By Vidya Rajan, Columnist, The Times
The story so far: The hot, dense singularity that was the source of the Big Bang exploded about 13.8 billion years ago (bya) and the energy in the explosion spread, cooled and became lumpy. The lumps were matter (~5%), dark matter (~25%), and dark energy (~70%).
To break it up into timelines after the instant of the Big Bang (A.B.B.):
- 10-43 seconds A.B.B.: Planck Epoch. This is the closest theoretical limit that we can get to the instant of the Big Bang. Temperatures were of the order of 1031 oC; all forces and matter were one in a point smaller than the size of a proton. Energy levels were of the order of about 1019 billion electron volts (GeV). This roiling energy was a soupy combination of all the stuff of the universe. Gravity was the first force to separate out, leaving the electroweak (electromagnetic + weak force) and strong forces still combined. “Time” began ticking.[1]
- 10-43 to 10-35 seconds A.B.B.: Grand Unified Epoch. The other forces (strong, electroweak) remained combined. Understanding this combination is the goal of the Grand Unified Theory.
- 10-34 to 10-32 seconds A.B.B.: Strong force separates out, powering the “Inflationary Period” which led to the expansion of the universe from the size of a proton to a grapefruit (the equivalent of from the size of a tennis ball to the size of the solar system) almost instantaneously. Temperatures around 1026 o Energy levels are around 1015 GeV.
- 10-12 seconds – 100 seconds A.B.B.: Quark Epoch. Universe chills quickly to a temperature around 1015 o Energy levels are around 100 GeV. “Electroweak” force splinters into two separate forces: the weak nuclear forces (transmitted by the W+, W– and Z0 bosons) and the electromagnetic force (carried by the photon aka the γ boson). Higgs field present. The energy of this epoch can be recreated in the Large Hadron Collider, which produced the Higgs boson in 2012.
- ~101 seconds A.B.B. to 13.8 billion years later: Matter Epoch. The current average temperature is a bitterly cold negative 270 oC: this is the ~3 K background temperature associated with the Big Bang called the Cosmic Microwave Background (CMB). Energy levels are around 0.25 eV. The initiation of the “matter period” reversed the e=mC2 equation to m=e/C2 creating the first fermions which make up matter. But the universe also contains ~70% dark energy; ~25% so fermions (protons, neutrons, electrons) are only a small part of the universe. There are probably more particles and forces beyond the Standard Model in dark energy and dark matter that we are unable to observe.
In this final installment of the description of the universe, I will describe the “force” carriers which allow exchanges of energy, and the push-pull between matter. To begin, let’s go back in time to 101 seconds A.B.B. This was the start of the “matter period”. There were no atoms because the incredible heat kept everything as a soup, but there were four fundamental forces in the universe that differentiated out, and remain to this day. In order of decreasing strength, they are:
- The strong nuclear force mediated by two sets of mesons. Inside the proton or neutron (collectively called nucleons because they reside in the nucleus) gluons (g) “glue” quarks together. Outside the nucleon (but still inside the nucleus) the “residual” strong force is a little weaker but mediated by massive mesons called pi (π) and rho (ρ).
The strength of gluons only manifests over the tiny (10-15 m) diameter of protons inside which quarks are whizzing about at the speed of light. Gluons come in many “colors” just like the quarks that they act on and, as they interact, both gluons and quarks can change a property that is termed “color” or “flavor” (Note: Physicists are even worse than biologists for naming stuff. I wish they’d used terms like “skarts” or “gipmos” that we don’t use in regular life for these forces, but we are stuck with what the terms they used. So color and flavor do not mean what they do in our perception. They are the ever-shifting properties of quarks and the gluons that bind them.) Color interactions between quarks and gluons is the field of quantum chromodynamics and you can get a flavor (haha) in the videos in the bibliography.[2] Gluons act like Velcro to hold quarks within a nucleon but also interact with each other through the color force. Gluon strength increases as they get farther apart within the nucleon (like a rubber band, which is floppy when relaxed and works best when it is stretched).
Because quarks whiz around at the speed of light, they sometimes escape their confinement within the nucleon. In this case, it simultaneously generates another quark-antiquark pair which becomes a pi (π) and rho (ρ) meson. The quark itself returns into the nucleon. The mesons disintegrate pretty soon but mediate the “residual” strong force. Therefore the outside-nucleon strong force is called the “residual strong force” because the (primary) strong force is mediated by the gluons confined inside the nucleon. The residual strong force forces protons (which have a positive charge and therefore repel each other) together in the nucleus of atoms with more than one proton. Despite shielding by neutrons, the electrostatic force of each proton-proton repulsion is calculated to be about 58 Newtons or about 13 pound-force[3]. That’s a lot of energy to overcome; releasing it is what makes the atomic (or nuclear) bomb so powerful.
