Speed of magnetism

Discussion in 'Physics & Math' started by Syzygys, Nov 20, 2006.

  1. Syzygys As a mother, I am telling you Valued Senior Member

    This is a similar question to the speed of gravity. When we turn on an electromagnet, how fast its effect takes place in its enviroment???

    Well, let's throw in this too: What is the speed of electricity? ( guess it depends on the wire or conductive material)
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  3. James R Just this guy, you know? Staff Member

    At the speed of light.

    The speed of an electrical impulse in a wire (e.g. when you turn on a light switch) is close to the speed of light. The speed of the actual electron flow in the wire is actually very slow - far less than walking speed.
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  5. CANGAS Registered Senior Member

    Right this time. You have been peeking in those textbooks, haven't you?

    The velocity of the electric wave in a conductor is determined by the permeability and permittivity of the conductor, and in most conductors amounts to about 2/3 of the velocity of light in vacuum.

    The drift velocity of a current electron in a conductor is dependent upon a number of variable factors and can have a very wide range. In normal human experience, such as the drift velocity of current electrons in the wiring of your car or your house, it is in the range of someting like one yard per HOUR.

    A very lazy snail could outrun the average velocity of the current electrons in the wires of your house.

    The thread starter question did not mention wiring, and so we must refer back to Maxwell's equations, and in particular, to his wave equation, for a satisfactory general answer. The speed of the disturbance of the electric, or, if you prefer, the magnetic, wave is provided by his formula taking into account the aforesaid permeability and permittivity of the medium conducting the wave. The wave equation is easily found in virtually any physics textbook published within the past century.
    Last edited: Nov 23, 2006
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  7. 68gto Registered Member

    Let's look at this a different way.

    How long does it take for light generated from the Sun to reach Earth? Minutes, 10, 15, let's say it takes 10 minutes.

    Now picture that somehow the Sun is suddenly taken away or destroyed in a second. If the speed of magnetism was the same as the speed of light, then the earth would continue to orbit NOTHING for 10 minutes. I don't think that is the case. I think the Earth would careen off into space immediately, so what is the speed of magnetism?
  8. kurros Registered Senior Member

    Unfortunately, despite what you may think, the earth will in fact continue to orbit nothing for 10 minutes. However, the sun will still appear to be shining brightly to all those on the ground looking up at it, so really this isn't such a weird thing. In fact thanks to special relativity we know that causal influences can't propagate fast than the speed of light, so there is no way the Earth could be 'aware' that the sun was gone any faster than this. If it could then you could use gravity to send messages backwards through time and cause all kinds of messed up paradoxes.
  9. 68gto Registered Member

    If gravity is no faster than the speed of light how come light can't escape from a black hole, yet the black hole itself has gravity that pulls in light. If gravity weren't faster wouldn't the black hole just disappear? I'm just an average person that thinks this is interesting, not trying to be argumentative.
  10. woowoo Registered Senior Member

    "The phenomenon lag is simply due to the limited mechanism of the brain.
    We have to wait for the afterimage in order to realize.
    The norm of Einstein is absolute speed instead of at rest. "At rest" was what we called instantaneous in our innocence of yesterday. We evolute toward ever lesser brain comprehension lags__ergo, toward ever diminishing error; ergo, ever diminishing misunderstandings; ergo, ever diminishing fear, and its brain-lagging painful errors of objectivity; wherefore we approach eternal instantaneity of absolute and total comprehension.
    The eternal instantaneity of no lag at all. However, we have now learned from our generalizations of the great complexity of the interactions of principles as we are disembarrassed of our local, exclusively physical chemistry of information-sensing devices__that what is approached is eternal and instant awareness of absolute reality of all that ever existed.
    All the great metaphysical integrity of all the individuals, which is potential and inherent in the complex interactions of generalized principles, will always and only coexist eternally."

