4 ms·

The other major example of a time-irreversible operation/object is much more mundane: the collapse of a wave function. I've never been able to find a decent lay

The other major example of a time-irreversible operation/object is much more mundane: the collapse of a wave function. I've never been able to find a decent layperson's explanation of what the wave function collapse really means, and why physicists seem to have no issue with it being time irreversible but seem quite concerned with black holes

KSS42 7y ago
Physicists do have a problem with it. See : https://backreaction.blogspot.com/2019/08/the-problem-with-quantum-measurements.html https://backreaction.blogspot.com/2019/08/the-problem-with-q... Sabine Hossedfelder also discusses it in her book Lost in Math. Two books on the subject are reviewed in the Booklab podcast, episode 20: http://booklabpodcast.com/booklab-020-what-is-real-and-beyond-weird http://booklabpodcast.com/booklab-020-what-is-real-and-beyon... Featured Books:What is Real? by Adam Becker; and Beyond Weird by Philip Ball. Quantum physics has been with us for more than 100 years – but what is it actually telling us about the world?
garmaine 7y ago
That’s because physicists do have a problem with it, and not all believe the wave function collapse is real.
DiogenesKynikos 7y ago
It is a problem. Quantum mechanics, as it's normally taught, basically has two components: 1. Schrödinger's equation, which governs how the wavefunction (or quantum state) evolves in time. This equation is time-reversible: given a state at time t, you can calculate what the state was at time t-T. Technically, that means that the "time evolution operator" is invertible. All the information about the history of how system's state is contained in the present state. No information is ever destroyed. 2. Observation. A quantum state looks like a_1 * psi_1 + a_2 * psi_2 + ... + a_n * psi_n, where psi_i are all the possible states of the system and a_i are complex numbers called "amplitudes." When you observe a state (I'm obviously leaving out some mathematical details here, so anyone with physics knowledge please forgive me), you observe it to be in one of the possible states, psi_i, with i between 1 and n. The probability of observing it to be in state i is proportional to |a_i|^2. This operation destroys information, because the state collapses to psi_i, and all the amplitudes, a_j, j≠i, are lost. You can no longer reconstruct the previous state of the system. I think most physicists who "seriously" think about quantum mechanics do not believe that step 2 above actually happens. It is a simplification of a much more complicated process called "decoherence." In order to understand decoherence, you have to change your perspective on what observation means. If you treat the observer as a system governed by Schrödinger's equation, which interacts with the system that's being measured, you find that the observer becomes entangled with the system under observation. The observer ends up in a superposition of states, each of which has observed a different outcome. It appears to each state of the observer as if there has been wavefunction collapse, but there actually is a larger quantum system containing both the observer and thing being observed, in which no information has been lost. The theory of decoherence and the "many-worlds interpretation" began to be developed in the 1950s by one of Wheeler's students, Everett. Somehow, it hasn't really made it into undergraduate physics yet, and most physicists can get by without thinking too deeply about what observation means. You can do most calculations assuming wavefunction collapse happens.

KSS42 7y ago

Physicists do have a problem with it. See : https://backreaction.blogspot.com/2019/08/the-problem-with-quantum-measurements.html https://backreaction.blogspot.com/2019/08/the-problem-with-q... Sabine Hossedfelder also discusses it in her book Lost in Math. Two books on the subject are reviewed in the Booklab podcast, episode 20: http://booklabpodcast.com/booklab-020-what-is-real-and-beyond-weird http://booklabpodcast.com/booklab-020-what-is-real-and-beyon... Featured Books:What is Real? by Adam Becker; and Beyond Weird by Philip Ball. Quantum physics has been with us for more than 100 years – but what is it actually telling us about the world?

garmaine 7y ago

That’s because physicists do have a problem with it, and not all believe the wave function collapse is real.

DiogenesKynikos 7y ago

It is a problem. Quantum mechanics, as it's normally taught, basically has two components: 1. Schrödinger's equation, which governs how the wavefunction (or quantum state) evolves in time. This equation is time-reversible: given a state at time t, you can calculate what the state was at time t-T. Technically, that means that the "time evolution operator" is invertible. All the information about the history of how system's state is contained in the present state. No information is ever destroyed. 2. Observation. A quantum state looks like a_1 * psi_1 + a_2 * psi_2 + ... + a_n * psi_n, where psi_i are all the possible states of the system and a_i are complex numbers called "amplitudes." When you observe a state (I'm obviously leaving out some mathematical details here, so anyone with physics knowledge please forgive me), you observe it to be in one of the possible states, psi_i, with i between 1 and n. The probability of observing it to be in state i is proportional to |a_i|^2. This operation destroys information, because the state collapses to psi_i, and all the amplitudes, a_j, j≠i, are lost. You can no longer reconstruct the previous state of the system. I think most physicists who "seriously" think about quantum mechanics do not believe that step 2 above actually happens. It is a simplification of a much more complicated process called "decoherence." In order to understand decoherence, you have to change your perspective on what observation means. If you treat the observer as a system governed by Schrödinger's equation, which interacts with the system that's being measured, you find that the observer becomes entangled with the system under observation. The observer ends up in a superposition of states, each of which has observed a different outcome. It appears to each state of the observer as if there has been wavefunction collapse, but there actually is a larger quantum system containing both the observer and thing being observed, in which no information has been lost. The theory of decoherence and the "many-worlds interpretation" began to be developed in the 1950s by one of Wheeler's students, Everett. Somehow, it hasn't really made it into undergraduate physics yet, and most physicists can get by without thinking too deeply about what observation means. You can do most calculations assuming wavefunction collapse happens.