What is “reality,’” according to physics?

I wanted to get a sense of what reality is really like, at least based on what scientists say. So I picked up copies of The Order of Time and Seven Brief Lessons of Physics, by the Italian physicist Carlo Rovelli. These were my takeaways.

An overview of what reality is

Reality.We are immersed in a flexible jelly of space and time that vibrates like the surface of an ocean. Everything is made up of fields, which are like layers. Everything around us is constructed from four elementary particles – electrons, quarks, gluons and photons. These elementary particles disappear and reappear, blinking in and out of existence.

Time. The idea that there is a well-defined now throughout the universe is an illusion, because time is local to us. Time does not move at the same speed everywhere, it passes at different rhythms according to how close we are to a mass and how fast we’re moving. The difference between past and future only exists when there is heat exchange. The world is a collection of events, not a collection of things. A kiss is an event; a stone appears to be a thing but it is actually a complex vibration of quantum fields and the momentary interaction of forces.

Perception. We group events around us in ways we understand. Where does a mountain end and the ground begin? We answer that question with our own definitions, but in nature there is no mountain, just vibrations of quantum fields, and we hardly see any of the variables at play. Time occurs because we are witnessing something that had low entropy in one state, and we’ve seen that entropy change. The direction of time is real from our perspective of it; we identify the growth of entropy with the passage of time. Our limited view of the world is sufficient for survival, but is insufficient for understanding the world.

An overview of spacetime

• The electromagnetic field is everywhere and vibrates; it carries electrical forces, and Einstein believed that gravity must be conveyed by a field similar to the electrical field.
  
  • But the gravitational field is not diffused through space like the electromagnetic field. The gravitational field is space itself. The gravitational field and space are the same thing.
• Space is not distinct from matter: It is one of the material components of the world. And space flexes and curves. We’re immersed in it.
  
  • Space bends around a heavy star, and light has to deviate. But time also curves.
  • Space moves like the surface of an ocean, and gravitational waves are observed with precision.
• A black hole bends space to such a degree that it plummets into an actual hole.
• Imagine the texture of the universe being moved by waves similar to those of the sea.
  

An overview of quanta

• Electrons don’t always exist. They only exist when someone or something watches them, or when they interact with something else. An electron is a set of jumps from one interaction to another. When nothing disturbs it, it is not in any precise place. It is not in a “place” at all.
  
  • In quantum mechanics no object has a definite position, except when colliding with something else.
  • It’s not possible to predict where an electron will reappear, but only to calculate the probability that it will pop up here or there.
• Quantum mechanics doesn’t describe a physical system. It only describes how a physical system affects another physical system. It’s not clear yet what this means.
  

Understanding particles

• Everything we see is made of atoms. Every atom is a nucleus (protons and neutrons) surrounded by electrons. Protons and neutrons are made up of quarks. The force that glues quarks inside protons and neutrons is generated by particles called gluons.
  
  • Electrons, quarks, photons and gluons are the components of everything around us. They are the “elementary particles” from which the entire material reality surrounding us is constructed.
  • Particles are the quanta of corresponding fields and disappear and reappear.
• Light is made up of photons (particles of light.)
• The world is a continuous, restless swarming of things, a continuous coming to light and disappearance of ephemeral entities.
  
  • There is no real void. The particles in the fields that form the world are continually created and destroyed. A handful of types of elementary particles, which vibrate and fluctuate constantly between existence and non existence and swarm in space, combine together to form everything.
• There are questions about the Standard Model. One question arises by dark matter which doesn’t seem to be described by the Standard Model. The Standard Model is the best we have for now; its predications have all been confirmed.
  

Understanding the grains of space

• General relativity and quantum mechanics work remarkably well, but they contradict each other.
• Loop quantum gravity says that space is not continuous and is not infinitely divisible: It’s made up of grains. These are called loops which are linked together and weaves the texture of space.
  
  • The loop quantum gravity equations have interesting connections for time.
• Collapsing stars become so dense that quantum mechanics exerts a contrary pressure, forcing the sun to rebound and expand again. When the universe is extremely compressed, quantum theory generates a repulsive force. The Big Bang may have actually been a big bounce when a preceding universe contracted under its own weight and was squeezed into a tiny space before bouncing out, like the expanding universe that we observe around us.
  

Understanding probability, time and the heat of black holes

• A hot substance is a substance in which atoms move more quickly. Heat moves from hot things to cold. Why doesn’t heat go from cold things to hot things?
  
  • Whenever there is heat exchange, the future is different from the past. Where there is no heat exchange (like a pendulum in a vacuum) the pendulum can swing forever.
  • The difference between past and future exists only when there is heat.
  • The reason that heat doesn’t move from hot things to cold things is because of probability, not because of a law.
    
    • It is statistically more probable that a quickly moving atom of a hot substance collides with a cold one rather than vice versa.
      

Notes about the nature of time

• A ball can come back up by rebounding, for example. Heat cannot. This is the only basic law of physics that distinguishes the past from the future.
  
  • If a sequence of events is allowed by the other equations, it is allowed backwards too. The arrow of time appears only where there is heat.
  • Whenever there is a difference between the past and future, heat is involved.
• The notion of a particular time in the universe only occurs if we see our beginning state a certain way. Otherwise, it’s not particular in any way.
• We can’t ask “what time is it now” on faraway bodies. There is no special moment there that corresponds to here.
  
