Thursday, 9 October 2014

space time - The multiverse spacetime paradox?

I'm not about to suggest that multiverse(s) do not exist, but merely that it is a fascinating, but highly speculative, topic. The Wikipedia article is a good start. From the wiki:




The structure of the multiverse, the nature of each universe within it and the relationships among the various constituent universes, depend on the specific multiverse hypothesis considered. Multiple universes have been hypothesized in cosmology, physics, astronomy, religion, philosophy, transpersonal psychology, and fiction, particularly in science fiction and fantasy. In these contexts, parallel universes are also called "alternate universes", "quantum universes", "interpenetrating dimensions", "parallel dimensions", "parallel worlds", "alternate realities", "alternate timelines", and "dimensional planes," among others.




From the way you phrase your question, you may want particularly to look at the sections on M-theory.

Monday, 6 October 2014

orbit - Do all planets rotate in the same direction in relation to each other?

In other words, does everything spin counterclockwise according to their axes (in relation to each other)? I suppose my question goes for stars, moons and other objects too.



But then again, all you have to do is look at any rotation upside down and it goes clockwise. So depending how you're looking at it, one could say everything rotates in the same direction whether they do or not. It's all a matter of perspective, right? And in space, there is no "right side up". Maybe my question is moot now that I think about it.



If we re-drew all of Earth's maps and globes upside down, New York or Sydney would be on the "west coast" and the Sun would rise in the "west" and set in the "east". Is it just by luck or maybe by the prominent early mapmakers, or that the majority of human population lives in the Northern Hemisphere, that it's given "dominance" on "top"? (I've heard that theory before)



Ugh. This is all so confusing.

Saturday, 4 October 2014

distances - Do astronomers, in principle, have more information about older phenomena than younger?

There are more stars and galaxies but not necessarily more information.



You might choose to measure the information content in terms of photons received, in which case, for a given star/galaxy, this decreases as the inverse square of the distance, so the two effects cancel out.



Astronomy is a constant struggle with the tension between these two competing dependencies. As we look further, not only do we see more of particular types of object, but we also get to see examples of rare objects - i.e. the tails of the distributions. On the other hand as we expand our horizons, so our grasp of what is going on becomes increasingly blurred in terms of spatial, spectral and temporal resolution. It is for that reason that bigger and bigger telescopes are built!



The issue you raise concerning redshift is interesting. The co-moving volume versus look back time in any direction will not be a simple function (i.e. doesn't just go as the square of the lookback time) and depends critically on the adopted cosmological parameters.



There is of course a limit in any case. Once we get back to the microwave background at $z=1100$, then further probing backwards with electromagnetic radiation is stymied by the optically thick nature of the universe at early times.

Friday, 3 October 2014

space time - Could the Earth use gravitational lensing / bending of light to see it's own bottom?

Whimsical question. Similarly, you could reason that someone on a planet far, far away would be holding up a mirror, such that we could see our own reflected past again.



Unfortunately, both options are practically infeasible, even if the light would be undistorted and redirected perfectly towards us. Remember that (1) Earth is small, (2) the reflected signal is weak as it is, (3) any object capable of reflecting this back towards us would be a black hole, the closest of which is further away than most known exoplanets, and the signal falls of with $r^{2}$ with $r$ twice (!) the distance to the object doing this lensing.



Now, if we somehow were able to observe it anyway, it would be straightforward enough to estimate the look back time - the distance to the lens would presumably be fairly well known.



How bright it would have to be is not really a sensible question - there are too many variables in play for that (which (hypothetical future) telescope are we using, what are the observing conditions, how far away is the source, what kind of source are we talking about...). And at the end of the day, the brightness of your source is not even the biggest problem; finding a suitable lens is.

the sun - Will a Ball placed close to Sun fall into it?

The answer depends on the size and mass of the ball. It also depends on its ability to reflect light (albedo $A$), but let's forget that for a moment.



Pressure vs. gravity



Solar pressure decreases with $R^2$ (the inverse square law). At Earth, which is located at a distance of $1,mathrm{AU}$ from the Sun, we receive an irradiance $S_0 = 1361,mathrm{W},mathrm{m}^{-2}$. Since the momentum of a photon of energy $E$ is $p = E/c$, the pressure at a distance $R$ from the Sun is
$$
P = frac{S_0}{c(R/mathrm{AU})^2}.
$$
If the ball's radius is $r$, this pressure will exert a force
$$
F_gamma = pi r^2 P = frac{pi r^2 S_0}{c (R/mathrm{AU})^2}qquad(mathrm{away,from,the,Sun}).
$$
Meanwhile, if the ball's mass is $m$, the gravitational force exerted by the Sun on the ball is
$$
F_g = frac{G M_odot m}{R^2},qquad(mathrm{toward,the,Sun})
$$
where $G$ is the gravitational constant and $M_odot$ is the mass of the Sun.



