Ari Siletz

Divaneh

When nations are viewed as planets going around the sun of History it has been observed that it only takes one revolution to go back 1400 years.

By the way, very well observed regarding the universal speed limit simply having to be updated to the faster speed of neutrinos instead of the speed of light photons. However, among the many issues with the idea is the following. The only types of particles that can travel at the maximum universal speed are massless particles. This is because any particle with mass going at this maximum speed would necessarily need an infinite amount of energy. Experiments have indirectly shown that neutrions have a tiny mass and therefore should never be able to go as fast as the massless photon--which can only exist at the maxiumum universal speed and at no lower speed (in vacuum) . Interestingly, it is theoretically imaginable to have particles with mass going faster than this speed, but for these particles, called tachyons, the universal speed becomes the miniumum possible speed. At the speed "limit," these particles would have infinite energy just like any other particle with mass. Prior to the CERN results some had speculated that neutrinos may have a tachyonic nature, so maybe these speculations were right. In any case, these theoretical tachyons can be used to send signals into the past. In other words simply the existence of a special limiting speed (whatever it is, and whether it is an upper bound or a lower bound for a particle) creates theoretical time travel possibilities. By the way, note that in a Newtonian universe where the speed of information can be infinite, cause and effect is safe from such considerations. 

MPD

Good question. We will get to the Alan Shepard case later in this comment. A simpler time travel application of the CERN/OPERA neutrino results in the context of urination is if someone wets his/her pants upon finding out that he/she has won the lottery. These faster than light (FTL) neutrinos can be used to send signals into the past--because in certain frames of reference they would arrive before they are sent! For example, you could check the winning numbers for today's lottery, then send yourself an FTL email with those  numbers. Your yesterday self will receive the email before the lottery drawing took place! A very problematic state of affairs, but if the CERN results turn out to be right then it's something we must learn to live with.

As for watching yourself pee backwards, consider that your memory and thought processes would also be running backwards in an "Alan Shepard" type of time machine, so that your mind would interpret the order of events inside the time capsule exactly as it would if time were going forwards. In other words, you wouldn't know you were in a time machine. In fact, the whole Universe could be going back and forth in time right now and we would never know it because our memories are going back and forth in time along with everything else.

The best design for a time machine into the past is where the traveller is isolated from the flow of time and the rest of the Universe is reversed. The "Alan Shepard" design, on the other hand, would have a younger Alen Shepard come out of his time capsule with no memory of how he got there (because his memories would have reversed in time).

By the way, here's the latest Nature article saying that the faster-than-light neutrino results have been repeated . Maybe they'll find a mistake somewhere, or it may turn out that for some reason faster than light neutrinos can't be used to send information into the past. It has been known for decades that faster than light effects occur that for subtle reasons can't be used to communicate with the past.

Relativity for idiots

It is all good Ari, but it now leaves me with the question.

Q1: How fast do you have to travel to go back 1400 years in time?

Q2: How fast do you need to travel to go back 32 years in time?

Thanks for this simple explanations of the relativity. I am not a physicist but I think it does not really matter if the Neutrinos travel faster than light. The relativity theory is based on the idea of a maximum speed above which nothing can travel faster. It is currently assumed to be the speed of light but if Neutrinos travel faster than light then that ceiling will be raised to the speed of Neutrinos. Interestingly at speeds close to the speed of light, not only the time slows down and eventually can go into negative, but the distance also shrinks. Therefore far away stars could become closer and human may travel to planets that never thought possible. I recently read a good book on the subject which I recommend to any interested person. The book by Brian Cox and Jeff Forshaw is titled "Why does E=MC2?"

Dear MPD,

That's a very good case. I think even more entertaining case would have been if you released some gas and that all got back in again.

Huuuummmmm!!! Let me see if I understand this correctly!

Let's say I am in a time machine, and it's taking too long to start up, and I can't hold my peepee anymore, as it happened to Alan Shepard on May 5, 1961, while he was waiting to be launch into space, so I begin to urinate, as he did, and as the stream of urine is coming out of my weewee the time machine fires up and it takes me to the past.  Are you saying, if it turns out to be true, that speeds faster than the speed of light is possible, implying that a time machine is a possibility, are you saying, in such a case as I describe above, in that case my peepee would go back into my weewee?!  Wow!  That's one hell of a thing to watch!