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Everything posted by swansont
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The wavelength in a medium is lambda/n, where n is the index of refraction
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vuquta has been banned for repeated thread hijacking, reintroduction of topics of closed threads, and soapboxing
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Except that it does — spacetime is bent by energy.
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Electricity question about p.d. and resistance
swansont replied to Histones to DNA's topic in Physics
V=IR So, yes, though normally the cause and effect are reversed — you raise the PD in order to increase the current. -
What is the difference between mass and energy?
swansont replied to needimprovement's topic in Relativity
Yes, they are different things. I meant energy conservation does not apply between frames or with multiple frames, since it is not an invariant quantity. -
I see nothing in D H's post that would lead one to conclude this. Yes, it did. It had a 100-day travel period. SR reciprocity assumes an inertial frame of reference, and SR does not account for gravitational effects. An orbit is not an inertial frame, and gravity is present. ——— Incidentally, the measurement of time dilation has been done in an accelerating reference frame, sans gravity. Mossbauer spectroscopy using a rotating cylinder. You can solve it with either the kinematic dilation or use a potential from acceleration, like you do in GR. Either way, you get v^2/2c^2 Measurement of the red shift in an accelerated system using the Mossbauer effect in Fe-57 Phys. Rev. Letters. 4, 165 (1960) H. J. Hay, J. P. Schiffer, T. E. Cranshaw, and P. A. Egelstaff Measurement of Relativistic Time Dilatation using the Mössbauer Effect Nature 198, 1186 – 1187 (22 June 1963) D. C. Champeney, G. R. Isaak and A. M. Khan
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IIRC, two neutrons cannot form a bound state. Even of they could, it would be unstable and decay, either splitting it up or forming Deuterium. Larger collections would likewise be unstable — there would always be unoccupied proton states representing a lower energy state. They might be formed temporarily in a reaction, but would not be a final product. Neutronium can form because of the large gravitational energy involved, but this will only manifest itself when you have a lot of mass.
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Have at it. (Moved this to a new thread so spammer could be banned)
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What is the difference between mass and energy?
swansont replied to needimprovement's topic in Relativity
! Moderator Note I don't think you've gotten bad information here, so if you are feeling frustrated with the concept, please don't take it out on others Yes; that's not being contradicted here. Mass is a form of energy. Photons are something else that can have energy. -
Why do the earth spin araund the sun?
swansont replied to jonsson's topic in Astronomy and Cosmology
No, not really. It's not like all of the rotation is along parallel axes throughout the universe. That's not even true in our solar system. -
If the supernova releases more energy than the gravitational well's depth, then it won't have to re-collapse, but even if it is less, there will be a distribution of energies given to the particles. Some would have enough energy to escape, and we know that a supernova leaves behind a core. The gravitational potential energy of a uniform sphere is -3/5 GM^2/r, and even though the sun isn't uniform, this gives us a starting point. This potential energy is of order 10^41 Joules. if you took the mass of the sun as Hydrogen and converted it all to Helium, you would release ~10^45 Joules, so it seems reasonable that a supernova of a sun-like star is capable of blowing itself apart.
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It will have a slightly smaller energy, because of the energy it has transferred to the target, which is p^2/2m, and since the photon's momentum is E/c, the energy reduction is E^2/2mc^2, so the fractional reduction in the photon's energy is its energy divided by twice the mass energy of the object it hits.
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However, the effect is noticeable on smaller objects — the forces and torques on satellites and asteroids has a measurable effect. http://en.wikipedia.org/wiki/Yarkovsky–O'Keefe–Radzievskii–Paddack_effect http://www.sciencemag.org/cgi/content/abstract/131/3404/920 http://repositories.lib.utexas.edu/handle/2152/6623
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The Crookes radiometers you can buy use heating, not radiation pressure — they rotate the wrong way for radiation pressure to be the cause. Absorption gives a momentum kick of p = E/c to whatever absorbed the photon. But if it's reflected, it gets an additional kick, since the photon goes back in the direction from which it came. This second kick will be slightly smaller since a tiny amount of momentum and energy are transferred to the target. But as an approximation, it will be 2*p
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Incomsistent results of MMX and the Earth's Totational Sagnac
swansont replied to vuquta's topic in Speculations
c being constant in all frames refers to inertial frames while the Sagnac effect is due to an accelerating frame, so consistency isn't expected. No competent physicist expects to measure rotational Sagnac using a MM interferometer; the null result from that experiment was not an attempt to measure rotation. -
And because of length contraction, you won't have clocks in agreement at other points — clock synchronization uses d=ct
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! Moderator Note This is ridiculous. Thread closed
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And you can't slow the spin down. It's intrinsic and quantized. You're stuck with it. The best you can do is arrange things so all the parts add to zero.
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What is the difference between mass and energy?
swansont replied to needimprovement's topic in Relativity
IOW, if spacetime is flat, energy is conserved, but once you have to go to another frame of reference (because of curved spacetime), energy will not be conserved — it is not invariant between frames. -
Solar sails also try and leverage the fact that reflection gets you about twice the momentum kick as absorption.
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Yes, we have been able to reach speeds to make these measurements, since we don't rely on human perception or other poor instrumentation like that. We use e.g. atomic clocks, which are sensitive to nanosecond-level amounts of time dilation. That permits us to see relativistic effects for speeds attainable with planes, trains and automobiles.
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When you divide by the area the earth presents to the sun, you get a pressure about 5 microNewtons per square meter.
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What happens if you scale by area instead of population? Transportation, among other aspects of infrastructure, has an area dependence.
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Even if it was a natural phenomenon, we still should be able to account for where the energy goes and why temperatures are increasing. You can't simply dismiss it as natural and be done with it. It's still a science problem, demanding a scientific answer. But "it's natural" lacks one.
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It's a different question to ask if it can be quantized as opposed to must be quantized. The values it depends on, classically — heat and temperature — are not. But in terms of number of states available to it, the answer is yes.