For centuries, scientists studied light to comprehend the visible world. […] But in the late 19th century all that changed […] the whole focus of physics—then still emerging as a distinct scientific discipline—shifted from the visible to the invisible. […] Today its theories and concepts are concerned largely with invisible entities: not only unseen force fields and insensible rays but particles too small to see even with the most advanced microscopes. […] Theories at the speculative forefront of physics flesh out this unseen universe with parallel worlds and with mysterious entities named for their very invisibility: dark matter and dark energy. […]

…the concept of “brane” (short for membrane) worlds. This arises from the most state-of-the-art variants of string theory, which attempt to explain all the known particles and forces in terms of ultra-tiny entities called strings, which can be envisioned as particles extended into little strands that vibrate. Most versions of the theory call for variables in the equations that seem to have the role of extra dimensions in space, so that string theory posits not four dimensions (of time and space) but 11. As physicist and writer Jim Baggott points out, “there is no experimental or observational basis for these assumptions”—the “extra dimensions” are just formal aspects of the equations. However, the latest versions of the theory suggest that these extra dimensions can be extremely large, constituting extra-dimensional branes that are potential repositories for alternative universes separated from our own like the stacked leaves of a book. Inevitably, there is an urge to imagine that these places too might be populated with sentient beings, although that’s optional. The point is that these brane worlds are nothing more than mathematical entities in speculative equations, incarnated, as it were, as invisible parallel universes. […]

Scientists, of course, are not just making things up, while leaning on the convenience of supposed invisibility. They are using dark matter and dark energy, and (if one is charitable) quantum many-worlds and branes, and other imperceptible and hypothetical realms, to perform an essential task: to plug gaps in their knowledge with notions they can grasp.

{ Nautilus | Continue reading }

related { How it works: An ultra-precise thermometer made from light }

In a paper published in the journal Science, physicists reported that they were able to reliably teleport information between two quantum bits separated by three meters, or about 10 feet.

Quantum teleportation is not the “Star Trek”-style movement of people or things; rather, it involves transferring so-called quantum information — in this case what is known as the spin state of an electron — from one place to another without moving the physical matter to which the information is attached.

{ NY Times | Continue reading }

The atomists held that there are two fundamentally different kinds of realities composing the natural world, atoms and void. Atoms, from the Greek adjective atomos or atomon, ‘indivisible,’ are infinite in number and various in size and shape, and perfectly solid, with no internal gaps. They move about in an infinite void, repelling one another when they collide or combining into clusters by means of tiny hooks and barbs on their surfaces, which become entangled. Other than changing place, they are unchangeable, ungenerated and indestructible. All changes in the visible objects of the world of appearance are brought about by relocations of these atoms: in Aristotelian terms, the atomists reduce all change to change of place. Macroscopic objects in the world that we experience are really clusters of these atoms; changes in the objects we see—qualitative changes or growth, say—are caused by rearrangements or additions to the atoms composing them. While the atoms are eternal, the objects compounded out of them are not.

In supposing that void exists, the atomists deliberately embraced an apparent contradiction, claiming that ‘what is not’ exists.

{ The Stanford Encyclopedia of Philosophy | Continue reading }

Scientists discover how to turn light into matter after 80-year quest.

{ The Stanford Encyclopedia of Philosophy | Continue reading }

Quantum physics is famously weird, counterintuitive and hard to understand; there’s just no getting around this. So it is very reassuring that many of the greatest physicists and mathematicians have also struggled with the subject. The legendary quantum physicist Richard Feynman famously said that if someone tells you that they understand quantum mechanics, then you can be sure that they are lying. And Conway too says that he didn’t understand the quantum physics lectures he took during his undergraduate degree at Cambridge.

The key to this confusion is that quantum physics is fundamentally different to any of the previous theories explaining how the physical world works. In the great rush of discoveries of new quantum theory in the 1920s, the most surprising was that quantum physics would never be able to exactly predict what was going to happen. In all previous physical theories, such as Newton’s classical mechanics or Einstein’s theories of special and general relativity, if you knew the current state of the physical system accurately enough, you could predict what would happen next. “Newtonian gravitation has this property,” says Conway. “If I take a ball and I throw it vertically upwards, and I know its mass and I know its velocity (suppose I’m a very good judge of speed!) then from Newton’s theories I know exactly how high it will go. And if it doesn’t do exactly as I expect then that’s because of some slight inaccuracy in my measurements.”

Instead quantum physics only offers probabilistic predictions: it can tell you that your quantum particle will behave in one way with a particular probability, but it could also behave in another way with another particular probability. “Suppose there’s this little particle and you’re going to put it in a magnetic field and it’s going to come out at A or come out at B,” says Conway, imagining an experiment, such as the Stern Gerlach experiment, where a magnetic field diverts an electron’s path. “Even if you knew exactly where the particles were and what the magnetic fields were and so on, you could only predict the probabilities. A particle could go along path A or path B, with perhaps 2/3 probability it will arrive at A and 1/3 at B. And if you don’t believe me then you could repeat the experiment 1000 times and you’ll find that 669 times, say, it will be at A and 331 times it will be at B.”

{ The Free Will Theorem, Part I | Continue reading | Part II | Part III }

When a coin falls in water, its trajectory is one of four types determined by its dimensionless moment of inertia I∗ and Reynolds number Re: (A) steady; (B) fluttering; (C) chaotic; or (D) tumbling. The dynamics induced by the interaction of the water with the surface of the coin, however, makes the exact landing site difficult to predict a priori.

Here, we describe a carefully designed experiment in which a coin is dropped repeatedly in water to determine the probability density functions (pdf) associated with the landing positions for each of the four trajectory types, all of which are radially symmetric about the centre drop-line.

{ arXiv | PDF }

What if the universe had no beginning, and time stretched back infinitely without a big bang to start things off? That’s one possible consequence of an idea called “rainbow gravity,” so-named because it posits that gravity’s effects on spacetime are felt differently by different wavelengths of light, aka different colors in the rainbow. […]

“It’s a model that I do not believe has anything to do with reality,” says Sabine Hossenfelder of the Nordic Institute for Theoretical Physics.

{ Scientific American | Continue reading }

According to the Standard Model of particle physics, the universe should be empty. Matter and antimatter, which are identical except for their opposite electric charges, seem to be produced in equal parts during particle interactions and decays. However, matter and antimatter instantly annihilate each other upon contact, and so equal amounts of each would have meant a wholesale annihilation of both shortly after the Big Bang. The existence of galaxies, planets and people illustrates that somehow, a small surplus of matter survived this canceling process. If that hadn’t happened, “the universe would be void,” Schönert said.

The explanation for the survival of some matter may lie in subatomic particles called neutrinos. These particles might have a special property that would give rise to neutrino-less double beta decay.

{ Quanta | Continue reading }

Speed of light experiments measure the average speed to a destination and back, leaving open the possibility that the speed may differ over each leg.

{ The Physics arXiv Blog | Continue reading }