    R. Buckminster Fuller


  11. AlexG Like nailing Jello to a tree Valued Senior Member

    None of which has anything to do with physics or the OP.
  12. CptBork Valued Senior Member

    The black hole's gravity has already been established before the light enters, so it doesn't matter that the initial ripples in the spacetime fabric travelled out at lightspeed when the black hole first came together, the aftermath of the ripples stays behind until the black hole is somehow removed again.
  13. temur man of no words Registered Senior Member

    Just to clarify, it is gravitation that matters here, not magnetism.

    Now for the main question, even when the Sun is out there like it normally is, the Earth is not orbiting the Sun because it somehow "feels" where the Sun is. The Earth can "feel" the gravitational field only in its immediate vicinity, and this gravitational field is so that anything moving only under the influence of it geometrically looks like orbiting around the Sun. If the Sun disappears in a second, then the gravitational field right at where the Sun was will be changed enormously, and this change will affect the gravitational field at the points very close to where the Sun was, and then this change in its turn will affect the points slightly further, and so on the change will propagate at the speed of light. Until this disturbance reaches the Earth, the gravitational field will be as it was before.

    This is also related to your other question, where the light only "feels" the gravitational field locally and move in accordance with it. The light does not "know" where the center of the black hole, and nothing needs to be transferred at faster speed than light between the point where the light is and the center of the black hole in order for the light to move the way it moves.
  14. Syzygys As a mother, I am telling you Valued Senior Member

    You are here dealing with gravity, not magnetism. But the question is still interesting.

    I am not sure gravity speed can be measured. (How fast gravity effects things)
  15. Motor Daddy Valued Senior Member

    Gravity as we know it is a result of an imbalance of a density order. The imbalance remains as long as there is a force opposing the order, ie, you hold a rock 2 feet above the surface of the Earth. When the force is released the rock goes towards the core and the density order starts to be restored, but the density order is not restored instantly, if at all. It takes time for change to occur.

    In your example, if the sun were to blow up, it would take time for the force to build on the Earth to send it outwards. You can't just remove the sun instantly from the solar system, it's impossible, as ALL change takes time to occur. All force takes time to build. Forces are built like 68 GTO's increase speed, you don't increase from 30 MPH t0 75 MPH instantly, change takes time to occur!
    Last edited: Nov 10, 2010
    Froilan likes this.
  16. James R Just this guy, you know? Staff Member

    Gravity is not caused by "density order". That's pseudoscience.
  17. defridgerator Registered Member

    Reply to Kurros

    Kurros: If I'm understanding you correctly, you're arguing that gravity equals the speed of light. I'm not exactly certain I can agree with you on that. While I can agree that the Earth would continue to see rays of the sun for about 8 minutes (on average 8 minutes and 20 seconds), I don't think you can say that the earth would continue spinning at the same speed around the sun.

    Anytime you remove power from an object, say a power drill (I know the analogy isn't even close), the drill if in use, will start slowing down the moment power is taken away.

    I don't believe the Earth would continue rotating at the same speed. The earth would continue to rotate around, but the trajectory of the Earth in relation to the Sun would change in relation to which direction the Sun went. Newton's first law states that objects in motion will continue on at the same speed unless acted upon by an unbalanced force. The Sun going "poof" or just moving would affect the Earth immediately. Newton's 2nd law - F=ma. Force = mass X acceleration. Removing the sun would cause a decceleration of all objects affected. All the planets would go careening off into the galaxy, and where the Earth ends up, is anyone's best guess. Assuming there is anyone alive to care.

    I'm not a physicist so maybe this is too simplistic, but believing that the Earth would continue on for 8 minutes and 20 seconds without slowing down or showing other reactions to the missing gravitational field is very far-fetched to me.
  18. billvon Valued Senior Member

    It would indeed. Observations of faraway stellar objects (binary pulsars) and of tides right here on Earth (specifically phase lag of the tides) confirms that the "speed of gravity" is very close to the speed of light.

    Not if your extension cord is 93 million miles long. The drill would continue to run at full speed for about 8 minutes.

