# Tag Info

73

It sounds like your confusion is coming from taking paraphrasing such as "everything is relative" too literally. Furthermore, this isn't really accurate. So let me try presenting this a different way: Nature doesn't care how we label points in space-time. Coordinates do not automatically have some real "physical" meaning. Let's instead focus on what doesn't ...

68

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.

24

It would be possible to see the progress of photons through space if the light pulse were exceedingly intense, and if the dust cloud from which they reflect were positioned and shaped to reflect the light toward us. Rather than shooting a beam from Point A to Point B, it would be better if the light source were between us and the dust cloud, as light ...

23

First: Maxwell's equations predict that the speed of light is absolute. The whole motivation for the special theory of relativity is to reconcile this with the notion that all motion is relative. In other words, you're worried about exactly the same thing that troubled Einstein. You just haven't understood how he solved it. The key to your confusion is ...

19

As far as we can tell, the local 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 ...

12

If some of the light is reflected off the dust at such an angle that it is diverted to reach the observer, the observer will see that light. However, those specific photons reaching the observer will not reach B (unless they are reflected there by the observer). Similarly, unless the observer is at point B (which is not the case in the question as asked), ...

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, ...

9

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. I.e. there should be no speed limit. If an invariant speed $c$ exists, then if an entity has speed $c$ relative to an inertial reference frame (IRF), the entity has speed $c$ relative to all IRFs. That's what ...

9

Using a camera that can capture "Motion at a Trillion Frames Per Second", this can be done at the laboratory scale. The technique used has been called femto-photography. (Image credit to Ramesh Raskar, Associate Professor, MIT Media Lab) Of course a camera that literally takes one trillion full frames per second is totally impossible with today's ...

8

Let's say you build a ping pong ball counter. It increments the count every time a ping pong ball hits the sensor. You throw a ball, and it hits the sensor: Detected! You throw a ball across the sensor from left to right... no detection, because you didn't hit the sensor. Your eyeball is a light sensor, which creates pictures from the light that hits ...

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 ...

7

It is an experimental fact that light moves at the same speed in every reference frame, no matter the underlying theory: see the experiment of Michelson and Morley. Every kinematic and dynamical quantity depends on the reference frame except the speed of light, which is the same for every observer. Besides the experimental result there is something deeper ...

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 ...

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 ...

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 ...

4

My masters project was on something like this (though with hydrogen alpha emission lines for gas clouds between galaxies rather than dust between stars) and the answer in that case (and almost certainly in this one too) is that you can't see it because it's just too dim, though using hundreds of telescope hours can get you close (maybe). Again, this is not ...

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, ...

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 ...

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 ...

3

Gennaro Tedesco's fantastic answer shows how the speed $c$ comes to mean the maximum speed that cause-effect links can propagate, relative to any observer. To, to sum Gennaro's answer up to answer your title question, the velocity concerned is the speed of cause-effect propagation relative to the observer's rest frame. It measures how long it takes to ...

3

(originally a comment, but it's getting a bit long, so...) Try the VSauce video again - it does actually explain the matter. Try the bit from 3:00 to about 4:30 a few times, and think hard. One of the tricky parts about Relativity is that you need to hold a lot of concepts in your head at once - you really need to understand every part to get rid of the ...

2

The energy of a relativistic body is given by: $$E^2 = p^2c^2 + m^2c^4$$ where $m$ is the rest mass and $p$ is the relativistic momentum, which is given by: $$p = \frac{mv}{1 - v^2/c^2}$$ Using this you can easily calculate the energy as a function of velocity. As you have already worked out, it doesn't make any difference whether you are ...

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 ...

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, ...

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. ...

2

The whole problem is really about knowing what the words mean. An event is a time and a place together as single object. For instance the event where a light sends its first, last, or only pulse. Or the event where a beam or particle touches something and bounces. Anything you can describe with a time and place together is an event. And different people ...

2

My fluid mechanics is not strong, but a steep increase in the pressure from a beam that you are heading towards as you approach the speed of light does indeed happen. You can reason this from several perspectives. Suppose you measure a plane wave in one frame and find out that the electric and magnetic field are $E_0\,\cos(\omega_0\,t)\,\hat{X}$ and ...

1

The speed of light is constant in a vacuum. However, it can change direction in the presence of gravity so in a sense it does accelerate.

1

Yash, Imagine S is the Sun sending two photons, P1 and P2, and the object O is represented by two asteroids, O1 and O2, equidistant from each other all the time and from S at $t_0$ - they are moving in the same direction at the same velocity (c/2). So for some time one will be moving towards the Sun and one away from it. So you are right that the speed of ...

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 ...

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