## Speed of Propagation of a Magnetic Field.

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Aelfyre
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### Speed of Propagation of a Magnetic Field.

Ok first the base question? Is the propagation of a magnetic field constrained by the speed of light?

Basically suppose you are way out in deep space and you have a electromagnet or some such that can generate a magnetic field when power is applied.
Now lets say we have some sort of instrument to detect magnetic fields.

Let's assume that once the magnet reaches full power it generates an magnetic field that the instrument can detect from a light year away.

Let's assume that the magnet can be brought from 0 to full power in 10 seconds.

Now when you crank the magnet up to full power will the detector pick up on the resultant magnetic field in 10 seconds or in 1 year plus 10 seconds?

My gut tells me that due to that whole causality thing that it will be 1 year and 10 seconds but then again a flux line is effectively massless right? It's just magnetic potential or some such.
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letterX
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### Re: Speed of Propagation of a Magnetic Field.

Remember: light is a propagation of electric and magnetic fields. So, if magnetic fields propagated faster than the speed of light, then light would also move faster than the speed of light. That is to say, they don't.

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### Re: Speed of Propagation of a Magnetic Field.

yeah I know that is what I am trying to wrap my head around. I know they call it electromagnetic radiation for a reason.. LOL but a it isnt like a flux line in a gauss field is made of photons or anything.. to be honest I have no idea what a flux line *is* made of if anything...
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Moose Hole
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### Re: Speed of Propagation of a Magnetic Field.

Fucking magnets, how do they work?

Edit: Another interesting question is, "What is the speed of propagation of gravity?" Wikipedia tells me that it is also c, because c has to do with the geometry of space time and stuff.

Aiwendil42
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### Re: Speed of Propagation of a Magnetic Field.

Aelfyre wrote:but a it isnt like a flux line in a gauss field is made of photons or anything.. to be honest I have no idea what a flux line *is* made of if anything

Short answer: It is made of (virtual) photons. Photons are the particles that carry the electromagnetic force in quantum electrodynamics.

Longer answer: Photons are actually excitations of a quantum field, analogous to the electromagnetic fields of classical mechanics. As you turn on an electromagnet, you can think of it as emitting virtual photons, which travel at the speed of light. Or, you can think of it as perturbing the quantized electromagnetic field at your location, and this perturbation causes ripples in the field that propagate outward at the speed of light. These two ways of thinking about it ultimately amount to the same thing.

Moose Hole wrote:Another interesting question is, "What is the speed of propagation of gravity?" Wikipedia tells me that it is also c, because c has to do with the geometry of space time and stuff.

Yeah, when you get down to it, c is basically just a units conversion, which we need because we measure time and distance in different units. The same goes for the gravitational constant G and Plank's constant [imath]\hbar[/imath]. In fact, theorists generally work in units where c = G = [imath]\hbar[/imath] = 1, and write things like E=m instead of E = mc2
Last edited by Aiwendil42 on Sat Jul 31, 2010 3:44 am UTC, edited 1 time in total.

LucasBrown
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### Re: Speed of Propagation of a Magnetic Field.

*Ahem*
Aiwendil42 wrote:In fact, theorists generally ... write things like E = m instead of E = mc2.
We would never write something like that. mc2 is the rest energy and gets denoted E0. If there's no subscript, then we're talking about the total energy, which is E=[imath]\gamma[/imath]mc2. But yeah... you're right about setting those constants to 1.

On a completely unrelated note, please use the format codes (like the [ sup ] [ /sup ] tags, etc) instead of imath when you can--imath renders very slowly on many browsers, and while it's rendering, scrolling is impossible. I mainly use it on individual characters (like just the gamma instead of the whole equation).

Aiwendil42
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### Re: Speed of Propagation of a Magnetic Field.

Well, I wanted to give an example with an equation a layperson would know. Actually the theorists I deal with would probably write E2 = p2 + m2.

Thanks for the tip on [ imath ] - I was completely oblivious to the [ sup ] tag.

PM 2Ring
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### Re: Speed of Propagation of a Magnetic Field.

I heartily endorse the use of natural units. They make equations so much neater, and if you understand what the equations are saying, you can always reconstruct the forms with the c's, G's & h-bar's in them if you really want to.

