How Fast Do Forces Like Gravity and Magnetism Act Compared to Light?

[pallidin]

'Cherenkov radiation', for example, is the blue glow that is seen in uranium storage pools and is caused by massive particles exceeding the speed of light in water.

True, however, the speed increase is relative to medium and NEVER exceeds the speed of light in a vacuum.
Therefore, the Cherenkov effects are superluminal with respect to the common mode within the medium, which IS LESS THAN C, but ARE NOT superluminal in a vacuum.

 

[pallidin]

In other words, the speed of light in a vacumm is approximately 299,792,458 meters per second. The Cherenkov effect DOES NOT exceed that. It only exceeds the speed of light common in that medium, which is about .75c.

 

[Danger]

Absolutely, pallidin. That's why I specified that it can only occur within a refractive medium. Nothing—unless tachyons actually exist—can exceed c in vacuum. Even in a refractive medium such as water or glass, the individual photons still travel at vacuum-level c. The atomic interactions limit the propagation speed, which can be exceeded by other particles that suffer less interference. (Not a proper scientific explanation, but the best that I can summon right now.)

 

[dextercioby]

A sphere embedded in $\mathbb{R}^{3}$ is very much 2-dimensional.

Daniel.

 

[masudr]

As Daniel correctly points out, the 3D sphere you speak of is embedded in $\mathbb{R}^{3}$ but you only require 2 co-ordinates to describe any position on the surface (e.g. $\theta, \phi$ form a common system). Since there are only 2 degrees of freedom in the tangent spaces of this system, there is no justification in calling the sphere 3D.

 

[Danger]

$\mathbb{R}^{3}$(e.g. $\theta, \phi$

Okay, that makes sense. I have no idea what the above quoted symbols are, though. So if a sphere is 2-D even though it occupies 3 dimensions, how many D's are planes and lines worth?

 

[DrGreg]

In other words, the speed of light in a vacumm is approximately 299,792,458 meters per second.

Not approximately. Exactly.

(By the http://www.bipm.fr/en/si/base_units/" .)

 

[George Jones]

In everyday informal discourse, the words "sphere" and "ball" are sometimes used interchangeably, but not in mathematics. For example, in R^3, a 2-sphere is the 2-dimensional surface of a 3-dimensional ball.

Regards,
George

 

[Danger]

Oh... okay. Thanks George. It's probably time to repeat my oft-mentioned fact that I have a grade 9 math level (if that) and never finished high-school.

 

[Son Goku]

I could be wrong, but isn't the speed of gravity only equal to 'c' in the perturbative regime.
In the full non-perturbative General Relativity its speed depends on local conditions and the observer's "position".

There is a quote between Einstein and Born (I think) about it.

 

[KY girl]

Help! I have a speed of light question

I just registered today! I am looking for a place to get some of my physics questions answered.

I am a complete novice (note to one who might reply), but I am fascinated with thinking about physics.

My question is - if nothing can go faster than the speed of light (and I know there is now some debate about that), how is it that we are able to SQUARE the speed of light in E= mcsquared? I know that we CAN because this equation is used all the time. But why is it that we can SQUARE something that we aren't supposed to be able to change?

Dumb question - I know - but I'd love for someone to tell me how I'm thinking about this wrong. THANKS!

 

[Danger]

In the equation, only the numerical value of c is squared. It has nothing to do with altering its physical properties. You could also square the value of your bank balance, but it won't make you rich.

 

[flotsam]

Space/time can travel faster than the speed of light can't it? There is nothing in GR to say it can't. Isn't that the reason for hypothetical galaxies being beyond our viewing? This being true, then surely we can't feel their gravitation. How is this effect calculated when estimating the size of the universe?

 

[HallsofIvy]

Space/time can travel faster than the speed of light can't it? There is nothing in GR to say it can't. Isn't that the reason for hypothetical galaxies being beyond our viewing? This being true, then surely we can't feel their gravitation. How is this effect calculated when estimating the size of the universe?

First I would have to know what you mean by "space/time" moving!

 

[DaveC426913]

The expansion of space is not limited by c, only the passage *through* space is limitd by c.

 

[Janus]

I just registered today! I am looking for a place to get some of my physics questions answered.
I am a complete novice (note to one who might reply), but I am fascinated with thinking about physics.
My question is - if nothing can go faster than the speed of light (and I know there is now some debate about that), how is it that we are able to SQUARE the speed of light in E= mcsquared? I know that we CAN because this equation is used all the time. But why is it that we can SQUARE something that we aren't supposed to be able to change?
Dumb question - I know - but I'd love for someone to tell me how I'm thinking about this wrong. THANKS!

When you square something with physical units the answer will be in different units and you are not changing the original units.

Example, assume you have a room that has walls of 10 ft length each. If I square the length of the walls, I get an answer of 100 square feet, a measure of area. By squaring 10ft I have not changed the length of the walls, but instead converted length into a new measure, area.

With E=mc², you are doing the same thing; you are taking the speed of light, one form of measure, squaring it, and then multiplying by a mass, another form of measure to calculate a third form of measure, energy. In doing so, you do not change the speed of light, any more than you change the length of the walls of a room when you calculate its area.

