# Tag Info

13

The link which NeuroFuzzy provided appears to be time lapse photography of a pulse of light from V838 Monocerotis, the most spectacular light echo in the history of astronomy, according to the European Space Agency. It is much brighter than even a supernova, but it is not exactly an explosion. The initial pulse happened in 2002. V838 Monocerotis didn't ...

22

Sometimes we do, and the phenomenon is called a light echo. What you're looking at there is NOT moving gas. It's an "echo" exactly as you describe. The problem is that you need a pulse of light. If you have a constant stream of light, the "light echos" will be exactly like what you see in fog on earth.

0

Michelson and Morley compare the speed detected when moving at two different speeds (going with the rotating earth and perpendicular to the direction of the rotating earth). It observed no difference. If an astronaut moved in two different ways and compared the speed detected when moving at those two different speeds they should also get no difference. ...

0

If you with your rods and clocks are in free fall (ie: your metric is the Minkowski diag(-1,1,1,1) ) in a vacuum and the light ray passes near you, you will always measure the standard speed c= 2.99792458 E+8 m/sec. However, the speed of light is observed to be different if the observer and his rods and clocks are in a different gravitational environment ...

0

The answer to your question depends on fine definitions. Locally the speed of light is always the same; more precisely, the universal, Lorentz invariant speed $c$ (which is also the maximum speed of a cause-effect relationship and experimentally observed to be the same as the speed of light) is constant. This means that any measurement of light speed in any ...

7

From Roemer and Light Speed: The orbital period of Io is now known to be 1.769 Earth days. The satellite is eclipsed by Jupiter once every orbit, as seen from the Earth. By timing these eclipses over many years, Roemer noticed something peculiar. The time interval between successive eclipses became steadily shorter as the Earth in its orbit moved toward ...

0

A Non-Relativistic Interpretation of Relativistic Results Million years ago, when self-studying special relativity, an exercise was created in order to understand non-relativistically the relativistic result that a force could not accelerate a particle to speed greater than $\:c\:$. The exercise, the Figure and the solution are already in LaTeX and are ...

0

Particles that have no mass, as photons, have momentum based on their energy, more especificaly on the relation $$p=\frac{E}{c}$$ Where $p$ is the momentum, $E$ is the energy and $c$ is the speed of light. You can think of it intuitively by noticing Einstein's famous equation, $E=mc^2$, and substituting it on the classical momentum formula $p=mv$, using ...

0

By inertia I assume you mean momentum. The momentum is related to the energy of the object by: $$E^2 = p^2c^2 + m^2c^4$$ and to the velocity by: $$p = \frac{mv}{\sqrt{1 - \frac{v^2}{c^2}}}$$ The momentum does indeed tend to infinity as $v \rightarrow c$, but note that it will never reach an infinite value because no massive object can travel at the ...

0

Let's say for argument's sake that light is a speeding car. That's quite a silly analogy, because a car is generally thought of as some thing which can be observed while it's driving along on the road (e.g. being observed by landmarks such as delineators along the road, or by other cars), and which may observe them in turn. A similarity to light is ...

0

the definition of a second wouldn't have an uncertainty when related to the transition of the Cs atom, The definition of the SI unit "second" does not refer to just any given sample of Cs atoms, and specificly, not to transitions between the two hyperfine levels of the ground state of just any given sample of caesium 133 atoms; but it refers to an ...

5

With respect to your question, the immediate thing you need to clarify is: constant with respect to what? How SR answers that question The speed of light is usually held to be constant with respect to reference frames. In other words, if we're both at the same place in outer space, but you're passing by me in your spaceship, then every photon in either of ...

2

Is the speed of light constant or does the math just happen to work out? None of the above. It's a tautology. What happens is that instead of having just one car, you count 9192631770 cars passing you by. See the defiition of the second which involves microwaves passing you by. Then you declare that a second has elapsed. If those cars are going slower, ...

0

The atoms of your body will be broken apart because they are glued by Electromagnetic Energy. Idem to the elementary particles. No material body then ... Admit for an instant that it is possible that your body is made of pure light, EM, i.e. not consolidated in atoms. If you go faster then light then you will be detached from your body. No EM body then ...

-3

Well, I'm not a scientist at all, but one thing I can think of off the top of my head, is that we could orbit a black hole and possibly find out everything from it. We could also hire William Shatner and Patrick Stewart to search for uncivilized worlds and go where no man has gone before... but back to reality, we could defiantly start the search for extra ...

-2

Is speed of light in vacuum always the same value? No. The speed of light varies with gravitational potential. You can see Einstein talking about this in the Einstein digital papers: Also see Shapiro's 4th test of General Relativity along with The Deflection and Delay of Light by Ned Wright and this PhysicsFAQ article by Don Koks: "Einstein talked ...

