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09.03.2002

# A physical question

I'm having a wand in a no gravity room which is long 1 light year. At the one end there is a person, at the other end there is a switch about one centimetre distanced of the end of the wand. That switch is connected to a hydrogen bomb. When the switch is pushed the bomb is ignited in that moment. Now the person moves the wand for that one centimetre in direction to the bomb.

When do you see the explosion when you are next to that person:
a) after one year?
b) after two years?
c) or even later?

3/9/2002: Andi has published the solution for this question. Thanks to all who send in their answers! You want to discuss this issue? Check out our forums!

from: Andi (6thfloor)
at: 10.12.99

My guesswork is only understandable if you look at the process of moving of that wand in the atomic realm. In my imagination the atoms which are placed in a special lattice structure that are the beginning of the wand were pushed against the next atoms.
This power of the lattice conducts to that the next row of atoms is pushed away.
Because of this, the movement is going on like a wave. In normal scale there is nothing to see of this because the way is too short to notice a delay.
So the possible answers are b) or c) while c) is more probable because the particles' moving doesn't happen with the velocity of light.

from: Andi (6thfloor)
at: 09.03.02

The answer is clear c. The wand which will have a great mass due to its length will have to be accelerated at first. This requires a large amount of energy that a human being cannot summon.
Let's say that we could release this energy and push this wand so that it moves one centimetre forward. The transmission or rather the information of this movement will be transmitted with the speed of sound which depends on the material.
Speed of sound because this acceleration is a disorder in the material, a regional change of the pressure. Such kind of disorders spread with the speed of sound. Sound waves are changes of the pressure in a medium. The speed of sound specifies the velocity with which these changes of pressure (therefore the change of the density in a medium that is limited in space and time) can spread. So the transmission of a disorder like the demanded movement is not possible instantly but only with the speed of sound.
If the wand is made of steel, only the transmission up to the bomb would take

Speed of sound in steel: 5100m/s = 5,1km/s

v=s/t

v*t=s

300 000km/s * 22 896 000s = 6 868 800 000 000km

t=s/v

6 868 800 000 000km/5,1km/s = 1 346 823 529 411,76 s = 42 707,5 years.

You could see the explosion of the bomb after

42 707,5 years + 1 year that the light of the explosion would take for the distance = 42 708,5 years.