Hot answers tagged

52

After they told me about their impressive "LHC Olympics" in which physicists (often hardcore theorists) were reverse engineering a particle physics model from the raw (but fake) LHC data, I proposed the same idea in a circle of physicists at Harvard, including Nima Arkani-Hamed, sometime in 2005 and we have worked on that LHC ideas in some detail. We were ...


32

Let me first mention that the LHC is in a way a text book experiment: you have a very good control over the experimental conditions and you can repeat your experiment as often as you like. You have, in way, full control over the signal. Results are reproducible in that you just redo the experiment. LIGO is "just" a detector: In particular, you have ...


9

You cut off the sentence that tells you what the numbers mean. "All uncertainties define a 90% credible interval". Crudely speaking, it means that there is a 90% probability of the parameters lying in the quoted range, with the most likely estimate being the headline number. It doesn't really make sense to translate these into Gaussian sigmas (it would be ...


9

Fake event injection is only one of several schemes for "blind" analysis. Other blinding schemes involve manipulating some parameter of the data as show to the analysis team by a reversible transformation of some kind, multiple independent analyses, and complete analysis dry-runs on simulated data. The thing to understand is what purposes are served by ...


9

The LIGO experiment consists of detecting individual events with a characteristic form that take place in less than a second. Given the high stakes and that conceivably, only one or a few events might be detected, it made sense to have dry runs of thedetection protocol to assess it's reliability and fidelity. The LHC monitors millions of collisions per ...


4

I don't think there has even been observational evidence of Kerr-specific effects. While gravitational lensing is well known these days I don't think any of the objects studied have been rotating fast enough for the difference between the Kerr and Schwarzschild metrics to be apparent. Well, not in lensing anyway - Gravity Probe B did measure frame dragging. ...


4

Experimental results can deviate from ideal. Outcomes depend sensitively on small differences in mass and alignment, and the extent to which kinetic energy is conserved. Alignment of non-identical balls is much more difficult. Considerable effort and expense may be required to achieve reliable results. The following sites agree with this view : https://...


4

Spin was assigned to elementary particles so that conservation of angular momentum would hold in the quantum mechanical framework of elementary particles and nuclei. The Stern–Gerlach experiment involves sending a beam of particles through an inhomogeneous magnetic field and observing their deflection. The results show that particles possess an ...


4

Online data analysis is cursory analysis done as the data is collected. It is often used for the purpose of selecting which events to save to disk or tape to be analyzed later (an event "filter"). Given that the current CERN experiments will be taking, in the next run, data at rates exceeding a terabyte per second, this notion is essential. In fact, the ...


3

The very flexibility that you mention in your post is a bit of a problem in an experimental context. In order to understand the signal that a Cf-Be calibration source would generate in your detector and tease useful information out of it, you're going to have to model all three channels that generate neutrons and the gammas that escape the source. This means ...


3

Electrons And Spin From Scientific American Unfortunately, the analogy breaks down, and we have come to realize that it is misleading to conjure up an image of the electron as a small spinning object. Instead we have learned simply to accept the observed fact that the electron is deflected by magnetic fields. If one insists on the image of a spinning ...


3

The trick is to convert to radians. You're mixing radians and degrees, which I think makes your error too big by $180/\pi$.


3

@CuriousOne is approximately right. Others have thought of it, the url below is a paper from one who concludes with slightly more careful calculations (but still relatively straightforward) that the most one could get in terms of a continuous power from it is 50 MW, and typically a lot less. 50 MW is 5% of a large power plant. http://www.electrostatics.org/...


3

Did you notice the caption of the table?


3

As you suggested in your comment, the $\mu^-$ and $\mu^+$ that stop in matter do not have the same lifetimes. The $\mu^+$ come to rest between the atoms of your stopper (eg: scintillator?) and decay into $\nu_{\mu}e^+\nu_e$ with the standard 2.2 usec lifetime. However, the $\mu^-$ get captured into Bohr orbits about the stopper nuclei. The $\mu^-$ then ...


