PV Gap

Stacy Said:

TONS of astronomy questions?

We Answered:

1. It's not quite so much an amalgam of those as much as it utilises specific knowledge from each. The physics is the most important bit (today the most vigorous research and interest is in astrophysics after all), followed by chemistry/geology (in analysing composition and structure of celestial objects) and even meteorology (when a planet's atmosphere is concerned for instance). Even other considerations are possible, such as the possibility of life, an ecology of some sort on a planet, but this is thus far a speculative branch.

2. Based on what's observable out there, the amount of mass in the form of stars, planets, asteroids, et al. is negligible when compared to the expanse of space they exist in, so the universe is very nearly empty. However when we look at the motion of galaxies for example, the stars within them tend to be moving much faster than the visible mass accounts for, thus we have 'dark matter' which exerts a gravitational influence despite not being observed (no interaction seemingly with electromagnetic radiation)! It turns out dark matter constitutes about four fifths of the matter in the observable universe with ordinary matter accounting for only a fifth!
Since the universe appears to be expanding at an accelerated rate, 'dark energy' has also been hypothesised and (since by relativity energy has mass) accounts for some three fourths of the mass-energy of the universe!

3. By considering quantum effects in a black hole, Stephen Hawking argued that black holes should emit thermal radiation. So black holes can indeed lose mass (which was an argument that popped up during the black hole paranoia that surrounded the LHC- that any micro black holes it created would almost instantly 'evaporate' in this manner!). A black hole dissipating because of this isn't too far fetched, seeing as space is largely empty as mentioned before so it may not encounter enough to consume.

4. Off into space in the form of particles (bearing note of wave-particle duality)

5. Light can be thought as made up of 'photons', these have null rest mass so while light is not quite a construct that can be thought of as 'tearing', the photons and hence it, have no chance of escape once the 'event horizon' is reached.
If a black hole closed in on the Earth, the most devastating effects will likely be during its approach and passing on to the event horizon. The singularity would then engulf the mass of the Earth and there's a definite 'unknowability' attached to what happens there.

6. We can indirectly observe quite a bit:
http://en.wikipedia.org/wiki/Black_hole#…
However, our assumption of an event horizon masking a gravitational singularity (a point or ring of infinite density) may be allayed by alternative hypotheses. General relativity makes no real claims about black holes beyond the event horizon (or even at it) since the theory breaks down. By considering quantum effects, it is expected that a theory of black holes which does not feature singularities or event horizons may result from quantum gravity.

You might like to read:
http://design.lbl.gov/education/blackhol…
http://imagine.gsfc.nasa.gov/docs/ask_as…
http://www.phys.vt.edu/~jhs/faq/blackhol…
http://casa.colorado.edu/~ajsh/singulari…
http://www.engr.newpaltz.edu/~biswast/bh…
http://www.nowpublic.com/tech-biz/mathem…
(this last is about a black hole not having to contain a singularity)

7. They usually rely on classifying by magnitude of a certain scalar (be it mass/size (like the Pluto fiasco a while back), luminosity, temperature (which corresponds to spectra in the case of stars) etc. depending on what's being classed).

8. The first question you need to ask yourself is, curiously, an etymological one- what is meant by 'universe'? We wish to capture an all-encompassing essence in the word, so why should we exclude actual events (the idea of occurrence of which has no meaning sans time) that occur in the continuum we call space?
The great insight of relativity due to Poincaré and (more famously) Einstein is that motion is local- depending on one's frame of reference (i.e- it makes sense only in relative terms). This leads to the idea of 'space-time' and, in the most general case, it being curved (on the whole) by the presence of mass (or equivalently energy). Mathematically, we have a smooth manifold (a space that seems Euclidean locally) of 4 dimensions and a pseudo-Riemannian metric on it gives curvature.
http://en.wikipedia.org/wiki/Causal_stru…

In fact, even usual Newtonian physics can be put in the context of a single 'space-time' continuum, but it turns out that the two are independent when considered this way!

Melinda Said:

multiple choice physics optics questions, please help!?

We Answered:

1. c)
If it is exactly 2 focal lengths away, the magnification will be 1 at the opposite 2x focal length.

2. a)
If the object is further than 2x focal length, it will be a smaller real image and it will be in front of the opposite 2x focal length but in front of the focal length.

3. Not sure, either a or b

5. I think it's standing waves.