# Communication back in time?

On the wikipedia page for time travel, it mentions that Robert Forward said (theoretically, unknown since we don't have a working theory of quantum gravity) that if a heavy nucleus was subjected to an intense magnetic field, it would elongate into a Tipler cylinder and allow gamma rays to be sent back in time. I was hoping someone could explain why this is and give me some specifics, or link me to the paper where he explains it.

While there are probably a lot more problems with that statements, here are a few :

• A Tipler cylinder is infinitely long, hence it would require the nucleus to stretch infinitely far (and due to limitations on the speed of light, for eternity) before actually qualifying as one. I am not quite sure a nucleus would even survive such a stretching.
• It is a static solution, meaning that it would have to have always existed
• Typically fields trying to cross closed timelike curves are blueshifted to such a degree that they would disrupt the spacetime, making it impossible to go back in time.
• NB the Tipler cylinder must be infinitely long. Hawking proved that any system generating CTCs must either involve exotic matter or be infinite in size. Just being very big won't do! – John Rennie Sep 16 '15 at 10:09
• Not exactly. The theorem states that a compactly generated Cauchy horizon must violate the NEC, though there is some wiggle room to what that can mean for time machines due to the way compactly generated is defined. – Slereah Sep 16 '15 at 10:15
• Time machines are science fiction. Sorry. – John Duffield Sep 16 '15 at 13:33
• @JohnDuffield Mainstream research. Tipler cylinders and co are mainstream, commenting "time machines are science fiction" on an answer that talks of mainstream physics is not constructive at all. Crowell's answer has nothing to do with your point. CTCs are world lines (well, in the general usage of the term), so yes, objects will have a time evolution along them. – Manishearth Sep 19 '15 at 14:48
• A paper (one amongst many) in Phys. Rev. D. sounds pretty mainstream to me. Conjecturing various CTCs and later proving them impossible is certainly something that qualifies as research worthy. – Manishearth Sep 19 '15 at 15:53

On the wikipedia page for time travel, it mentions that Robert Forward said

Yes, here it is:

"Physicist Robert Forward noted that a naïve application of general relativity to quantum mechanics suggests another way to build a time machine. A heavy atomic nucleus in a strong magnetic field would elongate into a cylinder, whose density and "spin" are enough to build a time machine. Gamma rays projected at it might allow information (not matter) to be sent back in time; however, he pointed out that until we have a single theory combining relativity and quantum mechanics, we will have no idea whether such speculations are nonsense."

(theoretically, unknown since we don't have a working theory of quantum gravity)

Quantum gravity remains elusive. Note that when the electron and the proton attract one another electromagnetically, they "exchange field" such that the hydrogen atom doesn't have much in the way of an electromagnetic field. Hopefully you can appreciate how the idea of virtual exchange particles arises from this. The trouble is that when two hydrogen atoms attract one another gravitationally, they don't exchange field like this. Instead the field doubles up. For this and other reasons to do with the nature of time and what clocks do$^*$, I'm afraid that I will say with some confidence that such speculations are nonsense. I'm sorry if this disappoints.

that if a heavy nucleus was subjected to an intense magnetic field, it would elongate into a Tipler cylinder and allow gamma rays to be sent back in time.

A Tipler cylinder is a speculation that "could only allow time travel if its length were infinite or with the existence of negative energy". We know of no infinities in nature and nothing that consists of negative energy. Some people will tell you that a gravitational field is negative energy, but it isn't true. See Einstein saying "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy" here.

I was hoping someone could explain why this is and give me some specifics, or link me to the paper where he explains it.

You won't get anybody doing that I'm afraid. There can be no backward travel through time because there is no forward travel through time, because there is no motion through spacetime. See relativist Ben Crowell saying the same in this answer. You move through space over time, and we depict this as your world line. But a world line is abstract thing. It's an abstract line in a static "all times" depiction of space and time. Have a look at A World Without Time: The Forgotten Legacy of Gödel and Einstein, where author Palle Yourgrau says Wheeler conflated a circle with a cycle:

You don't move along a worldline or along a closed timelike curve. In other words, there is no way you can move such that everything else in this universe not only moved back to where it was, but never moved at all.

• Clocks clock up some kind of regularly cyclical motion and show you some kind of cumulative display that you call the time. A clock is not some kind of cosmic gas meter that literally measures the flow of time.