relativity riddle

[enroger]

Say we have two electromagnet A and B a light year apart from each other. Someone turn on A and a magnetic field is created and propagate at c toward B. <br /><br />It takes a year for the mag field from A to arrive B, say someone turn on B just before mag field arrive. B is then attracted/repelled by the field.<br /><br />*****But since information can not travel faster than c, when B start accelerating A would not know!!! == /> No force is on A and A experience no acceleration!!!<br /><br />So we got an initially at rest system suddenly have non zero momentum, and the violation of conservation of momentum would last at least a year!!!! Did I miss something trivial or there is something profound in this?

 

[vogon13]

Anything B does takes a year to get back to A.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[origin]

<font color="yellow">So we got an initially at rest system suddenly have non zero momentum, and the violation of conservation of momentum would last at least a year!!!! Did I miss something trivial or there is something profound in this?</font><br /><br />I don't see the violation of the conservation of momentum. There is no time limit on the conservation of momentum. <br /> <div class="Discussion_UserSignature"> </div>

 

[enroger]

Exactly. So while B got accelerated, A feels nothing. Bye Bye Newton's third law.<br /><br />This could be used to build a reactionless drive:<br />step 1: A turn on, mag field out<br />step 2: just before A's mag field arrive, B turn on with a opposite field direction from A so B gets repelled. And B's mag field travel to A too.<br />step 3: just before B's mag field reach A, A reverse current direction to reverse field direction= /> A is attracted to B when the field arrive.<br />........... get the picture?<br /><br />as long as A & B switch field direction exactly out of phase, all the force acting on them are of the same direction.<br />

 

[vogon13]

No,<br /><br />you don't.<br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[SpeedFreek]

<font color="yellow">Exactly. So while B got accelerated, A feels nothing. Bye Bye Newton's third law.</font><br /><br />Newtons Third Law: <i>For every action, there is an equal and opposite reaction.</i><br /><br />So magnet A turns on and its field heads towards B, and a year later just before that magnetic field arrives, B turns on its own field, and is repelled. Of course, a year later A is repelled.<br /><br />A's action took a year to affect B and B's action took a year to affect A. Everything is equal and opposite.<br /><br />Think of "Newtons Cradle", those 5 chrome balls slung so they touch each other at rest. If you pull back the end ball and let go, it swings down and stops dead, the force travels through the balls and then the ball at the other end swings up, before swinging back down and sending the force back again.<br /><br />When the ball at one end is at rest, the ball at the other is moving. There is a delay in the reaction as the force propagates. <b>This is exactly the same thing.</b> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

 

[enroger]

"So magnet A turns on and its field heads towards B, and a year later just before that magnetic field arrives, B turns on its own field, and is repelled. Of course, a year later A is repelled. "<br /><br />Ordinarily, a year later A is repelled. But A is repelled due to the magnetic field B sent out right? What if A reverse it's magnetic field just before B's field arrive? Then it's attraction not repel! <br /><br />I think there is a slight difference between this and Newtons Cradle. As the "impact ball" hit the "middle balls", there is a compression wave travel at the speed of sound toward the other end, so if you add up all the momentum of the atoms in the system they will still conserve.

 

[dragon04]

Um.. No....... Newton's Cradle would behave the same way in 1g but in a vacuum. <br /><br />Further, in terms of your space magnets, Newton is okay with the time between "transmission of magnetism" and reaction.<br /><br />He only gets bent out of shape if something doesn't go his way once the two objects <b>are</b> interacting. What happens prior to that interaction is no concern to him.<br /><br /> <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>

 

[enroger]

"Newton's Cradle would behave the same way in 1g but in a vacuum. "<br /><br />By speed of sound I didn't mean the wave travel in air, what I mean is that all mechanical interaction/wave is limited by the speed of sound in the medium, in this case the balls. So the momentum is actually carried in the mechanical wave and is not lost. therefor the newton's cradle is momentum conserved AT ALL TIME.<br /><br />I see that all of you agree that the time lag between transmission of momentum is ok. Can some one please read my scheme of using this to build a reactionless drive?<br /><br />I am just being the devil's advocate here, I don't think the reactionless drive would really work but can anyone point out why?

