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bio website motls.blogspot.com
location Czech Republic
age 41
visits member for 4 years, 3 months
seen 1 hour ago

Hi, I am a string theorist and a publicist.


Jan
16
awarded  Nice Answer
Jan
15
answered What is the motivation behind the GSO projection in superstring theory?
Jan
13
awarded  Yearling
Jan
8
comment Intensity in rainfall and global warming
This is a physics questions-and-answer servers where people provide answers based on the accurate enough analysis of the science, not a computer modeling server where people are supposed to uncritically believe features of one computer program or another. By "textbook material", I meant objectively important basic information about a subject that someone should know. When the education system works, this physics-based classification is a sociological claim, too - people learn what's important. When the education system breaks down, the actual textbooks may deviate from the textbook material.
Jan
6
awarded  Nice Answer
Jan
6
comment What's the symbol behind Einstein's head?
By the way, on the blackboard, the symbol for the speed is really $q$, not $v$, see comments about Geschwindigkeit $q'$ here: einsteinpapers.press.princeton.edu/vol4-doc/71
Jan
4
comment Classical black holes?
The size of the Universe was not constant in the past but the size of the visible Universe actually is converging to a finite constant, very close to the current size, because in the asymptotically far future, and it's almost the case now as well, the Universe will be (close to) a de Sitter space with a fixed spacetime curvature radius (given by dark energy, i.e. the cosmological constant), and the size of the visible universe up to the event horizon is a fixed multiple of the curvature radius. All these radii are close to "dozens of billions of light years".
Jan
4
comment Classical black holes?
Dear Timaeus, every quantum mechanical theory will imply that the long-distance effective theory contains not just the Einstein-Hilbert term $R$ but also the higher-derivative terms of the form I mentioned. One may see the explicit form of these terms in string theory which is really the only consistent theory of quantum gravity we know but the arguments that these terms arise may be presented independently of string theory, just with the knowledge of the renormalization group etc. So what you want to learn is either "renormalization group" or "string theory" or both.
Jan
3
comment Tesla's theory of gravity
Dear @TomAndersen, the difference is that alchemy was the state-of-the-art chemistry of Newton's times, and astrology was a broader science about the implications of celestial bodies – a science that was later made obsolete mostly by insights that followed from Newton's works. On the other hand, Tesla's musings about gravity and many other things were not state-of-the-art descriptions of anything. They were wrong even relatively to the ideas of his time.
Dec
31
awarded  Good Answer
Dec
31
comment How were the ratios of distances between planets and the Sun first calculated?
For example, if the maximum angle between Venus and the Sun from our point of view is $\pm\alpha$, in radians, it follows that the ratio of the Venus-Sun and Earth-Sun distance is equal to $\alpha$. Well, $2\tan \alpha/2$ or something like that, which is the same for small $\alpha$. By measuring the angles separating two celestial bodies, we may deduce the information about their mutual distance relatively to our distance from them.
Dec
31
comment How were the ratios of distances between planets and the Sun first calculated?
Exactly, like Kyle, I just wanted to write that the ratios of distances are computed from the so-called "angles" which are defined as the amount of space in between two dots we see in some directions.
Dec
30
awarded  Nice Answer
Dec
22
comment How can we take pictures of something billion of light years away?
Dear Joshua, maybe Tolga wants to take a picture of someone this far, but she wants him to smile, so she first says "cheese", waits when the subject starts to smile, and press the trigger. Incidentally, the end of the visible Universe is currenly about 40+ billion light years from us, not 14, because those regions continued to expand between the moment when light was emitted and now.
Dec
21
awarded  Nice Answer
Dec
20
comment Is it acceptable to base a physical theory on a philosophical idea?
You can get the idea for a theory from any source, previous theories, philosophy, religion, drugs (this was a frequent source in recent decades), or something else. But to be successful, you must formulate the resulting hypothesis/theory in a quantitative language that makes some predictions about the phenomena in Nature and these predictions can't be true tautologically - there has to be something non-obvious about them. And these predictions have to agree with the empirical data. When they don't, the theory is killed. When they do, it is strengthened.
Dec
19
comment Does lunar module need the same amount of fuel for landing and take off?
Landing and taking off are opposite to each other so the signs of the energy change are opposite, right? Maybe you are asking about the amount of fuel but that's something different than energy. Fuel has to be spent - the sign is always decreasing - but the energy may change in both ways, to mention a difference between the two. The switching between the orbit and the surface are opposite processes, so the energies are opposite but the amount of fuel may be the same. To get to a low orbit is 1/2 of the energy needed to escape the gravitational field completely.
Dec
18
comment How do I calculate the perturbations to the metric determinant?
Hi, one may do the calculations in both ways - but your way would have to be fixed by adding the factor of 1/2 on the right hand side. My way assumes that $g_{\mu\nu}$ is a general tensor, not necessarily symmetric, and only treats the symmetricity as a "random feature of the initial conditions". Your way would effectively differ from mine by defining $g$ as the symmetric part of "my" $g$ but one must be careful about such things. My way is surely the standard way how the derivatives are calculated.
Dec
17
awarded  Necromancer
Dec
16
awarded  Enlightened