- Electromagnetism has 1/100th of the strength of the strong nuclear force, but its effect extends across distances. However, since atoms and large objects are not charged, its effect is not visible at large scales like gravity’s is. Electromagnetism’s force carrier is the photon (γ) which has no charge and no mass. Its motion is not affected by the Higgs field (which causes a drag on some particles, and gives them mass) but it is affected by gravity. Einstein showed that the photoelectric effect seen when photons striking a metal plate to release electrons is due to photons being quantized. It is the principle of how your camera works.[4] This meant light could morph from wave-like to particle-like behaviors and each particle had a specific amount (quanta) of energy. This discovery caused an earthquake in the physics community. It was for this discovery that Einstein won his only Nobel prize.[5]
Quantum electrodynamics explains how photons mediate magnetic attraction and repulsion between materials. Within an atom, electrons are constantly releasing and absorbing photons to move between lower to higher energy orbitals; chemical reactions between atoms are the consequence of photon-mediated interactions between electrons of different atoms. So photons are doing things within matter which we cannot sense except by experimentation, and their presence is essential to atoms sticking to other atoms to make molecules.
- The weak nuclear force is carried by electrically charged W+ and W– bosons and neutral Z0 It is about a millionth as strong as the strong nuclear force, and acts over tiny, quark-sized distances, about 1/1000th of that of the gluon’s strong force. It is this force acting between quarks that allows some particles to escape the nucleus as beta-decay. Beta minus (β-) decay was essential for the process by which the very first electrons in the universe were made, by the decay of neutrons!
β- decay: Neutron (Up-Down-Down quarks) Proton (Up-Up-Down quarks) + electron + anti-neutrino; here a Down quark changes into an Up quark
β+ decay: Proton (Up-Up-Down quarks) Neutron (Up-Down-Down quarks) + positron + neutrino; here an Up quark changes into a Down quark
Crucially, crucially, β- decay allowed electrons to form. This allowed the constituents of atoms to be made available about 100 seconds after the Big Bang. Matter started forming. The first atom (supposedly) to form was a hydrogen atom called a deuterium (1 proton + 1 neutron in the nucleus) which captured an electron, forming an atom. Two deuterium atoms combined to make helium. Subsequently, hydrogen molecules (H2) formed, abundantly, and make the fuel that powers fusion in the hearts of stars. Beta-decay due to the weak nuclear force also stabilizes the nucleus of large atoms by releasing energy (lower energies are more stable). Without the weak nuclear force, there would be a mess of protons and neutrons in the universe, but no electrons, no matter, no life, no you or me, and no essay to write or read.
- Gravity, whose operation at the atomic (quantum) level is a mystery. It extends and affects everything across the universe. Newton showed that gravitational force was directly proportional to the masses of two bodies and inversely proportional to the distance between them, and Einstein further refined gravity as being due to the warping of space-time by mass. At the quantum level scientists have posited gravity-carrying force particles called “gravitons”, but haven’t found any yet.
There are some other interesting terms/concepts to understand in understanding forces and matter, which describe the way interactions happen, and these are “wave function”, “quanta” “spin” and “fields”.
- Wave functions. A wave function is just probability of finding the object at a certain location (such as a high probability of finding you in your bed at night). The wave functions of varying energies of electrons in an atom are described as probability regions called orbitals. Prior to observation, the electron is a wave smeared over its probability area (called a superposition), and when its location is determined the electron’s wave function collapses and a particle is located at a specific location. The greater implication is that everything exists as waves, and collapse into particle form (wave-particle transformation) when observed. These waves travel in their dedicated fields, but fields can sometimes intersect and interact or not.
- Fields are regions of presence and waves of energy, and permeate the whole universe.[6] Some fields intersect with others, such as the photon field with the electron field, or the Higgs field with fermion fields, giving matter mass. But the Higgs field does not interact with either the photon or the neutrino fields, so photons and neutrons are massless and travel at the speed of light rather than slowing down. Fields are constantly vibrating with the appearance-disappearance of particle-antiparticle pairs, but larger waves are the actual particles made tangible, and energy can pass from one field to another. Thus colliding two protons at high velocities provides enough energy to pop a big wave in the Higgs field, making a Higgs boson visible for just a fraction of a millisecond, such as we saw in 2012 at the Large Hadron Collider. Its dissipation into other particles due to field-field interactions can be predicted and observed.