 

[alokmohan]

What is Newtons cradle?

 

[thebigcat]

*sniff sniff sniff*<br /><br />I smell troll.<br /><br />Watch your step, friend <b>alokmahan</b>. They tend to poop anywhere they want.<br /><br /><br /><br /> <div class="Discussion_UserSignature"> </div>

 

[Swampcat]

Newton's cradle.<br /><br />Google is your friend <img src="/images/icons/wink.gif" />. <div class="Discussion_UserSignature"> <font size="3" color="#ff9900"><p><font size="1" color="#993300"><strong><em>------------------------------------------------------------------- </em></strong></font></p><p><font size="1" color="#993300"><strong><em>"I hold it that a little rebellion now and then is a good thing, and as necessary in the political world as storms in the physical. Unsuccessful rebellions, indeed, generally establish the encroachments on the rights of the people which have produced them. An observation of this truth should render honest republican governors so mild in their punishment of rebellions as not to discourage them too much. It is a medicine necessary for the sound health of government."</em></strong></font></p><p><font size="1" color="#993300"><strong>Thomas Jefferson</strong></font></p></font> </div>

 

[vogon13]

{an aside to swampcat:<br /><br />the continuing aggravation of the loss of picture approval is even worse when I realize we are not seeing your fine work anymore, either}<br /><br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[SpeedFreek]

The reactionless drive wouldn't work because the <i>extent</i> or range of each field remains constant.<br /><br />So if B is repelled and is therefore further away from A than it was to begin with, then B's field will not attract A back to the original distance between them, A will end up further away, and so would repel B less on the next iteration.<br /><br />In fact, I'm not sure that A would be attracted at all, as A has repelled B away to the "edge" of where its magnetism has any influence on B, but it probably has a lot to do with the relationship between the strength of the fields and the original separation. <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

 

[enroger]

Yes, the field will get extremely weak at that distant, but it wouldn't be zero, they will always have a non zero force acting on each other.<br /><br />Assume we can dump unlimited amount of energy into the experiment, let say magnetic field strength at both sources are 10^100 Tesla <img src="/images/icons/smile.gif" /><br /><br />"So if B is repelled and is therefore further away from A than it was to begin with, then B's field will not attract A back to the original distance between them, A will end up further away, and so would repel B less on the next iteration. "<br /><br />One can always pass more current through B to make it's field stronger than A such that A will be attracted to their original distance.<br /><br />"I'm not sure that A would be attracted at all" <br /><br />Even if A is not attracted, as long as it's not repelled, momentum is already gained by the system out of nowhere. Even if only the first iteration works, it already breaks conservation of momentum. This is what the whole experiment is about.<br />

 

[Mee_n_Mac]

So B turns on his field and so reacts just as A's field arrives. B accelerates and moves. B's field travels towards A. A will eventually (1 year later) react and move equally but in the <b>opposite</b> direction. Momentum is conserved. If B shuts his field off then he stops accelerating and, 1 year later, A will reciprocate. Still, in the end, everything is conserved. <br /><br />In some regards all you have are two weights in space tied together with a spring. <div class="Discussion_UserSignature"> <p>-----------------------------------------------------</p><p><font color="#ff0000">Ask not what your Forum Software can do do on you,</font></p><p><font color="#ff0000">Ask it to, please for the love of all that's Holy, <strong>STOP</strong> !</font></p> </div>

 

[enroger]

So everybody get pass that no problem, what about the reactionless drive I mentioned above?<br /><br />"A will eventually (1 year later) react and move equally but in the opposite direction"<br /><br />A react because of B's magnetic field right? What is A change it's own field direction just before B's magnetic field arrive? Hence the reactionless drive scheme.

 

[alokmohan]

Swampcat is also friend.

