# Tag Info

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It depends on how you define mass. I like to think of it as mass is just rest mass. I mean the mass you weight on a scale when nothing is moving. On different media light moves slower not because it gains mass but because of its interaction with the atoms in the media. The photons get absorbed and reemited in such a manner that when you sum the waves for ...

0

First, we will look at the energy of a free relativistic particle of (rest) mass $m$ moving with velocity $v$: $$E = \frac{mc^2}{\sqrt{1-\frac{v^2}{c^2}}}$$ where $E=mc^2$ when $v=0$. We now consider a few cases: $m\ne0$: In this case, $E\rightarrow\infty$ as $v\rightarrow c$. Therefore, a massive particle that at any point of time is moving at less than ...

2

Which is more fundamental is probably a meaningless question, but you can think of the geometric notion of a manifold (the mathematical abstraction of spacetime) as being more general than Lorentzian manifolds, i.e. ones whose metric is locally like that given in John Rennie's Answer. So one could in principle conceive of universes that were described by ...

0

Every particle needs to have energy to be a particle (if it had none it wouldn't even exist). Since energy is equivalent to mass and therefore gravitates I would say YES, all particles that have a speed less than the speed of light must also have mass. Because the speed of the particle is less than the speed of light an observer could travel with the same ...

0

fundamental is a choice dependent term, its what one considers something to be more basic. spacetime is a consequence of both constancy of c + invariance of physical laws, which describes how the general nature of laws should be, which is called Lorentz invariance. spacetime is one consequence of Lorentz invariance, there are others like energy and momentum ...

2

It is an artificial distinction to say one is more fundamental than the other. The geometry of flat spacetime is given by the Minkowski metric: $$ds^2 = -c^2dt^2 + dx^2 + dy^2 + dz^2$$ and this is fundamental in the sense that all of special relativity is described by this equation. But it is also fundamental that the parameter $c$ in the equation (which ...

1

Randall Munroe answered this question in this article Let me (try) to quote: How fast would you have to go in your car to run a red light claiming that it appeared green to you due to the Doppler Effect? —Yitzi Turniansky As expected, quoting and mathjaxing are two things that do not go together well, the rest of this post should be considered a ...

-5

Your question is about what will be the speed of elektron? Is this sum of speeds of electron and the ship, or not? If it is a sum of these speeds, speed of elektron will be more than speed of light ? If not, then there will be a time which is slowing down, so,"can the electron reach to command in circuits? Is not it? Firstly the speed of electron won't be ...

2

But if heat consists of the speed of the molecule (which is an if) then shouldn't there be an Absolute Infinity as well as an Absolute Zero? This question's "if" is not correct. Temperature (not "heat", as we use this word in a specific technical way) consists of the energy, not the speed, of particles. While these two are obviously related, it's ...

0

There is a point of temperature called Planck temperature where are understanding starts to break down. Advances in quantum gravity will help us understand this incredibly high temperature and its effects on molecules.

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Is it correct to think that the speed of light does not depend on the speed of light source because photons have no mass, so they have no the kind of inertia that is associated with mass, so they can not "feel" (acquire) the speed of the light source? No. Photons have an energy E=hf or E=hc/λ where f is frequency and λ is wavelength. The frequency and ...

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To have particles that move faster than light requires violations of Lorentz invariance. So, to investigate the effects of this, one first needs to build a model in which Lorentz invariance is not an exact symmetry that is consistent with the known physics. This has been done here. Vacuum Cherenkov radiation is then indeed a predicted effect if charged ...

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When comparing light waves and sound waves in this fashion, we need to consider what is waving. In a sound wave, the position of air molecules are waving. In a light wave, the strength and direction of the electromagnetic field is waving. This does not exert any force on air molecules (actually it does, but that force is so small, and the frequencies are ...

0

Wavelength, speed and frequency of light: V=c/lambda, v being the frequency. Light emitted or radiated from a MONOCHROMATIC source has a range of wavelengths and velocity, the frequency is invariant, The speed of light actually is slowed down. That's why we define refractive index as c/v. Example: Light will travel in air having a refractive index 1.00029 ...