- Quantization means that the energy of the universe comes in discrete bundles, or packets. As described above, this would be like having stairs rather than ramps to go from one place to another. (Ramps are continuous and occupy every region between one energy level and the next, but stairs go in jumps from one level to the next. Stairs are quantized, and ramps are not. This has real implications for chemical reactions; for example, only energy packets of certain values of photons can be absorbed by electrons (whose values vary based on their orbital location, which is quantized).
- Spin is a quantum description of the particle which describes angular momentum, or rotation. Spins can be left-or right-handed. All fermions (matter constituents) have +½ or -½ spins. All bosons have a spin of 1 except the Higgs boson, with a 0 spin. The outcome of this spin is that fermions cannot have the same quantum number (formalized as the Pauli exclusion principle). So each particle of matter occupies a location and excludes another matter particle from being at the same location. So takes up space. Bosons are force carriers which can have the same quantum numbers. Therefore they can overlay/stack to make them stronger or weaker. Thus, light can have many identical units which can stack to have high amplitude (brightness). This is the principle behind lasers.
- Although Quantum Field Theory incorporates all the known information in the quantum realm, it is still missing a quantum description of gravity. There are two contenders for explaining gravity: String Theory and Loop Quantum Gravity.[7] (As I mentioned before, strings and quantum loops mean something altogether different than strings and loops in our daily life.) In LQG spacetime is pixelated, and the pixels are little loops with gravity. These gravity quanta have a minimum length that is possible (allowed) is called Planck length (l = 10-35m) and that also puts limit on minimum possible area (l2 = 10-70 m2) and volume (l3 = 10-105 m3). Time also is quantized and the smallest allowable value of time is 10-43 s, called Planck time. The fabric of space, then, is made of loops held by nodes which distort when other particles with mass interact with them. These distortions are space or time. In String Theory, all the fundamental particles of the Standard Model are made up of tiny vibrating strings. The problem is gravitons emerge from the strings but only when they vibrate in 9 dimensions, not just our 3 dimensions plus the 4th dimension of time.
And so physicists imagine, theorize, and then do very hard math to understand how they fit, no, if they fit, and do the experiments to test the theory and build a model of the universe that works at all levels. They are not yet there.
Bibliography
I recommend the following videos that make the topic a little more comprehensible with the use of animations.
[1]. Arvin Ash (2020). The Four Fundamental Forces of nature – Origin & Function. YouTube. Available at: https://www.youtube.com/watch?v=669QUJrF4u0
[2]. Fermilab (2016). QCD: Quantum Chromodynamics. YouTube. Available at: https://www.youtube.com/watch?v=df4LoJph76A
[3]. nanohubtechtalks (2014). Purdue PHYS 342 L15.2: Nuclear Structure and Decay: The Strong Force. [online] YouTube. Available at: https://www.youtube.com/watch?v=bdQUOChdafg
Fermilab (2016). QCD: Quantum Chromodynamics. YouTube. Available at: https://www.youtube.com/watch?v=df4LoJph76A
[4]. www.youtube.com. (n.d.). Quantum 101 Episode 8: Photoelectric Effect Explained. [online] Available at: https://www.youtube.com/watch?v=jWbwDTPju-M.
[5]. Relatively speaking, it is incomprehensible that he did not get other Nobel prizes! Einstein was the genius behind explaining that Brownian motion was due to the existence of atoms and molecules (yes! It was he who showed atoms exist!), mass energy equivalence (e=mC2), special theory of relativity of light being the theoretical maximum for speed, and general relativity about how space and time are related, Bose-Einstein condensates which explain how frictionless “superfluid” collectives of molecules form, for example, allowing supercold helium to flow up, against gravity, out of a container, giving a number to the cosmological constant, predicting the existence of black holes and quantum entanglement. His wave-particle duality for light gave the green light to the French aristocrat, Louis de Broglie, to suggest such a wave-particle duality for electrons, thereby opening up the field of quantum mechanics.
[6]. www.youtube.com. (n.d.). Quantum Fields: The Most Beautiful Theory in Physics! [online] Available at: https://www.youtube.com/watch?v=eoStndCzFhg
[7]. Arvin Ash (2020). String theory vs Loop quantum gravity: Wild hunt for Quantum Gravity: YouTube. Available at: https://www.youtube.com/watch?v=3jKPJa-f3cQ