 

[SpeedFreek]

No, everybody does not get past that, "no problem". <img src="/images/icons/smile.gif" /><br /><br />You are talking about a system where 2 magnets fields are being changed in turn, to cause one magnet to be repelled from the other, and then the other to be attracted back towards the first one. There <b>is</b> an equal and opposite reaction and however you manipulate the fields, Newtons third law is <b>not</b> violated.<br /><br />You are not gaining energy or momentum as such, as any gain is paid pack on the next iteration. If there is only one iteration, you cannot call it reactionless, as the second magnet <i>would</i> react and end up out of range of the first one. The magnets are either separated by the force or held together by the force. How would this produce a reactionless drive?<br /><br />Are you trying to create an electric motor with these magnets on a small scale, that fits inside a spacecraft?<br /><br />Are you trying to create something where giant magnets, positioned at the departure and arrival points, attract or repel a whole spaceship?<br /><br />Are you suggesting that the drive would be wholly within the ship it was to propel, and each magnet is somehow mechanically (or magnetically!) linked to the hull of the ship, <b>in turn</b>, so that the ship itself is attracting/repelling <i>itself</i> through space?<br /><br />By what mechanism would the thought experiment you propose be transformed into an actual working drive unit? <br /><br />Question: If the system is <i>reactionless</i>, how does it drive anything? <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

 

[Swampcat]

[OT]<font color="yellow">"Swampcat is also friend."</font><br /><br />Thanks, alokmohan. I appreciate that. I know I get on you about your cut-and-paste technique, but I'm only trying to help <img src="/images/icons/smile.gif" />.<br /><br />[/OT]<br /> <div class="Discussion_UserSignature"> <font size="3" color="#ff9900"><p><font size="1" color="#993300"><strong><em>------------------------------------------------------------------- </em></strong></font></p><p><font size="1" color="#993300"><strong><em>"I hold it that a little rebellion now and then is a good thing, and as necessary in the political world as storms in the physical. Unsuccessful rebellions, indeed, generally establish the encroachments on the rights of the people which have produced them. An observation of this truth should render honest republican governors so mild in their punishment of rebellions as not to discourage them too much. It is a medicine necessary for the sound health of government."</em></strong></font></p><p><font size="1" color="#993300"><strong>Thomas Jefferson</strong></font></p></font> </div>

 

[enroger]

The scheme exploit the time delay of field, if it really work then B will ALWAYS get repelled and A will ALWAYS get attracted. The force are in the same direction. I believe something like this should not happen, but can't find any flaw in the scheme yet.<br /><br />I'm not trying to build anything yet, but if someone give me a million dollar....... ok, here's the plan:<br /><br />Two coils(no core) placed 3 meter apart facing each other, with AC current at 100Mhz, signal at the coils are synchronized by a laser pulse with beam splitter placed exactly in the middle. As long as nothing get in the way of the mag field, you can link them to the hull anyway you want. And there you got a rocket that doesn't need fuel!<br /><br />Interested? Now whats your credit card number? <img src="/images/icons/smile.gif" />

 

[SpeedFreek]

<font color="yellow">Two coils(no core) placed 3 meter apart facing each other, with AC current at 100Mhz, signal at the coils are synchronized by a laser pulse with beam splitter placed exactly in the middle. As long as nothing get in the way of the mag field, you can link them to the hull anyway you want. And there you got a rocket that doesn't need fuel!</font><br /><br />Hang on... are <b>both</b> coils attached to the hull at the same time? <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

 

[enroger]

"Hang on... are both coils attached to the hull at the same time?"<br /><br />Ah, I see what you are getting at. Yes, both coils are attached to hull at the same time. It would not pose a problem because for the hull to transmit an acceleration from one coil to another, the transmission is far slower than the speed of light (In fact it is limited by the speed of sound of the hull material). Therefore the mag field will always get there first without any interference from hull movement.

 

[SpeedFreek]

So what we have here are 2 very powerful magnets, fixed in place within the hull of a spaceship.<br /><br />If we assume that the magnets are fixed in such a way that they cannot move, relative to each other within the hull of the ship, what would happen? If magnet B cannot move away from magnet A, are you saying that the whole ship would move instead? If magnet "A" is also fixed within the ship, how can the ship be repelled from it? <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>