# Tag Info

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As a theorist, one likes to invent new ideas of how things might work. One crucial component to theory-building is searching the connection to experiments: A theory is physically meaningless when we cannot test it, for then it cannot be falsified. A theorist should be able to come up with experimental tests for his theories. This requires a good ...

20

Special relativity and Lorentz ether theory (LET). From the linked Wikipedia article: Because the same mathematical formalism occurs in both, it is not possible to distinguish between LET and SR by experiment. However, in LET the existence of an undetectable aether is assumed and the validity of the relativity principle seems to be only ...

16

Because otherwise you are a mathematician. The point of Physics is to describe the nature using the language of maths, but the only ways to stay in contact with nature is to interact with it through experiments and observations. If you completely lose the ability to grasp how a process starts and develops, how much it can be influenced by external ...

14

Copenhagen quantum mechanics and DeBroglie-Bohm quantum mechanics, mathematically they are equivalent, "metaphysicaly" or "epistemologicaly" they are quite different

13

If I saw the word "amp" written as such in a paper in my field (astrophysics) it would strike me as a bit informal. I would expect to see the full "ampere" written. That said, it is rare to actually write out the full name of a unit; usually it follows a number and is given its standard abbreviation. When abbreviated to e.g. "$5\ \mathrm{A}$", I would ...

11

Technically, apparently, your teacher is correct. BIPM and NIST In the official brochure from the Bureau international des poids et mesures (BIPM, the keepers of SI units) in §5.1 Unit symbols we find: It is not permissible to use abbreviations for unit symbols or unit names, such as sec (for either s or second), sq. mm (for either mm2 or ...

10

For me, an experimentalist, the number of theoretically inclined people I have observed here, who are floundering with concepts that should be philosophy and who navel gaze about collapse of wavefunction, amazes me. I would order a course in particle physics, this will give an intuition of what it means to move in the quantum mechanical dimensions, a ...

6

As most people know, "let there be light" is a famous biblical quote, from Genesis. Now, on to the teacher's shirt. Those equations on his back are Maxwell's equations. "Let there be light" is a joke, because Maxwell's equations describe electromagnetic fields, and light is a form of electromagnetic radiation, so the equations can be used to describe ...

6

The other answers bring up some nice examples of theories in physics that are equivalent in prediction but different in interpretation. I just want to bring up a trivialized example to illustrate the flaw in the reasoning in the question. Suppose I have two theories which each predict the same quantity, say the maximum temperature at a given location on ...

5

Heavy water is an effective moderator for the production of $^{239}$Pu, which is a possible active ingredient of a fission bomb. The heavy water itself is not used in the final weapon. The allies realized that this might work - and they decided to set back any attempt by the Nazis to create a plutonium based atom bomb by depriving them of the moderator ...

4

Applications of matrices: Matrix (aka quantum) Mechanics, obviously Mechanics of deformable solids (where matrices describe stresses) Statics (most in engineering contexts), where matrices describe stresses. Symmetries (where matrices describe rotations/scaling/translations etc..) Coordinate transformations, where matrices describe the transformation a ...

3

Lie groups are fundamental for talking about anything related to symmetries in physics on a level of some rigor, and every finite-dimensional Lie group is a matrix group. Consequently, the trace as a basic matrix operation shows up anywhere where invariance on the adjoint action of the group is needed, and the matrices are everywhere. The Slater determinant ...

3

See http://en.wikipedia.org/wiki/Holographic_principle N-dimensional classical gravity can be mapped onto N-1 dimensional quantum field theory. Those worlds are very different. You'd think one or the other would have to be "true", but they're completely equivalent. If you think you live in one, the other is just a mathematical trick. But who's to say the ...

3

Well, I can make a guess for the future. Suppose that we establish that a specific string theory incorporates the standard model, thus gives the same predictions and descriptions as the standard model, and includes quantization of gravity. This theory of everything (TOE) will be identical as far as observations and predictions go as the standard model and ...

3

The reason for a collision is not because the nucleus of the atoms in both the stone and the glass 'collide', it is because the 'empty space' is actually a manifestation of the coulomb force (because of the opposite charge of both the electron and proton). It is this force that you would need to overcome in order to throw a stone through a window without ...

