jqi_news

AM: Entanglement means that there are correlations (that can not possibly happen in a classical world) between measurements made at the two ends. It's like rolling two dice, and when you roll them they always match. How is that possible without cheating? Entanglement is like there's some cheating going on to correlate the outcomes. To combat the chance that that's happening, scientists separate the entangled particles by long distances so that the cheating would have to happen faster than the speed of light (which isn't possible). Entanglement is a joint property -- a property of a system that contains at least two objects. It's only when you make a measurement on one object or the other that you collapse this jointness to properties of the individual objects. The individual objects aren't 1 or 0 or up or down before you make the measurement. Weirder still is that in some sense you can't say which of the two sides actually made the measurement! But after many repeated measurements at either end, the non-classical correlations survive.

AM: Observing means taking information about the thing you're trying to observe and putting it in something else (or the environment). So it has nothing to do with humans or animals. It's whatever mechanism carries the information away from the thing being observed into the rest of the universe!

JS: In principle observations in quantum mechanics amounts to making a copy of the information in one system on another system, and this involves no conscious observers. This is a reflection of the no-cloning theorem in quantum mechanics, which forbids making such a copy without interfering with the system.

JQI: One of us can recommend What Is Real?: The Unfinished Quest for the Meaning of Quantum Physics. We also recommend our own Quantum Atlas! (And we'll write back with more ideas later as new people filter in.)

AM: Umesh Vazirani taught a class on Coursera, but it seems to be gone now. It looks like this YouTube playlist has the videos from the course.

AM: If you created a system in which the individual objects were predetermined, then you cannot get the results seen in actual measurements of entangled particles. The fact that actual experiments give results that cannot happen if things are predetermined means that the individual objects are not a 1 or 0 or up or down before you make the measurement. We want to assume that when you make a measurement in quantum physics and get a result, that the object had that value before you made the measurements. Again and again, quantum measurement results tell us that's not the case. My son implemented an idea from Howard Wiseman in an Android app that tries to get some of the intuition behind this across.

JS: The short answer is we don't know. The quantum mechanical theory of light tells us that the vacuum is filled with so-called virtual photons, popping in and out with energies that are higher and higher at shorter and shorter wavelengths. When this is combined with Einstein's general theory of relativity (if we were to naively combine with QM), this suggests that the gravitational pull from these virtual photons would be strong enough to trap the photon as if it were in a black hole (the Schwarzchild radius for the experts). This is clearly nonsense.

AM: Or as Han Solo would say, "I have a bad feeling about this."

JS: What this means is that we do not have a consistent way of combining Einstein's general theory of relativity with quantum mechanics. This does not effect physics in the quantum world, which is small enough to evade the effects of gravity. This reflects in the fact that the Planck length is 10^-35 m, many orders of magnitude smaller than any length scale we access. For reference, the size of a nucleus of an atom is 10^-18 m, which is a billion billion times larger than the Planck length. So while we do not know what happens below the Planck length, we are very far from encountering this physics in real life.

AM: Of course, any time a physicist says it's impossible to do X, there's significant potential for embarrassment in the future.