# All Questions

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### Using delayed choice interference experiments as a computing device

I had an idea how to design a "quantum computer": How about designing interference-experiments where the design of the experiments itself represents algorithmical or mathematical problems that are ...
325 views

### Unusual particle effects at CERN

In 2010 there were press reports that CERN had identified unusual properties in particle behavour in collisions. One link here. Here is a partial quote: "In some sense, it's like the particles talk ...
617 views

### Is relativity necessary for the existence of life?

If the universe didn't have the relativity principle, would it be able to support life? Life consists of very complicated organisms. The operation of these organisms depends on the laws of physics. ...
382 views

### What is the quantum / Berry-Pancharatnam phase for a spin-j state with z-component m?

Quantum phase arises when a spin-j state is sent through a sequence of transitions that return it to its original position. For example with spin-1/2, a state picks up a complex phase of $\pi/4$ when ...
20k views

### Laptop self charge cable? [closed]

Ok this is a really silly question, but I was really curious as to why this wouldnt work. I'm just starting my electricity and magnetism course, so I really dont know much about electricity. I know ...
366 views

### What would the electromagnetic field of a massless electron look like

The Standard Model gives non-zero mass to the electron via the coupling to the Higgs field. Issues of renormalizability aside, this is fundamentally unrelated to the fact that the electron couples to ...
2k views

### Can anybody provide a simple example of a quantum computer algorithm?

Does anybody give a good textbook description of a quantum computer algorithm and how its different from an ordinary algorithm?
2k views

### gauss vs mW/cm^2: same thing?

A friend of mine is concerned about electromagnetic field negatively affecting their health and got a "DMF meter" to measure the field strength in their house in units of milliGauss. They are trying ...
187 views

### Non Classical mechanic answer to : length of time a thrown object spends in rest before falling down? [closed]

When an object is thrown upwards, when it eventually comes to rest and starts falling, for how long is it stationary? What about an particle in electric field having an initial velocity towards it's ...
753 views

### Relativistic object impacts the earth

A familiar trope within science-fiction is that of a large relativistic object hitting a planet such as the earth. This is normally an interstellar spacecraft or a kinetic weapon with a mass in the ...
478 views

### Is there an explicit angular momentum in Maxwell equations?

Electromagnetism implies special relativity and then the universal constant "c". And if we set c=1, the coupling constant has units of angular momentum (so in relativistic quantum mechanics we divide ...
3k views

### Visualizing Electromagnetic Waves in 3D Space [closed]

I did one module of physics for my GCSE one year ago which taught me about transverse EM waves & the EM spectrum, but since then, I do not understand how a wave would move in 3D space. Can someone ...
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4k views

### Zero probability of finding an electron in the nucleus

One and the same electron in a p orbital and taking part in a common π (pi) bond has two lobes visualized as connecting through the nucleus. There is however zero probability of finding an electron at ...
1k views

### How do Dirac fermions arise in graphene, and, what significance (if any) does this have for high-energy physics?

Graphene has a honeycomb lattice (in the absence of defects and impurities). By considering the low-energy limit of the half-filled Hubbard model used to model the strongly interacting electron gas we ...
560 views

### Evidence for black hole event horizons

I know that there's a lot of evidence for extremely compact bodies. But is there any observation from which we can infer the existence of an actual horizon? Even if we are able to someday resolve ...
222 views

### How small of a depletion signal can the best modern mass spectrometers detect?

So I know the answer to this question varies widely across mass spec techniques, masses, and, of course, budgets, but my question is about the best case scenario for all of these variables (although ...
489 views

### Electrical eddy current visualization or simulation

Eddy currents are induced in a metal plate when it experiences a changing magnetic flux. Is there a realistic visualization or simulation of eddy currents available? The only picture I found, on ...
5k views

### What is tension in string theory?

One often hears the words "string tension" in string theory. But what does it really mean? In ordinary physics "tension" in an ordinary classical string arises from the fact that there are elasticity ...
418 views

### Space-time filling D-branes in Type I superstring theory

It's known that gauge group for type-I superstring theory should be SO(32) for chiral anomalies to cancel. This is realized by presense of 16 space-time filling D9-branes, together with 16 space-time ...
3k views

### Is there a maximum possible acceleration?

I'm thinking equivalence principle, possibilities of unbounded space-time curvature, quantum gravity ...
263 views

### Basic Spin or Double Cover Experiment

We know that Spin is described with $SU(2)$ and that $SU(2)$ is a double cover of the rotation group $SO(3)$. This suggests a simple thought experiment, to be described below. The question then is in ...
759 views

### Optical explanation of images of stars?

Very often when viewing pictures of the cosmos taken by telescopes, one can observe that larger/brighter stars do not appear precisely as points/circles on the image. Indeed, the brighter the light ...
449 views

### Is Time Significant in the Double Slit Experiment

When doing the classic double slit experiment is the time between emitting photons significant at all? Say, a single photon is emitted, the scientist waits T seconds, then emits another photon. Are ...
958 views

### Misused physics analogies [closed]

Have you noticed that many questions and misconceptions in physics arise due to misuse of analogies, which were invented to "explain in simple words" some physical phenomena? For example there is a ...
85 views

