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    Do tidal forces mean the Equivalence Principle is BS?

    The equivalence principle is usually stated locally and that's where it's valid. It's not BS. The constancy of the speed of light is also only valid for a local observer and nobody says it's BS. If you were a zero-dimensional person you wouldn't be able to tell the difference between gravity...
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    Is physics about to hit a dead end, or am I jumping to conclusions?

    Oh I understand what the landscape and the anthropic principle are alright. That's why I call it nonsense. I never said I propose to stop all research. I just propose presenting what I just mentioned as nothing more than wild speculation that changes the way we do science, especially to the...
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    Is physics about to hit a dead end, or am I jumping to conclusions?

    Changing the definition of science and making unfalsifiable or tautological claims is not revolutionary. THAT is nonsense.
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    Is physics about to hit a dead end, or am I jumping to conclusions?

    I think theoretical physics has been more or less stuck at a dead for a long time, probably since the completion of the Standard Model in the 70s. And I'm not only referring to String Theory, but to most competing programs as well. However, I also think that's a good thing. The same happened...
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    An online reference for SM Feynman Rules?
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    Explain electromagnetic attractive and repulsive force via virtual photon exchange?
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    Faster than light?

    1) & 2) There's no frame of reference associated with something travelling at c. So you can't really say what they do relative to each other.
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    The end of uncertainty

    Apples are made of a gazillion particles. Electrons are not. Like ZapperZ says, scale has nothing to do with it. What makes an apple classical is decoherence. It doesn't matter if apples or electrons seem bigger or smaller to us.
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    Inline PhysOrg.Com Links

    I hope we don't get the kind of comments they get in their articles with that...
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    The Photon's Perspective Taboo

    OB50, how do you reconcile the existence of a rest frame for a photon with the second postulate of special relativity?
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    Coordinate Transformations in GR

    I'd say the reason is that one of the most important aspects of GR is the principle of relativity and the general covariance of its equations. Basically, that the laws of physics should be the same in any reference frame so naturally you'll want to perform coordinate trasnformations.
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    Problem from Intro to Quantum Mechanics by Griffiths (I know near nothing)

    use substitution and then:
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    Spin rotaion

    Well, yeah, obviously phases are physically meaningless. But the way it's usually presented to the popular audience, it's assumed that the ket is an object you can look at.
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    Heisenberg's principle

    It's the inequality obeyed by the deviation of the energy distibution of the states forming a wave packet that evolves with a wave function \Psi(t). In other words, a relation between the energy distribution of a wave packet and the characteristic time it takes to deform. The interpretation...
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    Coordinate Transformation

    Like they said before me, x^\mu are just the components of the four-vector. You need a basis to define the vector. The linearity comes from the fact that the new basis is a linear combination of the old basis. So the components of four-vectors transform in the same way...
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    Coordinate Transformation

    What difference would it make whether it's linear or non-linear? F_{\mu\nu} is a tensor and as such it follows the usual transformation rule for tensors: \displaystyle{ F{'}_{\alpha\beta} = \frac{\partial x^{\mu}}{\partial X^{\alpha}} \frac{\partial x^{\nu}}{\partial X^{\beta}}...
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    Spin rotaion

    I suppose that you're interepreting a revolution for spins as a rotation by 4pi? :wink: He's asking the reason why a particle's spin doesn't remain invariant when you rotate it by 2pi using the corresponding rotation operator. The easy answer is that spin doesn't live in normal 3D space so it...
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    Einstein Tensor

    That term doesn't appear in the equation. What you have is: g^{cd} g_{ab} Where the left metric tensor is contracted with the Ricci tensor. Remember not to repeat indices more than twice. It messes up the summation convention. Use different letters like George Jones said.
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    Tachyons and photons

    Yes, I see your point. Knowing about the future would violate causality because the cause of the emission would be in the future. And if there were a single emission I'd know about the future. But in the article I cited, a single event triggered an emission in a range of tachyon momenta. So...
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    Tachyons and photons

    I don't think that's a violation of causality because there's no causal loop. Yes, I will know about my own future, but I'd be unable to alter it. If I tried to send an FTL signal back to you (so it arrives before the atom decays and you can prevent it from happening) the obstacle would block it...
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    Tachyons and photons

    Actually, I still don't see the problem if tachyons were to transmit signals FTL. The whole point of banning FTL communication is that in some reference frames the causal order would be inverted. If this reinterpretation prevents that from happening, and observers would only disagree on the...
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    Tachyons and photons

    They used the same interpretation when they were quantized in".
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    Tachyons and photons

    But using this reinterpretation they can't be used to transmit information. A detector of tachyons from the future would actually emit tachyons instead of detecting them. I didn't invent this. It was actually published by Nature, American Journal of Physics and Physical Review...
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    Tachyons and photons

    Are you sure of this? It was my understanding that a tachyon sending information back in time can always be reinterpreted as an antitachyon sending information forward in time because of the impossibility to distinguish between creation and annihilation of tachyons. This reinterpretaion done by...
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    Argument with physics teacher

    More like 1897 before the Larmor formula. That's not even the Bohr model. In his teacher's model the electron would radiate and lose energy as it orbits the nucleus. No centrifugal force would be able to keep it from crashing. That guy shouldn't be a physics teacher.
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    Gravity = Megnetic Attraction?

    If you look at the the magnetic component of the Lorentz force you'll see that it's different from the gravitational force and it depends on the velocity of the particle. So, the answer is no.
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    Green's Fucntions in Cylindrical coordinates

    You can find it by solving the Laplace equation for the electric potential using separation of variables in cylindrical coordinates and then using a unit charge for the boundary conditions. The Green function should be a combination of Bessel functions for the radial coordinate and sines and...
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    Relativistic quantum mechanics and causality

    This looks like a very interesting discussion but I'm still an undergrad and haven't studied QFT yet, so I'm having problems following it. Let me ask a couple of questions: Is there agreement on the fact that non-QFTs, i.e the Dirac and Klein-Gordon equations for particles interacting via...
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    Relativistic quantum mechanics and causality

    I was told in class that both the Dirac equation and the Klein-Gordon equation violate causality, even though they're relativistic invariants, and that this wasn't surprising because the 2 postulates of special relativity don't imply causality. Is this true?
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    How are the formulas in QM derived?

    Well, linear algebra for one. Then calculus. And Fourier analysis. And usually solving the Schrödinger equation means solving a differential equation.