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Putting string theory to the test

In string theory, tiny bits of string replace traditional subatomic particles.
In string theory, tiny bits of string replace traditional subatomic particles. (Image credit: <a href="">Michael Taylor</a> | <a href="">shutterstock</a>)

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of "Your Place in the Universe." Sutter contributed this article to's Expert Voices: Op-Ed & Insights

String theory hopes to be a literal theory of everything, a single unifying framework that explains all the variety and richness that we see in the cosmos and in our particle colliders, from the way gravity behaves to whatever the heck dark energy is to why electrons have the mass that they do. And while it's a potentially powerful idea, which if unlocked would completely revolutionize our understanding of the physical world, it has not ever been directly tested.

There have been, however, ways to explore some of the underpinnings and potential consequences of string theory. And while these tests wouldn't prove string theory directly one way or another, they would help bolster its case. Let's explore.

Related: Why string theory persists — despite the knotty physics

A perturbing problem

First, though, we have to examine why string theory is so hard to test. There are two reasons.

The strings of string theory are stupendously small, thought to be somewhere around the Planck scale, a bare 10^-34 meters across. That's far, far smaller than anything we can possibly hope to probe even with our most precise instruments. The strings are so small, in fact, that they appear to us to be point-like particles, such as electrons and photons and neutrons. We simply can't ever stare at a string directly.

Related to that smallness is the energy scale needed to probe the regimes where string theory actually matters. As of today, we have two different approaches for explaining the four forces of nature. On one hand, we have the techniques of quantum field theory, which provide a microscopic description of electromagnetism and the two nuclear forces. And on the other we have general relativity, which allows us to understand gravity as the bending and warping of spacetime.

For all cases that we can directly examine, using one or the other is just fine. String theory only comes into play when we try to combine all four forces with a single description, which only really matters at the very highest energy scales — so high that we could never, ever build a machine to reach such heights.

But even if we could devise a particle collider to directly probe the energies of quantum gravity, we couldn't test string theory, because as of yet string theory isn't complete. It doesn't exist. We only have approximations that we hope come close to the actual theory, but we have no idea how right (or wrong) we are. So string theory isn't even up to the task of making predictions that we could compare to hypothetical experiments.

Related: The history and structure of the universe (infographic)

Cosmic blues

Even though we can't reach the energies needed in our particle colliders to really take an in-depth look into the potential world of strings, 13.8 billion years ago our entire universe was a cauldron of fundamental forces. Perhaps we might gain some stringy insights by looking into the history of the Big Bang.

One suggestion put forth by the string theorists is another kind of theoretical string: the cosmic string. Cosmic strings are universe-spanning defects in spacetime, leftover from the earliest moments of the Big Bang, and they're a pretty generic prediction of the physics of those epochs of the universe.

But cosmic strings might also be super-duper-stretched-out strings from string theory, which are usually so small that "microscopic" is too big of a word, but have been stretched and pulled by the incessant expansion of the universe. So if we found a cosmic string floating around out there in the cosmos, we could study it carefully and check if it's really something predicted by string theory.

To date, no cosmic strings have been found in our universe.

Still, the search is on. If we found a cosmic string, it wouldn't necessarily validate string theory — there would be a lot more work needed to be done, both theoretically and observationally, to tell apart the string theory prediction from the crack-in-spacetime version. 

Not so supersymmetry

Still, we might be able to pick up some interesting clues, and one of those clues is supersymmetry. Supersymmetry is a hypothesized symmetry of nature that links together all the fermions (the building blocks of reality like electrons and quarks) with the bosons (the carriers of the forces like gluons and photons) under a single framework.

The machinery of supersymmetry was first worked out by string theorists, but took fire as an interesting avenue for all high-energy physicists to potentially solve some problems with the Standard Model and make predictions for new physics. Within string theory, supersymmetry allows the strings to describe not just the forces of nature but also the building blocks, giving that theory the power to truly be a theory of everything.

So if we found evidence for supersymmetry, it wouldn't prove string theory, but it would be a major steppingstone.

We haven't found any evidence for supersymmetry.

The Large Hadron Collider (LHC) was explicitly designed to explore supersymmetry, or at least some of the simplest and easiest-to-reach versions of supersymmetry, by looking for new particles predicted by the theory. The LHC has turned up completely empty, without even a whiff of a new supersymmetric particle, wiping all the simplest supersymmetry ideas completely off the map.

And while this negative result doesn't rule out string theory, it doesn't make it look too great, either.

Will we one day have evidence for even one of the underpinnings or side predictions of string theory? It's impossible to say. A lot of hopes were pinned on supersymmetry, which has so far failed to deliver, and questions remain about whether it's worth it to build even-larger colliders to try pushing harder on supersymmetry, or if we should just give up and try something else.