Threshold for falling



The threshold for the ball falling into the Sun is found by equating the two oppositely directed forces:
$$
frac{pi r^2 S_0}{c (R/mathrm{AU})^2} = frac{G M_odot m}{R^2}.
$$
The first thing to notice is that $R$ cancels out; the reason being that flux density and gravity both follow the inverse square law. Second, re-arranging terms we see that the ball will fall if its mass per area is greater than this threshold (if I have calculated correctly):
$$
frac{m}{r^2} gtrsim frac{pi S_0}{c G M_odot } mathrm{AU}^2 simeq 2.4times10^{-4},mathrm{g},mathrm{cm}^{-2}.
$$
Now you can plug in your favorite numbers. You will find that for most macroscopic objects, such as a football, a rock, and even a sand grain, it will fall. On the other hand, small "balls" such as dust grains and atoms, will tend to be pushed away from the Sun. An example of a macroscopic object that won't fall is a solar sail which seeks to maximize area per mass.



For a given density, say $rho = 2.5,mathrm{g},mathrm{cm}^{-3}$ which is characteristic of rocky materials, you can also calculate the maximum size before it falls toward the Sun:
$$
r_mathrm{max} = frac{3}{4pirho} ,(2.4times10^{-4},mathrm{g},mathrm{cm}^{-2}) sim 0.1mathrm{-}1,mumathrm{m}.
$$



Albedo



The above calculations hold of all the radiation is absorbed by the object. If a fraction of it is reflected, the radiation will transfer more momentum to the object, and for a prefectly reflecting object, the radiation force will be roughly twice the amount above (the exact factor depends on the geometry of the object).



Extinction curves



It should be noted, though, that treating small particles as rigid spheres with a geometric cross section becomes imprecise when their size is comparable to wavelength of the light; rather, their absorption/scattering cross section should be treated quantum mechanically. In practice, extinction curves — i.e. the cross section as a function of the wavelength of the light for a given dust particle size distribution — are measured observationally by comparing the light from unobscured star with similar stars behind dust clouds, and subsequently fitted with various functional forms.

neuroscience - Does body mass have a bearing on reflex speed?

I went into this with the same assumptions that jello did, but I found two studies that had some interesting results.



Isojärvi (2010) found that in obese and under-exercised adults, physical fitness level predicted, among other things, a significant proportion of the variance of nerve conduction velocity and F-wave latency. So, these individuals may theoretically have a slower response to noxious stimuli.



While Type II diabetes is not always a result of obesity, there is often a high correlation between the two. Oltman (2005) found that, in "Zucker diabetic fatty" rats (an animal model of Type II diabetes), motor nerve conduction velocity in the sciatic was significantly slowed. This effect, too, would affect the rate with which an individual could withdraw a limb from a noxious stimuli.



So, I don't think that anatomical changes in the bodies of individuals who are obese would have any effect, but clearly there are physiological changes in the neurons of these individuals.





References:

Isojärvi H, Keinänen-Kiukaanniemi S, Kallio M, Kaikkonen K, Jämsä T, Korpelainen J, Korpelainen R. (2010). Exercise and fitness are related to peripheral nervous system function in overweight adults. Med Sci Sports Exerc. 42(7):1241-5.



Oltman CL, Coppey LJ, Gellett JS, Davidson EP, Lund DD, Yorek MA. (2005). Progression of vascular and neural dysfunction in sciatic nerves of Zucker diabetic fatty and Zucker rats. Am J Physiol Endocrinol Metab. 289(1):E113-22.

Thursday, 2 October 2014

cosmology - Is parallel universe possible

As much as parallel universes go, there's a theory for this known as the Multiverse theory, part of the String theory.




In String theory, the Multiverse is a theory in which our universe is
not the only one; many universes exist parallel to each other. These
distinct universes within the multiverse theory are called parallel
universes.




There are cosmic inflation models that support this as said here in Wikipedia. A quote from there:




It's hard to build models of inflation that don't lead to a
multiverse. It's not impossible, so I think there's still certainly
research that needs to be done. But most models of inflation do lead
to a multiverse, and evidence for inflation will be pushing us in the
direction of taking [the idea of a] multiverse seriously. Alan Guth[7]




Now to address your point if we can ever know if a parallel universe exists exists.



Opinion: Possible but unlikely. I don't think that we are close to achieving this.