# Tag Info

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Why do you say that the box doesn't shrink in a uniform gravitational field? The photons outside the box also experience the field! Thus, the photons "falling" on top of the box are slightly blue-shifted while the photons hitting it from below are slightly red-shifted. And the box squeezes in the same amount as its rocket analogue.

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@AlfredCentauri The condition of locality conventionally only requires a locally constant gravitational field during the time the experiment runs. For a stationary rocket in the gravitational field of e.g. the earth, this condition is perfectly satisfied, without requiring the gravitational field to be uniform throughout space. Clearly, the stars are not ...

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The problem only arises when considering energy eigenstates, which are completely delocalized. One of the tenets of the equivalence principle is that the equivalence between gravitational systems and accelerated frames is only true locally. All that the equivalence principle states is that there is a neighbourhood in any point small enough such that the ...

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This is an experimentalist's answer and yes, accelerated charged particles either in stable circular orbits or in linear acceleration do radiate. Classically, any charged particle which moves in a curved path or is accelerated in a straight-line path will emit electromagnetic radiation. Various names are given to this radiation in different contexts. For ...

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If you suspended a jellyfish in a bucket of water and accelerated the bucket upwards at say 100g, the jellyfish will remain stationary relative to the water. IE the jellyfish will not experience any acceleration, it is weightless as it normally is. All of the additional forces due to acceleration are imparted on the buckets bottom and sides. So by this ...

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The Weak Equivalence Principle, or WEP for short, states that under identical initial conditions, the motion of particles of different masses in a given gravitational field is identical. Or in other words, there are no physical effects that depend on the mass of a point particle in an external gravitational field. This is just the equivalence between the ...

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All the previous answers are correct. Let me add just some mathematics. Take a look at the equation for Fermi normal coordinates, for example in the original article by Misner and Manasse [1] or here. These coordinates provide an example of a local Lorentz frame, that is, a reference frame with a locally flat metric. As you can find in these articles, Fermi ...

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First of all your statement "because pseudo forces can (locally) be interpreted as gravitational fields and it is therefore impossible for the local experimenter to decide whether he is moving, or being accelerated, or motionless." is incorrect. I will paraphrase MTW's 'Gravitation', section 13.6, page 327: We have a very small man inside a very small, ...

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I'd like to answer as far as buckets are concerned but leave the CMBR to a cosmologist or a real relativity-ist. Mopping the floor up after the chaos left by my children, I think of myself as an expert on the former! In GR it is immaterial whether one describes a "force" as an "inertial force" or a gravitational field. All one "knows" is whether one is ...

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GR treats all free-fall reference frames as equivalent. Also, a reference frame is a local thing, as you said. A rotating bucket filled with water is a non-local thing. Anyway, you can go down a very deep rabbit hole by looking up Mach's principle, but I'm not sure I would advise this (I think it may be antiquated). Finally, assuming the universe is ...

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Is teleparallelism an alternative to the introduction of a metric? Teleparallel gravity still comes with a metric - just take the tetrad field as orthonormal basis and there it is. The main difference between GR and teleparallelism is that the former uses curvature, the latter torsion to model gravity. According to Kleinert, there's actually a type of ...

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There is a sense in which metric theories of spacetime are "general". I simplify to four dimensions, but the argument generalizes to higher dimensions. Consider a particle whose path is parameterized by four coordinates $x^{a} = (t(s),x(s),y(s),z(s))$. We wish to describe the motion of the particle, given that at s=0, each of these functions has a known ...

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A clock near the surface of the earth will run slower than one on the top of the mountain. Rather: the geometric (and kinematic) relations between two (or more) given, distinct, separated clocks must be determined and taken into consideration in order to compare intervals (from any one indication to any other indication) of each clock to each other, on ...

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What if I don't look at other stars, but the microwave background radiation, the "echo of the Big Bang", as it is sometimes called. Doesn't that give me a clue about my "absolute" motion, i.e. relative to "the universe"?

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The Earth orbital speed could be increased by bringing the Earth closer to the Sun. That would require orbital maneuvering of the whole planet (planetary retrograde burn to go into a transfer orbit and another retrograde burn to circularize the orbit) which would be felt by the Earth population as acceleration. After the final orbit would be achieved, the ...

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The faster the earths orbit, the further it would go form the sun until it reached "escape velocity". The speed itself would make no difference to you anything you can feel. However, the distance from the sun, you would feel. The sun rotates around our galaxy core at one tremendous speed (and us with it), and you don't feel that.

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First, it turns out that there are no uniform gravitational fields so the equivalence principle holds only locally. But, for the sake of argument, let's assume that a uniform gravitational field can exist. Now, consider the situation where an astronaut is in a rocket and the rocket's accelerometer reads a constant, non-zero value. According to the ...

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The stars you see at night are blue-shifted partly because they gain energy as they fall down Earth's potential well. Sorry, but Einstein wins this round :)

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English mathematician Sir Isaac Newton published Principia, which hypothesizes the inverse-square law of universal gravitation. He deduced that the forces which keep the planets in their orbs must be reciprocally as the squares of their distances from the centers about which they revolve. If he was reasoning this way about forces (F), he was also doing so ...

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