### Why is $\rho_m$ proportional to the deviation from critical temperature in critical phenomena?

In Peskin and Schroeder's chapter 12 about the renormalization group, it is stated that the parameter $\rho_m=m^2/M^2$, where $m$ is the mass and $M$ is the renormalization scale, is proportional to ...
1k views

### How are the calories in food calculated?

This is intended to be a fun question. Calorimetry used for calculating the heat generated from chemical changes has been around for centuries, however, I suspect the process for calculating food ...
222 views

### energy field of a newly created particle

The question is: How to calculate the energy of electrostatic field of a newly created particle. The usual formula involves integration 'all over space'. According to GR I believe that 'all over ...
308 views

### Gamma-ray bursts: A supernova connection

Can the gamma ray bursts connected to a new supernova prevent the black hole from forming? Quoting the abstract of the linked reference "...has become clear that probably all long GRBs are ...
4k views

### What is the world's biggest Schrodinger cat?

How big is it by a truly quantum measurement? I am thinking of comparing Science magazines "Breakthrough of the Year" (BYOT) with the Zeilinger buckyball. The BYOT is a piezoelectric mechanical ...
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### Reorienting a sensor axes according to particle displacement directions

Consider a sensor which is located inside the solid substance. This sensor is capable of detecting the substance oscillations along each of the three axes (usually orthogonal, but generally, any ...
1k views

### When is the force null between parallel conducting wires?

Consider two long wire with negligible resistance closed at one end of the resistance R (say a light bulb), and the other end connected to a battery (DC voltage). Cross-sectional radius of each wire ...
507 views

### Why is honeycomb lattice the ideal shape for solar cells?

Quoting this web site (which I haven't thoroughly verified for credibility yet): The material is graphene, also known as graphite, a naturally-occurring mineral that forms a one-atom-thick ...
731 views

### does current product push or pull effect [closed]

Since i was a child i was told that current has pushing or pulling effect. However, i never got a dc shock and i don't remember ac shock (i got it very long before). The day before yesterday, an ...
7k views

### Where's the best place to add weight to a Pinewood Derby car?

A little background: a Pinewood Derby car is a small wooden car that races down an inclined track, powered only by gravity. You are allowed to add weight to the car up to a certain limit. Here is a ...
554 views

### Big Bang snuffed by a black hole?

Wasn't the density of the universe at the moment after the Big Bang so great as to create a black hole? If the answer is that the universe/space-time can expand anyway what does it imply about what ...
183 views

### Size of our inflationary bubble

Once again, an attempt to connect popular and real science. I have read that inflation implies our universe should look flat, or really close to flat, because our observable universe is only a really ...
355 views

### Terminal velocity for falling in a shaft

One falls slower in a mine shaft than in free air. This is due to collisions with the walls. How should one model the terminal velocity in the presence of such collisions?
2k views

### Simple models that exhibit topological phase transitions

There are a number of physical systems with phases described by topologically protected invariants (fractional quantum Hall, topological insulators) but what are the simplest mathematical models that ...
638 views

### Thermal radiation spectrum of a blackbody

Why is it that thermal radiation of a black body usually described by a spectral distribution function rather than an intensity vs frequency curve? I have a vague explanation for this: Any measured ...
793 views

### What does second quantization mean in the context of string theory?

String field theory (in which string theory undergoes "second quantization") seems to reside in the backwaters of discussions of string theory. What does second quantization mean in the context of a ...
202 views

### Cooling via the quantum vacuum?

Suppose you had an isolated cloud of gas at a low temperature in a vacuum with no external sources of radiation (e.g. no CMB). The gas would clearly cool via the emission of low-energy photons. But ...
3k views

### What is the statistical likelihood of getting a job as a theoretical physicist? [closed]

I'm considering taking a hobby (taking math and physics classes, reading attempting to read arXiv papers) and turning it into a career change. However, I have 3 kids and a wife who doesn't make a lot ...
849 views

### Should a neutron fall faster than a proton?

If you drop a proton and a neutron in a gravitational field, they both fall, but the proton has a charge and accelerating charges radiate energy, so that leaves less kinetic energy for the proton and ...
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I'll be teaching a seminar for first-year undergraduates next year. The idea of my university's first-year seminar program is to expose students to exciting ideas and important texts in a somewhat ...
1k views

### Vortex in liquid collects particles in center

At xmas, I had a cup of tea with some debris at the bottom from the leaves. With less than an inch of tea left, I'd shake the cup to get a little vortex going, then stop shaking and watch it spin. ...
371 views

### Estimating Partition functions

I have a finite state ensemble with an energy functional (you can think of it as an ferromagnetic Ising model if you like), and I need very careful estimates of the partition function. What methods ...
746 views

### Selection Rules in electron spectroscopy

How to derive the selection rules $\Delta L= \pm 1$, $\Delta S=0$ for electron spectroscopy?
The textbook approach to explaining T-dualities is to show that a type of T-duality transformation "inverts the radius of the circle, that is, it maps $R\rightarrow\tilde{R} = \alpha'/R$ and it ...