# Tag Info

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1915 On November 25, nearly ten years after the foundation of special relativity, Einstein submitted his paper The Field Equations of Gravitation for publication, which gave the correct field equations for the theory of general relativity (or general relativity for short). Actually, the German mathematician David Hilbert submitted an article ...

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in the late 1960s, the strongly interacting particles were a jungle. Protons, neutrons, pions, kaons, lambda hyperons, other hyperons, additional resonances, and so on. It seemed like dozens of elementary particles that strongly interacted. There was no order. People thought that quantum field theory had to die. However, they noticed regularities such as ...

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In the following arXiv article Landau's Theoretical minimum, the author details personal experiences on being one of 42 students ever who passed it, as well as some details of the examination. He also mentions some questions in passing, from which I gather most of them were similar to the exercises and examples of his famous books. Quote: Landau gave ...

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There have only been 3 recorded deaths that occurred in space (that is, greater than 60 miles above the Earth). The crew of the Russian capsule Soyuz 11, died when their capsule depressurised during preparations for re-entry. It wasn't known they had died until the re-entry capsule was opened on Earth as communications had been lost with the capsule during ...

12

The Dirac equation implies negative energies as well as positive. This is due to energy-momentum relation $E=\pm \sqrt{m^2+p^2 }$. If we replace $E$ and $p$ by operators $E\to i\frac{\partial }{\partial t}$ and $p\to -i\nabla$ we get the Klein-Gordon equation $(\Box+m^2)\phi=0$ for scalar (spinless) fields $\phi$. The problem with this equation is that it ...

11

I will exaggerate a bit, but in physics, proof in the sense of mathematical proof is irrelevant. Even if all of Einstein's deductions of the formula were wrong, it still turns out that empirical evidence supports $E=mc^2$. Now, without the exaggeration, mathematical deduction is important in physical theories because it shows us how conclusions and ...

10

If you don't like complex numbers, you can use pairs of real numbers (x,y). You can "add" two pairs by (x,y)+(z,w) = (x+z,y+w), and you can "multiply" two pairs by (x,y) * (z,w) = (xz-yw, xw+yz). (If don't think that multiplication should work that way, you can call this operation "shmultiplication" instead.) Now you can do anything in quantum mechanics. ...

9

Newton's 1st and 2nd laws weren't particularly revolutionary or surprising to anyone in the know back then. Hooke had already deduced inverse-square gravitation from Kepler's third law, so he understood the second law. He just could not prove that the bound motion in response to an inverse square attraction is an ellipse. The source of Newton's second law ...

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I did not do more than read Newton, and a few commentators, so my insight on this is probably meager. But I am sure that you are right that the inertial frame interpretation of the first law is only a modern ex-post-facto justification for making it separate from the second law. Newton certainly never used the first law to define an inertial frame, he just ...

9

The list of topics can be found here (in Russian, of course). Nowadays students are examined by collaborators of Landau Institute for Theoretical Physics. Each exam, as it was before, consists of problems solving. For every exam there is one or several examiners with whom you are supposed to contact with to inform that you're willing to pass this particular ...

9

There were many models overturned throughout history, I will list some of the most salient ones. I will ignore the ones that predate modern science, the most prominent one being the geocentric model of the solar system, and I will confine myself to wrong ideas that were scientifically accepted as probably true at some point in history. Phlogiston: This is ...

9

Gugg's answer is thorough (+1 :-), and the answer to your question is probably "yes". However Hilbert acknowledged that he had merely added the last step to a long process and therefore that he had no claim to have invented General Relativity. It's tempting to think of GR being revealed to the world in a single stunning paper, but this isn't the way it ...

8

Tesla was an engineering giant but it is true that in most cases, he was just a crackpot when it came to theoretical physics. This "theory of gravity" is one of the major ones. It wasn't really connected with gravity - the attraction of objects to the Earth etc. with a universal acceleration - by anything else than a wishful thinking. As expected for the ...

8

Actually, heliocentric models were proposed in ancient times by many different people. The first of which was Aristarchus of Samos in about 270 BC. See the Wikipedia article on Heliocentrism for full details. One major reason geocentrism presisted, at least up through the time of Copernicus was due to the influence of the Catholic Chruch. They strongly ...