LucasBrown wrote:On a completely unrelated note, please use the format codes (like the [ sup ] [ /sup ] tags, etc) instead of imath when you can--imath renders very slowly on many browsers, and while it's rendering, scrolling is impossible. I mainly use it on individual characters (like just the gamma instead of the whole equation).

A single /gamma in math tags on a page forces the browser to load all the jsMath stuff, so I try to avoid even that, where possible. Unicode renders much faster: eg, γ. And for squares or cubes, I don't bother with sup tags, I just use ² and ³.

You can get around the scroll-blocking by changing your jsMath options to use asynchronous processing. This also allows you to abort jsMath processing if you don't need it on a particular page: once the jsMath stuff starts, hit the Esc key. I find this especially useful when creating or editing a reply on a page that uses jsMath.

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### Re: Speed of Propagation of a Magnetic Field.

voltardark
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### Re: Speed of Propagation of a Magnetic Field.

Imagine you tap a ball with a 1 kilometer pole and there is a timestamp that record when the ball started to move.
Will you see the ball move at the exact same time it started to move as recorded or will you see it at timestamp+the time it took the light to get to you ?

Force are instantenous and magnetism is a force...

gmalivuk
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### Re: Speed of Propagation of a Magnetic Field.

Forces are not instantaneous. A force travels through a rigid pole at the speed of sound through the pole's material, which is typically around a million or so times slower than the speed of light.
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LaserGuy
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### Re: Speed of Propagation of a Magnetic Field.

voltardark wrote:Imagine you tap a ball with a 1 kilometer pole and there is a timestamp that record when the ball started to move.
Will you see the ball move at the exact same time it started to move as recorded or will you see it at timestamp+the time it took the light to get to you ?

Both systems are at rest with respect to each other, so this problem is pretty easy. The timestamp would record the time that the ball started to move. The observer would record time when it moved as timestamp+light travel time. It's very much the same idea as, say, when you drop a stone down a well, the time it will take the sound to reach your ear is the travel time of the stone (generally several orders of magnitude greater unless it is a very deep well) and the travel time of the sound wave back up to you.

As gmal points out, though, this problem is also complicated by the fact that the time when the observer thinks she is tapping the ball is not actually the same as the time when the end of the pole taps it. So if the observer started her clock the moment he moved the pole, then he would have started his time some amount of time before the timestamp actually activated, and then would have to wait until the signal from the moving ball reached her. So her time would be off by an even greater amount. Remember that the pole is not truly a perfectly rigid object; it is made of molecules, and when you do something to one end of the pole, it takes time for the signal to transmit through the chemical bonds to the molecules at the other end.

A related problem that might help illustrate this point: Suppose you have a pole 101 km long that rotates around a point 1 km along its length. If a spaceship rotates the short end of the pole in circular motion at 0.5c, what speed does the long end travel at? What happens to the pole?

sardia
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### Re: Speed of Propagation of a Magnetic Field.

voltardark wrote:Imagine you tap a ball with a 1 kilometer pole and there is a timestamp that record when the ball started to move.
Will you see the ball move at the exact same time it started to move as recorded or will you see it at timestamp+the time it took the light to get to you ?

Force are instantenous and magnetism is a force...

Try imagining the scenario with the pole as a spring 1km long. It helps illustrate the fallacy of the initial description.

voltardark
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### Re: Speed of Propagation of a Magnetic Field.

Both of you are wrong...

Magnetism propagate at the speed of light but when when it cease to exist it's nearly instantaneous...

If the sun would vanish instantaneously, we would see it that fact 8 min later as it take 8 min to the sun's light to reach us but the earth would feel is effect right away... like making a yo-yo spin around and then cutting the thread.

Ciber
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### Re: Speed of Propagation of a Magnetic Field.

Um no.
Gravity travels at the speed of light too, so we would definitely not notice the sun was gone for eight minutes.
I feel like it helps to not think of lightspeed as the speed of light, and to instead to think of the speed of light instead as the speed at which changes in the universe propagate.

gmalivuk
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### Re: Speed of Propagation of a Magnetic Field.

voltardark wrote:Magnetism propagate at the speed of light but when when it cease to exist it's nearly instantaneous...
No, *all* changes in the magnetic field propagate at the speed of light, including the cessation of the magnetic field.
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FancyHat
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### Re: Speed of Propagation of a Magnetic Field.

voltardark wrote:Magnetism propagate at the speed of light but when when it cease to exist it's nearly instantaneous...