3

No, in perfect vacuum, photons do not slow down. Although, gravity of massive objects like stars or planets can bend the trajectory of photon (the Theory of General Relativity) like a lense. If you are referring to the fact that Black Hole is black because no photons can escape its massive gravitational force and you thought it is because the gravity of the ...

18

As far as we can tell, the speed of light in vacuum is indeed constant. Photons don't slow down or speed up as they fall into or rise out of a gravity well. However, just as a massive object's kinetic energy changes as the object falls into or rises out of a gravity well, photons also gain or lose energy. In the case of photons, this energy change manifests ...

-3

Is it true to say that all matter in the universe is travelling with velocity c through spacetime? Actually, no. Because there is no motion through spacetime. Spacetime models motion through space over time, but because it includes the time dimension, it's totally static. And whilst this model works very well, we live in a world of space and motion, ...

0

The problem is that special relativity uses Lorentz transformation and not Galilean transformation. If we consider an inertial reference frames $S$ it's perfectly fine to just use the vector sum to add two velocities. If we consider velocity $v_1$ and $v_2$, the velocity summation $s$ is so that $s = v_1 + v_2$. However, in special relativity $s$ takes the ...

0

Speed of 'c' remains constant in all inertial frame of references. Time dilution can be briefly explained as below- As Person 'A' is moving close to 'c', space around him expands w.r.t his frame.As Speed=dist/time, in order to keep this ratio constant time slows done for him,but for person 'B' everything seems to be normal, of course he would measure ...

0

I was just looking up your level of education in order to gauge my further replies. I bumped into this, and whilst it's an old question, but I felt moved to answer it. My understanding is that a photon propagating in vacuum has a small probability to spontaneously create a particle/antiparticle pair that will then quickly recombine to emit a photon ...

1

Start by considering a long pipe with water flowing through it. We'll assume the rate of flow is slow, so the current of water is small. This means water entering the pipe at one end will take a long time to flow all the way along the pipe to the other end. However suppose we generate a pressure wave at one end of the pipe. A pressure wave in water is just ...

2

We don't know that fundamental constants don't slowly change over time. Au contraire, you can find articles like Changes spotted in fundamental constant: "The researchers found that the fine-structure constant, known as α, has changed in both space and time since the Big Bang". The thing to note is that some "fundamental constants" aren't constant at all. ...

0

Let's say Bob is standing still while a one light second long photon formation flies past him. How long does the passing of the photon formation and Bob take according to us? Answer: It takes one second. 1 light seconds / c = 1 seconds. Let's say Jim is moving forwards at speed 0.1 while a one light second long photon formation moving to the opposite ...

0

I think other answers are insightful but I would like to elaborate them further. In a nutshell, the question in about an hypothetical brain which has physical dimensions comparable with one light years, and about its perception of time. Since information cannot travel faster than light, the time in which information travel from one side of the brain to the ...

0

A being with 1 lightyear arms or even 1 lightyear across and density similar to flesh would be sufficiently dense to create an enormous black hole. Even planet sized, a being of that size runs into gravity problems, where, lifting it's arm would require significant energy to resist it's on gravitational attraction on it's limbs, so too big doesn't work. ...

0

There is a unique numeric value of c in whatever units (eg: m/sec , or miles/hour, or furlongs/fortnight) you are using that correctly converts velocity to $\phi$ radians for doing boosts with the Lorentz Group where $\phi$ is called the Lorentz Boost parameter. $$v/c=tanh(\phi)$$ The number of radians for a particular boost is not random because ...

1

If there were an enormous being whose arm span is one light year across, how would that being perceive time? Wouldn't what we perceive as a year be virtually nothing to that being? It would probably have to have decenteralized brains spread throughout its volume here and there. Otherwise, yes, there is a distinct problem that a being with a single large ...

1

What happens if you first drive on a road at speed 0.99 c, and then accelerate to speed 0.999 c? You will pass milestones at much faster rate. That happens mostly because of length contraction, not because of larger speed, as the speed increase was only 0.009 c. Roughly the same contraction as above will happen to a line of photons approaching you, if you ...

0

For starters, Special Relativity only seems bizarre to someone who does not see it at work in its entirety. If its entirety is being seen, it becomes nothing but a simple single image. From that simple single image one can, in mere minutes, derive the entire collection of Special Relativity equations. To learn of Special Relativity by one's self, you simply ...

4

Do external forces affect light? Yes. See for example Faraday rotation: GNUFDL image by Dr Bob, see Wikipedia Can any external force make the light accelerate? Yes. See for example Compton scattering. The photon doesn't change speed, but it's accelerated in the vector sense: Image courtesy of Rod Nave, see hyperphysics And if it can, ...

