Dang, and here I was hoping for a few moments of peace and quiet, sans ringtones. (What if the two parties were on separate trains, headed in opposite directions? Would the speed requirement be halved?)
Question
How fast must a train be travelling for the Doppler effect to put all the mobile phones being used on it out of action?
- Philip Woodward , Malvern, Worcestershire, UK
Answer
The Doppler effect says that if an object emitting a wave is approaching the receiver of the wave, it is heard at a higher frequency by the following factor:
frequency heard = frequency sent x (1/(1-speed/c))
where c is the speed of your wave (practically the speed of light in this case).
If the emitter is moving away from your receiver, its frequency is lowered by the following factor:
frequency heard = frequency sent x (1/(1+speed/c))
In the UK, mobile phones use either the GSM900 protocol, broadcasting in the 890-915 megahertz range for uplinks and 935-960 MHz for downlinks, or the GSM1800 protocol, with equivalent ranges of 1710-1785 MHz and 1805-1880 MHz.
To ensure that one end of each of these ranges is Dopplered out the other end, one would need to travel at a speed of roughly 3 per cent of the speed of light for GSM900 signals, or 4 per cent for GSM1800. This amounts to a train speed of between 32 million and 43 million kilometres per hour.
Somehow I don't think that's likely on UK trains. Or indeed any others, if I'm being reasonable.
- Simon Scarle , University of Manchester Institute of Science and Technology, UK
Posted by thinkum at July 14, 2004 07:56 PMYou're on the train? Aren't you afraid of being rail-roaded? (ba-dum ching)
Yet another beautiful idea -- cell phone disfunction as a result of frequency shifting -- shot down. It would be a lot easier to simply make sure the train cars are Faraday cages.
Posted by: PyeCat at July 15, 2004 12:31 PMOkay, now you have to 'splain Faraday cages. *g*
Posted by: Thinky at July 15, 2004 02:03 PMFaraday cages: Long ago, beautiful young Princess Faraday was imprisoned by her evil stepsister in a cage built out of plutonium. Prince Katreen slew the stepsister, smashed open the cage, and freed the Princess, who took her rightful place on the throne -- only to die of radiation poisoning less than 24 hours after the coronation. And that's the tragic story of Queen Faraday.
Posted by: weaselchopsaw at July 16, 2004 12:04 AMCan't *quite* see Katreen as a guy... *g*
Posted by: Thinky at July 16, 2004 01:19 AMNeither could the Princess.
Posted by: weaselchopsaw at July 16, 2004 02:27 AMWell, radiation poisoning *is* a little hard on the eyesight...
Posted by: Thinky at July 16, 2004 02:31 AMA fairaday cage is a horrible prison where they make you attend gatherings to judge things the largest ear of corn or the alpaca with the nicest looking coat once every 24 hours. They only feed you on lemonade, kettle corn, corndogs, and cotton candy. It's a horribly sadistic form of imprisonment, and if your jailers are really nasty, there are clowns in there with you.
Or maybe it's a an enclosure with no apertures (holes, slits, windows or doors) made of a perfectly conducting material. No electric fields are produced within the Faraday cage by the incidence of external fields upon it or by currents flowing on the perfect conductor; that is, the perfectly conducting enclosure is a perfect electromagnetic shield.
Posted by: Noah at July 16, 2004 11:09 AMExcept for the clowns (and the pun), that first option doesn't sound too bad. The second option would make it just a tad difficult to breathe, though...
Posted by: Thinky at July 16, 2004 01:40 PM