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Topic: Challenging Einstein again... (Read 677 times) |
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phobos
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Challenging Einstein again...
« on: May 12th, 2003, 9:47am » |
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Alright my previous challenge was falsified by aero guy, but oh god don't I just love challenging established figure? And hence making fun of myself? um..okay.
This time there's a perfect conductor 300000000 meters long, with a X ohms resistor attached to one end. Apply a voltage at the other end and you'll see some current flowing, which immediately tells you the resistance of the resistor miles and miles away. So information can flow instantaneously. How can that be?
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| « Last Edit: May 12th, 2003, 9:48am by phobos » |
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ThudnBlunder
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Re: Challenging Einstein again...
« Reply #1 on: May 12th, 2003, 10:29am » |
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Quote:| This time there's a perfect conductor 300000000 meters long, with a X ohms resistor attached to one end. Apply a voltage at the other end and you'll see some current flowing, which immediately tells you the resistance of the resistor miles and miles away. |
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Er... no, if I remember correctly, the electricity travels at about 2c/3.
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BNC
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Re: Challenging Einstein again...
« Reply #2 on: May 12th, 2003, 10:31am » |
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Current is information. It travels at a finite rate, which is not higher than the c. 
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How about supercalifragilisticexpialidociouspuzzler [Towr, 2007]
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wowbagger
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Re: Challenging Einstein again...
« Reply #3 on: May 12th, 2003, 10:58am » |
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on May 12th, 2003, 10:29am, THUDandBLUNDER wrote:
Er... no, if I remember correctly, the electricity travels at about 2c/3. |
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I don't remember much about how fast "electricity" travels (apart from the upper bound c).
But this reminds me of the following, related question:
The particles carrying the electric current in metallic wires are known to move at a few cm/s (if I remember correctly). How come that a light bulb turns on almost instantaneously when you activate the switch, although there may be metres of wire between switch and bulb?
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redPEPPER
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Re: Challenging Einstein again...
« Reply #4 on: May 12th, 2003, 12:06pm » |
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on May 12th, 2003, 10:58am, wowbagger wrote:| The particles carrying the electric current in metallic wires are known to move at a few cm/s (if I remember correctly). How come that a light bulb turns on almost instantaneously when you activate the switch, although there may be metres of wire between switch and bulb? |
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That's because electricity is not the flow of electrons, it's the energy transmitted from electrons to electrons. I could compare it to waves in water: they go much faster than the actual water particles.
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James Fingas
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Re: Challenging Einstein again...
« Reply #5 on: May 12th, 2003, 1:59pm » |
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To transmit an electric signal, you need two wires, so let's assume we have 2 perfect conductors 300 000 000 meters long, separated by a certain distance.
The electrical signal is carried down the wire as an electromagnetic wave. If you apply it with a battery, you will get a step voltage and current going down the wire at approximately light speed (there's no dielectric except air). This step voltage and current will not be recognizeable at the other end (due to resistance in the wire and conductance of the atmosphere, the step spreads out). The step current drawn out of the battery will be related to the step voltage by the characteristic impedance of the conductor pair.
This behaviour is a direct result of Maxwell's equations. When the signal gets to the other end, it will be reflected back to the source, unless X happens to be the same as the characteristic impedance of the conductor pair.
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Speaker
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Re: Challenging Einstein again...
« Reply #6 on: May 12th, 2003, 8:52pm » |
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I don't have anything to say to James, but I would like to discuss the underlying question of simultaneous occurance.
Maybe there is a term used more currently.
I remember hearing about electron pairs that reacted simultaneously even though they were seperated. Specifically when the spin of one of the electrons was changed, the corresponding change occurred in the other electron.
I hope this is not too far off topic, just thought the genii might shed some light on this.
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ThudnBlunder
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Re: Challenging Einstein again...
« Reply #7 on: May 12th, 2003, 9:15pm » |
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Quote:I remember hearing about electron pairs that reacted simultaneously even though they were seperated. Specifically when the spin of one of the electrons was changed, the corresponding change occurred in the other electron.
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This is called the Einstein-Podolsky-Rosen (EPR) paradox.
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phobos
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Re: Challenging Einstein again...
