# Tag Info

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You are hung up on the word "experiment." Thought experiments are not replacements for empirical experiments, nor were they ever claimed to be. A thought experiment starts with assumptions about the laws of physics ("the speed of light is constant," "$F = ma$," etc.) and derives conclusions from these assumptions, analytically. The conclusions are ...

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Proofs exist only in mathematics where a theory starts with axioms and theorems are proven with a quod erat demonstrandum . Physics uses mathematics for modeling, imposing postulates that relate mathematical quantities and functions to measurements and observations. This has been an ongoing progress ever since Newton and Maxwell. Physicists work on ...

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It is a "crank and slider" or "slider-crank" mechanism.

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Quantum computing research already has improved our understanding of the basic laws of nature in what I think are several important ways. "Quantum" does not mean "microscopic" One of the types of system used to implement quantum bits (qubits) is a superconducting resonant circuit. These circuits are large enough to see with your naked eye, yet they exhibit ...

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The other answers are correct. I would like to add to them with an example. Take a spring, with spring constank $k$, with a mass, $m$, at one end and fixed to a large immovable object at the other. Let the only force acting on the mass be due to the spring and the difference from the equilibrium position to be $x$, which can be positive and negative. This ...

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Annihilation lines are spectral lines caused by the collision of particle-antiparticle pairs. In the case of $e^-e^+$, the emission is at 511 keV. (source (arXiv link)) However, because it is caused by a collision of particles, rather than an absorption-emission of a photon, the peak is Doppler broadened. This means that the peak is spread out over a few ...

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The word 'Physics' comes from the Greek Word for 'Nature' (written as 'φύση'). From Google: 'etymology of physics' physics - ˈfɪzɪks noun: physics the branch of science concerned with the nature and properties of matter and energy. The subject matter of physics includes mechanics, heat, light and other radiation, sound, electricity, magnetism, ...

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If a theory is not built on a solid foundation of (semi-rigorous) mathematics and a well-defined physical idea, the chances of it being accepted by the majority of physicists as a valid physical model are extremely small. If one wants to build up a theory of physics purely from philosphy, one will into some significant problems. After all is said and done, a ...

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If a theory never connects with experimental reality then no matter how neat it is it will eventually be dropped. This is the problem that much of theoretical physics based around the various incarnations of String Theory finds itself. A theory is in one very real sense a data compression and predictive algorithm. It needs to join the experimental dots in ...

3

I have finished reading a great book called "The Theoretical Minimum" by Leonard Susskind (a famous string theorist) and George Hrabovsky. It's about classical mechanics but mainly talks about both the Lagrangian formulation and the Hamiltonian formulation of classical mechanics. It is great for beginners in physics or just about anyone. It also reviews the ...

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A multi-body problem consisting of $N$ objects requires $N$ coupled differential equations that need to be solved simultaneously (if you want to find the objects trajectories in time with known initial positions) When you solve $x + 2y = 3 \text{and} x + y = 2$, this is what is known as mathematical analysis. The exact solution can be found: x = 1, y = ...

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The American Journal of Physics has in its archives a couple hundred "Resource Letters," which are mini-reviews of some interesting topic with several hundred references each. The recent resource letters usually group the references by their complexity, making it easy to find "simple" or "thorough" treatments of a topic. The journal is published by the ...

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Both $h$ and $\tilde{h}$ are usually called weights. Their sum, $\Delta=h+\tilde{h}$ is the (scaling) dimension of the operator, while the difference, $s=h-\tilde{h}$ is called the spin. This is due to their association with scale transformations (dilatations) and rotations, respectively. To see this, note that the dilatation operator is given by ...

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Angular speed is a vector (a pseudovector actually), and as such it changes relative to you under a rotation of coordinates. Angular speed is defined as the vector $\boldsymbol \omega =\boldsymbol r$ x $\boldsymbol v/|r|^2$, and it will change direction in an angle of $\pi$ as, expected, if your system of coordinates rotates by $\pi$ too (as it was in your ...

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In 1988, Weinberg used an equivalent relation as an historical starting point for the weak anthropic principle, in his "The cosmological constant problem". I quote, page 7: An example is provided by what I think is the first use of anthropic arguments in modern physics, by Dicke (1961), in response to a problem posed by Dirac (1937). In effect, Dirac ...

