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# Hello, I'm on the train

Helen Thomas
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Posts: 1759
How fast must a train be travelling for the Doppler effect to put all the mobile phones being used on it out of action?

(While browsing some scientific site I came across this question which I found very intriguing. It IS possible to put all those mobile phones on the train out of action. )

Fletcher Estes
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Posts: 108
The speed of sound!

Stan James
(instanceof Sidekick)
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Posts: 8791
Well, that would make it hard for people standing by the tracks to hear the phones, but folks moving their air along with them wouldn't be bothered. What kind of frequency ranges do cell phones work on? You'd have to shift the low end up beyond the high end (and vice-versa). Or just each phone outside of its own operational range. And it would depend on how high the cell towers are ... satellite phones would be a real challenge.

Helen Thomas
Ranch Hand
Posts: 1759
Stan's nearer the mark if you follow the logic Speed of sound

Altitude Temperature m/s km/h mph knots
Sea level 15�C (59�F) 340 1225 761 661
11000m-20000m
(Cruising altitude of commercial jets,
and first supersonic flight) -57�C (-70�F) 295 1062 660 573
29000m (Flight of X-43A) -48�C (-53�F) 301 1083 673 585

This logic is beginning to look a bit dubious to me and nigh impossible.
So mobile phones on trains will be with us for a looong time/ for ever.

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

Dan Maples
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Posts: 153
is the 32 million speed the speed at which point the train is traveling at the same speed as the wave? because if that is the case, then the phone would stop working at a much slower speed, as long as the frequency gets distorted enough before the point of the standing wave. Or is the 32 million speed the point were the distortion becomes to great for the phone to overcome?

Helen Thomas
Ranch Hand
Posts: 1759
the speed of the wave is practically the speed of light.

33 million km/h is 3% of that.

Anyway that's what I understood from the logic given above.

Dmitry Melnik
Ranch Hand
Posts: 328
I have an intuition that factors like maximum range phones can connect a tower from, distance between towers and time required for a phone to establish connections would be more important for finding the maximum speed of train. Not the Doppler's effect. And the resulted speed would be much slower