I admit, it's been a few years since I've studied physics, but the following question came to me when I was listening to a talk by Lawrence Krauss.

Is there any knowledge of from where matter that exists today originated? I recall that the law of conservation of mass asserts that matter cannot be created nor destroyed, but surely the matter we see today had to be created at some point? Perhaps I am applying this law in the wrong fashion.

The reason I ask, is because Krauss mentioned that the elements of organic matter where created in stars, not at the beginning of time (whenever that may have been), but I ask, where did the building blocks for these elements arise? Were they too created in stars? If so, from where did their constituent building blocks come?

Please forgive me if this off topic, it is my first post on this particular stackexchange site. Thank you.

If naturally occurring ⁴⁰K is responsible for a dose equivalent of 0.16 mSv/y of background radiation, calculate the mass of ⁴⁰K that must be inside the 59 kg body of a woman to produce this dose. Assume that each ⁴⁰K decay emits a 1.30 MeV β, and that 40% of this energy is absorbed inside the body. The half life of ⁴⁰K is 1.25 × 10⁹ years.
_______ g

(a) Compute the derivative of the speed of sound in air with respect to the absolute temperature, and show that the differentials dv and dT obey dv/v=1/2 dT/T. (b) Use this result to estimate the percentage change in the speed of sound when the temperature changes from 0

I want to create a stream of water that emits only a droplet of water, waits a few milliseconds, and then continues. The important thing is that I need to create a visible gap between drops.

Considering the desire to have droplets created with a consistent size and shape (asked in this question), how would one go about creating a stream analogous to a morose-code-stream of droplets?

[Edit]

I'm considering having a solenoid valve connected to a tube pointed downward, where capillary action holds the water in place. The solenoid releases the amount equal to one droplet of water. What I'm having trouble with is making the drops look semi-uniform as they fall.

Here is an artistic rendering of what I'm trying to accomplish:

Clearly there will be differences like air resistance; I'm not interested in that. It seems like you're working against gravity when you're actually running in a way that you're not if you're on a treadmill, but on the other hand it seems like one should be able to take a piece of the treadmill's belt as an inertial reference point. What's going on here?

Although I doubt somewhat whether this question is really appropriate for this site, I hope it gets answered anyways. I guess, what I'm wondering is:

1. How does one get to work as a theoretical physicist and - probably more importantly - what do theoretical physicist actually do all day long?

2. How are theoretical physicists distinguishable from mathematicians? Does a physicists day look very different from that of a mathematician?

3. I have a great interest in physics, but I'm not really much interested in doing experiments: Would it be advisable to do my bachelor in mathematics and try to get into theoretical physics later on?

4. Is there a real chance of getting into research afterwards? (not that any kind of answer to this question would ever stop me from trying...)

Well, I hope this question is acceptable.

I think 1) might for example be answered by giving a link to a blog of a working theoretical physicist, who gives some insight into his or her everyday life, or some kind of an essay on the topic. Of course any other kind of answer is greatly appreciated.

Thanks in advance!

Kind regards

I am sending a couple of questions which seem a bit more specific than others on this site, partially to probe if there is a point in doing so. Not sure what is the range of expertise here, and no way to find out without trying. This one is also not terribly focused, but nonetheless here goes:

I am wondering if there are some well-known and well-studied examples of large N matrix models (in which the fields are adjoint rather than vectors) which are of use in describing some condensed matter phenomena.

There are lots of applications of matrix models in anything between nuclear physics to number theory, and there are well-known vector models which are useful in CM physics, but off the top of my head I cannot think about matrix models which are used to solve some condensed matter problems. Quite possibly I am missing something obvious...References or brief descriptions will be appreciated.

Is it possible to counter-act g-force for a jet-pilot, by him putting on a scuba-diving suit and filling the cockpit with water? On earth we are constantly pulled down, or accelerated with one g. In this situation, if we put the jet-pilot in a pool, he would neither sink nor float.

If we could increase Earth's gravity to say 9 g's, the pilot in the pool would float even more.

Is this correct? Thanks...

in recent questions like "How are classical optics phenomena explained in QED (Snell's law)?" and "Do photons gain mass when they travel through glass?" we could learn something about effective properties of matter interacting with a force field in terms of the path integral and quasiparticles.

Surely, both approaches must be equivalent but come from a different philosophy. Widely used is the quasiparticle approach in solid state physics e.g. calculating dispersion relations of phonons.

