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What is String Theory?

By R. Kayne
Updated: May 21, 2024

String theory, sometimes called the Theory of Everything, is thought by some to be the unifying field theory Einstein sought before his death. It is the first mathematically sound theory that reconciles the world of the infinitesimally small, with the world known at large. It unites Einstein’s Theory of Relativity with quantum physics and offers a potential explanation for the Big Bang.

Prior to string theory, subatomic particles were envisioned as tiny balls or points of energy. This theory works on the premise that the tiniest subatomic bits that make up the elements of atoms actually behave like vibrating strings. The strings are so small that physicist Brian Greene has analogized that, if a single atom were enlarged to occupy the footprint of our solar system, a string would still be no larger than a tree.

Because these tiny vibrating strings are responsible for the properties of all matter, the cosmos has been likened to a cosmic symphony of superstrings. While poetically appealing, the strength of string theory is that it accounts for all four known forces in one elegant theory. These fundamental forces are gravity, the strong and weak nuclear forces; and electromagnetism.

One of the surprising elements of this theory is that it requires extra dimensions to be free of mathematical anomalies. Scientists added an extra six dimensions, initially, for a total of ten. The six dimensions were predicted to be contained in tiny curled up formations at every point within our three-dimensional space.

There was a problem, however: string theorists came up with several theories that all seemed to be correct. Ultimately, scientists found that adding an 11th dimension mathematically explained all of the seemingly different theories as different aspects of the same one. The one theory to rule them all is known as M-theory.

The 11th dimension of string theory predicts a new kind of string, stretched infinitely long to create what is termed a floating membrane, or brane. According to this theory, infinite branes exist that each support a separate but parallel universe. In this wildly exotic neighborhood, the “problematic” force of gravity was also explained.

While the Standard Model of physics had already united three of the known forces, gravity remained elusive. Part of the problem was that gravity was such a weak force relative to the others. String theory mathematically predicts that gravity is weak because it is only leaking here from a parallel universe.

This is possible, string theorists explain, because strings can be open or closed. Open-ended strings have one endpoint attached to the brane on which they reside, keeping matter contained within that brane. Human bodies are believed to be made from open-ended strings, which explains why people can’t reach into or interact with other dimensions. Close-ended strings, however, are like tiny rings, unattached to their brane, able to “leak” away from it.

Gravity is thought to be transferred via massless, hypothetical particles called gravitons. If gravitons were made from close-ended strings, scientists theorized, gravity might be leaking off our brane. It sounded good, but it didn’t work mathematically. The opposite hypothetical did work, however: gravity appears to be leaking to our brane from a parallel universe. Fantastically, this notion is mathematically sound.

String theory also offers a possible explanation for the Big Bang. It had long bothered scientists that although they could plot the stages of the Big Bang backward to the singularity, the initial cause for the event was without explanation. Now string theorists believe that two branes colliding could have caused the event.

The biggest challenge to the theory is that much of it is not provable. Scientists can’t test other dimensions, study migrating gravitons, or peek between the curtains of floating branes to witness a Big Bang event. For this reason, string theory has many detractors and critics. Some scientists believe that without the ability to prove the theory, it is not true science at all. Nevertheless, proponents seem confident that proof of various sorts will come with technological progress and time.

All The Science is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
By anon992689 — On Sep 24, 2015

Anon, hahaha, I thought it was somebody until just now. Because religion and God kept coming up as string theory is to prove God's nonexistence. In reality it actually shows the opposite without error.

All math has never been able to work out because everything you see is, in fact, "God."

The number of theoretical physics scientists going nuts is astronomical, pun intended. Why not look at where it all breaks down? How many numbers of brains? Compare to animal dermis. The other: take a really long good look at the pictures, beautiful pictures of the human circulatory system, overlap the layers and bam! There it is. We and everything you see are part of a bigger being and if right. looking like us.

Solar systems, and all space dark matter, light matter, all matter is running through the body. The cells dropping away to black holes, i.e. different veins and arteries of this being then lost out of sight. All the solar systems, ' cells, running this body. Humans themselves being a virus have been lucky so far we haven't caused a cough yet except for some carriers. Wow. Another answer to where some biologicals come from I have space. There you have it.

I was saving this for someone special to put together and realized it won't happen in my lifetime. So I am giving to all. Just remember to give some credit to Dr.Tim1111.

By anon923949 — On Jan 01, 2014

Was there a logical basis for string theory or were mathematicians simply playing around with formulas, trying to make something happen? What was the overriding logic?

By anon340167 — On Jun 30, 2013

String theory might not be able to disprove the god argument (because it is a logical fallacy,) but it does prove that people may not have always been god's chosen creation, and that fact probably also means that any one religion/ religious text also does not make sense.

