Physicists have struggled to marry quantum mechanics with gravity for decades. In contrast, the other forces of nature have obediently fallen into line. For instance, the electromagnetic force can be described quantum-mechanically by the motion of photons. Try and work out the gravitational force between two objects in terms of a quantum graviton, however, and you quickly run into trouble—the answer to every calculation is infinity. But now Petr Hořava, a physicist at the University of California, Berkeley, thinks he understands the problem. It’s all, he says, a matter of time.

More specifically, the problem is the way that time is tied up with space in Einstein’s theory of gravity: general relativity. Einstein famously overturned the Newtonian notion that time is absolute—steadily ticking away in the background. Instead he argued that time is another dimension, woven together with space to form a malleable fabric that is distorted by matter. The snag is that in quantum mechanics, time retains its Newtonian aloofness, providing the stage against which matter dances but never being affected by its presence. These two conceptions of time don’t gel.

The solution, Hořava says, is to snip threads that bind time to space at very high energies, such as those found in the early universe where quantum gravity rules. “I’m going back to Newton’s idea that time and space are not equivalent,” Hořava says. At low energies, general relativity emerges from this underlying framework, and the fabric of space-time restitches, he explains.

Hořava likens this emergence to the way some exotic substances change phase. For instance, at low temperatures liquid helium’s properties change dramatically, becoming a “superfluid” that can overcome friction. In fact, he has co-opted the mathematics of exotic phase transitions to build his theory of gravity. So far it seems to be working: the infinities that plague other theories of quantum gravity have been tamed, and the theory spits out a well-behaved graviton. It also seems to match with computer simulations of quantum gravity.

Hořava’s theory has been generating excitement since he proposed it in January, and physicists met to discuss it at a meeting in November at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario. In particular, physicists have been checking if the model correctly describes the universe we see today. General relativity scored a knockout blow when Einstein predicted the motion of Mercury with greater accuracy than Newton’s theory of gravity could.

Can Hořřava gravity claim the same success? The first tentative answers coming in say “yes.” Francisco Lobo, now at the University of Lisbon, and his colleagues have found a good match with the movement of planets.

Others have made even bolder claims for Hořava gravity, especially when it comes to explaining cosmic conundrums such as the singularity of the big bang, where the laws of physics break down. If Hořava gravity is true, argues cosmologist Robert Brandenberger of McGill University in a paper published in the August Physical Review D, then the universe didn’t bang—it bounced. “A universe filled with matter will contract down to a small—but finite—size and then bounce out again, giving us the expanding cosmos we see today,” he says. Brandenberger’s calculations show that ripples produced by the bounce match those already detected by satellites measuring the cosmic microwave background, and he is now looking for signatures that could distinguish the bounce from the big bang scenario.

Hořava gravity may also create the “illusion of dark matter,” says cosmologist Shinji Mukohyama of Tokyo University. In the September Physical Review D, he explains that in certain circumstances Hořava’s graviton fluctuates as it interacts with normal matter, making gravity pull a bit more strongly than expected in general relativity. The effect could make galaxies appear to contain more matter than can be seen. If that’s not enough, cosmologist Mu-In Park of Chonbuk National University in South Korea believes that Hořava gravity may also be behind the accelerated expansion of the universe, currently attributed to a mysterious dark energy. One of the leading explanations for its origin is that empty space contains some intrinsic energy that pushes the universe outward. This intrinsic energy cannot be accounted for by general relativity but pops naturally out of the equations of Hořava gravity, according to Park.

Hořava’s theory, however, is far from perfect. Diego Blas, a quantum gravity researcher at the Swiss Federal Institute of Technology (EPFL) in Lausanne has found a “hidden sickness” in the theory when double-checking calculations for the solar system. Most physicists examined ideal cases, assuming, for instance, that Earth and the sun are spheres, Blas explains: “We checked the more realistic case, where the sun is almost a sphere, but not quite.” General relativity pretty much gives the same answer in both the scenarios. But in Hořava gravity, the realistic case gives a wildly different result.

Along with Sergei M. Sibiryakov, also at EPFL, and Oriol Pujolas of CERN near Geneva, Blas has reformulated Hořava gravity to bring it back into line with general relativity. Sibiryakov presented the group’s model in September at a meeting in Talloires, France.

Hořava welcomes the modifications. “When I proposed this, I didn’t claim I had the final theory,” he says. “I want other people to examine it and improve it.”

Gia Dvali, a quantum gravity expert at CERN, remains cautious. A few years ago he tried a similar trick, breaking apart space and time in an attempt to explain dark energy. But he abandoned his model because it allowed information to be communicated faster than the speed of light.

“My intuition is that any such models will have unwanted side effects,” Dvali thinks. “But if they find a version that doesn’t, then that theory must be taken very seriously.”

Note: This article was originally printed with the title, "Splitting Time from Space."

 

Although such dualistic accounts are emotionally reassuring and intuitively satisfying, they break down as soon as one digs a bit deeper. How can this ghost, made out of some kind of metaphysical ectoplasm, influence brain matter without being detected? What sort of laws does Casper follow? Science has abandoned strong dualistic explanations in favor of natural accounts that assign causes and responsibility to specific actors and mechanisms that can be further studied. And so it is with the notion of the will.

