String Theory: Worth the Wait or Just Stringing Us Along? (Part 1)

Author: Molly Kozminsky

Editors: Theresa Mau, Jimmy Brancho, and Alisha John

July 2016 was all about taking potshots at string theorists. First, Bryan Moyers pointed out that people questioned if their field is really science. Then they showed up in the Ghostbusters reboot as the villains.

But what is it about string theory that inspires such vitriol? String theory suffers from a number of problems that inspire strong feelings and entire books. Over forty years of research have passed without yielding the promised “Theory of Everything,” with many scientists questioning whether it is even possible to confirm the theory. But before we write off string theory entirely, it might help to think about other long-shot theories such as the Higgs boson and gravity waves, and more generally about string theory itself.

So, what is string theory anyway?

String Theory: Another Way of Looking at the Universe

What makes up matter? The answer depends on how small you want to go. Matter is made of atoms. You, for example, are made mostly from the atomic elements oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorous.

What makes up atoms? As previously discussed on the blog, atoms are composed of the subatomic particles protons, neutrons, and electrons.

What makes up subatomic particles? A particle physicist will point you in the direction of the elementary particles of the Standard Model (Figure 1), which fall into the categories of quarks, leptons, gauge bosons, and the Higgs boson (more on this later). These particles are zero-dimensional (i.e. are points that lack height, width, or depth) and can be characterized by properties such as spin and color charge. This model describes both matter and the four fundamental forces of nature: strong nuclear, weak nuclear, electromagnetic, and gravitational.

 

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Figure 1. Matter is made of atoms, which are made of subatomic particles, which are made up of quarks from the Standard Model.

 

But we can move beyond the Standard Model and go smaller still, at least according to string theorists. String theory describes matter, forces, and their respective particles, using one-dimensional “strings,” which are characterized by their vibrations. Strings may be open, like a piece of string, or closed, like a hair tie.

To adequately describe how strings move and interact, we need to use more dimensions of spacetime than we observe. We usually think about three spatial dimensions and one dimension of time, or four dimensions in total. Different string theories (yes, there are several) describe strings in varying numbers of dimensions, but all such theories may be described as specific instances of M-theory, which uses up to 11 dimensions to unify the lower-dimensional string theories.

The identity or location of those dimensions can be explained by a concept called compactification. Compactification can be compared to an ant on a garden hose, or as the Netflix show “Stranger Things” has mentioned, a flea on a tightrope. In the first case, think about how an ant sees a garden hose. On the scale of the ant, the hose is two-dimensional: the ant can crawl up and down the length of the hose (one dimension), but also around the circumference (another dimension). However, a human looking at the garden hose from far away might only see the one dimension of length. Similarly, other dimensions may only be visible at different scales or can “loop up,” becoming too small to detect like how the human can’t “detect” the circumference of the garden hose. An example of the latter is the Calabi-Yau manifold, a six-dimensional space (Figure 2).

 

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Figure 2. A visual representation of the Calabi-Yau manifold, or one way of compactifying the extra dimensions of string theory

 

String theory has already been useful in both addressing open questions in physics and mathematics. It is also a candidate for a “theory of everything” to unify the four fundamental forces (strong nuclear, weak nuclear, electromagnetic, and gravitational), while the Standard Model has only unified the first three.

But the pursuit to validating the theory is not without drawbacks.

“The Test of All Knowledge Is Experiment.” (Richard Feynman)

The problem is that even with forty years of research, scientists have not been able to experimentally verify components of string theory—it actually may not be possible to test some parts of the theory. Given that research is largely funded by the government, some may see the ongoing research as a pretty big leap of faith on the part of funding agencies that receive an incredible number of proposals for worthy causes.

Is the pursuit worth funding? Will it ever yield fruit? It turns out that long pursuits like the one after string theory are not new in physics research; the pursuits for Higgs boson and gravitational waves are good examples.

The Higgs boson, mentioned earlier as a category within the Standard Model, was theorized to unite electromagnetism with the weak force. However, they ran into a snag after their experiments: some particles that were predicted to be massless actually had mass.

In 1964 six physicists addressed the contradiction by proposing a mechanism involving an omnipresent field called the Higgs field. The field imparts mass onto a particle based on how much the particle interacts with the field. If a particle doesn’t interact with the field, it is massless. Particles with similar sizes might have drastically different masses depending on their interaction with the Higgs field, as is the case with the electron (relatively low interaction with the Higgs field and therefore low mass) and the Top Quark (high interaction, high mass).

The Higgs field was used to make a number of accurate predictions in physics but still lacked experimental verification. To validate the existence of the Higgs field, scientists looked for evidence of the particle that comprises it – the Higgs boson. While scientists had an idea of what properties this particle would have, there were two major experimental challenges in the search for the Higgs boson: 1) they did not know its mass and 2) the particle itself is highly unstable and decays quickly – on the order of 10-22 seconds. Since the particle was so unstable, the physicists had to study the products of the particle’s decay—which allowed them to narrow down the range of the true mass. Physicists used increasing large particle colliders to study the products of decay, which can be visualized in a diagram that is reminiscent of a pizza (Figure 3). About fifty years after the original papers proposing the particle’s existence, scientists confirmed the theory in 2013.

 

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Figure 3. Delicious “particle accelerator”

 

The next post will continue to discuss the researchers’ effort to validate theories, specifically gravitational waves, and whether string theory research is worth the wait.

 

About the author

molly

Molly is a PhD student in Chemical Engineering at the University of Michigan. Her research involves the use of microfluidics (tiny channels) to isolate and study cancer cells traveling in the blood stream. Molly’s undergraduate degree is from MIT, and her master’s degree is from the University of Michigan. Outside of the lab, Molly enjoys reading, running, and optimizing her baking experiments.

Read all posts by Molly here.

 

Image Sources:

Figure 1: Adapted from https://en.wikipedia.org/wiki/Standard_Model#/media/File:Standard_Model_of_Elementary_Particles.svg

Figure 2: https://en.wikipedia.org/wiki/Quantum_gravity#/media/File:Calabi-Yau.png

Figure 3: https://home.cern/sites/home.web.cern.ch/files/image/inline-images/stpandol/higgs-pizza-day-set.pdf

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