# Tag Info

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If you are 1 light year from Earth, and the Earth time is 1/1/2020 12:00:01 AM, then you are seeing Earth's light that was emitted on 1/1/2019. If you are stationary with respect to Earth, then that light is "one year old" for you, and lags Earth's clock by one year. How you got there doesn't matter. If you are moving at the time of detection, then ...

2

It always takes light in a vacuum 1 year to travel 1 light year, so if you were 1 light year away from anything, including Earth, you would see it as it was 1 year ago when the light you are seeing left it. In fact since light never travels instantaneously, everything you see has already happened even if only microseconds ago.

1

There's a sense in which mass does increase with speed. However, to increase something's speed, you have to put energy into it. That energy also has mass, it also figures into center-of-mass calculations, and it's exactly equal to the mass gained by the object when it speeds up. So no scheme of this sort can work.

0

General relativity doesn't have a fundamental notion of distance, but the same is true of special relativity. The big problem being that, unlike inertial observers in special relativity, there's no "canonical" foliation of the spacetime into spacelike hypersurfaces. In other words, for some inertial observer $\gamma$ in special relativity, there is ...

0

The answer to your question is yes. You can check the Fraunhofer diffraction out of a rectangular slit. You will notice how, according to the dimensions of your slit, you have diffraction over two directions or not.

0

Actually, diffraction occurs whenever there is a spatial or temporal alteration of the shape of a wave field. However it is easier to observe if it is caused by something with small features or by something that changes very rapidly. Note that the edge of a large hole or of a razor blade is a very narrow feature, nearly infinitesimally narrow. Light ...

1

I think you took anna v's answer too literally. Atoms and molecules are not so much "single entities" as the elementary particles are. They are far from elementary. This means that, although the individual constituents like electrons and nuclei (or deeper, electrons, quarks and gluons) don't have their single-particle quantum states, they still do ...

0

The light is in principle going to be present in a three-dimensional region. In whichever directions the light is restricted, in those same directions it will subsequently spread out, and we call this diffraction. Usually we think of spreading out sideways (the transverse direction) for a beam of light. If we have a pinhole then the light spreads out in two ...

2

Colliding stars and merging black holes produce gravitational waves that we have detected. Occasionally we have been able to locate the source and correlate it with electromagnetic emission. By comparing time of arrival of the two signals and knowing the distance to the source - and knowing the composition of the intervening space - we can compare the ...

2

You need to have a changing quadrupole moment in order to see gravitational waves. A spherical explosion would not produce any gravitational waves at all, so exploding is not the answer. What you would want is to collapse the sun into two half-suns and then have the two half-suns spin around each other very rapidly. Of course, that wouldn’t conserve angular ...

1

Photons are massless elementary particles, as defined in the Standard Model, and always travel at speed c in vacuum, when measured locally. Now there are two ways to explain/interpret on this site why the speed of light itself if slower then c in a medium, and individual photons always travel at speed c when measured locally: 1. Now in a medium, it is true ...

1

Hope I understood your question. About the speed of light, since you can' t see any object travel faster than light, you cannot be observed as faster than light as well. Thus you can' t reach speeds higher than $c$. About your second question, it is somehow true that Lorentz transformation do not apply to references as fast as light is. We can see what light ...

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Additional to @Dale's answer, probably I have a picture which demonstrates that the momentum change should create a recoil pressure on the matter. Imagine the prism refraction as in the picture: Here, the exiting light is turned compared the entering, so its momentum changes as it passes the prism. So, the prism should experience an oppositely directed ...

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Light (photons) always travel with the speed of...guess...light! To put it lightly. When photons travel through a medium, the effective speed associated with the effective momentum of light can be less than c. How do photons interact with the medium? See for example the comment made to the answer above in which it is stated that photons in a dielectric ...

0

You can not prove all of SR. You can derive the Lorentz transformation using those two postulates plus linearity. The Lorentz transformation then gives you time dilation, length contraction and relativity of simultaneity. But this is not all of SR. You can not get the relativistic formula for momentum and the well-known formula $E=mc^2$ without also ...

