From Classical Relativity to Particle Physics: String Theory’s Possible Bridge
Updated: Oct 10, 2024
By Alexandra Sendrea
Gravity, is the force within our world that seems stable and constant, keeping everything in its place. In maths we often simplify it as 9.8 or just g, easily cancelling it out when it appears on both sides of an equation. This seems straightforward, right? But what if this fundamental force is not as familiar as it seems, what if it holds more complexity than we ever thought and ideas that we have yet to uncover?
Sir Isaac Newton worked on some groundbreaking work in the 17th century, suggesting that space and time are absolute and fixed; however, he was unaware that his laws would break down under extreme speeds or greater objects. In Principia (1687), he proposed that the same force making objects fall to the ground also governs the motion of planets and stars. Yet, Newton’s perspective on gravity, though revolutionary, was incomplete. He did not fully understand the nature of this force or how gravity works the way it does.
1 Centuries before Newton, Galileo hinted at the concept of relative motion, the idea that the characteristics of motion (speed and direction) depend on the observer’s frame of reference. In 1632, he famously wrote, “It is impossible by mechanical means to say whether we are moving or staying at rest.” 2 To illustrate this www.science.org imagines being in a smoothly moving boat, in a closed room with no windows. If the boat is traveling at a constant velocity and you throw a ball, it would behave the same way as if you were standing on solid ground. This principle explains why we do not perceive the Earth's movement around the Sun. Since no external resultant force is detected, Galileo’s insight forms the basis of what we now understand as inertia—an object in motion remains in motion unless acted upon by a net external force.
Einstein’s special relativity revolutionized our understanding by showing that space and time can be linked by a single term – spacetime – in a way that keeps the speed of light constant for all observers, but distances and time themselves can stretch and compress. 3 Time dilation is a phenomenon predicted by Special Relativity, which states that time is not an absolute measure; rather, it is relative and can vary depending on the relative speeds of observers. According to the theory, as an object moves closer to the speed of light, time for that object slows down relative to a stationary observer. If you take the famous twin paradox where Twin A and Twin B's experiences of time illustrate how motion affects aging and the passing of time. Similarly, if Observer A is in an aircraft traveling at speeds exceeding 0.1c, Observer A will perceive Observer B's clock as ticking slower due to its relative velocity. Building upon this, general relativity introduced the concept of gravity not as a force but as the bending of spacetime caused by massive objects. His idea was that “matter tells spacetime how to curve, and curved spacetime tells matter how to move”. Imagine spacetime as a flexible sheet; placing a heavy ball on it creates a dip that alters the path of other objects.
When unobserved, particles behave like waves, but when measured or observed, they appear as fixed points, behaving like particles. This phenomenon is known as "wave-particle duality” and is what describes quantum Mechanics. The connection between Quantum mechanics and special relativity led to the creation of quantum field theory (QFT). For example, the famous equation E=mc2 from special relativity plays a crucial role in QFT by explaining how energy and matter are interchangeable, which is essential for understanding particle creation and annihilation at the quantum level.
Particle physics experiments, like the Large Hadron Collider (LHC) study rare, fleeting particles that are produced. In 2012 the Higgs boson was discovered, which gave particles mass. This caused open questions to arise, like the nature of dark matter and how to incorporate gravity into quantum physics, general and particle physics still haven’t been linked. One of the theories that try to link this is string theory.
In string theory, the idea is that all fundamental particles (such as quarks and leptons) are not point-like objects, but rather different vibrational modes of tiny, one-dimensional "strings." The type of particle, its charge, mass, and other properties correspond to the way the string vibrates. Each vibration creates a different particle, and since strings can vibrate in many ways, string theory predicts more particles than the Standard Model of Fundamental Particles (up, down, charm, strange, top, bottom quarks, and leptons like the electron).
This is one of the challenges of string theory—it gives rise to many possible particles that we haven’t observed yet, which are called string excitations. Some of these particles might have masses or charges beyond the reach of current experiments, which is why they haven’t been detected.
If you want to describe quantum mechanics through string theory, we arrive at particle physics. To fully understand these particles, we must mathematically simulate their behaviours. In fact, string theory suggests the existence of 11 dimensions. To make the theory more manageable, you apply compactification to simplify it and reduce it to 4D, which we can measure.
Imagine a sheet of paper that you roll to create a cylinder; in one direction, there is infinity, while perpendicularly, there is a defined circle. If you roll it so much that it eventually becomes a line, that illustrates how we compact dimensions into shapes. An infinite number of dimensions can help explain gravity leaking into areas that Einstein's theory cannot measure. Each theory is more precise than the previous one, with Einstein being more precise than Newton.
In the pursuit of a unified theory, the concepts of "swampland" and "landscape" in string theory highlight the challenges and possibilities of finding a theory that is consistent with both quantum mechanics and gravity. The graviton, a hypothetical quantum particle responsible for transmitting gravitational forces, represents a potential key to unifying General relativity and Quantum Mechanics. This is why the graviton is theoretically described as the highest level on a roller coaster.
In conclusion, we still struggle to formulate questions that reach the level of the theory. Attending a lecture at the Institute of Physics by Davide De Biaso, who holds a PhD in string theory, inspired me to write this article.
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