Scientific American Editors - Quantum Universe

Quantum Universe

From the Editors of Scientific American
Cover Image: Mark Garlick/GettyImages
Letters to the Editor
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ISBN: 978-1-948933-20-9

On the Heels of a Light Beam

As a 16-year-old boy, Albert Einstein imagined chasing after a beam of light in the vacuum of space. He mused on that vision for years, turning it over in his mind, asking questions about the relation between himself and the beam. Those mental investigations eventually led him to his special theory of relativity. Such thought experiments, which Einstein referred to by the German term gedankenexperiment, continue to nourish the heart of physics today, especially in the field of quantum mechanics, which he helped to establish.

In quantum mechanics, things dont happen, theoretical physicist Stephen L. Adler tells our reporter Tim Folger, referring to the probabilistic nature of quantum reality.

Philosophically, this may be true, but it hasnt stopped researchers from testing quantum concepts. Using lasers to excite electrons into emitting photons, a group at Delft University of Technology in the Netherlands ruled out the existence of hidden variables, which Einstein believed were controlling so-called entangled particlesone of the main tenets of quantum theory. Without these mysterious forces, bizarre dynamics could indeed be at work in the quantum world, defying our notions of space and time. Physicist Lee Smolin argues that the fabric of the cosmos is a vast collection of atomic interactions within an evolving net- work of relations where causality among events is complex and irrespective of distance.

Despite the theoretical mysteries of quantum theory, its real-world applications are growing. Researchers are cooling atomic systems to near absolute zero for use as quantum simulators to study applications in superconductors and superfluids. Others are using table-top experiments to monitor the gravitational fields around entangled objectsminuscule gold or diamond spheres, for examplelooking for signs that gravity itself is quantized into discrete bits. At a larger scale, tools such as the Event Horizon Telescope, which recently took the first picture of a black hole, and gravitational-wave detectors could help resolve long-standing, vexing contradictions between quantum mechanics and general relativity.

These quantum insights are fueling tremendous innovation. A team of researchers in China successfully tested superposition over a distance of 1,200 kilometers, paving the way for an unhackable quantum-communications network. Computer scientists are using quantum algorithms to enhance traditional systems, ratcheting up progress toward the heralded quantum computing era. Such applications are still immature, as Elizabeth Gibney reports, yet its not stopping investors from pouring money into quantum start-ups.

Science historians have argued about whether Einstein accepted the elements of quantum theory that conflicted with his own theories. Who knows whether he could have imagined the applications his ideas engendered. In any case, the thought experiment continues.

--Andrea Gawrylewski
Senior Editor, Collections

Space: The Final Illusion

by Lee Smolin

Many of the great advances in science are marked bythe discovery that an aspect of nature we thought was fundamentalis actually an illusion, the result of the coarseness of our sensoryperceptions. Thus, air and water appear to us to be continuous fluids,but we discover on deeper experiment that they are made ofatoms. Earth appears to us motionless, but a deeper understandingteaches us that it moves relative to the sun and the galaxy.

One persistent illusion is that physical objects only interactwith other objects they are close to. This is called the principleof locality. We can express this idea more precisely by thelaw that the strength of forces between any two objects fallsoff quicklyat least by some power of the distance betweenthem. This can be explained by positing that the bodies donot interact directly but only through the mediation of a field,such as an electromagnetic field, which propagates from onebody to the other. Fields spread out as they propagate, with thefield lines covering a constantly greater areaproviding anatural explanation for the laws that say the forces betweencharges and masses fall off like the square of the distancebetween them.

Locality is an aspect of an even more compelling illusion:that we exist within an absolute space, with respect to which wemark our positions as we move through it. Thus, Isaac Newtonopined that motion is ultimately defined as change of positionwith respect to absolute space. If this seems obscurebecause no measurement can establish a relation of a physicalobject to this imagined absolute spaceNewton assured us thatabsolute space is seen by God, making your location relative toit an aspect of the divinity of the world. We humans must makedo with relative positions and motionswhich are defined relativeto physical objects we can see.

Gottfried Wilhelm Leibniz broke the mystification by declaringthat all that exists is relative positions and motions. He proposedas a matter of principle that any acceptable science ofmotion must be formulated in terms of relative motions alone.And this, after two centuries of waiting, is what Albert Einsteindelivered to us in his general theory of relativity. In this gloriousconstruction, space is subsumed into spacetime, which isexplicable as a dynamically evolving network of relations.

And what defines those relations? Nothing but causality. Theelements of spacetime are eventsthe ultimate expression oflocalityand each of these is caused by events in their past.Each event will also become a cause of events in the future.Most of the information in the geometry of spacetime is actuallya coding of the relations of causality that relate the events.

Thus, we see that the idea that physical forces must actlocally is a consequence of a deeper principle, which is thatphysical effects have causal processes. And the basic principlesof relativity theory insist that causes can only propagatethrough space at a finite speed, which cannot exceed thespeed of light. We call this the principle of relativistic causality.