That's a contradiction. A particular magnetic field 'ceasing to exist' would just mean it changing, perhaps very rapidly, but that change still propagates at only the speed of light.

Imagine an electromagnet with a current passing through its coil. There's a magnetic field around it. Suddenly, the current is reduced to zero very, very quickly. The magnetic field reduces very rapidly, but that change still only propagates away from the electromagnet at the speed of light. Even if you could stop the current instantly, the sudden change in magnetic field would still only propagate at the speed of light.

Edited to add the following:-

voltardark wrote:If the sun would vanish instantaneously, we would see it that fact 8 min later as it take 8 min to the sun's light to reach us but the earth would feel is effect right away... like making a yo-yo spin around and then cutting the thread.

How fast does sound travel along a string? Sound along a string, tin can telephone style, is variation of tension along its length. How fast do such variations of tension propagate along the string? A lot slower than the speed of light! So, if you cut the string, rapidly reducing the tension at one end to zero, that reduction in tension will propagate along that string at far less than the speed of light. Once that reduction in tension has reached the other end of the string, the yo-yo will stop travelling in a circle, and will start travelling in a straight line.

Even if changes in tension along your string propagate at the speed of light, a 93 million mile string (the same length as the distance from the sun to the earth) would take about eight and a third minutes to propagate that loss of tension from one end to the other.
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voltardark
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### Re: Speed of Propagation of a Magnetic Field.

The answer is the relationship between the quantum model and the newton model.
Propagation is newton and cancelation is quantum.
Cancelation of magnetic field is relative to quantum entanglement.
So when a nova explode its effect is instantaneous on earth even if we can see the explosion much later.
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### Re: Speed of Propagation of a Magnetic Field.

voltardark wrote:I know its hard to accept, but its true.

[citation needed ]

gmalivuk
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### Re: Speed of Propagation of a Magnetic Field.

Dammit, when I approved that first post, it looked like it was a typical long-rod relativity question, posed byan intelligent layperson willing to learn what they didn't know about physics.

Now it appears as though voltardark may simply be another Internet Crackpot.
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snow5379
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### Re: Speed of Propagation of a Magnetic Field.

To be fair I'm a crackpot and can still contribute! Also sometimes when you explain things it helps people with crackpot theories understand better. I used to have some crazy notions about relativity before I understood it.

Gravity and electromagnetic waves travel at c. If the source is moving some weird things can happen with said waves but they still only ever travel at c. Moving magnets or gravitational sources that go off and on are part of a number of larger more complex experiments and it's pretty much confirmed that you can't send information faster than c.

One experiment that comes to mind is the measurement of which direction you fall towards in accordance with a moving star or something. Do you fall towards where the star is or where it was when it transmitted the gravity? What if the star disappears between the time it transmits the gravity waves and the waves reach you? Such experiments prove that all gravitational information travels at c even if that information is the absence of gravity after a star explodes.

voltardark
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### Re: Speed of Propagation of a Magnetic Field.

Future studies will prove if i'm that wrong...
Well i'never thought i would be insulted for speaking my mind...

In the above spinning yo-yo experiment what would happen if instead of cutting the rope, make it simply vanished. Propagation? how much time the yo-you would keep spinning without a rope ?
Cancellation of force is or is nearly instantaneous.

FancyHat
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### Re: Speed of Propagation of a Magnetic Field.

voltardark wrote:In the above spinning yo-yo experiment what would happen if instead of cutting the rope, make it simply vanished.

Well, that's cheating!

You're obviously just trying to force the outcome you want, by instantaneously removing all the rope instead of allowing time for the change in tension to propagate along it.

You're like a really bad stage magician who makes a volunteer disappear by putting them in a cabinet, closing the door, opening the door again, leading them out and off stage for all the audience to see, closing the door, and then opening the cabinet door to reveal that the volunteer has vanished!

You must be trolling, and are just rubbing our noses in it now.
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### Re: Speed of Propagation of a Magnetic Field.

voltardark wrote:Cancellation of force is or is nearly instantaneous.

No it isn't.

• Find a slinky and a big step-ladder (or anywhere were you can fully extend it by holding it up).
• Hold the slinky at a height and let it fully extend.
• Let go.

