Theoretical physics is a fascinating and an amusing field. While, most people wouldn’t claim to know much about this field of research, numerous of its more advanced concepts come-up in popular culture all the time. In fact, words such as “nuclear”,” amount” & “multiverse“ are crucial to the plot of our favorite Television shows & pictures.
On the other hand, numerous of the more advanced concepts in theoretical physics sound more like philosophy & metaphysics than science. In fact, several theories also manage to blur the lines between science & religion, and are generally met by either awe or dismissal (depending on who is listening).
Consider the thought of “extra dimensions,” which numerous people would suppose refers to the existence of dimensions parallel to our own, where things are slightly or very different, aka. “multiverse” theory. In truth, theory of extra-dimensions deals with the possible existence of beyond the bones we’re immediately aware-of.
While this type of talk may sound like something far-fetched or truly speculative, it’s really a vital part of our understanding on how our Universe works. However, we will finally have a Theory of Everything (ToE), and know how it all fits together, if & when we measure how many dimensions our Universe has (and what each of them does).
To break it down, term “dimension” refers to mathematical measurement. This can generally-relate to a physical measurement (object/space) or a temporal dimension (time). There are 3 dimensions that we experience every day, which define the length, range & depth of all objects in our Universe (x, y & z-axis, independently).
However, scientists want that to understand laws of nature, one must include a “fourth dimension,” which is time. Without this coordinate, position, velocity & acceleration of objects in our Universe can’t be correctly measured. It is not enough to know where an object is in terms of 3 spatial coordinates. You even need to know when the object was where.
Beyond these 4 dimensions, theoretical physicists ventured that there might be more at play. The number of dimensions changes, but the purpose behind extra dimensions is to find ways of unifying known laws of the Universe, which theoretical physicists are trying to do for about a century.
The reason has to do with 2 too-interesting fields of study: Quantum Mechanics (QM) & General Relativity (GR). These fields emerged during early 20th century and were nearly concurrent with each other. Whereas, QM has numerous forebearers (Planck, Heisenberg, Schrodinger), GR owes its existence, at least initially, to Albert Einstein, though numerous of his ideas were refinements on earlier theories.
For record, Einstein even contributed for the development of QM by his research on the behavior of light. In any case, whereas Quantum Mechanics (QM) describes how energy & matter behave at the atomic & sub-atomic levels, General Relativity (GR) describes how matter, energy & space-time behave on larger-scales in the presence of gravity.
The funny thing is, our greatest scientific-minds are trying to figure-out how these 2 fields fit together for nearly a century. Both appear to work just-fine on their own, but where they come together into a single coherent system, that still largely a mystery.
Four fundamental forces
After thousands of years of research into nature, and the laws that govern it, scientists determined that 4 fundamental forces govern all matter-energy interactions. These forces & fundamental particles that make-up all matter (quarks, leptons, hand bosons, & scalar bosons), are part of The Standard Model of particle physics. These forces are:
• Weak Nuclear Force
• Strong Nuclear Force
The first 3 forces are completely described by field of Quantum Mechanics and are associated with specific sub-atomic particles. Electromagnetism is associated with electrons (a lepton), which are responsible for electricity, magnetism and all forms of electromagnetic radiation, which includes visible light, heat, microwaves, radio waves, ultraviolet radiation & gamma rays.
The weak nuclear force deals with the interactions between sub-atomic particles responsible for radioactive decay of atoms and is associated with particles that are smaller than a proton (bosons). At higher energy, this force merges with electro-magnetism, which has given rise to the unified-term “electro-weak force.”
The strong nuclear force governs particles that are the size of protons & neutrons (hadrons) and is so-named because it’s nearly 137 times as strong as gravitation, millions of times stronger than weak nuclear force and 1038 times as strong as gravitation. It causes quarks to come together to form larger protons & neutrons and binds them to make atomic nuclei.