I would really like to know if there are simple examples for explicit calculations of the properties of photon-quasiparticles coming from a rigorous approach like a matter description via QED and finding an effective action e.g. using the Wetterich equation (see e.g. Introduction to the functional RG and applications to gauge theories).

Any calculations and/or references would be very nice.
Thank you in advance, sincerely,

A stone is dropped into a river from a bridge at a height h above the water. Another stone is thrown vertically down at a time t after the first is dropped. Both stones strike the water at the same time. What is the initial speed of the second stone? Give your answer in terms of the given variables and g.

Except for Mercury, the planets in the Solar System have very small eccentricities.

Is this property special to the Solar System? Wikipedia states:

Most exoplanets with orbital periods of 20 days or less have near-circular orbits of very low eccentricity. That is believed to be due to tidal circularization, an effect in which the gravitational interaction between two bodies gradually reduces their orbital eccentricity. By contrast, most known exoplanets with longer orbital periods have quite eccentric orbits. (As of July 2010, 55% of such exoplanets have eccentricities greater than 0.2 while 17% have eccentricities greater than 0.5.1) This is not an observational selection effect, since a planet can be detected about equally well regardless of the eccentricity of its orbit. The prevalence of elliptical orbits is a major puzzle, since current theories of planetary formation strongly suggest planets should form with circular (that is, non-eccentric) orbits.

What is special about the Solar System that orbits of planets here are nearly circular, but elsewhere they are moderately or highly eccentric?

Earth's perihelion passed about nine hours ago. How accurately do we know the moment of closest approach of the Earth to the center of the sun? How do we make this measurement?

Many light sources like LEDs and lasers only emit a single wavelength of light.

Is there a light source that emits all wavelengths of visible light at the same time?

EDIT: I edited the question to reflect Moshe's objections. Please, look at it again.

It's apparently a black hole time around here so I decided to ask a question of my own.

After a few generic questions about black holes, I am wondering whether string theory is able to provide something beyond the usual semiclassical Hawking radiation talk. Feel free to provide an answer from the standpoint of other theories of quantum gravity but AFAIK none of the other theories has yet come close to dealing with these questions. That's why I focus on string theory.

So let's talk about micro black holes. They have extreme temperature, extreme curvature, and I guess they must be exceptional in other senses too. At some point the gravitational description of these objects breaks down and I imagine this kind of black hole could be more properly modeled like a condensate of some stringy stuff. So let's talk about fuzzballs instead of black holes.

What does that microscopic fuzzball model look like?
What does string theory tell us about the evaporation of those fuzzballs? Is the Hawking radiation still the main effect (as for the regular black holes) or do other phenomena take over at some point?
Also feel free to add any other established results regarding black hole decay (as Jeff did with information preservation).

Here's an proposal on how to get from point A to point B in zero-gravity without using any propellant and the question why it wouldn't work:

A closed tube, filled with water and a round (solid) object. If you need equations, the volume of enclosed water is the same as the round object, but the round object is 10 times lighter. (imagine a glass with water plus a ping-pong ball).

On earth the round object will float on the water inside the tube (subject to one G).

In zero-gravity the round object has no preferred position.

If we accelerate the tube in zero-gravity by one G, the situation is the same as on earth, the round object "floats". In this example we are accelerating the tube from the left side to the right side. The round object will consider floating to the right AS LONG AS THERE IS acceleration.

Now consider adding a pipe to the bottom of the tube and connecting it to the top, a loop. Inside the pipe is a small water pump.

If we give this apparatus a push, say one G in a zero-g environment, the round object will move "up", but now we start the water pump and spray the water on the round object, we try to submerge it. It will resist and impart a impulse on the water. like trying to hose down a air balloon floating on the pool. The pump will feel a resistance and hence the whole apparatus will move.

Just running the pump at constant speed, same volume of water per second, will do nothing. but if we run the pump faster and faster the whole apparatus will start moving:

the amount needs to be geometrical. The point is that we need to keep the apparatus feeling an acceleration, since only then will the round-object "float" and resist the incoming water at the top, hence we have something to "push on".

Before you blow the "foul whistle": consider the situation if there were no round floating ball in the apparatus.

(the pump runs on solar power or pre-charged battery)

(disclaimer: i know standing on a sailboat and blowing into the sail will not get me anywhere, action<->reaction)

thanks a bunch Sklivvz for the edit. sometimes the idea just needs to get out, never the mind how it looks like : P