# What Types of Radiation Emanate in the Future and are Perceived in the Present?

Years ago I came upon a scientific text which mentioned different types of radiation and how they are perceived. One such form of radiation was described (from our perspective and understanding of space-time) to originate from the future and travel backwards through time.

I cannot for the life of me remember what this type of radiation was called and I would love to know it.

One concept of antiparticles (see Feynman) is that they're regular particles travelling backward in time. If you go with that, then by extending the meaning of "radiation" to include physical particles -- which is common usage, e.g. $\alpha$ and $\beta$ particles -- then antiparticles such as positrons came from the future.

Nothing that is known to exist

Relativity does not allow for a massive particle to travel at the speed of light, but it doesn't prevent a particle from travelling faster than light. Such a particle has been called a Tachyon. No such particle has ever been observed. There are good reasons for believing that they don't exist.

Such a particle would be extremely strange. The mass of such a particle would not be positive, it would be imaginary! However you could not use such a particle to send a message to the past. The particles can't be localised, which means that you can't detect them as being "at" a particular place at a particular time.

In conclusion, such particles have never been detected, probably don't exist, and couldn't be used to send messages.

• Thank you! I thought it was a tachyon (I remember hearing the term in Star Trek) but I wasn't sure if that was correct or not. – tmp Feb 16 '17 at 1:35
• I'm not convinced a tachyon travels backwards in time. It gets "somewhere" a lot sooner than light does, but, e.g. a 10Xc tachyon still takes 1000 years to go 10 000 light years. (you may convert into Kessel Runs if you like) – Carl Witthoft Feb 16 '17 at 14:09
• @CarlWitthoft That's what I think too, but I've read arguments to the contrary, which I don't understand. Perhaps that's a subject for another question though. theory.uwinnipeg.ca/mod_tech/node142.html – userLTK Feb 16 '17 at 18:17
• @userLTK you are correct -- depending as always when one talks high speed, on frames of reference. There's no simultaneity and hence all sorts of wacky things can theoretically happen. So in fact I just invented a whole new particle :-) ; one which exceeds $c$ but is immune to special relativity inertial frame rules. – Carl Witthoft Feb 16 '17 at 19:02