The bottom of the slinky will not drop when you let go; the 'information' that you let go has to reach it first, and that takes time. Specifically, it takes same length of time as it takes a pulse to traverse the length of the slinky, which is also the time it'll take for the bottom of the slinky to leave the ground if you picked up the top end at speed.
There are videos demonstrating this on youtube if you lack the high-tech equipment to perform this groundbreaking experiment, or are just plain lazy.
Now stop spouting bullshit.

voltardark wrote:In the above spinning yo-yo experiment what would happen if instead of cutting the rope, make it simply vanished. Propagation? how much time the yo-you would keep spinning without a rope ?
Cancellation of force is or is nearly instantaneous.

Cutting the rope will result in a delay: the pulse has to travel the length of the remaining rope.
Making the rope vanish completely (through magic) is, from the yo-yo's point of view, is the same as cutting the rope arbitrarily close to the yo-yo (although there's still a delay for the person holding the yo-yo). But that isn't relevant, because that isn't what happens when you remove a source of any form of radiation.

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### Re: Speed of Propagation of a Magnetic Field.

Well i see we can't talk freely here without being insulted.
Openness of mind is what make science go forward.
You are behaving like the one who told humanity that the earth was flat for so long and burned the heretics...
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### Re: Speed of Propagation of a Magnetic Field.

voltardark wrote:In the above spinning yo-yo experiment what would happen if instead of cutting the rope, make it simply vanished. Propagation? how much time the yo-you would keep spinning without a rope ?
Cancellation of force is or is nearly instantaneous.

The problem with this reasoning is that cutting the rope and vanishing the rope are two very different things. Let's consider each case in detail, and then think about which one corresponds to the sun vanishing:

If you cut the rope, that is an event that takes place at one location. At the instant you cut the rope, none of the molecules have moved significantly yet, they have merely become disconnected at one place. As time progresses, microsecond by microsecond, the two newly disconnected ends of the rope leap apart, under tension from the rest of the rope. However, as the piece of rope going to the yo-yo accelerates outward, conservation of momentum dictates that the rest of the rope and yo-yo must accelerate inward. In effect, the yo-yo doesn't notice that the string has been cut until the wave of rope losing tension reaches it. This wave travels at the speed of sound in the rope. Notice that if you cut the rope at the yo-yo end, the yo-yo would notice the change immediately, but the finger spinning it would have to wait for the tension to be released.

Now suppose the rope vanishes instead. This is equivalent to cutting the rope in two places simultaneously*: at the finger, and at the yo-yo. They both instantly feel the tension release, because it was instantly released at both ends.

And now the vanishing sun. When the sun vanishes, the gravitational "rope" is cut at the sun. The "release of tension" in the "rope" travels at the speed of gravity (which is equal to the speed of light), and the earth feels it several minutes later.

*Also simultaneity is relative, so a rope can only be said to "vanish" in a single frame of reference. Pick your favorite.

edited to increase emphasis of footnote at suggestion of eSOANEM.
Last edited by scarecrovv on Thu Apr 25, 2013 4:32 pm UTC, edited 1 time in total.

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### Re: Speed of Propagation of a Magnetic Field.

voltardark wrote:Future studies will prove if i'm that wrong...

No. Current studies have proven that you're wrong.

You haven't given some argument which is consistent with current observations but gives different predictions from accepted theory (which could still be crackpotish, but would at least be intellectually honest); what you have done is make a statement which directly conflicts with observation. It cannot be correct.

voltardark wrote:Well i see we can't talk freely here without being insulted.
Openness of mind is what make science go forward.
You are behaving like the one who told humanity that the earth was flat for so long and burned the heretics...
So it's goodbye for me and i'm sure i won't be missed.

Openness of mind != accepting anything anyone says.

Furthermore, the cornerstone of science is observation. Observation disagrees with you. As has been said, their are videos of slinkies being dropped in slow motion which directly test and disprove what you are claiming to be true.

The people who correctly realised the earth was round did not make a statement contradictory with observation. They made a statement consistent with observations at the time (that the earth looks pretty flat at a human scale) but gave different predictions (that the earth doesn't look flat at a larger scale). This is completely different from what you are doing.

scarecrovv wrote:Now suppose the rope vanishes instead. This is equivalent to cutting the rope in two places simultaneously*: at the finger, and at the yo-yo. They both instantly feel the tension release, because it was instantly released at both ends.