0

There is no mistery here: the solution of Maxwell equations include all the space $\square A = j$. All points of space take part in forming a wave. The problem arises when we think of photon as of a localized point particle which is obviously not always a fruitful approximation.

87

This question is a very long-standing one, and is sometimes known as the Abraham-Minkowski controversy. Both Abraham and Minkowski derived expressions for the energy-momentum tensor of electromagnetic waves in matter. Each author’s tensor is based on sound theoretical arguments. Unfortunately, they disagree. Abraham’s tensor shows that the momentum decreases,...

3

No faster than light communication or quantum mechanics needed to describe the interference that occurs when light reflects off of a piece of glass. The glass is a Fabry-Perot etalon. An etalon is an optical component which has two glass surfaces parallel to each other. Suppose the etalon has thickness $L$ and speed of light $v = \frac{c}{n}$ where $n$ is ...

2

The photon does not reflect off the surface. What is a surface anyway? It is two-dimensional, infinitely thin, a mathematical construct. It does not exist. The things that exist are the atoms and electrons of the glass. The photon is interacting with all that, and it is of course not possible to do a full treatment of this in quantum electrodynamics. It can ...

14

In Feynman's QED The Strange Theory of light and Matter, he devotes a large amount of time explaining how one goes about calculating the probability of reflection/transmission of a photon from a thin layer of glass. Feynman's account of thin film interference can be found on pages $69$-$72$. Briefly speaking, he uses the idea of a clock hand as a revolving ...

1

It is very important to understand that this experiment was done using a light ray consisting of many photons, not just shooting a single photon at a glass slide. Actually when they did this experiment, they did only check the part of the wave that was refracted by way of checking whether that part of the ray exited the glass slide on the other (far) side. ...

5

The sinusoidal pattern of the reflection rate as a function of thickness is due to constructive/destructive interference between reflections off the near and far surfaces of the glass, and it's correctly explained by Maxwell's electromagnetism without quantum mechanics. As Ruslan said, the "detection" of the thickness happens at the speed of light: ...

7

I can only comment on the basic problem: photon scattering off definite field conditions. As an example here is a lowest order scattering of a photon with an electric field, represented by virtual photons, The on mass shell photon enters on the top left and leaves on the top right. The diagram will give the probability of scattering when calculated and ...

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Actually, the photon doesn't have to know the thickness. Moreover, if we speak of a wave with a well-defined "beginning", like e.g. $\psi(x,t)=\sin(\omega t-kx)\theta(\omega t-kx)$ (with $\theta$ being Heaviside function), incident on the glass, part of this wave will reflect as if the glass were semi-infinite. But then the reflection from the far ...

2

Having c as a speed limit is not sufficient to make causality hold. In general relativity, you can have things like naked singularities and closed, timelike curves that violate causality. It's not really even true that Newtonian mechanics has causality: https://en.wikipedia.org/wiki/Norton%27s_dome In special relativity, causality isn't the only thing ...

5

The extra term in the field equations, $\lambda g_{\mu\nu}$ is often written with a capital lambda, i.e. $\Lambda g_{\mu\nu}$. $\Lambda$ is the cosmological constant. The second part of your questions asks why the speed of light is apparently being omitted, this is because we are using natural units in which $c=1$ and is also dimensionless. Geometrized units ...

0

Just think of the rotation of the quark inside a proton as the number of times it takes the highest position on an imaginary circle i.e. 90 degrees (up) to the center of mass of the proton. Now, let's push the proton to reach a relativistic speed. The quark must oscillate around the center of the proton and doing so it describes a path. If it collects the ...

0

This question has led to at least one totally incorrect statement, namely that the speed of light in a medium can exceed the speed of light in vacuum. Although the phase velocity, $\omega / k$, can be larger than $c$, this is not true for the group velocity, $d \omega / dk$. Information and energy are travelling at group speed. Secondly, there is no ...

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