• Question: If I have a stick that is a light year long, and I push one end, the other end will move before light can reach it, wont it? So, would the stick be moving faster than light?

    Asked by bleddy to Angela, Gabriele, Karen, Maria, Shane on 11 Nov 2013.
    • Photo: Karen McCarthy

      Karen McCarthy answered on 11 Nov 2013:

      Ah this classic question! The answer is basically no, because when you poke someone, you are pushing an energy wave through a stick via the atoms in the stick. If this stick happens to be a light-year long stick, it would take a long time for that energy to be sent to the end of the stick, so the pokee wouldn’t feel the poke for a long time.

      This article explains much better, and offers some more examples too.


    • Photo: Shane Mc Guinness

      Shane Mc Guinness answered on 11 Nov 2013:

      Wow, Bleddy, that is an amazing question and one that, to be honest, had me stumped for a quite a while this morning over my cornflakes! It’s something that I’ve been puzzling about and I think I can explain. The article Karen gave seems to cover it, but rigidity is the key: basically nothing can be “stiff” enough so that when you give it a little poke the other end of it will move instantly. In other words, everything is springy! Even diamonds here on earth. If the stick was infinitely rigid, then it would be breaking one of the most basic rules of the universe by theoretically being faster than light! So the shockwave of you pushing the stick would still have to travel at the speed of light (though probably a lot slower!). Mind bending question, but hopefully the answer wasn’t…..!

    • Photo: Gabriele De Chiara

      Gabriele De Chiara answered on 12 Nov 2013:

      Well, I think Karen and Shane explained the question very well. The key concept here is that no massive object, including messages or the deformation on the stick can travel faster than light. I would leave you with a puzzle: if I’m on a space ship travelling at half the speed of light with respect to the solar system and I shine a laser in the forward direction, how fast is the speed of light with respect to the solar system?

    • Photo: Angela Stevenson

      Angela Stevenson answered on 12 Nov 2013:

      That’s a really neat question bleddy! I was very puzzled by it for a while.. I think Gabriele sums it up very well though: nothing can travel faster than light! In fact, it’s so fast that it only takes about 1.3 seconds for light or radio waves to reach us from the Moon! now that’s fast hey. Hope we answered your question 🙂