Learn more by listening to the episode "Is String Theory worth it? (Part 6: We Should Probably Test This)" on the Ask A Spaceman podcast, available on iTunes and on the Web at Thanks to John C., Zachary H., @edit_room, Matthew Y., Christopher L., Krizna W., Sayan P., Neha S., Zachary H., Joyce S., Mauricio M., @shrenicshah, Panos T., Dhruv R., Maria A., Ter B., oiSnowy, Evan T., Dan M., Jon T., @twblanchard, Aurie, Christopher M., @unplugged_wire, Giacomo S., Gully F. for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and

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Paul Sutter

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe, to the earliest moments of the Big Bang, to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space," and frequently appears on TV.

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  • rod
    Admin said:
    String theory hopes to be a literal theory of everything — a powerful idea that could completely revolutionize our understanding of the physical world. But it has never been directly tested.

    Putting string theory to the test : Read more

    Interesting test(s) proposed, some using cosmology and the primordial universe conditions, another with particle colliders. It seems after reading about string theory from different reports, I cannot use a telescope and record stellar spectrums showing the various dimensions and strings that light passes through to reach the telescope. This morning I viewed Mars and Jupiter close conjunction in Sagittarius using my telescope at 71x with a true FOV a bit larger than 1-degree. Did the light from Mars (orange-red hue), Jupiter with cloud belts visible, Europa, Ganymede, and Callisto moons visible, pass through the required strings and dimensions, perhaps 11 or so on its way to the telescope while I enjoyed the view? Interesting how cosmology and strings are tested or proposed test.
  • rod
    Okay, another report shows X-rays using Chandra could test string theory. The particle, axions. So far the tests have not confirmed the axion particle in string theory. Chandra data tests 'theory of everything' "Clearly, one possible interpretation of this work is that axion-like particles do not exist. "

    Looks like my telescope observations this morning of Jupiter and Mars, the telescope will not show axions that the optical light passed through :)
  • sunstance
    What if I told you that our atomic structure is really just a crystal that is resonated by external forces tour universe. the crystal uses a spectral prism and the displacement energy resonates it so fast that internally there is a reaction that you call neutrons. the protons are really the tips of the crystals . each elements is really just a crystal. that I s being stimulated by a blue star external to our universe. there are three giant suns one white it produces effects a blue star it tends to be the stimulation part of energy and one orange sun. . the string theory to me is like you said how things are connected. but it can also be how things evolved. and most of that is a natural process over time. But time is all connected each moment is connected to another . There is no way to understand everything before that would happen you would find yourself in an infinite understanding even though it would only ever be greater then self. and in that all you could do if turn back to self to realize things about yourself and then better yourself. It would only be later that you could turn back out from self to attempt to explain what you have learned. and in that you would of evolved yourself and be able to ask question greater then that you understood prior. In truth all you would be doing is formatting yourself to have greater logic and analytical understanding it is the simplest under . That all correct answers in a IQ test would prove that the mind is well formatted.
    Looking at time over long periods can help with string theory as it is the time that you can then assign things to actual or real which must be truth in creation and provide the physical world in evolution can be simplified into a known assigned term.
  • rod
    *But is has never been directly tested* something very important. Here is the report on a binary brown dwarf system < 7 light-years away. The astrometric measurements reported I can take to the bank compared to string theory confirmation :)
  • sunstance
    maybe no-one is string theory has looked at it. I can look at it as well . string theory can also be used to engage further into the situation. it is clear that science has looked at this there is no dispute on matters pertaining to accurate information. such as banking results compared to a string theory. banks are law and string theory is theory. it goes hypothesis , theory and then law in all science first ( hypothesis) generally agreed upon by the scientific groups or a few . then theory something that has been put out there for other scientist academic and citizen scientist to review . if it passes the last test then it can become law considered in science as law. or referred to as a law.
  • Catastrophe
    I am sorry, I find it difficult to understand this. If you get few responses then maybe others have same problem. Can you please make clearer post?
  • sunstance
    ok so imagine a coil that leaves the sun and passes all the way out into space. this coil touches every planet .
    it is spinning and every planet touches this coil at different points. some planets touch the coil on top and spin one way, some touch the coil underneath and spin backwards, some touch the coil on the side and spin sideways.
    so the first thing to do is reduce everything down to a simple process. and then see what is happeneing before starting to form a theory with assigned terms.
  • Catastrophe
    sunstance said:
    ok so imagine a coil that leaves the sun and passes all the way out into space. this coil touches every planet .
    it is spinning and every planet touches this coil at different points. some planets touch the coil on top and spin one way, some touch the coil underneath and spin backwards, some touch the coil on the side and spin sideways.
    so the first thing to do is reduce everything down to a simple process. and then see what is happeneing before starting to form a theory with assigned terms.
    There are few exceptions to the general rule. It is much more likely that the exceptions were caused by hits (mainly during Late Heavy Bombardment),

    Incidentally, your coil would have to be a disk to include all motions of all planets, otherwise (if linear) it would have to be sentient to find where each target planet was. Also, why doesn't it return and knock Uranus, or another planet, sideways - or slow down the rotation of Venus or another planet.
    No. These phenomena are 99.999% more likely caused by impacts during LHB.

    Cat :)