1

So the basic mass/energy/momentum relation in relativity can be phrased as$$\left(m_0 c^2\right)^2 = E^2 - c^2 p^2$$and the conventional explanation here is that $m = 0$ so $E = c p$, a formula known more or less since Maxwell showed that light is an electromagnetic wave. It has also been known that light comes in packets with angular frequency $\omega = E ... 2 1. Will eventually the two objects will have the same velocity Newton No, in the Newtonian model the longer you apply a force to an object the faster it travels (assuming no other forces apply, in an atmosphere for example, there might be a terminal velocity regardless of the continuing application of force). However Newton's laws are only valid for ... 4 One has to think broader in order to answer those questions. Sure you can imagine a magical 'spring' being attached to a baseball, but there is no way to attach it to the light ray. There is simply not enough magic in this world. Instead, let's focus on what forces could be actually applied to light. Currently, we are aware of four different types of ... 0$F=ma$is a classical approximation to a more complete expression$\vec{F}=\dfrac{d\vec{p}}{dt},$so it doesn't work well at speeds about approximately 0.1c (c is speed of photons). That means we find the change in momentum, $$\Delta\vec{p}=\int_{t1}^{t2}\vec{F}dt.$$ To continue the simplified version you started, let the force be constant. That means that ... 11 You are right in that the speed of light doesn't change. It is a completely different effect to the rain drop analogy. If you had only light hitting you directly from the front and directly form the back, you would observe the same intensity in the moving frame (only blue/red shifted). But for light coming at you from an angle$\theta_s$in the rest frame, ... 3 Light will never be completely at rest, but we have succeeded in slowing it down significantly. (See this for example) In a medium, particles can move faster than the speed of light. (The speed of light in that medium) In fact, this is used in some particle accelerators to detect certain particles. When a charged particle travels faster than the speed of ... 0 It's relative to any observer. This is possible because perspectives moving at different speeds experience time differently, which allows for light to be seen moving at the same speed regardless of perspective and the motion of that perspective. -1 If all motion is relative, how does light have a finite speed? Because of the wave nature of matter. Check out the The Other Meaning of Special Relativity by Robert Close. When you and your rods and clocks are all made out of waves, you calibrate your rods and clocks using the motion of waves, then use them to measure the motion of waves. It doesn't ... 1 I'd like to add a little to GreenBeans wonderful answer that not everything is relative. He makes a few points at the end of his/her already long answer hurriedly (not meant as a criticism): ...it is a choice[, ]a choice of coordinate system[?] There are many coordinate systems in which the speed of light is not constant, or even depends on the ... 0 You can, in non-perfect gasses (which is basically all real-life gases). Your higher push, will result in different frequency sound and the speed will be different by a very small and negligible amount. The example you provided, is an extreme case of a non-perfect gas, that is why you notice the change so much. The speed of iron balls is almost zero and ... 0 The molecules in air are not sitting still waiting for your push. Because of thermal energy, they are moving quite rapidly (close to 1,000 miles per hour). Your push will not significantly increase the speed of sound unless you push hard enough to raise the gas temperature. Pushing that hard would be very unpleasant on your ear drums. 1 What Brian Greene is on about here is that in relativity, we assign to any pair of events an interval value,$s^2$. If there had been (at least) one participant (say,$A$) present ot both events (say event$\varepsilon_{AP}$, the meeting of$A$and participant$P$; and event$\varepsilon_{AQ}$, the meeting of$A$and participant$Q$) then the interval is ... 0 I'm no expert but: you have to assume that you stand still in your initial system. And also because of these contractions it doesn't sum up straight v1+v2 (like the pre-answered said). There are other thought experiments like when you turn yourself and you look out in the universe, things can then pass way faster than light but only because your angular ... 0 I think I see your mistake: it's the classic mistake. You would think that if you travel in a rocket moving at the speed of light and flash your flashlight out from it in the same direction the speed of the light would be$2c$(where$c$= speed of light), but in truth, due to time dilation and length contraction etc, etc, relative velocity would still be ... 1 Although there are several excellent answers, perhaps my answer will remove your confusion. In your statement "if all motion is relative... finite speed" you need to pay special attention to the words motion and speed. Motion is just a space displacement. How fast it is accomplished, is irrelevant. Speed is a rate of change of space displacement per unit ... 0 Time in special relativity is measured as a distance, obtained by multiplying a time interval by$c$. What Brian Greene is on about here is that in special relativity, we assign a special spatial coordinate$w = ct$to every event and consider it as a "four-vector"$[w, x, y, z]\$. Now, what is a four-vector? Well, they have one key role in special ...

0

This is a very good example for how tv programs for laymen are doing more harm than good. There is no such thing as "time speed". Time is that which a clock shows. Period. Please repeat after me: Time is that which a clock shows. Repeat after me: Time...is...that...which...a (as in ONE)...clock...shows. Any one clock, anywhere, anytime. Resting ...

6

By my reckoning, if all speed is relative, then no mater how fast you go light should always race away from you at the same apparent speed. It does. Thats the clever bit! No matter where you are or how fast you are going, you will always get the same measurement of 3*10^8 meters per second for the speed of light (in vacumn) as every one else. Of ...

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