Cosmic Strings and the Time Machine
A way to travel faster than light
The Earth has been described as a marble; a spirited blue with gold-bearing continents and yet hidden demurely by flourishing white clouds overhead. It’s smooth and self-contained, from a distance seeming so perfectly round even if one knows about the daunting pits and cliffs at its surface. And though these trenches and these mountain ranges are a normal result of the planet’s mechanisms, it’s easy to see how someone on another surface — even as close as the moon — would mistake it for being nothing but a flat blue dot. It’s only when one arrives on its surface and samples the tropical sandy beaches or the thick foliage on the rainforest floor that one understands just how textured Earth truly is. The universe, on a fundamental scale, may be exactly the same way.
Cosmic strings are a kind of texture for the universe. They’re topological defects that may have occurred 10-³⁵ seconds after the Big Bang when the universe went through a phase transition. This phase transition is similar to the one exhibited by water as it changes from a solid to a liquid to a gas. In the case of the universe, the phase transition would have occurred in the fluid-like quantum fields that inhabit all of space. The particles we’re familiar with — photons, gluons, quarks, electrons — all come from ripples in the quantum fields, which in turn make the seas, the cosmos, and every human on Earth. If the universe cooled too quickly, spacetime would have cracked into hairline fractures just like the sharp white veins often seen in blocks of ice. This would happen because the fields wouldn’t always be aligned with one another. The resulting cracks are cosmic strings.
One of the most remarkable features of these strings is the huge amount of energy stored within them. Even though they’re thinner than a proton and thus invisible even to our most advanced technology, they have an immense weight. About half a mile (1 km) of a cosmic string would have the mass equivalent to the entire planet. During the phase transition, as the surrounding universe dropped to a lower energy, higher energy would have gotten trapped inside the strings until they were a billion times more energetic than manmade particle collisions. In essence, cosmic strings can be seen as one dimensional black holes. Instead of being spherical around a point, the strings are symmetrical around an infinite line. These strings (about a dozen or two of them) stretch the length of the observable universe. However, smaller cosmic “loops” would number in the billions, with that number constantly replenishing itself as new loops formed. This happens when an oscillating cosmic string crosses itself and thus slices off a lightyear-size loop. These loops, in turn, can be self-intersecting.
In early cosmic string research, it was believed loops may have given rise to galaxies since the number of loops present in the early universe would have closely matched the number of galaxies also present at that time. But data on the cosmic microwave background didn’t support this idea, instead showing no signs of the ever elusive strings.
To many scientists they became fanciful, or nothing more than a casual thought. In pop culture they’ve been treated much the same way. Time travel is done by using Alcubierre warp drives or by opening up grand blustering wormholes connecting us to another place and time. And yet mathematical models and theories of particle physics continue to predict cosmic strings time and time again. Physicists like Stephen Hawking and Tom Kibble (responsible for discovering the Higgs boson) supported the idea that somewhere out there, at the edges of the universe, these strings exist even if they are a difficult find.
Interest in cosmic strings came back in the 90’s. With it, a proposition for time travel.
In theory, to go backwards in time all one needs to do is go faster than the speed of light. General relativity tells us nothing can go faster than the speed of light, but it also tells us the fabric of space can be manipulated and warped. It’s this manipulation that could give us a shortcut for superluminal speeds.
Stretching the strings to such unfathomable size places them under a lot of tension (about a hundred billion billion billion tons). Because of this tension, the strings accelerate very close to the speed of light, warping the spacetime around them into the shape of a cone. This gives rise to gravitational lensing. With gravitational lensing, light from an object can take more than one path. Research into light traveling from quasars reveals that two separate light beams, having taken two different paths, can vary from one another by over a year (417 days, in this case). If two cosmic strings approached and passed each other, the warping would be so great that a person moving around them could take the shorter path, traveling faster than light and thus going back in time. After circling the cosmic string, you’d meet yourself before you ever left.
But there are certain limitations for this time machine. For one, the gravitational forces involved could cause a black hole. And if that isn’t an issue, there’s also the fact that you cannot go back to a time before the machine existed. If we built one this year, humans of the year 2098 could come back to visit us in 2019 but none of us could go to the more exotic and antique era of the 1800’s. Richard Gott, who proposed the idea in 1991, emphasizes that time travel is an activity only super civilizations would undertake. The energy and manipulation needed far exceeds anything we’re capable of today.
For now, we concentrate on first detecting cosmic strings using the young science of gravitational wave astronomy. If the strings do exist, their main way of losing energy is by radiating gravitational waves. As they oscillate in space, sections of the string whip at the speed of light, giving off signals we may be able to detect using LIGO or VIRGO (gravitational wave observatories). These signals can also be given off as the loops rotate. But scientists speculate the signals are too low a frequency for us to hear. Our best chance to detect these marvels will come in 2034 with the Laser Interferometer Space Antenna (LISA).
There is one last unsettling thought that cosmic strings bring. If they did form soon after the Big Bang, they may have dissipated too quickly to have left any trace at all. They would be a part of the universe’s history that’s lost to us, just as certain sights of the cosmos will be lost to future humans as space continues to expand. However we may try to fully understand the universe — or multiverse — in which we live, it may remain to us always a marble. Colorful and polished, but one whose true landscapes are kept tucked away beneath a protective layer of glass.