« Reply #8 on: May 12th, 2003, 9:30pm » |
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on May 12th, 2003, 8:52pm, Speaker wrote:| when the spin of one of the electrons was changed, the corresponding change occurred in the other electron. |
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This is the famous Einstein-Podolsky-Rosen (EPR) pair thought experiment, which, in brief, involves exciting a spin zero system and let two electrons travel in different direction. After they'd travelled for a far enough distance, measure the spin of one of the electron. By doing so, you'll immediately know the spin of the other electron (which may well be light years away) because you started with a spin zero system. So information concerning the spin state of an electron light years away is transmitted instantaneously, which results in a paradox if you believe in relativity.
This is an interesting topic and had been debated for years, but my 'challenge' is not so subtle or philosophical confounding. 
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phobos
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Re: Challenging Einstein again...
« Reply #9 on: May 12th, 2003, 9:40pm » |
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Jesus Thud, you're one step ahead of me.
Quote:| the electricity travels at about 2c/3 |
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Well, it doesn't really matter how fast electricity travels. I'm not trying to get information transferred using electricity. All I want is to know the value of the resistor one-light-sec away. I apply a voltage and measure the current, and I know the resistance immediately.
Quote:| Current is information. It travels at a finite rate, which is not higher than the c. |
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Quote:| To transmit an electric signal |
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BNC and James, I'm not relying on current or EM wave to get information transmitted.
I like James post though - the real stuff involving solid EM theory. That's how signal is actually transmitted through copper wire/coaxial cable. 
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| « Last Edit: May 12th, 2003, 9:46pm by phobos » |
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ThudnBlunder
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Re: Challenging Einstein again...
« Reply #10 on: May 12th, 2003, 9:47pm » |
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Quote:Jesus Thud, you're one step ahead of me.
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Sorry about that - but your explanation was better. 
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Speaker
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Re: Challenging Einstein again...
« Reply #11 on: May 13th, 2003, 12:30am » |
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Regarding measuring the resistance of a wire at a distance.
Is this like how they can measure the resistance of the Trans-Atlantic telephone cable and discover where a break has occured?
How do they do that, measuring the distance to the break?
Wasn't there a short story by Asimov about this, putting the insurance companies out of business...
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phobos
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Re: Challenging Einstein again...
« Reply #12 on: May 13th, 2003, 1:11am » |
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Um..I didn't know anything about Asimov, but to the best of my knowledge most inter-continental telephone cables run on fiber optics nowadays. To detect 'break' or defect ultra short light pulses are sent through the fiber, and the returning light pulses are plotted against distance (2nL=ct). The 'break' is indicated by the peak of the reflected light pulse, because of Fresnel reflection and TIR.
But light takes finite time to travel (so the measurement takes time), while the act of knowing the resistance at the end of the long conductor seemed to be instantaneous.
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James Fingas
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Re: Challenging Einstein again...
« Reply #13 on: May 13th, 2003, 9:46am » |
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As part of what I was saying before, the current that comes immediately out of your battery is determined entirely by the characteristic impedance of the conductors. If you use 50 Ohm cable, you get 0.18 A out of the battery instantaneously, regardless of what the value of X.
When the current/voltage pulse reaches the end, then you get a reflected pulse if X is not 50 Ohms. Over time, these pulses travelling back and forth establish the final current and voltage on the wires: 9V, with a current of 9V/X.
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Chronos
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Re: Challenging Einstein again...
« Reply #14 on: May 16th, 2003, 12:06am » |
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Speaker, the short story you're thinking of is "Lifeline", and it's by Heinlein, not Asimov. A fellow invents a device which can reliably tell you when you're going to die, regardless of the cause. He uses the analogy of detecting a break in a wire to explain it.
And phobos, how do you figure that you're not transmitting information? Initially, the current you measure won't have anything to do with the resistor at the other end. After the information about the resistor has had time to propagate, you'll eventually measure the "correct" current.
Also, the version of the EPR experiment you just described doesn't require any communication between the electrons, or any quantum effects at all, to explain it. You could do the same thing with envelopes and the USPS. In order to see the quantum weirdness, you need to be able to measure the spins along arbitrary axes, and do a statistical analysis.
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phobos
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Re: Challenging Einstein again...
« Reply #15 on: May 16th, 2003, 11:29am » |
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Well done James, that's really smart to use transmission line model to explain the stuff. I was actually thinking along a simpler line, that for a really long conductor, one has to consider its equivalent inductance and capacitance as well. As a result the voltage developed as well as the current measured will change over a period of time. So although initially we can measure a current that flows through the conductor, it gives us no information about the resistor on the other end until a much later time.
Quote:| And phobos, how do you figure that you're not transmitting information? |
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Er..I suppose I'm cooking up a small puzzle (a flawed argument) for others to figure out?
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