Hacker News Reader

Comment by jsemrau | Hacker News Reader

If you pull a string that has the length of 1au, would it take 8 minutes for the other side to react to the tug, or would it move as soon as you pull the string?

View on HN

It would take 1 au / speed of sound in string. The 8 minutes comes from the (much faster) speed of light.

Based on some googling it seems it would take roughly 1.8 years.

What if the string is completely non-elastic?

At those sizes there is no such thing as "completely non-elastic" (aka "rigid").

This. Diamond is one of the stiffest materials in existence. It has a sound speed of 12 000 m/s. Lightspeed is 300 000 000 m/s.

How about observing a quantum-entangled particle 1AU away from its partner? Can the speed of information surpass the speed of light? I'm a physics noob so I wouldn't know about research in the field.

As far as we can tell, the effect of measuring an entangled particle is at least 10000 faster than the speed of light. According to theory, it's instant. You can't use it to transfer any information, though.

At any size, there is no such thing as "completely non-elastic", but at those sizes, pretending that some things are completely non-elastic is no longer a tolerable approximation.

What if the string isn't actually made of matter but is made up of some hypothetical substance that doesn't follow any laws of physics?

Same question.

If you can persuade people that you can make that hypothetical substance, there are a few pissed off investors around at the moment who will throw money at you.

Since it doesn't follow any laws of physics: it actually moves the opposite way than the way you tugged it ;-)

> What if the string isn't actually made of matter but is made up of some hypothetical substance that doesn't follow any laws of physics?

If it doesn't follow any laws of physics, then there is no predictable relationship between any input and its action (since if there was such a predictable relationship, it would be a law of physics that the string was obeying.)

I assume that complete non-elasticity of something would violate the (presumed) light speed limit on causality.

Even rarefaction obeys the speed of light. :)

But does hype?

If it doesn't, we could use it to send messages back in time. Isn't that exiting?

I used to think so.

Speed of sound in the respective medium, actually. EM interactions obey speed of light, mechanical interactions obey the speed of sound.

Reply on news.ycombinator.com