Hi all,

I've heard it said that if I could send a message faster than light, then another inertial observer would think it was going back in time. If the message was to hire an assassin to kill me before I sent the message, we'd have a paradox.

I'd like to understand the details of this. Suppose I ask some Italian to drive a bunch of neutrinos to Mars at e.g. 2c with my suicide note. Major Tom is also driving to Mars at .99c and sees the message arrive before I sent it. But this Martian assassin can't do much to me from there. He's gonna have to bounce the neutrinos back to Earth to ask one of his human friends to do it.

Tom now has a choice. If he keeps driving in the same direction, he'll perceive the return message as incredibly slow. If he turns around, he's encroaching on the twins paradox. I'm not sure where the latter option leads to, but his erratic driving certainly casts doubt on his validity as an inertial observer. As far as I'm concerned, the assassin arrived when I expected him to, long after I sent the message.

So what's the problem?

Anyway, wouldn't it be nice if we had zero-energy tachyons to go at infinite speed - that would solve Einstein's biggest objection to QM, namely that observations are presumed to change the wavefunction at all points in space instantaneously.

Hi all,
I've heard it said that if I could send a message faster than light, then another inertial observer would think it was going back in time. If the message was to hire an assassin to kill me before I sent the message, we'd have a paradox.
Why do you think that is a paradox?
After all the assassin clearly failed his mission because you are alive!

Fredrik
Staff Emeritus
Gold Member
You might be interested in my replies in this thread, and also this post. (Make sure to scroll down to #138 to see the correction of a typo).

Hi Passionflower,

It's not me, I just heard a rumour that everybody else thinks so.

Hi Fredrik,

I guess the Alice-Bob thing is indeed a paradox.

In the meantime, I thought of another aspect: you lose the 2nd law of thermodynamics. Suppose you punch out some morse code on your 2c neutrino torch towards a receiver on Mars. The spaceman is going to be a bit suspicious that what looks like a receptor on Mars is shooting out neutrinos that are getting hoovered up by a torch on earth, but let's suppose he thinks no more of that. The problem is if there's a bit of space dust in the way and the beam gets blurred. Then he's gonna see a messy beam come out of Mars and get tidied up along the way to Earth. That's a direct violation of the 2nd law and amply serious enough without any paradoxical suicides.