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A thought on light cones

  1. Jan 17, 2015 #1
    Hello to everyone. I used to be very passionate about physics when I was a high school student, but I have taken a different path andless than 10 years later here I am, struggling to remember the basics...all this to say that you will probably able to point out the flaw in my reasoning. Here it is:

    I was recently reading the first chapters of "The Large, the Small and the Human Mind" by Roger Penrose and I was facing the apparently simple concept of light cones and the concept of casuality. To my understanding, if I am on planet Earth and my cat is napping on planet Cat, which happens to be about 300 000 km away from Earth (not the best distance to keep in touch with your cat I guess), if I want to wake up my cat I will only be able to do so 1 second in my future (in the best case).
    Now what if on planet Cat there were a gong, placed close to my cat's hears, and right next to it a mallot with a 300 000 km long handle, so that I could grab the mallot from planet Earth and strike the gong on planet Cat. Assuming the mallot is made of some non deformable material, wouldn't its movement transfer instantly from planet Earth to planet Cat thus allowing me to immediately wake up my lazy? Obviously not, as that would violate some pretty important laws, as far as I know. Thank you for taking the time to read, could you please tell me where the flaw is?
  2. jcsd
  3. Jan 17, 2015 #2


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    There is no such thing as a perfectly rigid material. The speed of light forbids it.

    Movement can only be transferred through a material object at the speed of sound of the material that object is made of. (Diamond, the hardest natural substance known, has a speed of sound of about 12km/s)
  4. Jan 17, 2015 #3
    Well there probably isn't a planet Cat either ;)
    So would the deformation of the mallot account for the delay?
  5. Jan 17, 2015 #4


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    Perhaps, but the universe does not explicitly forbid it. So it is a valid component in your thought experiment. :)

    Yep. Quite so. A physical connection - being necessarily much slower than the speed of light - would be a very poor way of transferring the signal.
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