3

One can calculate a lot more in quantum field theory if one goes beyond asymptotic computations into thermal field theory. I recommend that you look at the book ''Nonequilibrium Quantum Field Theory'' by Calzetta and Hu.


2

The small number of "conceptually independent types of processes and calculations" is exactly a symptom of the theory's being fundamental! Even in classical physics, all calculations could have been mathematically reduced to the calculation of the final state that evolves from an initial state (or a state that is stationary etc.). In quantum mechanics, this ...


2

The mirrors in Advanced LIGO are not cooled. Future detectors may utilize cryogenic temperatures to reduce thermal noise, but we aren't there yet.


2

Water evaporates if it has a higher temperature and or if the humidity of the air isn't at 100%. This is a nonlinear process as the vapour pressure of water is nonlinear with temperature and the rate of evaporation is linear with partial pressure difference (the partial pressure in the water is equal to the vapour pressure). The evaporation reduces the ...


2

From the links I provided in the comments below your question it should become clear that entropy "meters" do not exist, you calculate it from other measured variables. If this does not satisfy you requisites for an experimental measurement, then your conclusion that the claims are only theoretical is justified. However, having said that, with your ...


2

Charged particles can't have Majorana masses of any type because they would violate the charge conservation law. The Majorana mass is really a term that is converting a particle into its antiparticle. It implies that the particle must be considered "physically indistinguishable" from its antiparticle. The Majorana mass term violates the lepton number or its ...


2

This is a physics question and answer site. Laws in physics are a distillation of experimental observations, and a foundation stone in the theories, mathematical models, built up in order to describe data and predict new behaviors. In a very real sense they are the physics axioms that connect mathematical formulae to physical measurements. History of ...


2

Please explain by what means electrons extraction can be done. Hot enough plasmas have all the electrons in the plasma leaving the nuclei positive. How person can focus activity on single atom (from precision point of view) to do so? One cannot deal with individual atoms. It is a statistical phenomenon and one can get a beam of ions without any ...


2

This shows a schematic illustration of the Pound-Rebka experiment: At the bottom we have $^{57}$Fe source that emits gamma rays upwards with an energy of 14.4 keV. The frequency of the gamma ray is $3.48 \times 10^{18}$ Hz, but let's just call this $\nu_0$ to avoid messing around with figures. The gamma ray travels upwards, and as it travels it is red ...


2

We don't need to separate electrons out in order to observe them. The structure of an atom, as revealed in electron transitions (atomic spectroscopy) is clearly based on orbitals at specific energy levels, with a two-electrons-per-orbital limit. And, the collective behavior of unpaired electrons that gives rise to ferromagnetism, and subtle spectroscopic ...


1

Materials that seem homogeneous often have internal strains, or voids, or even inclusions. Under stress, rather than uniform deformation (bending), those flaws may undergo brittle fracture, or stretch excessively, or become chemically active. A cosmic ray can create internal damage, a particle decay track. So, after some kinds of handling (bending, ...


1

The meter will give a value less than what? Less than +300mV or less than -300mV? What voltage reading the meter gives depends on what type it is and what setting is used. Perhaps the book is suggesting that the meter is on an AC setting and will show the RMS voltage. This is $\frac{300}{\sqrt2} mV$ for a sinusoidal waveform, which is less than the peak ...


1

In reverse order, and to echo CuriousOne's comments, as you know already I'm sure, the second "law" isn't a law, more a general principle regarding the probabilities of the existence of certain quantum states. My biggest question about this is about proof through experimentation. If one were to prove a law through experimentation, what is deemed a ...


1

I know that you are explicitly asked about not equipment related answers. But when I learned something from experimental physics then that you should always consider equipment flaws. I could imagine a scenario where the events on which you trigger to start/stop the clock have different rise times depending on where they take place in the scintillator, ...


1

More of an extended comment, but here are two thoughts: 1) I'm not sure I completely agree with the statement Textbook discussions of the Second Law of Thermodynamics (SLT) often stress that this law applies only to "closed systems". Or, differently stated: if the system is not closed, its entropy can go down. Okay, this is correct, of course, ...



Only top voted, non community-wiki answers of a minimum length are eligible