# Tag Info

4

Yes, light can be brought to a complete halt under the right conditions. To understand how this happens you need to understand what is going on when light slows down in a medium. Light is an oscillating electromagnetic field, and when it passes though anything that contains charged particles (i.e. any matter made from electrons and protons) the electric ...

2

I think the main confusion is here is the difference between a scientific measurement and a pedagogical demonstration. You're right that the microwave and chocolate method has to assume the frequency on the back of the microwave is actually the frequency of radiation inside the cavity. A proper measurement of the speed of light would include a measurement of ...

1

Dispersion is not the only one phenomenon, where photons of different energy are acting different. In the case of reemission of photons in materials as well as in the case of dissipation of photons in materials the visible for us result is a transparent material like glas. But due to this two effects the speed of light in materials is different. As you see ...

0

This could be the best visual explanation of my question and it's answer: https://www.youtube.com/watch?v=IXxZRZxafEQ

0

The first postulate is satisfied by Galilean relativity with an infinite speed of light, but this violates the second postulate. Therefore the second postulate does not follow from the first. Of course experiment tells us that the speed of light isn't infinite, and if we combine the first postulate with a finite speed of light we find they are inconsistent ...

1

If you seriously think the second principle goes without saying, then Galileo should be credited with discovering special relativity. The second principle basically asserts that the laws of electromagnetism are physical laws valid in all frames, not just laws that hold in the frame of a medium. And since that was an actual view back then, it needed and ...

0

My teacher told me that speed of light is constant in the universe. Your teacher is mistaken I'm afraid. The speed of light varies with gravitational potential. You can see Einstein talking about this in 1920 in the Einstein digital papers: Also see Irwin Shapiro saying the same in his Shapiro Delay paper dating from 1961: There's also ...

2

The equations that describe how light, and all other electromagnetic radiation, works have a couple of constants which mean that the speed of light is constant. The equations don't depend on where you are, what temperature it is, which way you are looking etc so we assume they apply everywhere. It is entirely possible (although experimentally unlikely) that ...

2

From the perspective of a photon: There is no such perspective. I was trying get an understanding of the universe from the photons perspective. There is no such perspective. Consider the following excerpt from I am driving my car at the speed of light and I turn on my headlights. What do I see?: Sometimes people persist: What would the ...

0

The big problem with imagining that you are the photon is that you simply cant. As mentioned in the comments there is a strict distinction between massive and massless particles. For massive ($m\neq0$) particles, you can always find a reference frame where the particle is stationary. For massless ($m=0$) particles this is impossible. Another interesting ...

1

speed is $$\frac{\text{distance}}{\text{time interval}}$$ but at the event horizon of a black hole, time interval becomes $0$. Imagine a flashlight flashes periodically 1 flash/s (in flashlight's reference frame). As flashlight getting close to the event horizon, someone far away from the event horizon will see flashlight flashing $0.1$ flashes/s, ...

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It means that for ultra near-c-speed particles, the closest to c the less time and space exist. For instance, transactionalists consider the emission then absorption of a photon by distant electronic layers as a simple handshake between them (the photon jumping instantaneously from one to the other in his "frame"). This also gives some clue about the ...

1

As I know, Field Theory, that to what appeals the topic creator cannot explain the very powerful gravitation fields . So trying to understand what happens with a photon there are inside the Black Hole in meaning of Field Theory, or Special Relativity, isn't a good idea. The Nature has no the alone space , and the alone time , you can abstractly image ...