 

[Danger]

re: Dave's post
That's the basis of one theoretical approach to 'starship' design. If a ship can generate a bubble of artificial spacetime around itself, and remain sub-c relative to it, the bubble itself could be accelerated to superluminal speed within the 'real' spacetime. I'm not too confident about it ever being achieved, but at least it offers some hope for interstellar travel.

 

[flotsam]

Yes I mean the expansion of space.

 

[Gokul43201]

Okay, that makes sense. I have no idea what the above quoted symbols are, though. So if a sphere is 2-D even though it occupies 3 dimensions, how many D's are planes and lines worth?

Planes are 2 dimensional (you can describe every point on a plane using 2 co-ordinates); lines (and other curves) are 1 dimensional.

 

[pervect]

Okay, that makes sense. I have no idea what the above quoted symbols are, though. So if a sphere is 2-D even though it occupies 3 dimensions, how many D's are planes and lines worth?

$\mathbb{R}$ is the set of real numbers, $\mathbb{R}^3$ is a 3-dimensional vector space, the space represented by a list of 3 real numbers.

 

[KY girl]

To Danger and Janus:

Thanks for your responses. I got all caught up in looking at E=mcsquared as mass must travel at the the rate of the speed of light squared in order to become energy...and well, I knew that couldn't be right, but I couldn't figure it out. So THANKS!

Also - in the recent Nova special about Einstein they showed his "thought" experiment of looking into a mirror while traveling at the speed of light. He theorized that you would never see your reflection because the light couldn't travel faster than you in order for your eyes to see the reflection. But here's my QUESTION:
If you and a friend are holding a mirror BEFORE going the speed of light, and you both see your reflection...what happens to that reflection once you REACH the speed of light and the friend moves out of the way of the mirror? Will her reflection still be there and stay there until you decelerate?

Thanks in advance for your responses! I live in a rural area...and I can't find anyone willing to ponder this stuff with me!

 

[Danger]

Thanks, Gokul & pervect. I can see that my confusion arose because I was considering the volume of the sphere rather than just the surface area. The third coordinate that I had in mind was the radial distance from the centre.

KY girl, I don't really have much of a clue about your last question. The light itself should continue to pass back and forth between you and the mirror at c relative to you, since it's a constant, but I don't know what effect time dilation and Doppler shifting would have.

 

[lightgrav]

I got all caught up in looking at E=mcsquared as mass must travel at the the rate of the speed of light squared in order to become energy...and well, I knew that couldn't be right,

The Energy that is localized in a region, especially if congealed around an object so that the Energy has to be moved if the object is moved - shows up as a wee bit of equivalent mass.
The kinds of Energy that can be so localized are (negative) Potential Energies, so a proton and a neutron combined into a nucleus (or an electron trapped by a proton to make an atom) have a mass deficit m = E/c^2 .
Some folks used to talk as if all mass was localized Energy (before quarks).

If you and a friend are holding a mirror BEFORE going the speed of light, and you both see your reflection...what happens to that reflection once you REACH the speed of light and the friend moves out of the way of the mirror? Will her reflection still be there and stay there until you decelerate?

Well, 1st, we ponderous objects (we're "slow" because we have mass, not because we think too much) can't travel as fast as light. But as you accelerate up to high speed (if very quick a), the light that had been reflected from your friend already would strike the mirror farther back than usual and keeps drifting backward. So the reflected light from them that enters your eye would be coming from farther forward than before.

The nuclear forces are mediated by massive exchange particles.
I daresay that these Forces do not propagate at the speed of light,
but I don't recall taking time-delay into account for nuclear calculations.

 

[IPHO]

I'm a fresh registered student but i also know smth. about light.
I know that there are Quasarsthan have an energy which is ten times than our solar system. No one knows anything about them ...:

 

[Danger]

No one knows anything about them ...:

Well, we know a bit. There are still a lot of questions about them, though.

 

[heusdens]

Gravity propagates the speed of light in Einsteins theory, as is argumented here.

But is this not problematic considering that nothing can escape a black hole, not even light? How can gravity itself escape a black hole?
Wouldn't it be expected that once a star collapses and forms a black hole, not only light but also gravity can not escape the black hole?

 

[ranger]

How can gravity itself escape a black hole?
Wouldn't it be expected that once a star collapses and forms a black hole, not only light but also gravity can not escape the black hole?

But if you look at gravity as the curvature of spacetime, which is what Einstein's theory states, then there is no reason why it shouldn't "propagate".

 

[tehno]

Let's assume Gravitational waves could propagate FTL.
Does that necessarily imply the violation of causality in GR?

 

[healey.cj]

I don't know whether this has been said or not yet but there is such a thing as superluminal communication... ie. data can be transmitted faster but at short distances there's an interesting effect...
Eg/ If you were to flick a light switch for something superluminal you would see the light on before you had realized you had flicked the switch...

 

[ranger]

I don't know whether this has been said or not yet but there is such a thing as superluminal communication... ie. data can be transmitted faster but at short distances there's an interesting effect...
Eg/ If you were to flick a light switch for something superluminal you would see the light on before you had realized you had flicked the switch...

If this where possible, then problems with causality will occur. SR confirmed that cause must precede its effect. Therefore it is impossible to influence distant objects by FTL signals.
http://en.wikipedia.org/wiki/Causality_(physics )

Interestingly enough, wikipedia has an article about superluminal communication. Its actually a pretty interesting article. Have a look:
http://en.wikipedia.org/wiki/Superluminal