Rethinking how things could have been

Alex was a brilliant kid who lived in a tech-driven city. He loved technology and believed it was the engine that drives humanity forward. He graduated from a top-tier college in his country and wanted to become an entrepreneur like Elon Musk. As a contrarian thinker, he constantly came up with bold and unconventional ideas. One such idea was to build a car that required no external resources—no fuel, no electricity—to run. He loved taking risks and decided to bet his career on this vision. He believed that if successful, his idea could create a massive positive impact on the economy. He pitched the idea to venture capitalists and successfully secured substantial seed funding to start his car company. Determined to make it work, he put in eighty-hour workweeks, experimenting with different approaches. However, despite his relentless efforts, he was unable to make the idea succeed. One day, he invited his close friend John to help him figure out the problem. John listened carefully and quickly identified the issue. He was an extraordinary thinker who approached problems using counterfactual reasoning—the science of what is possible and impossible. He believed that reality is not just about what happens, but also about what can and cannot happen. John explained to Alex that he was trying to create something out of nothing, which is fundamentally impossible. He pointed out that Alex was violating one of the most fundamental principles of physics: the law of conservation of energy. This law states that energy can neither be created nor destroyed; it can only be transformed from one form to another. He further added that success is like a vector quantity—it requires not just effort and speed, but also the right direction. Working hard is not enough if the underlying idea is flawed. John advised Alex to abandon the idea and instead focus on solving challenging problems that do not violate the laws of physics.

You might be thinking about how one should think in terms of counterfactuals to avoid grave mistakes that cost both money and time. Let's explore together the deepest ideas in physics through the lens of counterfactuals.

Fundamental physics is stable. Elementary particles like electrons obey unchanging laws. They don't decay and fall apart easily. When we observe the reality around us, it feels fragile, temporary, and dreamlike. Everything we care about is unstable. People die, buildings collapse, and civilizations disappear.

If the laws of physics are stable, why is everything else so fragile?

Some laws don't guarantee the survival of complex things and don't protect life, structure, and knowledge. The universe doesn't care about preserving you. It doesn't ensure anything complex survives. Complex things like cities, life, and books are not built into the laws. A rock can erode, break, and eventually disappear. Nothing in physics says protect this rock forever.

The universe moves towards disorder, and its default direction is towards decay. It's because complex systems require specific arrangements. There are many more ways to be disordered than ordered. For instance, a glass staying intact is a very specific arrangement, and breaking has many possible outcomes. So breaking is more likely.

How does life exist at all if everything tends to decay? Why do we see living organisms, technology, and organized systems?

Knowledge is a physical force that can keep things going and prevent decay. A human builds a house, repairs it, and maintains it; DNA replicates a cell; and an engineer fixes machines. Knowledge allows systems to resist decay. Without knowledge, everything collapses quickly, and with knowledge, things can persist, repair, and improve. Random processes destroy structure and increase disorder, whereas knowledge-based processes maintain structure, create order, and enable survival. A book left alone decays, and with humans, it can be preserved, copied, and printed. Knowledge keeps it alive.

The existence of life and knowledge means some tasks are possible, like copying information, repairing systems, and building structures, whereas some are impossible, like perfect stability without intervention and preventing decay forever. Reality is what can be prevented(decay) and what can be achieved through knowledge. All evils are caused by ignorance and can be eradicated through right knowledge.

The universe doesn't guarantee meaning or survival. Stability must be created, not given. Humans and life are special because they contain knowledge that fights entropy. The world feels like a dream because more structures are temporary and stability is rare, but some things persist because they embody knowledge.

For centuries, physics has followed a simple structure:

1. Initial Conditions(Starting State)

2. Laws of Motion(Rules of Evolution)

For example, if you throw a ball, the initial conditions are speed, direction, and the laws of motion(Gravity, Newton's Laws). Then physics predicts where the ball will go. This is called the prevailing conception of physics. This approach is limited. It can answer what will happen, but it struggles with what could happen, what can't happen, and why certain transformations are possible. It can't fully explain why we can't build a perpetual machine, what makes information physically real, how life can exist, and why computation is possible. These are not just motions but about possibilities.

Early navigators like Columbus made predictions about reaching new lands. They used maps, instruments, and knowledge, but some predictions were wrong, yet still useful. Prediction alone is not enough. What matters is the quality of explanation behind the prediction. A vague prophecy may be correct, but useless. A Scientific explanation tells you why. Physics needs better explantion not just predictions. A true explanation is hard to vary while still explaining what it purports to explain.