-1

is there anything that says that speed has to be a max limit of any kind? Yes, the so-called chronometric distance definition; whereby the (mutually equal) value of distance between two participants, $A$ and $B$, at rest to each other is evaluated as $$\ell[~A, B~] = \ell[~B, A~] := \frac{c_0}{2} ~ \tau A[~\text{signal}, \circledR B \circledR ... 1 A key to understanding this is realizing that it's not always true. In fact, at x-ray frequencies, refractive indices are typically less than 1, so that the phase velocity is faster than the vacuum speed of light. The key difference is that x-ray frequencies are well above the natural frequencies of most of the electronic excitations that are involved in the ... 2 For better clarity, let's define the following: Axial direction = the direction the person & light beam are drawn into the BH. Radial direction = the direction perpendicular to the axial direction. If we, looking in the same direction as the person & light are being drawn into the BH, watch the light beam as it is drawn into the BH, we will see the ... 4 Now there is a light ray moving outward at the speed of light. I'm afraid that isn't the case; within the event horizon of a Schwarzschild black hole, the radial coordinate is timelike and so, moving 'outward' toward the horizon is as impossible as moving 'backward' in time. This plain to see in the Kruskal–Szekeres coordinates: Image credit See ... 2 I think a possible analogy would be to imagine that the singularity is a waterfall. By emitting light, you are trying to send a signal upstream using a tame fish. Outside the event horizon the fish is able to make headway against the current. But the river flows so fast within the event horizon as it approaches the waterfall, that your fish ends up going ... -9 How does light behave within a black hole's event horizon? It doesn't behave at all. If the event horizon of a black hole is the distance from the center from within which light cannot escape, imagine a person with a flashlight falls into the black hole. I've explored this with a variety of relativists, and posed this question. The answer comes ... 0 Calculating this is based on the concept of retarded time and retarded variables. The retarded time is:  t_r = t - \frac{|\vec{S} - \vec{s}|}{c}  The retarded field would then be:  \vec{f} \left( t \right) = a \int _{\vec{s} \in C} \frac{x\left(\vec{s}, t_r \right)}{{|\vec{S} - \vec{s}|}^2} \hat{\left( \vec{S} - \vec{s} \right)} \, |dC|  Or ... 5 A tensile pulse travels through the rope at the speed of sound. This speed depends on the density and the bulk modulus of the material - a rope strong enough to support its own weight would probably have a very high bulk modulus. The equation is$$v = \sqrt{\frac{K}{\rho}}$$Where K is the bulk modulus and \rho the density. 0 I am assuming you have 2 particles facing each other, and that they are approaching each other ? First, as mentioned elsewhere on this page, "..a particle moving at the speed of light does not experience time, and thus is unable to make any measurements." Instead, let's change the particle #1 that you are sitting on to having a specific velocity that is ... 1 I don't know the "formal" proof, but here is my proof: Time dilation and length contractions are given to us by the Lorentz transformations by: t’ = t/(1-v2/C2)1/2 and d’ = d/(1-v2/C2)1/2 (in other words “same” or proportional to each other) where: t = distance/length traveled through the T dimension in observers own frame of ... 1 I am sure there was a delay when Nixon talked to the Apollo crew. But how long was the delay? The distance from the Earth to the Moon is about 360000 km, if I remember correctly, so the one-way delay is about 1.2 s (plus maybe a smaller but possibly comparable delay for processing and retranslation) - that is noticeable, but it is not a showstopper. As for ... 2 What you are confusing here is speed and velocity. Light speed is constant, but the velocity, which takes into account the direction as well as the speed is not. As an example of how something can accelerate without changing speed, consider the case of circular motion, where the acceleration of an object moving at a speed v in a circle of radius r is ... 0 The disconnect is between the first and second clauses of your first sentence: Light speed is constant, therefore experiences no acceleration Yes, the speed of light is a constant, but it experiences no acceleration in its direction of travel. Light definitely accelerates laterally when gravity pulls on it, which is why it curves when passing near ... 2 No your argument is not correct. Firstly, velocities do not add linearly like 3-vectors in Euclidean space: the relativistic sum of two velocities always has a speed of less than c if both the velocities' magnitudes are less than c (no matter what their direction). Secondly, photons have no rest frame: that's a basic property of things that have zero ... -1 According to the special relativity, there is a formula describing the relation of particle mass with its speed:$$m = {m_0 \over \sqrt {1- ({v \over {c}})^2}} where $m_0$ is the mass of the particle when it is still----rest mass. So as long as a particle moves, no matter how large or small the velocity is, the mass of the particle $m$ will vary and be ...

1

Take as an example the electron positron annihilation Feynman diagram: Before time t' there exists no photon. After that the photon exists conserving energy, momentum and angular momentum for the reaction, and has to have velocity c in special relativity, because of its zero mass. The concept of acceleration needs the photon to exist before it does, ...

-2

Wow 'll try and sort this out. Light travels at a velocity that is dictated by a relationship of properties of 'freespace' (empty) called permittivity and permeability. it is the ability to reach and pull for lack of a simplier explaination ... Lasers 'cut' because they heat up and either melt or disintegrate complex molecules. Once melted the excess can be ...

2

Determining what is happening "right now" on a planet 63 light years away is exactly the same as determining what will be happening 63 years in the future on Earth. Both problems are, technically speaking, impossible. We can't know exactly what the future holds 63 years from now, as things may change in surprising ways, just as we can't know what is ...

0

Time appears to slow down as light passes a strong object of gravity. Time is not really a physical property, time is simply a unit of measure invented by earthlings. Time appears to change because gravity bends light and it actually travels further to get to the observer so it takes longer to get there. Time is actually distance travelled. The light that ...