8

Physicists never work this way, this is an extreme caricature which would be a joke if it weren't tragic. Veneziano was not fitting experimental data to a mathematical function, he was using general principles to deduce what form a tree-level self-consistent scattering amplitude in a theory of infinitely many particles on straight line Regge trajectories, ...

8

Halley's method requires one to measure the timing of the beginning of the transit and the end of the transit; both pieces of data have to be measured at two places of the Earth's globe whose locations must be known. The picture by Vermeer, Duckysmokton, Ilia shows that the two places on Earth have differing locations in two different directions (the ...

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The complex numbers in quantum mechanics are mostly a fake. They can be replaced everywhere by real numbers, but you need to have two wavefunctions to encode the real and imaginary parts. The reason is just because the eigenvalues of the time evolution operator $e^{iHt}$ are complex, so the real and imaginary parts are degenerage pairs which mix by rotation, ...

8

The excitement behind various claims is somewhat excessive. First, the Mayan astronomers, see e.g. Mayan astronomy at this page, didn't use any armillary spheres or sextants as others did. Their observations were made with naked eye and they were depicting positions of planets with crosses. The accuracy of the Venus' position after a synodic 584-day cycle ...

8

This right description of multiparticle states via tensor product spaces may have been surprising for folks like Schrödinger and from the viewpoint of "wave mechanics", but it has been incorporated from the very beginning in "matrix mechanics", Heisenberg's and pals' approach to quantum mechanics. After all, the wave functions for a single particle in 3 ...

8

(Attn: non-seriousness ahead.) Since Helen, whose face could launch a thousand ships is the unit of beauty (as in a millihelen is a face that could launch one ship), perhaps Edison could be the unit of jerkishness. I base this, of course, on an entirely unbiased source.

7

Historical issues I suppose; indeed current definition of ampere is rather stupid (force between two cables in vacuum) in light of the fact it could be done with number of elementary charges per second. Candela is even worse, because it involves properties of average human eye (so called "luminosity function") -- so in principle it changes instantaneously as ...

7

First of all, it would be preposterous to think that there was a simple recipe that Newton followed and that anyone else can use to deduce the laws of a similar caliber. Newton was a genius, and arguably the greatest genius in the history of science. Second of all, Newton was inspired by the falling apple - or, more generally, by the gravity observed on the ...

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The argument is summarized on the Wikipedia page "Mass/Energy equivalence", and it goes like this: imagine a body at rest which then emits two equal photons, one to the right and one to the left. In the rest frame, the body is still at rest because the photons have equal and oppsite momentum. Shifting to a frame moving to the right. In this frame, the ...

7

You'll find Dirac's 1933 Nobel lecture on the Nobelprize.org website. The pdf is quite brief (5 pages long) and speaks on the antiproton at the end (p4). The argument is the following : In any case I think it is probable that negative protons can exist, since as far as the theory is yet definite, there is a complete and perfect symmetry between positive ...

7

Much of Physics is now far better understood than when it was first formulated. For example, if you wanted to learn Maxwell's equations you are far better off using a modern textbook than going back to Maxwell's original work. Likewise Einstein's General Relativity. You mention Newton, but he used a notation that would flummox most modern students. There ...

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which particular observation, made us think that it could be the other way around Retrograde motion must be a prime candidate. As seen from Earth against star background, Mars occasionally slows down and goes backwards. Our moon doesn't. It probably became clear to people constructing orreries that heliocentric models were enormously simpler and more ...

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A nautical mile is the length of one minute of arc (1/60 deg) along any meridian If you are navigating by measuring the angles of the sun and stars then it's a simple and obvious unit to use since it avoids a lot of calculation and it's close enough to a normal mile to be understood. It's also been an internation standard for quite a long time - unlike ...

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Einstein's proof did not rely on having a rigid body. It relies on having a body with mass (obviously). To be more clear. The paper only says body. It does not rely on any rigid body property (like the size) It does not rely on any relativistic speed or condition on the body The proof merely involve how energy is measured by an observer stationary with ...

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The first estimate of Avogadro's number was made by a monk named Chrysostomus Magnenus in 1646. He burned a grain of incense in an abandoned church and assumed that there was one 'atom' of incense in his nose at soon as he could faintly smell it; He then compared the volume of the cavity of his nose with the volume of the church. In modern language, the ...

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