Finally, there’s gravitation, which is the weakest of the 4 forces and deals with the interactions between massive objects (asteroids, planets, stars, galaxies and the large-scale structure of Universe.) Unlike other 3 forces, there’s no known sub-atomic particle that describes gravitation or gravitational interactions.
This is why, scientists are forced to study physics in terms of QM or GR (depending upon the scales involved), but generally not both-combined. Because of this, scientists are trying to come-up with a theoretical framework for unifying gravity with other forces. Attempts to-do-so-generally fall under the heading of “quantum gravity” or a Theory of Everything (ToE).
How many dimensions are there?
Attempts to form a unified field theory of gravitation & electromagnetism can be traced to German physicist, Theodor Kaluza (1885 – 1954). In 1921, he published a paper, where he presented an extended interpretation of Einstein’s Field Equations. This theory was formed on the idea of a 5D Universe, which included a dimension beyond the common 4D of space & time.
In 1926, the Swedish theoretical physicist Oskar Klein offered a quantum interpretation of Kaluza’s 5D theory. In Klein’s extension, fifth dimension was curled-up, microscopic and could take the form of a circle that had 10-30cm radius. In 1930s, work was under-taken on the Kaluza field theory by Einstein & his colleagues at the Princeton. By 1940s, theory was formally completed and the given name was Kaluza-Klein theory.
The work of Kaluza & Klein predicted the emergence of String Theory (ST), which was first proposed during 1960s. By 1990s, many interpretations emerged, which include Superstring Theory, Loop-Quantum Gravity, M-theory & Supergravity. Each of these theories entails the existence of “extra dimensions”, “hyperspace” or something similar.
To summarize, ST states that point like particles of particle physics are really one-dimensional objects, called” strings.” Over distances more than the string-scale, they resemble ordinary particles by their mass, charge and other properties are determined by string’s vibrational state. In one state, string corresponds to graviton, which is what causes gravitation.
Superstring theory, a variation on ST, needs the existence of 10 space-time dimensions. These include the 4 dimensions incontinently apparent to us (length, range, depth, time) and 6 further that are not.
These extra 6 dimensions are curled-up into a compact space. On order string scale (10-33cm) we would not be able to detect the presence of these extra dimensions directly because they are just too small.
According to the theory, fifth & sixth dimensions deal with possible worlds that began with same initial conditions.
The 5th dimension encompasses worlds with slightly-different outcomes than ours, while sixth dimension is where a plane of possible worlds would be visible. The seventh dimension is where one could see possible worlds that begin with different initial conditions and then branched-out infinitely, hence why the term “infinity” is used to describe them.
The eighth dimension would similarly give-us a plane of these “infinities,” while in ninth dimension, all possible universes & laws of physics could be seen. In tenth dimension, anything & everything possible in terms of cosmic evolution are accessible. Beyond that, nothing can be seen-by living creatures that are a part of the space-time continuum.
M-theory, which combines 5 distinct super-string theories, posits the existence of 11 dimensions which include 10 spatial and 1 time. This variation on super-string theory is considered attractive, because of the phenomena it predicts. For one, M-theory predicts the existence of the graviton, which is consistent with string theory as an entire and offers an explanation for quantum gravity.
It even predicts a phenomenon similar-to black hole evaporation, where black holes emit “Hawking radiation“ and lose mass over time. Some variations of super-string theory even predict the existence of the Einstein-Rosen bridges, aka. “wormholes”. Another approach, the Loop Quantum Gravity (LQG), posits that gravity is entirely different from other fundamental forces and space-time itself is made of quantized, discrete bits, in the form of small, one-dimensional loops.
Some versions of super-gravity theory even promote an 11-D model of space-time, with 4 common dimensions & 7 hyper-space dimensions. There is even “brane theory, “which posits that the Universe is made-up of multi-dimensional vibrating “membranes” that have mass & charge and can propagate through space-time.
To date, there’s no experimental evidence for the existence of “extra dimension”, “hyperspace” or anything beyond the 4 dimensions we can perceive.
Why cannot we see them?