And now the vanishing sun. When the sun vanishes, the gravitational "rope" is cut at the sun. The "release of tension" in the "rope" travels at the speed of gravity (which is equal to the speed of light), and the earth feels it several minutes later.

*Yes, I know that simultaneity is relative. Pick your favorite frame of reference.

That footnote deserves to be bigger because it illustrates an important point (so I made it normal size).

The fact that it requires to spatially separated events to be simultaneous when simultaneity is relative demonstrates that in any relativistic setting (which, seeing as relativity (particularly SR which is relevant here) has been confirmed at many different scales over the last 100 or so years, any future physics must be) this situation cannot possibly exist. It is impossible to cut the rope at both ends at the same time just as it is impossible for the force to disappear instantly.
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### Re: Speed of Propagation of a Magnetic Field.

voltardark wrote:Future studies will prove if i'm that wrong...
Past studies have *already* proved that you're that wrong.

This is what differentiates between a crackpot and an honest layperson willing to learn: You apparently have already made up your mind in the face of contrary evidence, and nothing we say or do or show you is likely to correct your false understanding.

But fine, if you prefer to think of yourself as the lone genius heretic leaving a forum full of dumb close-minded sheeple, go for it. I hope your astounding arrogance helps you sleep at night.
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### Re: Speed of Propagation of a Magnetic Field.

Ciber wrote:Um no.
Gravity travels at the speed of light too, so we would definitely not notice the sun was gone for eight minutes.
I feel like it helps to not think of lightspeed as the speed of light, and to instead to think of the speed of light instead as the speed at which changes in the universe propagate.

You guys are obviously way more into this stuff than me, so pardon if this is a n00b question but isn't gravity the result of a curvature of space-time? And if so, is the curvature of space-time limited by the speed of light? I thought the whole notion of the Alcubierre drive was that the expansion and contraction of space would not be bound by the speed of light?

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### Re: Speed of Propagation of a Magnetic Field.

Well, the point of the Alcubierre drive is that nothing actually exceeds the speed of light in the local reference frame. Space is lengthened and contracted to make that happen. In terms of space itself being curved along a line that expands faster than light moves through that space, I think it's like the example of moving a laser pointer dot faster than light, by turning it across a wide, quick arc across a distant target, even though the photons are bound by c, as are changes in the path. But the ship can't leave its bubble at the destination precisely because that would allow information to travel faster than c between two points.
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### Re: Speed of Propagation of a Magnetic Field.

Copper Bezel wrote:But the ship can't leave its bubble at the destination precisely because that would allow information to travel faster than c between two points.

This sounds like something Douglas Adams would have imagined.
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### Re: Speed of Propagation of a Magnetic Field.

voltardark wrote:Well i see we can't talk freely here without being insulted.
Openness of mind is what make science go forward.
You are behaving like the one who told humanity that the earth was flat for so long and burned the heretics...
So it's goodbye for me and i'm sure i won't be missed.

Two plus two is equal to the square root of milk to the four-thirds-pi-r-green power. All you fools who think it's four are just stuck in the "old" math. Sure, I have no actual evidence of this fact, but you should believe it immediately because having an open mind requires you to entertain bullshit. I mean, you people are like those "teachers" who keep trying to tell me there's no such thing as phlogiston. IF THERE'S NO SUCH THING, WHAT'S BEING RELEASED IN A FIRE? Spark goes in, fire-stuff comes out. Never a miscommunication. You can't explain that.

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### Re: Speed of Propagation of a Magnetic Field.

It was going good until the last three sentences, and then I nearly died. = )
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### Re: Speed of Propagation of a Magnetic Field.

Copper Bezel wrote:Well, the point of the Alcubierre drive is that nothing actually exceeds the speed of light in the local reference frame. Space is lengthened and contracted to make that happen. In terms of space itself being curved along a line that expands faster than light moves through that space, I think it's like the example of moving a laser pointer dot faster than light, by turning it across a wide, quick arc across a distant target, even though the photons are bound by c, as are changes in the path. But the ship can't leave its bubble at the destination precisely because that would allow information to travel faster than c between two points.

But my question is, if the expansion and contraction of space can "move" a ship from point A to point B faster than light, then why doesn't the gravity from a massive object (for example the sun) move faster than light, since it is essentially the result of space itself being bent?