2

I will elaborate on @RedAct 's answer, to eliminate the coherence problem. Let the stone be a crystal. A crystal can be described quantum mechanically with a state function and there is no coherence problem as the positions of the atoms are defined quantum mechanically. Let the glass be of crystal too, again described by a single coherent wave function. ...

2

As was stated in the comments, your question can't be answered precisely. Here's my reactions, I hope it could help you : From a biological (evolution) point of view, a creature can be considered as "good" or "bad" only in a given environment. It means that there can't be a "best creature", because you will always find environments in which the "best" ...

2

I've always found the establishment of the Maxwell speed distribution from an initially random distribution interesting. Set up a bunch of particles with some initial speed distribution of your choosing, and as time goes on, collisions will tend to change the distribution to a well-known result. This is like billiards on drugs, though, and would involve ...

2

Excellent points you make. All true too. The written statement you gave is the way one should understand the rest-energy equation. To reconcile the fact that $E=mc^2$ is not always true when $E$ is the total energy (or rather, rest and kinetic energy), we write the full form of the energy equation, which is generally true: $$E^2=m^2c^4+p^2c^2$$ where $p$ is ...

2

I am not an expert on nuclear technology or weapons but the Wikipedia page on the plant and it's destruction provides some clues. Ultimately there were many potential methods that might be used to design a weapon. It was known at the start of the war that bombarding Uranium with neutrons resulted in nuclear fission which could be chained together. Heavy ...

2

Some cases with examples from my field (just because I know it best), but are applicable to others: Be aware of the observables. They provide starting and ending points for a theory. An example, if you are modelling 3D structure of proteins, you may be interested in generating contact maps (basically, all the pairs of atoms that are close to each other) ...

1

Physics literature is, usually, not strict in using mathematical rigour. The overlap of mathematics and physics is mathematical physics, and people in mathematical physics are usually found in mathematics rather than physics departments. If you look at papers published in journals in this field, like e.g. Comm.Math.Phys, J.Math.Phys., Lett. Math.Phys., ... ...

1

Humans weren't designed, they evolved in a series of small incremental modifications that each at the time imparted a little overall survival advantage. Such a method results in a effective design, but not necessarily a optimal one. This process is ongoing, and there are still modifications that could be made that would be advantageous in some ...

1

Yes if you open it :-) Joking aside... The reason why solids interact when contacted is the Pauli exclusion principle. It says that two electrons cannot fill the same place if they are in the same state. That means their energy levels are same, or more technically, their wave functions are not orthogonal. To make wave functions of the stone's electrons ...

1

I'm the developer of a project called the Physics Derivation Graph. https://sites.google.com/site/physicsderivationgraph https://code.google.com/p/physics-equations-graph/ My intention is to develop a set of derivations into a graph which would capture the current state of knowledge in Physics. Although I consider automated reasoning outside the scope of my ...

1

Consider classical electrodynamics. Here you can introduce potentials $\phi$ and $\vec A$ from which you can derive the electric and magnetic field. However they are not uniquely defined by the electric and magnetic field, but different potentials can generate the same fields. Unlike electric and magnetic field, those potentials are usually not considered ...

1

For me in my own field (optics, where one most often comes across it in engineering considerations), "amps" is common spoken usage, particularly for compound words such as milliamp or microamp. For written usage, I'm afraid I like to see the full name - it is, after all, recalling a very great man of science André-Marie Ampère. Even so, curiously, the SI ...

1

Your question and this answer are really better suited to the Meta, and I suspect a moderator will be along some time soon to migrate them. But while your question is still here ... An intuitive understanding of GR is extraordinarily difficult to attain. I've been studying GR (as an interested amateur not a pro) for a decade and I still make naive errors ...

1

According to the Wikipedia page, amp is acceptable, but is not a correct SI unit. I think your instructor is being thorough and making certain that you know the correct term to use.

1

More use of matrices: The moment of inertia tensor needed to describe the rotational motion of rigid bodies The Pauli matrices for spinn-1/2 (but that example is perhaps included in the Lie group example already mentioned).

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