  • The notion of the present refers to things that are close to us, not to anything that is far away. We have a “bubble” around us that extends to how precisely we measure time.
  • Our future is the moments in front of us; our past is the moments behind us. Between the past and the future there is an interval that has a duration. On Mars it is fifteen minutes; eight years on Proxima B.
  • The idea that a well-defined now exists throughout the universe is an illusion.
• Why did we even get it in our heads that time moves at the same speed everywhere?
  
  • It’s not our personal experience – we know that time is subjective.
  • It’s the regulation by mechanical clocks. Before then time was expressed other ways, like the passage of the seasons or the length of the day.
  • We measure time by the amount of change – the swings of a pendulum, for example. What if nothing changes?
• The world is like a set of layers – the electromagnetic field, the gravitational field, etc. These fields flex and stretch.
• Time passes at different rhythms according to place and speed. The difference between past and future does not exist in the elementary equations of the world; its orientation only appears when we neglect the details. There is nothing that we can call present. Time is an aspect of a dynamic field, that stretches and jumps.
• The world is not a collection of things; it is a collection of events.
  
  • A kiss is an event; a stone is a thing.
  • But a stone is also an event, just a longer one. A stone is just a complex vibration of quantum fields and a momentary interaction of forces.
  • The world is less made of stones but of waves moving through the sea.
• What things are there?
  
  • The atoms are made up of smaller particles.
  • The elementary particles are only the ephemeral agitations of a field.
  • Think of the world as a network of events. Simple events and more complex events that can be broken down into simple events.
• We describe the world as it happens not as it is.
  
  • Our equations tell us how events happen, not how things are.
• The absence of time doesn’t mean that everything is frozen and unmoving.
  
  • It means that the happenings of the world are a disorderly network of quantum events.
• There is no objective global present.
  
  • The most we can describe is a present relative to a moving observer. But then, what is real for me is different than what is real for you.
• Changes are not arranged in a single orderly succession. This does not mean that it is non-existent or illusory. The distinction between past, present and future is not an illusion. Change is not an illusion, but it doesn’t follow a global order.
• Past and future do not have a universal meaning. They have a meaning that changes between here and there.
• Loop gravity theory says that the fields manifest themselves in a granular form (elementary particles). These elementary grains from space and the world consists of their interactions. The grains interact incessantly with each other, and only exist in terms of these interactions.
  
  • Quanta manifest themselves in the interaction of what they interact with.
  • They interact probabilistically.
• At the smallest scale, there is only the frenzied swarming of quanta that appear and vanish.
• Everything appears blurry close up. Where does the mountain end and the ground begin? We cut the world into slices that are meaningful to us at a certain scale.
  
  • We see the sky turning but we are the ones who are turning. Our observation involves our relation with the sun and stars.
  • Real things – a cat, a mountain, the clouds – emerge from a world that at a much simpler level has not cats, mountains or clouds.
  • Time emerges from a world without time, in a way that is similar.
• Not all the variables of the world interact with us, only the segment of the world to which we belong – a very minute fraction of the variables.
  
  • That’s why our interactions are blurred – we don’t see all the variables, like the movement of molecules in a glass of water.
• We see time from within it, just as we see a slice of the world that we call a mountain, but it’s only because we are ignoring so many variables.
• At the fundamental level, the world is a collection of events not ordered in time. These events manifest relations between physical variables that are at the same level.
• Among the small systems of variables that exist for us, there are a few special ones where the flow of thermal time entropy happens to be low at one of the two ends of the flow.
  
  • The change of entropy in these systems is what we perceive as time.
  • NOTE: Does this mean a rock, which does not have a low end of thermal time entropy in the segment which we see, does not appear to be affected by time?
  • This is the best theory so far.
• The world needs low entropy sources so that heat can flow into it, increasing entropy. When the entropy of these sources increases, it cannot be turned back. This is what is perceived of as time.
• Understanding our identity:
  
  • We identify with a POV in the world. This POV is a rich structure of correlations essential for our survival.
  • We organize the world into a way that allows us to interact with it.
  • We have memory.
• Time:
  
  • There is no common present throughout the universe.
  • Events are not ordered in past present and future. They are partially ordered.
  • The present is localized, not global.
  • The difference between past and future issues from the fact that in the past, the world was in a state that appears particular to us from our vantage point.
  • Locally time passes at different speeds according to where we are at and what speed we are moving.
  • The closer we are to mass, or the faster we move, the more time slows down.
• How does the concept of time emerge?
  
  • We only see the world in a blurred way. (We don’t see all the variables).
  • The direction of time is real in the sense that it’s real for our perspective.
  • We identify the growth of entropy with the passage of time.
• The structure of the world is different than our perception of it.
  
  • That image is suitable for our daily life.
  • It’s not suitable for understanding the world.
    

Understanding light

• Our eyes distinguish certain frequencies of electromagnetic waves, which produces a physical reaction.
  
  • We only see vibrating lines. If we see a child on a beach, we only see vibrating lines that “transport his image to us.”