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### Re: Speed of Propagation of a Magnetic Field.

Yeah, actually, I think you're right, but in reverse. The Alcubierre drive seems to require that the mass ahead of the bubble is either laid out ahead of time, or sent ahead faster than light, since its effects on its environment could still only propagate at c. I had thought that there was a cheat to this, but apparently there isn't.

Wikipedia wrote:Yet another problem is that, according to Serguei Krasnikov,[6] it would be impossible to generate the bubble without being able to force the exotic matter to move at local faster-than-light speeds, which would require the existence of tachyons. Some methods have been suggested which would avoid the problem of tachyonic motion, but would probably generate a naked singularity at the front of the bubble.[7][8]

I also like the fact that no information beginning in the ship can ever reach the front of the bubble, which I guess is effectively advancing at c relative to the ship, meaning....

Wikipedia wrote:A paper by José Natário published in 2002 argued that it would be impossible for the ship to send signals to the front of the bubble, meaning that crew members could not control, steer or stop the ship.[18]

It really is a little bit Adams. And I guess that's built in; I mean, it started from the math, but it seems like a crazy thought experiment from there in trying to find every possible way in which this thing could never, ever work.
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### Re: Speed of Propagation of a Magnetic Field.

Umm... guys... I am not spam. I am still not spam. This is an original thought... what gives?

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### Re: Speed of Propagation of a Magnetic Field.

I wrote 4 paragraphs, not crazy long, topic relevant, with no links whatsoever. Why would it say "This message was flagged as spam and has been denied." Do you not welcome new members?

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### Re: Speed of Propagation of a Magnetic Field.

Maybe you wrote something that looks like the innocent nonsense spammers sometimes post before they unleash all the spam links. Your username doesn't inspire confidence either - again, it looks like spammer nonsense.

But hey, repost it and see what's up.

Also: don't double-post. It was 3 minutes between your two posts, you could have just edited the first one.
(defun fibs (n &optional (a 1) (b 1)) (take n (unfold '+ a b)))

davidstarlingm
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### Re: Speed of Propagation of a Magnetic Field.

cphite wrote:You guys are obviously way more into this stuff than me, so pardon if this is a n00b question but isn't gravity the result of a curvature of space-time? And if so, is the curvature of space-time limited by the speed of light? I thought the whole notion of the Alcubierre drive was that the expansion and contraction of space would not be bound by the speed of light?

Others have already explained, but another good way is to think of an Alcubierre drive as if you're standing on a surfboard in a pool:
Spoiler:

This happens to be a wave pool, so there are wave generators all around it (excuse my very crudely drawn wave generators):

Unfortunately, these wave generators are not very strong:

So the waves they produce can't move any faster than 5 mph:

This is unacceptable. So, in order to increase your travel speed, you program the wave generators on the sides to fire in sequence:

And you make sure that the timing of the firing sequence moves much faster than 5 mph:

....so that when the two waves meet, their combination point moves at the sequence speed, rather than the wave speed:

....boosting you along much much faster than the 5 mph of any individual wave:

Weeee!

We can do the same thing with the curvature of space. Any individual curvature wave can only move at the speed of light, but the gravitational potential at the merge of two waves is an effect independent of the wave speed. You can set up the sequence for any speed you like.

Unfortunately, this has some drawbacks. Just like you had to program the wave generators to fire in advance, you have to set up the production of the Alcubierre curvature waves in advance, and at normal (sublight) speeds. So even if you could acquire the appropriate exotic matter needed to produce the curvature waves you want, you still could only use it if you set up the path at normal speeds. More like a trolley than a spaceship.

gmalivuk
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### Re: Speed of Propagation of a Magnetic Field.

OCCOMBILL wrote::( I wrote 4 paragraphs, not crazy long, topic relevant, with no links whatsoever. Why would it say "This message was flagged as spam and has been denied." Do you not welcome new members?
We welcome new members all the time, and most of them never use the words in our spam filter, which automatically flag a post for rejection. If you inadvertently said anything that is extremely common in spam posts, your post was mistakenly flagged. Sorry about that, but everything on the list gets used in spam attempts multiple times a day, so we'll keep the list as it is and deal case-by-case with the legitimate users who accidentally set off the spam detectors.

Either now or after you've made a few more posts around the forum, try posting the same thing here again and it won't be rejected.
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