All Questions

59 views

is cosmic expansion related with IR divergencies?

This question is related to renormalization, but in the IR limit. It is assumed that unitarity does take care of IR divergencies in interacting theories like QED. But how would one interpret ...
248 views

How many stars within 5 parsecs?

I've ran some scripts on some star catalogs, which counted the number of stars with a parallax greater than 200 mas, which should mean everything closer than 5 pc. The results are as follows: ...
32k views

Gravity on the International Space Station

We created a table in my physics class which contained the strength of gravity on different planet and objects in space. At altitude 0(earth), the gravitational strength is 100%. On the moon at ...
3k views

Determine the acceleration and angular acceleration of a disc

The question is: A 90kg disc is floating in a frictionless vacuum. A 150N force is applied to the outer rim of the disc. The disc has a radius of 0.25m and a radius of gyration of 0.16m. What ...
1k views

Why isn't the symmetric twin paradox a paradox?

Two twin sisters synchronize their watches and simultaneously (from the earth frame) depart earth in different directions. Following a predetermined flight plan, each sister accelerates identically to ...
41 views

bond energies database [closed]

I’m looking for a public database of bond energies of biochemical compounds. Do you know about any? Otherwise, which is the standard computational methodology to calculate them? Any references would ...
542 views

Is Schrodinger's Cat a real conceptual problem or just a problem with approximations?

In this thought-experiment a cat is placed in a box set with a bottle poison that will release and kill it depending on whether or not a certain radioactive particle decays. The box is kept closed and ...
165 views

How is extra information encoded in digital radio?

Earlier today, I was listening to my digital radio, and was intrigued as to how they are able to encode the information displayed on the LCD screen (which was basically a short string of text giving a ...
78 views

Unitarity and quantum cosmology

By studying quantum cosmology I was asking myself if the fact that the universe is expanding, so space is expanding and with it I would say that phase space is also expanding, so it's a non-unitary ...
11k views

How can I theoretically describe the potential between two capacitors in series?

Suppose to have two capacitors in series: The voltage in the middle point will be: $$V_X = V_1 \frac{C_1}{C_1+C_2}$$ How can this be explained? It's been asked in electronics, and explained in ...
354 views

Two identical rockets, time dilation, and possible weirdness [duplicate]

Possible Duplicate: Why isn't the symmetric twin paradox a paradox? Suppose there are two identical rockets, each carrying one of two identical clocks and one of two identical ...
13k views

Why do people recommend wider tyres in car for better road grip?

Tyre companies boast of their wider tires for better grip on road. Also, the F1 cars have broad tires for better grip. But as far as I know Friction does not depend on the surface area of contact ...
1k views

Why is the center-of-mass of 2 bodies at the focus of their elliptical orbits?

Why is the center-of-mass of 2 bodies (which interact only via Newtonian gravity) located at a focus of each of the elliptical orbits? I know that when there are no external forces, the center of ...
461 views

How is traveling back in time possible in theory according to some scientists? [duplicate]

Possible Duplicate: Is it possible to go back in time? Is time travel possible? I get the idea of traveling to the future and it makes perfect sense as we'd be somehow trapped in a ...
70 views

spectral eigenvalue staircase and quantum system

in a d-dimensional system of Quantum physics , does the Eigenvalue staircase $N(E)= \sum_{E_{n}\le E} 1$ determine ALL the properties of Quantum System ?? for example, let us assume that the ...
134 views

Change in the Higgs mechanism

The most important thing about the Higgs mechanism is that the higgs mechanism is one of the simplest mechanisms (certainly the simplest way to break the electroweak symmetry) of elementary particles. ...
4k views

Spring compression and Momentum

I am asked to rate a series of elastic collisions in order of greatest time of max compression to least time of max compression for several vehicles with varying masses and velocities, which strike a ...
363 views

Why can i replace a gauge field by the current it couples to in the calculation of a greens function?

I am reading about anomalies in QFT at the moment and have a related question. Often people calculate the time ordered expectation value of some fields (in QED for example) by replacing the field ...
622 views

How much thrust would be needed to turn a hobbyist weather balloon into a deep space probe?

I was reading the article Weather Balloon Space Probes that says you can put your own balloon probe at 65,000 ft temporarily. Is it even remotely possible to raise the probe high enough using ...
2k views

Make a semi transparent mirror with copper

The question: How would you make a semi transparent mirror (50% reflection, 50% transmission) with glass with a layer of copper. For light $\lambda$ = 500nm Try to be as realistic as possible What ...
231 views

Propagator of the Klein-Gorden equation

Does this integral converge? My question is related to this one: Free particle propagation amplitude calculation I am reading the book of Peskin and Schroesder. In the second page of their chapter ...
768 views

Finding instantaneous speed in MPH given acceleration, RPM and gear ratio

I am still trying to figure out how to (semi) accurately model instantaneous speed after having found acceleration. I have found that at higher RPMs, the resultant acceleration will be lower. I was ...
236 views

How did ancient physicists around 500 BC decide that the earth is spherical?

One of the standard stories that is given for this (and is mentioned in Aristotle) is that the hull of a ship disappears first as it sails towards the horizon on a calm sunny day. Is this a myth, or ...
2k views

Why/how does an electron emit a photon when decelerating?

I've had two special relativity courses so far but none really gave me a clear description of the process.
223 views

Applying $\nabla\times\mathbf{B} = \mu_0\mathbf{J}$ in the presence of magnetic shielding

2012-06-13 - Revised question in experimental format (This is a thought experiment for which RF experts may have an immediate answer.) I'll assume (I could be wrong) the possibility of creating a ...
284 views

How do particle scattering cross sections scale with energy in colliders?

