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6

In the context of bosonic string theory, the ground state with no oscillator excited, has a mass, $$M^2 = -\frac{1}{\alpha'}\frac{D-2}{6}$$ where $\alpha'$ is the Regge slope, satisfying $\alpha' = 1/2\pi T$, where $T$ is the tension of the spring, and $D$ are the spacetime dimensions. It seems it has an imaginary mass (providing $D\geq 3$). You may have ...


8

In addition to the Hypnosifl's answer: in lots of modern theories tachyons do not violate causality. For example, in QFT you can choose the mass-squared term in the action to have a negative sign, that would make your field's quanta tachyons. You can check it by looking at the propagator, which oscillates at space-like region and decreases exponentially ...


10

The conclusion derives from looking at how tachyon signals would behave as seen in slower-than-light inertial frames, not from trying to consider a tachyon's own "time" (if you can call it that, since a tachyon's worldline would have to be space-like, not time-like)--basically, it's a consequence of the relativity of simultaneity. It can be shown that any ...


1

Since this is also true for sound propagating in air No, actually, it isn't. There's something crucial that you've left out. Look more closely at the 2nd postulate: light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body. Now, this holds for the emitter of the light ...


1

If one thinks in terms of spacetime and world lines, the answer is plain to see. While each particle has a world line which must be within the light cone of any event along the world line the distance between the two particles is not a physical entity and does not have a world line. Thus, it is not directly constrained by the 2nd postulate. Put another ...


1

(Edit) Notice one thing, Geremia. When two particles finally meet, or when a material body (spaceship in my second example below) and a photon meet (or even when two material bodies meet), none of them separately have exceeded the speed of light. Each of them travel at either c or $v<c$, and they simply meet on their way toward each other. So the closing ...


0

I won't try to defend Feynman's derivation, which seems strangely non-relativistic. (A similar argument is used by Schwartz in his "Principles of Electro-Dynamics".) However, I will defend the result (the Lienard-Wiechert potentials), and specifically claim that they are not in conflict with your discrete charge example, at least for the case of uniform ...


0

The "relativity of simultaneity" is not a statement about what "exists"--that is a matter for philosophy--but just about the fact that the laws of physics obey the same equations in all the inertial coordinate systems related to one another by the Lorentz transformation, which dictates that different inertial systems define simultaneity differently (i.e. two ...


0

The question seems to imply that the OP has a fundamental problem with existential questions. Those are for psychology and philosophy to ponder. Physics simply observes that one can build very precise clocks that agree with each other reasonably well under certain circumstances and not at all under others. The theory of special relativity clarifies when ...


3

First of all, physics does not ever talk about the question of existence, but about useful descriptions and predictions of observations. No physicist will ever prove to you he is not just a figment of your imagination but he can prove to you that Newton's law works pretty well for what you see. In the scientific method, a theory is indeed used until it ...


1

The aim of special relativity and of spacetime (in particular: the Minkowski space time) is not to know about what time is. Spacetime is showing a relation between space and time from an observer's view only - and this whatever time is in reality (including the question if time exists or not). The result is that time (i.e. the value measured by clocks) may ...


0

Your statement "I read that if we travel near to the speed of light,time will move slow." misses some important information. "Moves slow" relative to what? It doesn't say it, but it makes people think there is some universal time to measure the perception of time relative to. There is not. An observer in the lab (at rest in his own frame) sees fast ...


0

Your statement "after traveling one year, both rockets had reached back to Earth" means different things to the two ships, because the clocks on the two ships keep time at different rates. After the fast ship has travelled one year according to the fast ship's clock, a clock on Earth will say that 50 years have gone by. But if the slow ship returns after a ...


0

I don't think the increase in potential due to the moving charge leading to an "overcounting" IS in disagreement with Feynman's result. In fact, the "overcounting" is what leads to the 1/(1-v/c) enhancement factor in the potential equation precisely accounts for the fact that the slow moving charge has some of it's charge contributions "overcounted" because ...


0

There is no perfect evidence you can give that either you are moving or at rest becouse there is nither absolute rest nor absolute motion.I give an example, if you say that i am really at rest, then an observer on another planet moving with velocity v would measure your velocity equal to his own velocity, considering himself at rest, this is called relative ...


0

As @Chris White points out velocity (and speed) is relative, but I'll add a caveat that the Universe does have a 'special' set of local frames of reference, these are the frames in which the CMB (cosmic microwave background) appears at it's most isotropic (the same in all directions) and are called comoving frames. The speed of an object relative to it's ...


0

Velocity is always relative. So if the Earth could be considered stationary, and yet some other bodies were moving, the Earth would be again in motion wrt. these bodies. Therefore you would have time dilatation again. In Einstein's SR Theory there is no time dilatation only if everything in the Universe is still.


2

There is no true velocity in special relativity, only relative velocity between two different frames. I think part of the confusion is in the interpretation of phrases like "time slows down." Time never slows down for you according to your own watch. Consider Earth's motion around the Sun at $30\ \mathrm{km/s}$, $0.01\%$ the speed of light. As far as ...


2

Don't worry too much about the word "matter": the modern view afforded by GTR, quantum field theory and much more means that the word "matter" has become very vague. If you look up the "matter" Wikipedia page, this seems to agree that the word "matter" is very vague now indeed, so as a useful concept in physics, the word seems clearly to have passed its ...



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