My son and I were listening to RadioLab in the car – as we often do -and heard an interesting short on ‘What the Slinky knows‘. What an interesting podcast. Check out the attached video of the point in question. Here we see a slinky being held up at the top, but allowed to dangle until it settles and becomes still. At this point, the top is released, but the bottom doesn’t move:
The question posed in the podcast is: Why does the bottom of the slinky seem to levitate until the top reaches it and then the whole things begins to move together. The basic question is relatively simple if you think about the situation and model all the forces in operation (1. gravity – down; 2. 3. hand holding slinky-up; force of the spring’s tension – up.)
When the slinky is released, you eliminate force #2, leaving only #1 and #3 in operation. The interesting thing is that force #3 will continue to operate and pull up until the top of the slinky collapses on itself and then only force #1 remains.
But then Neil DeGrass Tyson was brought into the discussion and said something I didn’t expect. He started with what you might expect him to comment on – an immediate application to space, which seems completely appropriate. His example was about the sun. Given that the sun is several light minutes away from the Earth (just over 8 minutes), if the sun were to somehow disappear, we wouldn’t know for those intervening eight minutes.
During that time, we would be seeing light that had left the sun eight minutes earlier and everything would look fine. Then, at the 8:20 mark (give or take), we would finally see that the sun had gone missing….
…. and here’s the good part: He also said that it wouldn’t be until then that the Earth would be released from its gravitational attraction to the sun and go flying off at a tangent into space. This begs a question that I don’t know the answer to: what is the ‘speed of gravity’? Would it take eight minutes for the Earth to be released and go tumbling into space… or does gravitational attraction operate at a different speed than light?
I’m serious, I don’t know the answer – and even more frustrating, I don’t know how this question could even be asked. I’m all ears for the answer to this one.