How do particle scattering cross sections scale with energy in colliders? Particularly photons, electrons, protons, and gold or lead nucleii? (If necessary, break this into four separate questions.) ...
358 views

How to find an old Soviet physics paper?

I am interested in an old physics paper by Viktor Ambartsumyan and Dmitrii Ivanenko: Doklady Akademii Nauk SSSR, ser. A, No. 6, p.153 (1930). It concerns the structure of the atomic nucleus and ...
358 views

Why does magnetic field lines go from plus to minus?

My question is why the magnetic field lines goes from plus to minus, if there was two charge. Is it true or isn't it true.
371 views

Modeling a bar magnet's magnetic flux inside a coil

A bar magnet and a coil arranged alongside their common axis is a common setting in teaching. But how does the magnetic flux H(x) (where x is the distance of the bar magnet's center relative to the ...
51 views

strength calculations - scooter [closed]

I need some help to compute strength calculations of my homemade scooter. the plate (where you stay on) are 20mm x 500mm x 10mm In the back there are 1 wheel, and in the front there are 2 wheels ...
2k views

Question on Shockley's diode equation

We owe a debt to Bell labs with Shockley, Bardeen and Brattain for inventing the semiconductor. All have now passed on. I remember seeing my first transistor radio in the early 60's. Shockley ...
284 views

Are cylindrical mirror telescopes possible?

Is it possible to use a set of cylindrical mirrors and software correction to build an optical reflector telescope, instead of using a paraboloid reflector? By 'cylindrical' I actually mean a planar ...
269 views

The role of dark matter in black holes and star formation

In my understanding, there exists a critical mass for which a star needs to be in order for it to collapse into a black hole. This also applies to a certain critical density of gas in order for stars ...
379 views

How fast do you need to travel to go 35 light-years in 2 (apparent) years?

Time slows the faster you go, so perhaps going 35 light-years in "2 years" is actually do-able, but how fast would you actually be going? If $$t' = \frac{t}{\sqrt{1-\frac{v^2}{c^2}}}\\$$ If we say ...
220 views

Why doesn't relativistic momentum appear conserved in this frame?

Suppose I have an inelastic head on collision between two idential particles of mass $m$ that come to rest in the centre of momentum frame where relativistic momentum is obviously conserved. If I now ...
388 views

Do objects with mass “suck in” spacetime?

I don't really understand the general theory of relativity (GTR) really deeply, but according to my understanding, the GTR say that gravitation is caused by the curvature of spacetime by objects with ...
1k views

Does Newtonian mechanics predict the bending of the course of light by objects with mass?

$F=Gm_1m_2/d^2$ $F=ma$ $a_g=Gm_{other}/d^2$ In Newtonian mechanics, the acceleration of object A toward object B is not dependent on the mass of object A but on the mass of object B and the ...
144 views

Do magnetic field shift away from the coil that generates it?

I read about magnetic induction communication on Lockheed Martin's wireless mining communication system In this interview Warren Gross said: We generate a signal and send it through a loop that's ...
186 views

Helium Fusion and the Origin of Elements

In the fifth of his Messenger Lectures Feynman mentions the fact that Carbon-12 has an energy level at 7.82 MeV. He then states that this is what enables the fusion of three Helium-4 ions into Carbon ...
3k views

Calculating the Uncertainty for an Average Value [closed]

How would I calculate the uncertainty for the average of this set? $32.5 \pm 0.1$ $32.0 \pm 0.1$ $32.3 \pm 0.1$
143 views

What's the amount of deviation of cellestial orbits from perfect ellipses

It's well known that the planets don't orbit the sun in perfect circles and the characteristics of the elliptical orbits which serve as better approximations to their motion have been calculated ...
132 views

Cosmic Background Radiation: How did planets form before the CBR could reach us?

I've understood that the Cosmic Background Radiation(CBR) is an electromagnetic wave that originated from the big bang. However, we now live on a planet which that is also originating from the big ...
58 views

Hipothetical universe history with power law distributed matter [closed]

As every physicist knows that the history of universe has been dominated from light, matter and vacuum. All of three are supposed uniform distributed at very large scale. What would been happened to ...
715 views

How does Lunar Orbit Injection (LOI) work?

Just what the title states. I read that LOI was used by the Apollo program (and possibly others) to achieve escape velocity. How does it work? Is it merely a matter of centrifugal/centripetal force?
356 views

Fundamentals of Physics vs Classical Mechanics book and physics undergraduate course

There are books like Halliday Fundamental of Physics and there are books titled Classical Mechanics. To me, these two types of books seem to cover the same thing. Is there any difference? (If ...
884 views

Vortex street and Reynolds number

Its been stated in Wikipedia regarding Reynolds number(Re) that "laminar flow occurs when Re<2300 and turbulent flow occurs when Re>4000. In another wiki file related to Kármán vortex street it has ...
265 views

If I lift a submerged hose over 35 feet above the water's surface, what's in the top foot?

I've been arguing with a friend about the whole "perfect vacuum" concept. He and I agree that the most powerful vacuum pump in the world couldn't pump water more than ~34 feet above the surface of a ...
518 views

Help me understand the first equation in Landau & Lifshitz's Quantum Mechanics

While I've covered a basic course in Quantum Mechanics, I'm self-studying Landau & Lifshitz's book to help me understand what's going on. Unfortunately, I'm stuck on the very first equation in ...