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The Landau series is less modern than many of the Griener books from the series, however its much more concise and of very high quality. If you are comfortable with the maths and just need some physics insight, Landau is a fantastic choice. If you want some more modern approaches with more thought put into application and examples, then the Greiner books ...

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Let's be specific. I don't believe Galileo ever dropped two balls from the Leaning Tower of Pisa. It was a thought experiment. Suppose you believe a 1 lb iron object will fall slower than a 2 lb one, but that two 1 lb iron objects would fall at the same speed as a single 1 lb iron object. OK, take a 2 lb iron ball. It falls at a certain speed. Now, you ...

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Good question. When we build a theory we start from a couple of assumptions that we BELIEVE that are valid, i.e. in the everyday life, and/or in previous experiments, they were confirmed. From this point we use mathematics and obtain other results. As long as we need our new results for proving even more theorems, things may be fine. But a physical theory is ...

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Newton described his theories with maths, and they worked great, until Einstein came along and found that as objects approached the speed of light, Newwton's maths broke down. Empirical evidences trumps theory.

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Astronomy talks about celestial objects (such as stars, galaxies, nebulae etc) and celestial phenomena (such as gamma ray bursts etc), their position, motion, evolution, chemistry, physics. Astrophysics is a sub-branch of astronomy to deal with physics of celestial objects and phenomena. The first encounter: A guy put microscope in front of rainbow spectrum ...

1

Richard Feynman has nice words about science. It is not bad to read chapter 3 of "Feynman Lectures on Physics". The main point of his lecture is that "there is no strict boundary between different fields of science", "nature doesn't concern what we call its parts!" So, we can't look for a line that divides celestial works into astronomical or astrophysical ...

1

Your toy theory is i) local and ii) sounds like it's counter-factual definite, that is, every measurement that could be or could have been performed would result in a single, definite outcome (roughly speaking, an object's properties are pre-existing or real). You want to know whether you can evade Bell's inequalities [that permit i) or ii) but not both], ...

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If you were looking at a real person, as you move, their images changes: from the left, you see their left side image, from the right, you see their right-side image. This phenomenon is call parallax, and is partly responsible (in addition to binocular vision), for depth perception. In monocular vision, parallax is the only cue. In a painting, there is ...

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The name you're looking for is ubiquitous gaze, or also pursuing eyes. This is an optical illusion (which means that it is exclusively a function of how our brain interprets its visual inputs) and it can be realized on completely flat canvas by appropriate handling of perspective. For more information see e.g. this HowStuffWorks page.

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A theory in physics is a way of "joining the dots" (experimentally determined facts) to make a picture. In the past, that picture was almost literally that. It was a sort of "what really happens is..." notion. That has been superseded by theories which are essentially data compression algorithms to give the most economical fit to the data. "What really ...

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A rather recent book is An Introduction to Tensors and Group Theory for Physicists. It also speaks of vectors and tensors at a good level. In my opinion it clears up the confusion physicists tend to make when speaking of these topics. Moreover the book is disseminated with examples and applications from mechanics, EM and QM, so is a great introduction to ...

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This is d'Alembert's principle. The basic, very general idea is to take Newton's second law applied to an accelerating mass, and write it as $F-ma=0$. That is, we take the $ma$ term and pretend it's another force balancing the $F$ term. This allows us to think about the dynamic, accelerating mass as if it's a static system. The $ma$ term is what's referred ...

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Well, that's the basic essence of science. Testing theory by observations and experiments. This is also how knowledge goes on, usually first assumptions are wrong or at least partially and proof of a theory to be wrong help to improve the understanding and encourage one to find a better one describing better our universe.

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This is a usual term about solving differential equations. By "analytic" (or mathematical analysis), we mean finding an algebraic expression like $y=f(t)$ which satisfy the desired differential equation. But sometimes we solve the equation only at some special points. The latter method is called "numerical". Since the Newton's law (and other principal ...

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Are theories that are based on the philosophy acceptable in Physics? Sure, but only if backed by maths and experiment. The prime example would be General Relativity, the development of which was guided by a whole bunch of principles (Mach's principle, equivalence principle, principle of covariance). In contrast, Einstein failed to develop a unified ...

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