Alas, question remains. However, why cannot we confirm their existence? If additional dimensions are needed for the laws of physics to make sense. There are 2 possibilities one, what we suppose we know about physics is wrong, or two, dimensions of space-time beyond 4D we experience are so-subtle or small that they are invisible to our current experiments.
On its face, first possibility seems largely unlikely. After all, ongoing particle experiments such as those conducted with Large Hadron Collider (LHC), have confirmed that Standard Model of particle physics is correct. Similarly, General Relativity has been verified numerous times over since Einstein formally-proposed it in 1915.
That leaves us with the 2nd possibility that extra dimensions can’t be measured or characterized using current methods & experiments. A well-studied possibility is that dimensions are “curled up” at small-scales, which means their properties and influence on space-time could only be measured at sub-atomic levels.
Another possibility is “compactification,” where some dimensions are finite or temporal in nature. In short, this theory posits that curled up dimensions become too small or close in on themselves to make circles. If this is true, then 6 extra dimensions would probably take the form of a Calabi–Yau manifold (these are shapes that satisfy the needed requirement for the 6 “unseen” spatial dimensions of string theory.
For astrophysicists & theoretical physicists, compactification & the idea that extra dimensions are small explains, why the Universe remains exists billions of years after its emergence. If these dimensions are larger, they would accommodate sufficient matter to trigger gravitational collapses & formation of black holes (which would consume the rest of the Universe).
The fact that cosmos remains exists after 13.8 billion years and shows no-sign of being torn apart, would suggest that this theory is sound. Alternately, laws of physics may operate differently in these extra dimensions. Either way, there is still the unanswered question of how we might observe & study them.
How do we find them?
So, if Universe really have extra dimensions that are imperceptible to us, how are we going to find evidence of their existence, and determine their properties? One possibility is to look for them by particle physics experiments like those conducted by European Organization for Nuclear Research (CERN), operators of LHC and other particle accelerator labs.
At CERN, the scientists boost particles to high energies before smashing-them together and determining the resulting cascade of sub-atomic particles. Detectors gather clues about particles, like their speed, mass & charge, which can be used to work-out their identity.
Theories involving extra dimensions predict that there must be heavier-versions of standard particles recurring at higher & higher energies as they navigate smaller dimensions. These would exactly the same properties as standard particles (so be visible to detectors like those at the CERN) but at a greater mass. However, this might suggest the presence of extra dimensions, if evidence of these were to be found.
Another way is to look-back through time towards the period called “Cosmic Dawn,” nearly 100-500 million years after Big Bang, when first stars & galaxies formed. Even-if extra dimensions are imperceptible to detection today, they would influence the evolution of the Universe from the too beginning.
To date, astronomers are unable to see this far-back in time, since no telescopes are sensitive enough. This will vary in the near future, because of next-generation instruments, like James Webb Space Telescope (JWST), Nancy Grace Roman Space Telescope (RST), Extremely Large Telescope (ELT) and Giant Magellan Telescope (GMT).
This coincides nicely with existing dark matter & dark energy surveys that are observing early comic history in the hopes of determining their influence on cosmic evolution. Since several theorists venture that existence of extra dimensions could help to explain “Dark Universe,” these observations could address some mysteries at once.
This dual approach isn’t unlike our current understanding of the Universe, which scientists can merely understand in one of two ways, the largest (GR) & smallest of scales (QM). By observing the Universe with a too wide & too tight-angle lense, we may be able to account for all forces governing it.
Much like other ToE candidates, the thought that the universe is made-up of 10 dimensions or greater is an attempt to take all the physical laws we understand and find-out how they fit together. In that respect, it is like assembling a puzzle, where every piece makes sense to us, but we’re unaware of what the bigger picture looks-like.
It is not enough to put pieces together, wherever they seem to match. We also require to have an overall idea of what the framework is, a mental picture of what it’ll look like when it’s finished. This helps to guide our efforts, so we can anticipate how it’ll all come together.