    Not until the change in gravity propagated to the Earth. A solar sail would see the same thing - it would continue generating thrust for 8 minutes or so.
    It does seem that way; a lot of the implications of the speed of light are indeed odd.
  19. billvon Valued Senior Member

    Or it's just Motordaddy.
  20. Robittybob1 Banned Banned

    So what do you want to happen? The removing the Sun will also remove the Earth. (i.e.there is a permanent wire rope between the two). Or would you prefer that the moment the Sun disappears the Earth accelerates radially.
    No I think it will continue on tangentially, so there would be no sudden change in direction.
    Delay in action - yes I think so.
  21. Froilan Registered Member

    Actually I think that makes sense. As we all know, according to general relativity and as opposed to Newton's concepts, gravity is not a force but an effect of the distortion of space-time caused by the mass of objects. I think such distortion of space-time may well be called an "imbalance of a density order" without it being pseudoscience, it's actually an interesting point of view, as to me. So, assuming we are talking under the paradigm of general relativity, the question here is: how long does it take for that distortion (imbalance) to propagate and affect it's surroundings?

    Actually I think something odd: what if the speed of light wasn't constant but dependent on the density of the "media" it is traveling by? Because light is one thing, and another thing is the media it travels by, right? but.... what if that wasn't the case? what if what we call light, the waving electromagnetic field, is actually the media itself? So, is the speed of light affected by the gravitational field (read: space-time distortion) of the objects it is traveling next to?

    Galileo used to think objects fall at a constant speed, because he could not sense their acceleration when falling from short distances, but Newton then realized objects actually accelerate when falling into a gravitational field. What if the exact same happens to light?

    I think that's an interesting thought too. The accepted fact is that light is affected by gravity to a point where there's no escape, right? So, is it affected in a way different than the other objects would? I wouldn't think so. I'd say the speed of light also changes depending on the intensity of the gravitational field it is being affected by. So what is this gravitational field anyway? What is space-time? how fast does it travel? or... does it travel at all? because I would say it is the distortions (waves, pulses, etc) that travel, not the media itself. So, if the media does not travel, and gravity (whatever it be) is the media, then gravity does not travel, the distortions (density imbalances) in it do at an speed that depends on the intensity of the field they are traveling by.
  22. paddoboy Valued Senior Member


    Every massive object has an escape velocity. The escape velocity at the EH of a BH is simply "c"...So therefor light does not escape.
    Gravity exists as we all know and the best description we have of it is spacetime curvature or warping.
    The earth has gravity caused by it warping the spacetime within its vicinity.
    The Earth's spin also affects the surrounding spacetime much as stirring a cup of coffee is affected. This has been measured by GP-B.
    The speed of light is always "c" from its frame of reference or in that vicinity.
    It may appear to go slower from an outside frame of reference, but the curved spacetime means it has a longer path to travel.
    What is spacetime? Spacetime is that which first evolved from the BB. And from it the four forces we are familiar with, and matter evolved through a recognised process.
    Experiments continue to show that there is no 'space' that stands apart from space-time itself...no arena in which matter, energy and gravity operate which is not affected by matter, energy and gravity. General relativity tells us that what we call space is just another feature of the gravitational field of the universe, so space and space-time do not exist apart from the matter and energy that creates the gravitational field. This is not speculation, but sound observation.
  23. Russ_Watters Not a Trump supporter... Valued Senior Member

    Gravitational changes (gravitational waves) propagate at the speed of light.
    That's a hodgepodge of separate issues. Light does not require a medium to travel on (or you could say it generates its own), but when traveling through a medium, it gets slowed down. And gravity's effect on light is to alter its path or energy.
    You may be confusing Galileo and Aristotle and you have the thing that was thought wrong (I don't believe that Aristotle thought objects fell at constant speed, but rather believed acceleration depended on mass). Galileo is the one who proved that all objects accelerate at the same rate due to gravity.
    Essentially, it is.
    Not correct.
    Those are largely unscientific questions. In science, "what is...?" is simply answered by listing the properties and behaviors of the thing you are describing.

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