By anon301398 — On Nov 03, 2012

I understand gravity, em energy,and the universal cycle and strangely I haven't discovered the big bang, string theory, an expanding universe or god. I have discovered the lengths people will go to to craft science fiction in the guise of science though. Everything is entirely explainable without extra dimensions and floating branes. Physicists who challenge science will discover the solution for themselves. If I can do it, so can you.

By anon170814 — On Apr 27, 2011

When we finally allow ourselves the realization that there is only one string with infinite superposition, a theory of everything will properly emerge.

By anon122043 — On Oct 26, 2010

Firstly I am a novice here and would appreciate your understanding that I may request explanation on what you may consider obvious.

I have recently started independently studying Quantum Physics and astronomy. Although my research has just started, I am already beginning to get overwhelmed. M theory is indeed fascinating but I don't quite understand it on a small scale, not literally but figuratively. I get the concept but how does it work?

So these giant branes float around, each its own dimension with open ended strings attached to them creating matter that exists only in that brain as close-ended strings vibrate in and out of different dimensions, or at least have the ability to. Got it. But I am left with a lot of questions:

1.So how does this relate to the infamous cold spot?

2. what are these dimensions, why are the necessary in order to reconcile quantum physics with the theory of relativity, the macro-physics, if you will?

3. When this article says it is mathematically sound, what does that mean? Are there mathematical expressions you can post here for me to play with and get a feel for how this works?

At this point, string theory merely sounds like a pipe dream, I want to know not just that it could work but how. With the information I know now (which is admittedly very limited) I could write my own theory, the quantum-pubic-lice theory involving tiny interdimensional pubic predators on quantum hamster wheels powering the universe. And this theory would have as much a possibility of being true to me.

It seems to me that whenever I look for a meaty article to explain this theory further I run into one of two problems. Either it is way over my head or it is as basic as the above article, offering little to no mathematical proof or reference to an outside study. I am not critiquing anyone but myself and my inability to educate myself on this, let that be known. I am just frustrated and want to know more. Any suggestions?

By anon117721 — On Oct 11, 2010

I'm really puzzled that so many comments on this article are really about 'God', who isn't mentioned in the article at all. Is there a string theory that has 'God' in its equations, and which is omitted in the article above? Or is this just general bigotry?

By anon115746 — On Oct 04, 2010

The Standard Model has united the strong force and the electroweak force? I think not.

By anon113885 — On Sep 26, 2010

Actually, to those that say "God can't exist because of the string theory" or those of you that say "The string theory can't exist because of God," I strongly disagree.

As a matter of fact, I am completely in support of the string theory. It so utterly makes sense to me. However, this actually makes it *easier* for me to believe in God. First, if something as fantastic as alternate dimensions exist, why can't God? And furthermore, what if these other dimensions compose the afterlife? Is that so impossible? Couldn't God have *caused* the collision of two branes, specifically for the reason of creating our universe? The two coincide so beautifully to me. That's just my opinion. -YaleGal

By anon75678 — On Apr 07, 2010

Seems to me that no theory on how the universe can best be described (evolution, relativity, string theory, etc.) ultimately solves the question of who put it there or who holds it together.

Ultimately every force and every action, every relationship among matter and energy, every thought and emotion of mankind, every single thing that exists in our universe as we know it - no matter how we choose to describe it - all these point to the Creator as the one who holds them in His hands.

He is the Infinite Solution you seek. I would encourage you all to look to Him today.

By anon64840 — On Feb 09, 2010

It's amazing how now that string theory exists, people still believe the universe was made by some magical man in the sky.

By anon64490 — On Feb 07, 2010

How does the string theory fit into the Big Bang theory?

By anon62155 — On Jan 25, 2010

Article says: "string theorists believe that two branes colliding could have caused the Big Bang event"

Where did the two branes come from?

Then concludes: "As many string theorists have quipped, "Something this elegant can’t be wrong!"

"God created" -- that's more elegant than this proposed string theory, so therefore it can't be wrong!

I cannot believe the lengths to which people will go to try to deny the existence of God.

By anon58496 — On Jan 02, 2010

Wow, thank you so much element92!

By anon58358 — On Jan 01, 2010

i at the present moment do not believe in the string theory because of some complications. at present it seems to me that light/mass/gravity and molecular structure are in a dance with one another, even though not necessarily in this order.

The hole puzzle falls apart if one aspect is removed. We see this constant factor time and time again time itself is just a beginning and ending point. As we interpret it the brain may seem to slow the clock, but time is still a measurement problem. it leaves too many holes in this theory.

By anon43068 — On Aug 25, 2009

Very well written article-one of the best summaries of string and M theory I have come across. Where does dark matter fit into this?

By element92 — On Nov 01, 2008

Actually, you see, your reservations towards the Big Bang Theory and its implications go not without explanation.

