## Thursday, August 02, 2012

### Temporal recipes

kw: book reviews, nonfiction, physics, time travel, time machines

Brian Clegg is becoming one of my favorite explainers. I reviewed two of his books already (last week and late last year). This one is in the middle, published December 2011: How to Build a Time Machine: The Real Science of Time Travel. In his fluid and entertaining way, Clegg starts with the philosophy of time (there's barely any), segues to the physics of time (primarily the general theory of relativity), and discusses all the approaches to controlling one's progress along the time axis that have a scientific basis, and a few that don't.

H.G. Wells was the first to write of time as a fourth dimension, and his Time Traveler  constructed a mechanism to move along that dimension. For Wells, time travel doesn't remove one from one's physical location on Earth's surface. But I wonder what Wells was thinking when he wrote that after the Time Traveler dragged his machine outside the Morlock cave and returned "home", there were skid marks on the floor. Is it just a hint that the whole adventure was the TT's dream?

Let's cut to the chase. Do we learn how to build a time machine? If the theories of physics about how gravity and mass/energy in motion affect time are true, then yes, indeed, a time machine is possible. Possible, but difficult in an engineering sense. Here is one recipe:
1. Procure a spinning black hole. The bigger the better, because tidal effects are less dangerous near the event horizons of the largest black holes. However, though the "escape velocity" from just outside the event horizon of any black hole is c, the energy needed to make a powered ascent against the gravity is much greater the more massive the hole is. Also, the faster the spin rate, the better. Faster rotation means you don't have to get as close to the event horizon. A few million revolutions per second ought to do it.
2. Procure a spaceship with sufficient fuel to allow you to dive into the gravity well of the black hole, swoop above the event horizon, and return.
3. Rig some kind of shielding sufficient to protect you and the ship from radiation and impacts from light and matter particles that have dropped down the gravity well and are approaching infinite intensity.
4. When you perform your swoop, you'll take advantage of the frame-dragging effect of the rotating gravity field. Supposedly, swinging by in the direction of rotation takes you forward in time, and swinging by the other way takes you backwards. The limit into the past you can go is the date of formation of the black hole.
The challenge here is not to build the hole, but to build the spaceship that can safely take you into its vicinity and back out again. Other schemes of similar scale involve infinite, or really near-infinite, rotating cylinders of massive "dust", and spinning neutron stars. The key issue is the difficulty of traveling about 300 light years to the nearest neutron star, or thousands of light years to the primary black hole at the Galaxy's core. While Clegg does discuss one scientist's table-top device that ought to produce a measurable amount of frame dragging (within the next decade or so, and we hope the 65-year-old scientist lives to see it finished), it is clear that the time shift it produces will be at most a few milliseconds. Not enough to get you to the year 2400 for a look around.

By the way, if your spaceship can exceed the speed of light, it is already a time machine; you don't need the black hole. But we don't know how to make such a space drive, while we do know what it would take to utilize a black hole's frame dragging.

Two points were new to me. I don't know why I haven't read or heard of either one before, given all the reading I do. Firstly, that the inside of a black hole is bigger than the outside. Because of the way that mass warps spacetime, the "distance" from the event horizon to the singularity at the center is infinite, if it is a true singularity. If instead, space is granular and the "singularity" gets no smaller than the Planck length, the distance is still huge. It is like the difference between the diameter of the bell of a trombone, and the length of the entire horn, were it straightened out. Indeed, Clegg discusses the shape known as Gabriel's Trumpet, formed by the function y = k/x, cut off where x=1, and rotated about the x axis; it has a maximum radius k but goes infinitely along the x axis, getting narrower as it goes.

Secondly, the conundrum of mass balance. If you could jump to a chosen date, say, 100 years either past or future, and you were to stay there for a few days before returning, your mass, and the mass of whatever part of the time machine accompanied you, would be added to the universe in that other time, and subtracted from whatever period in your "home" time you were gone. Is the conservation of mass/energy a requirement for every moment in time, or only for all spacetime? This is not known, but it may be a reason for time travel to be impossible. This could mightily complicate general relativity.

One point I didn't find discussed. If someday a machine could take someone to any point in time, would they return to the time that they left, or to that time plus the duration of their visit elsewhen? Could you "return" to a time earlier than home time plus trip duration?

Some of the points discussed by the scientists Clegg consulted are more metaphysical, such as the notion that nature somehow conspires to keep a time traveler from making any paradoxes (like killing your own grandfather, the classic paradox); or the idea that making a wormhole that incorporates a time shift would result in catastrophic feedback as radiation from the futureward end exits the pastward end and re-enters the futureward end and so forth, causing an energy pileup that effectively explodes the wormhole and everything in its vicinity. Depending on how long this takes, it could result in a detonation similar to a supernova. Remember the tongue-in-cheek proverb: Time is what keeps everything from happening at once. It may be more true than we'd like to think.

So, now I know a few ways a time machine could be made. Most of them do not permit travel into a past that is before the machine was made. So if time travel is possible, and it is of that sort, it explains why nobody has dropped by from the future to visit. No such machine exists, just yet. The day one is invented, perhaps we'll find many events thereafter clogged with time tourists. Could true time travel be discovered during my lifetime? I hope not. I am not sure I am ready to be interviewed by a genealogy-seeking great-great grandchild!