Traditional physics focuses on predicting outcomes. Our focus should be on why things are possible or impossible. For instance, the prediction is that this machine will not work. The explanation is that this machine can't work because it violates energy conservation. Many important scientific principles are already about impossibility. For example, in thermodynamics, you can't create energy from nothing. You can't convert heat completely into work. These are can't laws not motion laws. Some of the most powerful laws are already constrained on what can't happen.

The laws of motion are not enough. They describe how things evolve and work well for simple systems. But they fail for complex systems like Information, Knowledge, and life. All of these depend on counterfactuals what can be done and what transformations are possible. A computer works not just because electrons move but because it can compute, copy information, and errors can be corrected. These are not captured by motions alone.

The new proposal goes beyond the laws of motion. Physics should include possible and impossible transformations. It leads to the Constructor theory, where the focus is not on trajectories but tasks. It was proposed byDavid Deutsch, with major development and collaboration from Chiara Marletto.

Instead of asking, what will happen to the comet? We ask what transformations of this system are possible? Can we redirect the comet? Can we change its orbit? This shift turns physics into a theory of capabilities.

Life requires self-reproduction and error correction. These tasks must be possible. Information requires copying and transformation. These are also tasks. To explain life and information, physics must describe what transformations are allowed. The traditional view is that the universe is like a movie, and physicists predict the next frame. The modern view is that physics is like a rulebook of possibilities, and it tells you what can and can't be done.

We use information in computers, DNA, language, memory, etc., but physics doesn't define it clearly. Information seems abstract, like numbers or words, but it is always stored in physical systems like brains, hard drives, and DNA.

How can something abstract be physical?

Shannon's information theory focuses on communication, measures information as bits, and ignores meaning and physical reality. It is a useful but incomplete view. Information needs to be defined in terms of what transformations are possible in physical systems.

What is information from the perspective of Constructor Theory?

According to the Constructor theory, a system contains information if it can exist in multiple states and those states can be transformed easily. For instance, a bit can be 0 or 1.

Can we switch 0 to 1?

Can we copy 0 to another system?

If these tasks are possible, then the system carries information. An information variable is a set of possible states that a system can take. For example, a light switch can be turned on and off. This is an information variable because it has distinct states, and we can manipulate them. The key condition is that the states must be distinguishable. If you can't tell them apart, then they don't carry any information.

Information exists only if it can be copied because knowledge spreads by copying. Memory and Communication also depend on copying. A file can be duplicated, sent, and stored. That's why it contains information. Some physical states can't be copied perfectly, like unknown quantum states. This is not classical information. According to the quantum theory, we can't perfectly copy an unknown quantum state. This is known as the no-cloning theorem. It implies that not all physical states can carry copyable information; only certain states qualify. The key insight is that information is constrained by what is physically possible.

How is computation understood within the framework of constructor theory?

The traditional idea about computation is that it is an abstract mathematical operation. According to the constructor theory, computation is the physical transformation of information. A computer takes input(bits), transforms them(calculations), and produces output. These are tasks like adding numbers, sorting data, and encoding information. These tasks must be possible under physical laws.

Information is also related to living systems. DNA stores genetic information and gets copied during reproduction. It matters because information can be stored, copied, and transformed. Life depends on the possibility of information tasks.

Distinguishability and Interoperability are the two important characteristics of information. The states must be distinguishable for information to exist. If two states look identical, we can't tell them apart and can't use them to encode information. So, information depends on measurement and distinguishability. Interoperability means information can be transferred between different physical systems. The text on paper can be typed into a computer and also spoken aloud. The same information is present in different forms. Information is independent of the medium, provided physical laws allow transformation between media.

A system is an information medium if it allows all the required tasks:

1. Copying

2. Transferring

3. Distinguishing states

Computer memory, DNA, written language, etc., are all examples of information media.

The old idea is that information is abstract and physics deals with matter and energy. The modern view is that information is a physical property defined by possibility. Information exists only where physics allows certain tasks. It is not separate from the physical world.

How does the constructor theory account for quantum mechanics?

