How Can We Test for String Theory?
Though it is a huge gamble for many physicists, string theory has its devotees because of the elegance of the mathematics involved. Simply put, string theory is the idea that all that is in the universe is just variations of the modes of “tiny, vibrating strings of energy.” Nothing in the universe can be described without the use of these modes, and through interactions between objects they become connect by these tiny strings. Such an idea runs counter to many of our perceptions of reality, and unfortunately, there is no evidence for the existence of these strings yet (Kaku 31-2).
The importance of these strings cannot be understated. According to it, all forces and particles are related to each other. They are just at different frequencies, and the alteration of these frequencies leads to changes in the particles. Such changes are usually brought on by motion, and according to the theory the motion of the strings causes gravity. If this is true, then it would be the key to the theory of everything, or the way to unite all the forces in the universe. This has been the juicy steak that has been hovering in front of physicists for decades now but thus far has remained elusive. All the math behind string theory checks out, but the biggest problem is the number of solutions to string theory. Each one requires a different universe to exist in. The only way to test each result is to have a baby universe to observe. Since this is unlikely, we need different ways to test for string theory (32).
Waves of Gravity
According to string theory, the actual strings that make up reality are a billionth of a billionth the size of a proton. This is too small for us to actually see, so we must find a way to test that they could exist. The best place to look for this evidence would actually be in the beginning of the universe, when everything was small. Because vibrations lead to gravity, at the beginning of the universe everything was moving outward. Thus, these gravitational vibrations should have propagated at about the speed of light. The theory tells us what frequencies we would expect those waves to be, so if gravity waves from the birth of the universe can be found we would be able to tell if string theory was right (32-3).
Several gravity wave detectors have been in the works. In 2002 the Laser Interferometer Gravitational Wave Observatory went online, but by the time it was terminated in 2010 it had not found evidence of gravity waves. Another detector that has yet to launch is LISA, or the Laser Interferometer Space Antenna. It will be 3 satellites arranged in a triangle formation, with lasers being beamed back and forth between them. These lasers will be able to tell if anything has caused the beams to sway off course. The observatory will be so sensitive that it will be able to detect deflections up to a billionth of an inch. The deflections will hypothetically be caused by the ripples of gravity as they travel through space-time. The part that will be interesting to string theorists is that LISA will be like WMAP, peering into the early universe. If it works correctly, LISA will be able to see gravity waves from within one trillionth of a second post Big Bang. WMAP can only see 300,000 years post Big Bang. With this view of the universe, scientists will be able to see if string theory is right (33).
Another avenue to look into for evidence for string theory will be in particle accelerators. Specifically, the Large Halidron Collider (LHC) at the Switzerland-France border. This machine will be able to get to the high energy collisions that are needed to create high-mass particles, which according to string theory are just higher vibrations off of the “lowest vibration modes of a string,” or as is known in the common vernacular: protons, electrons, and neutrons. String theory in fact says that these high-mass particles are even the counterparts to protons, neutrons, and electrons in a symmetry-like state (33-4).
Though no theory claims to have all the answers, the standard theory does have a few problems attached to it that string theory thinks it can resolve. For one, standard theory has over 19 different variables that can be adjusted, three particles that are essentially the same (electron, muon, and tau neutrinos), and it still has no way to describe gravity on a quantum level. String theory says that’s okay because standard theory is just “the lowest vibrations of the string” and that other vibrations have yet to be found. The LHC will shed some light on this. Also, if string theory is right, the LHC will be able to create miniature black holes, though this has yet to happen. The LHC may also reveal hidden dimensions that string theory predicts by pushing the heavy particles through, but this also has yet to happen (34).
Flaws in Newton's Gravity
When we look at gravity on a large scale, we rely on Einstein’s Relativity to understand it. On a small everyday scale we tend to use Newton’s gravity. It works great and wasn't a problem because of how it operates at small distances, which is what we primarily work with. Hwoever, since we don’t understand gravity at very small distances, maybe some flaws in Newton’s gravity will reveal themselves. These flaws can then be explained by string theory.
According to Newton’s Theory of Gravity, it is inversely proportional to the distance between the two of them squared. So, as the distance decreases between them, the force gets stronger. But gravity is also proportional to the mass of the two objects. So if the mass between two objects gets smaller and smaller, so does gravity. According to string theory, if you get to a distance smaller than a millimeter, gravity can actually bleed into other dimensions that string theory predicts. The big catch is that Newton’s Theory works extremely well, so the testing for any flaws will have to be rigorous (34).
In 1999, John Price and his crew at the University of Colorado in Boulder tested for any deviations at that small scale. He took 2 parallel tungsten reeds 0.108 millimeters apart and had one of them vibrate at 1000 times per second. Those vibrations would change the distance between the reeds and thus change the gravity of the other. His rig was able to measure changes as small as 1 x 10-9 of the weight of a grain of sand. Despite such sensitivity, no deviations in the theory of gravity were detected (35).
Though we are still not sure about many of its properties, dark matter has defined galactic order. Massive yet invisible, it holds galaxies together. Even though we do not have a way to describe it currently, string theory has a sparticle, or a type of particle, that can explain it. In fact, it should be everywhere in the universe, and as the Earth moves around it should encounter dark matter. That means we can possible capture some (35-6).
The best plan to capture dark matter involves liquid xenon and germanium crystals, all at a very low temperature and kept below ground to ensure that no other particles will interact with them. Hopefully, dark matter particles will collide with this material, producing light, heat, and movement of atoms. This can then be recorded by a detector and then determined if it is in fact a dark matter particle. The difficulty will be in that detection, for many other types of particles can give off the same profile as a dark matter collision (36).
In 1999, a team in Rome claimed to have found such a collision, but they were unable to reproduce the result. Another dark matter rig in the Soudan mien in Minnesota is 10 times as sensitive as the set-up in Rome, and that has no detected any particles. Still the search goes on, and if such a collision is found it will be compared to the expected sparticle, which is known as a neutralino. String theory says these were created and destroyed after the Big Bang. As the temperature of the universe decreased, it caused more to be created than destroyed. They should also be 10 times as many neutralinos as normal, boson matter. This also matches with current estimates of dark matter (36).
If no dark matter particles are found, it would be a huge crisis for astrophysics. But string theory would still have an answer that would be consistent with reality. Instead of particles in our dimension holding galaxies together, it would be points in space where another dimension outside of our universe is in close proximity to ours (36-7). Whatever may be the case, we will soon have answers as we continue to test in multiple ways for the truth behind string theory.
Kaku, Michio.“Testing String Theory.” Discover Aug. 2005: 31-7. Print.
© 2014 Leonard Kelley