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Interplay between classical and quantum physics

6 min read

From: mk_thisisit

The challenge of understanding the interplay between classical and quantum physics is a central theme in modern physics [02:29:00]. Professor Wojciech Żurek, a renowned physicist, is recognized for his contributions to this area, particularly through his theories of decoherence and quantum Darwinism [02:06:00], [02:11:00].

The Challenge of Bridging the Quantum and Classical Worlds

Classical physics primarily describes the forces between objects [05:19:00]. In contrast, quantum physics largely addresses how the classical world emerges from the quantum realm [05:31:00], [05:36:00]. Albert Einstein was deeply concerned by the implications of quantum mechanics, particularly the concept of “superposition,” where an object could exist in multiple states simultaneously [03:07:00], [03:21:00], [03:27:00]. He found this inconsistent with observed reality [04:19:00], [04:23:00], expressing these concerns in a letter to Max Born [04:30:00].

Decoherence

Decoherence is the key mechanism that explains why quantum superpositions are not observed in the macroscopic world [02:34:00], [04:53:00]. It describes how the interaction of a quantum system with its environment causes the “coherence” (the ability to be in multiple states simultaneously) to break down [06:38:00], [06:39:00].

  • The Problem of Superposition: In quantum mechanics, an object can exist in a superposition of states, meaning it can be “here and there” at the same time [00:00:00], [03:27:00], [04:38:00]. Einstein questioned why this is not observed in everyday life [04:06:00].
  • The Role of the Environment: When an object interacts with its environment—such as air particles or photons—information about its state is transferred to the environment [06:00:00], [06:03:00]. This acts as a “measurement,” causing the quantum system to “cease to exist” in a superposition and “decide” on a specific state [06:34:00], [06:46:00], [06:49:00]. Even observing an object, like photons falling into our eyes, causes decoherence [06:26:00], [06:31:00], [06:34:00].
  • Selective Survival: Decoherence selectively eliminates superpositions, allowing only certain “pointer states” that are resistant to environmental interaction to survive [08:29:00], [09:34:00], [09:37:00].

Quantum Darwinism

Quantum Darwinism builds upon decoherence by explaining how information about these surviving quantum states is replicated in the environment, making them observable [10:38:00].

  • Replication of Information: The environment not only causes decoherence but also replicates information about the robust “pointer states” into many copies [10:59:00], [11:49:00]. This allows observers to gain knowledge about an object without direct interaction, simply by interacting with these environmental copies (e.g., photons) [11:05:00], [11:13:00], [11:40:00].
  • Analogy to Darwinism: The theory is named “quantum Darwinism” because there’s a “selective information” process [12:52:00], [12:55:00], similar to natural selection, where certain quantum states survive and multiply their information copies in the environment [12:33:00], [12:36:00], [12:44:00].
  • The No-Cloning Theorem: This principle, also developed by Professor Żurek, states that an unknown quantum state cannot be cloned [13:08:00], [13:16:00], [13:18:00]. However, quantum Darwinism doesn’t involve cloning an unknown state; rather, it describes how the environment creates multiple copies of already determined information about a state through interaction [13:32:00], [13:35:00].

Broader Implications

Unification Theories vs. Interplay between classical and quantum physics

Professor Żurek’s work focuses on understanding how the classical world emerges from quantum mechanics, rather than solely on the “grand unification theory” sought by some physicists [15:47:00].

Quantum Computers and Decoherence

The development of quantum computers is a groundbreaking area in quantum physics, but it faces significant challenges, primarily decoherence [24:25:00], [24:29:00], [24:52:00].

  • Maintaining Quantum States: For a quantum computer to function, its quantum bits (qubits) must interact strongly with each other to perform calculations, but interact minimally with the external environment to preserve their delicate quantum states [25:37:00], [25:42:00], [25:52:00], [25:56:00]. Even a weak interaction with the environment can “derail quantum processes” [25:59:00], [26:01:00].
  • Quantum Simulation: While general-purpose quantum computers are still some time away, simulating complex quantum systems using quantum devices is a more promising area [26:16:00], [26:19:00], [26:36:00], [26:44:00]. These simulations connect quantum mechanics with solid-state physics [26:58:00], [27:07:00].

Kibble-Zurek Mechanism

This mechanism describes how topological defects are formed during phase transitions in the early universe and in laboratory experiments [40:51:00], [41:58:00].

  • Phase Transitions and Symmetry Breaking: When the universe cooled after the Big Bang, it underwent phase transitions (e.g., like water crystallizing), where symmetries broke, leading to the emergence of different physical laws or properties in various regions [41:43:00], [42:06:00], [42:08:00], [42:10:00].
  • Topological Defects: The Kibble-Zurek mechanism predicts the formation of “topological defects” when these broken symmetries cannot be reconciled continuously [43:33:00], [43:37:00], [44:27:00]. These defects are very stable and, if abundant, would have dominated the evolution of the universe [44:44:00], [45:01:00].
  • Laboratory Experiments: The density of these defects can be calculated and observed in laboratory experiments, such as with liquid Helium or using quantum computer simulations of phase transitions [45:37:00], [45:41:00], [45:48:00], [45:51:00], [45:53:00], [46:06:00].

Other Philosophical Questions

Physicists also ponder fundamental questions that touch upon the nature of existence, information, and consciousness.

Scientific Journey and Mentorship

Professor Żurek’s research journey was significantly influenced by his mentor, Professor John Archibald Wheeler [33:53:00], [33:57:00]. Wheeler encouraged exploration into the classical universe and measurement theory, even when others considered it a “waste of time” [34:11:00], [34:13:00], [34:16:00], [34:28:00], [34:32:00]. This openness to diverse ideas and the emphasis on realizing and correcting mistakes, even for great scientists, shaped his approach to physics [36:00:00], [37:15:00], [37:20:00], [37:25:00]. As poet Wisława Szymborska noted, a willingness to “wander” and acknowledge errors is crucial for discovery [37:29:00], [37:31:00], [37:34:00], [37:36:00].

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