5

The light from the observation point that hit the mirror and returned would be two years old by the time it returned to the observation point, but there is a very big problem with this set up. The mirror would have to be huge and curved to reflect enough light from the observation back again so that it could be seen. (Imagine tyring to brush your hair in a ...

-4

Sonic booms happen when an object crosses the sound barrier. Light leaves its source at the speed of light, so it never creates the effect, even in a circumstance where the light is strongly interacting with the air, like when there is fog.

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There are many differences between light and sound waves noted in other answers, such as the impossibility of any object with nonzero rest mass reaching lightspeed. However, there is one likeness that I don't think has been noticed yet and that is the following: a sound wave travelling at the speed of sound does not make a sonic boom! This is because the ...

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A sonic boom is produced when a macroscopic object (say, roughly: larger than the average spacing between air molecules, $\approx 3\,\mathrm{nm}$) moves so fast that the air has no time to “get out of its way” in the usual way (linearly responding1 to a pressure buildup, which creates a normal sound wave that disperses rather quickly, more or ...

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I know that when an object exceeds the speed of sound[340 m/s] a asonic boom is produced .Light which travels at 300000000m/s [much more than the speed of sound] doesn't produce a sonic boom right? Why? The answer is already in your own question: just because light is not an object. Sound "is a vibration that propagates as a typically audible ...

1

Is it correct to think that the speed of light does not depend on the speed of light source because photons have no mass In a certain sense, yes. The Lorentz transformations guarantee that the speed $c$ is invariant; an object with speed $c$ in one inertial reference frame (IRF) has speed $c$ in all IRFs. But an object with invariant mass $m$ cannot ...

1

the photons will travel at the speed of light relative to both the moving light source and an object in another frame of reference. time dilation will bridges the gap so that both may co exist.

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photons have inertia. They move in a straight line when no net force act on them,their momentum and energy also remains constant.(Frequency too). Otherwise they donot move in a straight line which can be seen in gravitational lensing.( http://en.m.wikipedia.org/wiki/Gravitational_lens ). So your reasoning is incorrect. But you are indirectly correct because ...

-3

If light(to be taken here) travels @ a velocity >c, you won't be able to see or apprehend it. So, you need an observer having a relative velocity =

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See,the philosophy of relativity is that all the observers will agree with the natural phenomena, or what is happening; like, if one observer sees two particles to collide or a lightning causing damage then all the observers will agree with the collision and lightning causing damage. But there will be disagreement in positioning and timing (and in interval) ...

0

Special Relativity is based on two assumptions: any reference frame (viewpoint) is valid and light moves at the same speed in all reference frames. In the first picture picture, light is trapped between two mirrors. If the distance is 300,000m then it takes 2 seconds to go up and down. As you can see, when the mirrors moves (with the light) it makes a ...

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What matters in principle is not if information can travel faster than c, rather if this leads to causality violations. As pointed out in this article, the Scharnhorst effect is the only known effect where light is expected to travel faster than c. However, in that case you cannot use this to create a causality paradox.

2

The local speed of light is always $c$. Local in this sense could mean that for each observer there exists a neigborhood of that observer such that, if we call $v_c$ the "observed speed of light", $|v_c - c| < a$ where $a$ is arbitrarily small. However $v_c$ is a slightly nebulous concept as beyond the inertial frames in special relativity, there ...

2

A stellar mass black hole usually forms during a core collapse supernova of a very massive star. Our understanding of star formation is that most stars will have some angular momentum, and some of this angular momentum will be passed on to the black hole which is produced when the core collapses. This means that, as you mention, most black holes will be ...

0

I'm not sure I'll get everything right, but I can give some of these questions a try. I think, in general, it's better to stick with one specific question. You asked several. and, while mass can't travel at the speed of light, light or no-mass particles can't travel slower than the speed of light, though light does slow down through a medium, but that's ...

