- Quantum Mechanics: An Introduction - CRC Press Book
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- Introduction to Theoretical Physics
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He started from the fact that when photons are red one at a time through the experiment with two holes they still build up an interference pattern on the detector screen. But according to the CI, if a device is placed between the two holes and the detector screen to monitor which hole the photon goes through, the interference pattern will vanish, showing that each photon really did go through just one of the holes. Of course, human reactions are not fast enough to do this. But experiments have been carried out with automatic monitoring devices to do exactly this, switching the monitors on or off after the photons have passed the holes.
They show that the interference pattern does indeed disappear when the photons are monitored, meaning that each photon or the probability wave only goes through one hole — even though the decision to monitor the photon was made only after it had passed the holes. Wheeler pointed out that you can imagine a similar experiment on a literally cosmic scale. In a phenomenon known as gravitational lensing, light from a distant object, such as a quasar, is focused by the gravity of an intervening object, such as a galaxy, so that it follows two or more paths around the gravitational lens.
This makes two images of the object in detectors here on Earth. In principle, instead of making those two images it would be possible to merge the light coming different ways around the gravitational lens to make an interference pattern, caused by waves going both ways round the lens. A cosmic version of the experiment with two holes. But then we could monitor the photons before they get a chance to make the interference pattern to see which way round the lens they have come.
In that case, according to the results of the laboratory-scale experiments, the interference pattern would disappear.
Quantum Mechanics: An Introduction - CRC Press Book
The quasar might be 10 billion light years away, the galaxy acting as a gravitational lens might be 5 billion light years away. But according to everything we know from experiment, what the photons were doing billions of years ago and billions of light years away is affected by what we choose to measure here and now. What is going on? Not so wonderful, then. In essence, the Copenhagen Interpretation says that a quantum entity does not have a certain property — any property — until it is measured.
Which raises all kinds of questions about what constitutes a measurement. Does human intelligence have to be involved? Is the Moon there if nobody is looking at it? Does the Universe only exist because human beings are intelligent enough to notice it? Or does the interaction of a quantum entity with a detector count as a measurement?
Updating his example, imagine that a detector in the room measures the spin of the electron. If it is up, the device is triggered and the cat dies. If it is down, the cat is safe. The electron is in a superposition of states before it is measured.
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But there is nobody in the room to see what happens when the detector is triggered. So does the wave function collapse, or not? Is the cat also in a superposition of states, both dead and alive, until someone opens the door of the room to look in? That is their privilege. The capsules are connected by a tube, and in the middle of the tube there is a box which contains a single electron.
The electron wave fills the box evenly. A partition is slid down to divide the box in two and separate the two capsules, each now connected to a box containing half an electron wave. The two capsules are now taken on separate long journeys, in opposite directions at exactly the same speed, until they are a couple of light years apart. Each one has a detector to monitor the presence of an electron. If there is an electron in it, the now grown- up cat dies. If not, the cat lives. But there is no intelligent observer to know what is going on. So are the cats now each in a superposition?
An intelligent alien in a passing spaceship captures one of the capsules and looks inside, to see either a dead cat or a live cat. Is it at that point that the wave function in each capsule collapses, so that what the alien sees determines the fate of the other cat two light years away? Yes, according to the not so wonderful Copenhagen Interpretation. So what is the alternative? There are many, although you may find them just as laughable as the CI, and the first off the rank is the one that started to emerge at the same time as the Copenhagen Interpretation, was nearly smothered at birth by Bohr, but lived to fight another day.
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Women in Science: 50 fearless pioneers who changed the world. This view of the world is incompatible with the conventional interpretation of quantum mechanics, in which key features are unpredictability and the role of observers in the outcome of experiments.
Thus, Einstein never accepted that quantum mechanics was anything but an impressive placeholder for a more fundamental theory conforming to his realist credo. Smolin agrees. He conducts his search for other ways of setting out quantum mechanics in language intelligible to a lay audience, with scarcely an equation in sight. Smolin is a lucid expositor, capable of freshening up material that has been presented thousands of times.
Non-experts might, however, struggle as he delves into some of the modern interpretations of quantum mechanics, only to dismiss them. The book is, however, upbeat and, finally, optimistic. Unapologetically drawing on historical tradition and even modern philosophy, Smolin proposes a new set of principles that applies to both quantum mechanics and space-time. He then explores how these principles might be realized as part of a fundamental theory of nature, although he stops short of supplying details of the implementation.
Smolin concludes with the implications of all this for our understanding of space and time. He suggests that time is irreversible and fundamental, in the sense that the processes by which future events are produced from present ones are truly basic: they do not need to be explained in terms of more basic ideas.
Space, however, is different. He argues that it emerges from something deeper.
Introduction to Theoretical Physics
In my opinion, Smolin downplays the extraordinary success of this conservative approach. Despite the impression that Smolin gives, modern theoretical physics is thriving, with potentially revolutionary ideas about space and time emerging from a combination of the standard quantum mechanics and relativity theory taught in universities for generations. Maybe the upheaval in physics that Smolin yearns for is simply unnecessary. To do that, he will need to present his ideas more rigorously than he could reasonably do in a popular book.
There have been no surprises concerning the inner workings of atoms for some 20 years. It is experimental results that will decide whether Smolin is correct, or whether he protests too much. After all, although quantum mechanics might not satisfy the philosophically minded, it has proved to be a completely dependable tool for physicists — even those who have no interest in debates about its interpretation.
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