Faster than Light thought experiment

About the ends of light beams and the cutting points of scissor blades and the intersections of lines -

They can move at greater than c. They aren’t objects and don’t carry mass or information. You can’t hang a message on one going by to get it to carry your message to someone else. All that’s correct.

But more significantly, these matterless agencies not only can move faster than c, they can reach their destination before they start traveling, and they can exist in multiple locations at once, and do other weird things, depending on the motion of whoever is observing them. Now, of course, this observer understands that whatever light he is seeing as part of his observation process has a travel time - he is accounting for that correctly. The bigger message here is that these matterless agencies REALLY DO accomplish these surprising things. It isn’t some artifact of observation limits.

We have in English the word “now”, which carries with it no qualification that we are applying it in only one location in space. That is, you can wonder what the temperature is on Pluto “right now”. But there isn’t a “now” on Pluto, or anyplace other than “here”. Missing this is just about the biggest relativistic error and limitation you can have.

The problem isn’t that the energies needed are so large; the problem is that the energies needed are so small. If it were just a matter of converting the mass of Jupiter to some other form, well, we could probably find somewhere in the Universe where that’s occuring. Not in our laboratories, of course, but somewhere. But when you need to get an amount of energy that’s less than zero, well, that’s where things get tricky.

Can you give some examples or illustrations please?


If I understand correctly (and using the same analogy), a pair of scissors with a curved blade has two contact points at once.

Wikipedia has a (rather poor IMHO) article on superluminal motion, which can be astronomically observed on occasion in various structures. The best example I can think of is an expanding sphere (say, from a nova, etc), which intersects at some point with the flat plane of a gaseous nebula, which then lights up. Observing the nebula, you would see first a point of light, then a circle that could (if the geometry was right) appear to expand at speeds far greater than the speed of light.

Hmm. What if there were no thought experiments? :wink:

Maybe I’m just not imagining the “superluminal scissors” thought experiment correctly, but how exactly CAN’T a cutting point of scissors convey information?

The cutting point seems to travel from the original user to the end-point of the scissors. It would be rather trivial to make it convey information in morse code.

In the other thread cited above, it was shown that it turns out that the intersection point accelerates such that as it approaches the endpoint, it is “moving” FTL, but the average speed over the course of the acceleration is lower than FTL. This acceleration is due to the blades of the scissors bending as they move, such that the ends of them kind of “snap” into place at the end of the movement.


>…how exactly CAN’T a cutting point of scissors convey information…

If you are holding the giant handles of the scissors and you want to signify that the British Are Coming by making them snip, you squeeze the handles and some time later the tips cut. Scissoring, in this case, is messaging. The message can’t travel faster than c. In fact, it can’t even travel faster than the speed of sound, as some have noted.

Suppose you were able to order giant scissors with any curve you liked on the cutting edges. You could arrange for a great long length of the edges to do all their cutting with only the slightest bit of angling. Two people standing next to the scissors at either end of this great long length will note that the edges pass almost in an instant. So, you squeeze the handles, and the people see the snipping happening almost simultaneously along this area - but if the person closer to you wanted to change something about the message (which could only be to stop it, because we only have two states and the scissors are in the cutting state now), he would have to try to grab the blades and prevent them from snipping. He’s in the same predicament you are with the handles - he can only influence the other end of the long length at the speed of sound. The cutting point on the scissors is moving along, and he can’t change that faster than the handles already could change it.

Imagine you got the great long length curved a little too far, so that the scissors actually met out near the tips before they met at your end. Now the cutting point is actually coming toward you. Clearly, there’s no way you could alter the cutting point that is now passing you to change what it did a moment earlier, out there.

I think superliminal behaviors in astronomy are a rare real world example of these kinds of weirdnesses.

Another analogy - you can aim a laser pointer at the floor, and make the bright red dot zip around. With a big enough floor and a powerful and tight enough laser pointer you can make the red dot move at faster-than-c. So you get two of your friends to stand millions of miles away where they can see the dot going between them at greater-than-c. You can aim the laser, or turn it on and off, and signal your friends that way, at c. But when the dot goes past one of your friends and towards the other one, the first friend has no way of changing what the other friend is about to see.

Somebody traveling between the friends at the right speed will see 3 dots, one at each friend and a third one traveling in the direction from the friend who got the dot last and toward the friend who got the dot first.

It is confusing to do these thought experiments, but if you look up “world lines” and coordinate transforms in cosmology, they are used there.