1

Have a read through my answer to What is so special about speed of light in vacuum?. From popular science articles and TV programmes it's easy to get the impression that the speed of light is, well, just some speed. However it's intimately related to the geometry of the universe, and as such is one of the most fundamental properties of our universe. In ...

2

Jon Custer hinted at something, which I think is best explained via an analogy. Imagine you can walk along a pavement at 4mph. When the pavement is empty, it takes you an hour to travel four miles. But when the pavement is crowded, you're dodging around people and bumping into them. You're still walking at 4mph, but it takes you an hour and a half to travel ...

2

Feynman: The correct picture of an atom, which is given by the theory of wave mechanics, says that,so far as problems involving light are concerned, the electrons behave as though they were held by springs. So we shall suppose that the electrons have a linear restoring force which, together with their mass $m$, makes them behave like little oscillators, ...

2

The light emitted before the shut off of the sun will definitely reach the earth and it will take 8 minutes. Which means, you will not know for 8 minutes that the sun had stopped producing light.

-2

$c_0^2 = \frac{1}{ε_0μ_0}$ in vacuum. Permitivity and permeability (in materials) depend on frequency, in general. + In material you have $\epsilon=\epsilon_r \epsilon_0$, $v^2 = \frac{1}{εμ}$, where $v$ is a phase velocity of the light. - see http://en.wikipedia.org/wiki/Permittivity, where is a also a picture of frequency dependence of $\epsilon$.

1

Is the speed of light affected by all mediums it travels in? Yes. Including space, but don't get distracted by virtual particles. The speed of light varies with gravitational potential. Search the Einstein digital papers on "speed of light" or "velocity of light" for examples like this: Also see Shapiro's 4th test of General Relativity along with The ...

1

It is a standard exercise in most quantum field theory books that the 2-point function does not vanish outside the light-cone of a particle. Less technically; the probability that some particle $r$ away from a source feels the effect of the source quicker than light to travel would $r$ is non-zero. That is a good definition of superluminal motion. See for ...

0

Light speed being invariant is necessary for there to be "quantum effects" (if by that, you mean probabilistic phenomena). Since light speed is invariant, fixed at a rate of 1 lp per 1 tp, and no measurements are possible below the Planck scale, therefore any speed slower than c must be described as a probability at that scale. Even if you plot the ...

1

The experiments of John Bush on pilot-wave hydrodynamics have evidently proven that macro-scale replication of quantum effects is entirely possible, the Copenhagen interpretation may be in trouble, and the answer to your question may end up being "yes," after all, to the surprise of most all of us. That being said, until more experiments are done, I doubt ...

1

I don't really see a difference between the microwave measurement and the definitions of the SI units. We have Unit of time: second: The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. Unit of length: meter: The ...

0

A quantity or geometric relation being called "proper" can be understood as "referring to those participants who are thereby (directly, intrinsically) characterized". Consequently we can consider for instance the "proper length of a given train", as the distance of its two ends ("tip of the locomotive, $A$" and "ETD, $B$") between each other, provided ...

3

It's a mixture of $c_\infty = c_0 = c$ and "the question doesn't make sense". So, first, how it does not make sense: What's the "speed" of a quantum object? It has, in general, no well-defined position, so $v = \frac{\mathrm{d}x}{\mathrm{d}t}$ is rather ill-defined. Instead, we should probably look at the mass of the photon, since all massless objects ...

0

As far as I know for a photon that's moving at the speed of light (obviously) time comes to a halt and space contracts to a point, making all travels instant from its perspective. Now this is the part I might have understood wrong, so please correct me if what I've said isn't true. It isn't true I'm afraid. Imagine you were travelling though this universe ...

2

There is no contradiction. From your quote: The proper length of an object is the length of the object in the frame in which the object is at rest. and The proper time between two events - such as the event of light being emitted on the vehicle and the event of light being received on the vehicle - is the time between the two events in a frame ...

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