Quantum mechanics can be understood entirely in terms of what information can and can't be processed. In classical physics, information can be copied perfectly, measured without disturbance, and stored reliably. For example, a bit that can take the value 0 or 1, or you can copy a file infinitely many times. In quantum physics, things change drastically. Information can't always be copied, measurement disturbs the system, and some properties are fundamentally unknowable simultaneously. These are not accidents - they are deep physical constraints.

Quantum theory introduces new impossibilities.

1. No-cloning principle: It is impossible to copy an unknown quantum state perfectly. This is known as the No-cloning theorem. Copying is forbidden at the quantum level. This single impossibility defines much of the quantum behaviour.

2. Measurement disturbs reality: In classical physics, you can observe without changing the system, whereas in quantum physics, measurement changes the state. For example, measuring position affects the momentum, which connects to the Heisenberg Uncertainty principle.

3. Complementarity: Some properties can't both be known precisely, like position and momentum. These are mutually exclusive tasks.

Information becomes Superinformation at the quantum level. It describes information that exists physically but can't be fully accessed or copied. For instance, A quantum bit(qubit) can be 0, 1, or in a superposition. You can't extract all the information at once. You can't clone it. So, it's richer than classical information but also more restricted.

Instead of saying quantum theory is “strange,” a better approach is to say that quantum phenomena place constraints on what tasks are possible. For instance, a particle can exhibit both particle-like and wave-like behaviour. According to Constructor Theory, certain transformations—such as predicting an exact trajectory—are impossible, while others—such as producing interference patterns—are possible.

Entanglement is a concept in quantum physics that states that when two particles become linked, measuring one affects the state of the other. From the perspective of Constructor Theory, information is not stored in individual components but in the relationships between them.

In classical systems, information directly flows from A to B. In quantum systems, information can be localized, hidden in correlations, and not directly observable. Even though quantum systems restrict what you can do, they also enable new possibilities like quantum computing(solving problems faster than classical computers) and quantum cryptography(security guaranteed by physical laws, and eavesdropping is detectable). Quantum theory is not about behaving strangely -it's about which information processing tasks are possible or impossible. Quantum reality is defined by limits on information.

How does the constructor theory account for the theory of knowledge (epistemology)?

Knowledge is something that rises, spreads, and persists like the wind. It is invisible but powerful. It moves through systems(people, machines, DNA) and survives even when individuals don't. Knowledge is information that can cause itself to keep existing. This is very different from the usual idea of knowledge as ideas in your mind. It can be copied, preserved, and can guide transformations. For example, a recipe for building an airplane, DNA instructions in a cell, and software code. These are not just information but are active casual things in the physical world.

In the constructor theory, knowledge acts like an abstract constructor. It tells physical systems what transformations to perform and also ensures those transformations can happen reliably. Reliable transformations need something like a catalyst that remains unchanged. That catalyst is often knowledge. Knowledge enables repeatability. Without knowledge, things happen randomly, and no complex system persists. With knowledge, tasks can be done again and again, and the system becomes reliable. A factory builds planes repeatedly because it has instructions(knowledge). A cell replicates because it has DNA knowledge.

Knowledge is a physical property of matter. This means it exists in systems, follows laws of physics, and can be created, copied, and destroyed. Knowledge is resilient information capable of reproduction. Knowledge is an abstract catalyst. A catalyst is something that enables a reaction but is not used up. Abstract catalyst means knowledge acts like a catalyst and enables transformations without being destroyed. For example, a manual for building a machine, a genetic code. They guide processes, stay intact, and can be reused infinitely.

Knowledge allows progress to continue. Information is passive, just data, and may disappear. Knowledge is active, causes things to happen, and is designed to persist. Not all information is knowledge. Only information that can survive and replicate counts as knowledge. Knowledge makes complex things possible. There is no life, no technology, no civilization without knowledge. However, we can build machines, cure diseases, and explore space with knowledge. The universe is not just matter and energy. It is also shaped by knowledge. A system that contains knowledge can resist disorder and create complexity.

How can thermodynamics be understood through the lens of constructor theory?

Traditional physics faces a major puzzle: microscopic laws, which govern atoms and particles, are reversible, whereas macroscopic processes, such as heat flow and engines, are irreversible. For example, heat generated by friction cannot be fully converted back into motion. This is known as the problem of irreversibility.

Rather than asking, “What happens to energy?”, constructor theory shifts the focus to identifying which energy transformations can occur and which cannot, offering a novel approach to thermodynamics.

