speed of like

[skilliyay]

why is it not possible to break the speed of light using a series of gears. I calculated that a motor spinning at 5k rpm, would need a ration of 1:60K to do it

[mibn2cweus]

1. Rotation does not translate directly to velocity:
The velocity for a given rate of rotation depends on the circumference of the gear/wheel or outer radius of the spoke, etc.
2. The maths is wrong:
300.000.000 rpm is only 5,000,000rps so the circumference of the wheel turned by the final gear would require a circumference of 60 metres or a radius of 60/π metres.
3. No material could withstand a tiny fraction of the g stresses: One by one the gears would disintegrate from the outward angular momentum as the rpms increased.
4. Time dilation would grow exponentially:
Time dilation would become apparent as the velocity of the outer edge of each gear approached the speed of light. As the speed of the drive motor increased many time over in an attempt to compensate, time dilation would simply not allow any part of the gear train to reach the elusive velocity of light. Meanwhile the motor itself would disintegrate from excessive rpms in its attempt to defy the laws of physics.

[R1Geezer]

It is possible to slow c which is only constant in a vacuum. In other mediums the speed varies considerably, for example on a recent rerun of QI they claimed light has been slowed to 38mph in Sodium at close to 0 degrees K.

[jake-the-peg]

Yes that's right. At such low temperatures you get a different phase of matter - the Einstein-Bose condensate. In this state atoms stop behaving as individual entities and can do wierd tricks like this.

The real purpose isn't a party trick though. Laser light is actually used to cool the medium those last few billionths of a degree.

There's an article on it here

http://www.hno.harvard.edu/gazette/1999/02.18/ light.html

[chakka35]

mib, I think yours is a superb answer to the original question, which was about everyday practicalities.

[mibn2cweus]

Thanks Chakka :o)

On second glance it appears (as I'm sure you noticed) I too got the maths wrong regarding the relationship of the radius to the circumference which is actually:

circumference = 2π x radius

So it should have read " . . . or a radius of 60/2π metres."

Please feel free to deduct 2 points ;o)

[beso]

R1Geezer. You can slow light but you can't actually change the constant c. It is defined as the speed of light in a vacuum.

[R1Geezer]

yes silly me, I suppose I left myself open for pedantry there.

[Whickerman]

I'm out of me depth. I'll get my coat.