First of all, the microwave background radiation, regardless of stellar distribution, appears strikingly uniform-containing deviations in temperature no more than one part in a hundred thousand. Were you to be correct in asserting starlight be a determining factor, we would observe considerably more dramatic temperature differences distinguishing intergalactic voids from their matter-filled counterparts.

While not as exacting as my first description, this point too can be explained clearly. The Standard Model, arguably the most "correct" theory ever to be formulated, contains a staggering 19 adjustable parameters. Despite this rather embarrassing ambiguity, the theory remains so precise in its description of nature that scientists have had little qualms over accepting its predictions. So you see, requiring more than an arbitrary, aesthetically pleasing number of adjustable parameters is not necessarily a demerit for any theory.

The answer to your third assertion is actually in the form of an appendage concept to the original Big Bang Theory. Less than a fraction of a second after the initial singularity, there emerged an incredibly brief period of hyperinflation, during which the universe ballooned from the size of a single proton to a cosmic colossus one thousand times the size of our solar system.* This explains why the universe appears to be so much larger than a period of 10-20 billion years of constant expansion would allow.

As for the apparent brightness of various quasars as related to their respective redshifts, the incomprehensible distances at which we are viewing these objects would have an ultimately dampening effect on our observations, since their perceived variations in luminosity would be incalculably miniscule, causing them to appear at identical apparent brightnesses. Therefore, we need no longer necessitate that the actual luminosity of any quasar decrease with respect to time in order to compensate for the relatively slight variations in redshift relative to each other. Their huge distance from us (and comparatively negligible distances from each other) account for the perceived relationship.

The oldest globular cluster in the Milky Way Galaxy is measured to be approximately 10 billion years old (with a relatively narrow margin of error). The most accurate measurements yet, collected by NASAs WMAP satellite, of the current age of the universe stands at 13.7 billion years, well outside the possible deviation in age for our oldest globular clusters.

Your next point can actually be addressed from two angles. First, our universe is not necessarily finite in space, and, by all accounts, is only finite in our one temporal dimension, hence its age of 13.7 billion years. Secondly, in order to capture the uniformity of our universe, it must be observed on truly cosmological distances. The distances involved when observing the Local Group, spanning about 6 million light-years, are much too minute in order to appreciate the large-scale uniformities characteristic of the observable universe.**

This does not pose any inconsistency with the Big Bang Theory. It is able to describe the origin and evolution of our universe, meaning that it is consistent even with the presence of dark matter. Our own ignorance need not be invoked.

Quasars, contrary to popular belief, are not characteristic of the very earliest galaxies, since they had not yet accumulated sufficient gas and dust to sustain such an AGN (active galactic nucleus). Consequently, having existed prior to quasar formation, it is deducible that they would in fact retain higher redshifts.

Again, while not as exacting as my previous arguments, this too, can be explained. Put bluntly, you would be hard pressed to find a cosmologist susceptible to even a single flinch at this ratio. The cosmic microwave background radiation contains variations in temperature of one part in a hundred thousand-a significantly more impressive ratio, considering that it alone gave rise to the staggering density deviations we observe now! It seems to me that if a trifling amount such as this could have laid the foundation for such a magnificently diverse cosmos, then a difference of one part in a mere thousand or so would certainly be capable of having the even more consequential effect of creating an open universe.

*You would be correct, though, to pose the question of what in fact drove the phenomenon of inflation and what happened to shut it off. The Big Bang Theory does in fact say absolutely nothing about this.

**In cosmological jargon, this is referred to as The Horizon Problem.

By harvardgurl — On Jun 02, 2007

hi, i was just wondering how does string theory prove the Compton effect and cosmic background radiation that is simultaneous with the Big Bang Theory...because there are many problems with the Big Bang theory as well

here are some:

he microwave "background" makes more sense as the limiting temperature of space heated by starlight than as the remnant of a fireball.

Element abundance predictions using the big bang require too many adjustable parameters to make them work.

The universe has too much large scale structure (interspersed "walls" and voids) to form in a time as short as 10-20 billion years.

The average luminosity of quasars must decrease with time in just the right way so that their mean apparent brightness is the same at all redshifts, which is exceedingly unlikely.

The ages of globular clusters appear older than the universe.

The local streaming motions of galaxies are too high for a finite universe that is supposed to be everywhere uniform.

Invisible dark matter of an unknown but non-baryonic nature must be the dominant ingredient of the entire universe.

The most distant galaxies in the Hubble Deep Field show insufficient evidence of evolution, with some of them apparently having higher redshifts (z = 6-7) than the faintest quasars.

If the open universe we see today is extrapolated back near the beginning, the ratio of the actual density of matter in the universe to the critical density must differ from unity by just a part in 1059. Any larger deviation would result in a universe already collapsed on itself or already dissipated.

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