Work is a reversible energy transfer. It can be undone without extra resources, like lifting a weight or compressing a string. Heat is an irreversible energy transfer. It can't be converted back into useful work, like friction producing heat or a hot object cooling down. Once energy becomes heat, it spreads out and becomes unusable. The difference between work and heat is not about the type of energy but about whether the transformation is possible to reverse.

There are three types of irreversibility:

1. Statistical Irreversibility: It is based on probability. The system could reverse, but it is extremely unlikely. It is not the fundamental problem, but just a likelihood. Example: a gas spreading in a room. In theory, all the gas molecules could return to one corner, but the chance is astronomically small.

2. Forgetful Irreversibility: It happens because we ignore microscopic details. Irreversibility comes from limited knowledge. The problem depends on observers, not on physics itself. For example, we observe only temperature and pressure, not each particle’s motion. If we had complete information, the process could, in principle, be reversed.

3. Counterfactual Irreversibility: Some transformations are fundamentally impossible to reverse. It's not about probability or knowledge(ignorance). This gives a precise objective meaning to the second law. For example: heat spontaneously flowing from a cold object to a hot object without external work—it cannot happen, by the laws of physics.

How does information linked to work?

Work media are the physical systems that can reliably store and transfer energy reversibly, like weights, springs, batteries(idealized), etc. They allow multiple states, like high or low energy, and the transition between these states is without any loss. These are the building blocks of work. Systems that can perform work also encode information because a system with a distinguishable state, like up/down, charge/empty, etc., can store both energy and information. This links thermodynamics and Information.

Traditional physics states that entropy always increases. From the perspective of the constructor theory, certain transformations are impossible. You can't fully convert heat into work, and you can't build a perfect heat engine. This law becomes a statement about impossibility. A universal constructor is a machine that can perform any possible transformation. It's like a perfect 3D printer for physics. It helps define what tasks are possible and what are forbidden.

The universe is like a maze of transformations. Physics tells us which paths are open or closed. Not all imaginable processes are allowed. The structure of reality is defined by the constraints on possibility. Thermodynamics is not about the flow of energy. It's about which transformations are possible and which are impossible. Heat is the energy trapped in an impossible-to-reverse transformation, whereas work is the energy in possible reversible transformations.

Suppose you travelled through a world with strict physical rules. Your goal is to go from point A -----> B------>A again. A complete round trip and back again.

Can every journey be reversed perfectly?

At first glance, it seems obvious that if you can go somewhere, then you should be able to come back, but physics forbids it.

Reversible transformations can go forward and back without loss, and no information is destroyed. For example, an ideal pendulum motion and a perfectly elastic collision. In the constructor theory, both directions of the task are possible. Irreversible transformations can't return to the original state exactly, and information is lost in the process. For example, burning paper and mixing milk into coffee. You can't unburn or unmix perfectly.

Irreversibility is really about the loss of information. If every detail is preserved, then you can return exactly, but if the information is lost perfect return becomes impossible. A journey is reversible if nothing about the system is forgotten.

Knowledge is physical and must be stored in physical systems. It can be preserved, copied, or destroyed. Why does this matter for you?

To return to A, you need a record of how to go back and a system that preserves that knowledge. Without it, the return journey becomes impossible.

A constructor can perform a task repeatedly and encode knowledge about how to do it. For example, a GPS storing routes or DNA encoding how to build an organism. These are knowledge-bearing systems. To complete the round trip, a constructor must exist that knows how to go forward and knows how to reverse the journey. If such a constructor can't exist, the reverse journey is impossible. Work is a reversible process, and heat is an irreversible process. Once information spreads out like heat, it can't be fully recovered.

Conclusion: Reversibility is a counterfactual property about possibility. Instead of saying, this process happened? A constructor theory asks Can the reverse process happen? If yes, then the process is reversible, and there is no fundamental limitation. If not, the process is forbidden by the laws of nature. Nature is asymmetrical, and some processes are fundamentally one-way. It leads to the idea of the time arrow. The past is fixed because it has already happened. The events that have already occurred have left traces in the world—they are recorded in the states of physical systems, and many processes are irreversible, meaning they cannot be undone. The future is open because multiple outcomes are possible, constrained by laws of physics, but not yet realized.

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Photo by David Emrich on Unsplash