual result contained in the paper is that you can make a BEC whose vorticity field has regions of nonzero divergence. It is mathematically similar to (but fundamentally unrelated to) actual magnetic monopoles. Nonetheless, the paper contains the phrase "Dirac monopoles" in the title and then spends half the abstract talking about actual magnetic monopoles, as if the result is somehow relevant to actual magnetic monopole searches (which it isn't).
A minor point: it's "the Newton equation" or "Newton's equation", but not "the Newton's equation" (in the same way that one could write "the Eisenhower presidency" or "Eisenhower's presidency", but not "the Eisenhower's presidency").