0

...what will happen if it attains the speed of light? That's not something you have to worry about. Saying that it takes an infinite amount of energy is the same thing as saying, "No matter how much you increase the kinetic energy of a particle, you will never observe it to move as fast as light." Nothing in the universe has an "infinite" amount of ...

1

How can we say that difference in energy between 99.99% and 100% of speed of light is infinite? The relationship between energy and velocity of an object is not linear but instead follows this equation $E^2 = m_0 ^2c^4 + p^2c^2$ (1) where $p = \gamma m_0 v$ and where $\gamma$ $\gamma = \frac{1}{\sqrt{1 - (v/c)^2}}$ and as you see, as $v$ ...

0

The geometry for a spinning ball changes. If you consider the circumference as small line segments, the fact that they are spinning means that the line segments are moving in the direction of motion and exhibit Lorentz contraction while the radii are not foreshortened since they are moving perpendicular to the spin. You are now dealing with non Euclidean ...

0

If you're thinking in terms of waves, yes the mathematical formalism used in both cases are quite similar. Gravity does rid itself from the black hole, because the kind of wave (gravitational one) yielding gravitational radiation, what can be measured outside the event horizon. This gravitational irradiation moves on the speed of light.

1

According to Einstein and many others, light speed c0 in vacuum is universal and measures about 299,792,458 m/s. So it is never possible to change the light speed in vacuum, which is the absolute upper limit for everything. Gravity does not affect a light ray, but the space and time through which the ray travels. In a strong gravitational field, the time ...

1

I would suppose the short answer is no, but photons are affected by gravity. What is happening is naturally quite relativ to the observer. Suppose you are sitting on the photon, travelling past the gravity source with the speed of light. As has been argued above you would experience the force of the gravitational pull as an acceleration toward the source. ...

23

You don't feel acceleration. When onboard the ISS, you are accelerating towards the earth (down) due to gravity: if you didn't, you would just fly away from the planet. Because you and the ISS are accelerating exactly the same way, you don't feel a thing. You don't feel a force if it's accelerating you: you feel pressure caused by opposing forces. Here ...

3

To talk about acceleration in space is a little bit dangerous without exact definition. One has to separate free fall and acceleration from an impulse. Imagine, you are inside the ISS during an orbit correction. The impulse from the rocket engine you could feel, you get some weight, and and this is an acceleration. In all other time you are weightless and ...

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Photons are blue-shifted when attracted by gravity (I mean - moving towards a mass, not moving at right angles to the gravitational field like in an orbit). They can't go faster, but their energy goes up.

3

Let's say we're sending a scouting mission to an Earth-like planet 100ly away to see if it's suitable for colonization. We could send the scouts out at near light speed, and due to time dilation they could easily survive the trip without dying of old age. If we send them out at .9c then the entire round trip will only take 20 years in their frame of ...

4

Size is relevant. Even weak photons can be absorbed by knocking an electron to a higher energy level. If you were that small you would be ripped apart.

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It's because photons are massless and so they can and must move at the speed of light with any none zero amount of momentum. This means that even though they move at the cosmic speed limit they can have very little amounts of momentum.

2

thats an interesting question, but photonic fields are fundamental and cannot really be compared to say a baseball. If you throw a baseball at near the speed of lightit would probably break a lot of stuff, but photons don't really effect things very much unless they are high energy. Note that high energy photons like x-rays actually do have the ability to ...

1

Let me turn your question around. A radio wave photon, with thousandths or millionths of an eV in energy, travels at $c$. Why should it be able to rip anything apart? Its energy is very small compared to any chemical bonds. That says it has only three options-be absorbed, reflect, or scatter. Reflection imparts the most momentum into the object, but it ...

0

Theoretically speaking, of course, if your true linear speed with respect to the true center of the universe was zero, you would be experiencing true time. Even on Earth, who's movement is what we base our time (e.i. seconds, days, years, etc) off of, we theoretically would be experiencing time dilation based on Lorentz, assuming that at a true zero ...

1

A magnetic sphere with a negative core with a positive surface and then encased in a positive shell creating a free floating entrapped sphere while also providing enough magnetic pressure on the sphere to reverse the gravitational effects from the spin allowing it to spin faster. If pushed by laser through a hole in the vacuumed encasing with high velocity ...

1

One thing you have to note is that speed is relative, Clock A would see clock B moving from A's point of reference, and B would see A moving in B's reference, so you shouldn't be using the word "stationary" in this context. Both the clocks would see the other clock tick slower, B would see A's future only if it returns back to A, this makes it obvious to A ...

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