# light travels a maximum speed… /? [duplicate]

Possible Duplicate:
Is it possible for information to be transmitted faster than light?

we know that speed of light is an unconquerable term in physics..light takes about 1 year to travel 9500000000000000 metres that is light year....suppose i have a needle of length 95000000000000000 metres long....suppose that one end of needle is at my hand and other end is at distance 95000000000000000 metres apart. . now if i push gently the end of needle at my hand then at the same time the other end at a distance of 95000000000000000 metres will move forward.....now light has taken 1 year to travel a distance of 95000000000000000 metres..and i have made a mechanical disturbance to travel the same distance with in no time from one end of the needle to other end placed at a distance of 9500000000000000 metres apart. . in other words i have made a mechanical disturbance to travel faster than light.. . .

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## marked as duplicate by Qmechanic♦, David Z♦Jan 23 '12 at 9:12

The needle is not infinitely rigid. When you push the end of the needle you will compress the metal that the needle is made of. This creates a compression wave that will travel down the needle at the speed of sound (i.e. the speed of sound in metal) and it will reach the far end of the needle some time later.

To make it easier to picture this imagine replacing the needle by a long spring. When you press one end of the spring you can see the compression wave travelling along the spring to the other end. Obviously a needle is a lot stiffer than a spring, but the principle is the same. Your press on the end of the spring won't instantaneously cause the far end to move.

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There are no rigid bodies in relativity. The result of the experiment you described would be a displacement of parts of the needle that travels at a very high speed along the needle. This is due to the fact that the forces which keep the needle seemingly rigid are not instantaneous and need time to propagate through space.

Other interesting things you could do with such a long needle is to rotate it around an axis passing through one of its ends and perpendicular to the long axis of the needle.

In both experiments the needle would appear to be non-rigid. The concept of rigid body is a convenient approximation of reality which is only consistent with experiments in which the size of the body is very small compared to ct where t is the time scale of the experiment and c is the speed of light in vacuum.

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