Sensation and Action
Over the past decade psychologists such as Daniel M. Wegner of Harvard University amassed experimental evidence for a number of conscious sensations that accompany any willful action. The two most important are intention and agency. Prior to voluntary behavior lies a conscious intention. When you decide to lift your hand, this intention is followed by planning of the detailed movement and its execution. Subjectively, you experience a sensation of agency. You feel that you, not the person next to you, initiated this action and saw it through. If a friend were to take your hand and pull it above your head, you would feel your arm being dragged up, but you would not feel any sense of being responsible for it. The important insight here is that the consciously experienced feelings of intention and agency are no different, in principle, from any other consciously experienced sensations, such as the briny taste of chicken soup or the red color of a Ferrari.

And as a plethora of books on visual illusions illustrate, often our senses can be fooled—we see something that is not there. So it is with the sensation of intentionality and agency. Decades of psychology experiments—as well as careful observation of human nature that comes from a lifetime of living—reveal many instances where we think we caused something to happen, although we bear no responsibility for it; the converse also occurs, where we did do something but feel that something or somebody else must have been responsible. Think about the CEO of a company who takes credit—and bonuses worth many millions—if the stock market price of his company rises but who blames anonymous market forces when it tanks. It is a general human failing to overestimate the import of our own actions when things go well for us.

Lest there by any misunderstanding: the sensations of the intention to act and of agency do not speak to the metaphysical debate about whether will is truly free and whether that even is a meaningful statement. Whether free will has some ontological reality or is entirely an illusion, as asserted forcefully by Weg­ner’s masterful monograph, does not invalidate the observation that voluntary actions are usually accompanied by subjective, ephemeral feelings that are nonetheless as real as anything else to the person who experiences them.

Telling Clues from Surgeries
The quiddity of these sensations has been strengthened considerably by neurosurgeons. During certain types of brain surgery, neural tissue must be removed, either because it is tumorous or because it gives rise to epileptic seizures. How much tissue to remove is a balancing act between the Scylla of leaving remnants of cancerous or seizure-prone material and the Charybdis of removing regions that are critical for speech or other near-essential operations. To probe the function of nearby tissue, the neurosurgeon stimulates it with an electrode that passes pulses of current while the patient—who is awake and under local anesthesia to minimize discomfort—is asked to touch each finger successively with the thumb, count backwards or do some other simple task.

During the course of such explorations in 1991, neurosurgeon Itzhak Fried, now at the University of California, Los Angeles, and his colleagues stimulated the presupplementary motor area, part of the vast expanse of cerebral cortex that lies in front of the primary motor cortex. Activation of different parts of the motor cortex usually triggers movements in different parts on the opposite side of the body, for example, the foot, leg, hip, and so on. The medical team discovered that electrical stimulation of this adjacent region of cortex can, on occasion, give rise to an urge to move a limb. The patient reports that he or she feels a need to move the leg, elbow or arm.

This classical account was elaborated on by a recent study from Michel Desmurget and his colleagues at the Center for Cognitive Neuroscience in Bron, France, that was published in the international journal Science. Here it was electrical stimulation of the posterior parietal cortex, gray matter involved in the transformation of visual information into motor commands—as when your eyes scan the scene in front of you and come to rest on the movie marquee—that could produce pure intentions to act. Patients made comments (in French) such as “It felt like I wanted to move my foot. Not sure how to explain,” “I had a desire to move my right hand,” or “I had a desire to roll my tongue in my mouth.” In none of these cases did they actually carry out the movement to which they referred. But the external stimulation caused an unambiguous conscious feeling of wanting to move. And this feeling arose from within, without any prompting by the examiner and not during sham stimulation.

This was different from the cortical sector explored by the earlier Fried study. One difference between the two stimulated regions was that, at higher current levels, the patient actually moved the limb when the target site was the presupplementary motor area. Parietal stimulation, on the other hand, could trigger a sensation that actual movement had occurred, yet without any motion actually occurring (illusion of movement).

The take-home lesson is that the brain has specific cortical circuits that, when triggered, are associated with sensations that arise in the course of wanting to initiate and then carry out a voluntary action. Once these circuits are delimited and their molecular and synaptic signatures identified, they constitute the neuronal correlates of consciousness for intention and agency. If these circuits are destroyed by a stroke or some other calamity, the patient might act without feeling that it is she who is willing the acting!

In the debate concerning the meaning of personal freedom, these discoveries represent true progress, beyond the eternal metaphysical question of free will that will never be answered.

 

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How can you tell the difference between a French baby and a German baby? No, it’s not that one is wearing a saucy little beret while the other is tucked into tiny pair of lederhosen. Well, maybe that’s part of it. But a new study in the journal Current Biology shows that the babies actually sound different. Because the melody of an infant’s cry matches its mother tongue.

We all know that babies start eavesdropping while they’re still in the womb. So when they come out, they know their mother’s voice. When they’re older, they start to imitate the sounds they hear. Eventually they babble, and then start to speak, and then you never hear the end of it. But long before that first burble or coo, babies are learning the elements of language.

A team of scientists recorded the cries of 60 newborns: 30 born into French-speaking families and 30 that heard German. And they found that French infants wail on a rising note [baby cry sound] while the Germans favor a falling melody [baby cry sound]. Those patterns match the rhythms of their native languages. So next time you hear a baby cry, listen closely. He could be telling you where he’s from.

—Karen Hopkin

What are the odds that intelligent, technically advanced aliens would look anything like the ones in films, with an emaciated torso and limbs, spindly fingers and a bulbous, bald head with large, almond-shaped eyes? What are the odds that they would even be humanoid? In a YouTube video, produced by Josh Timonen of the Richard Dawkins Foundation for Reason and Science, I argue that the chances are close to zero (www.youtube.com/watch?v=JKAXrmkx12g). Richard Dawkins himself made this interesting observation in a private communication after viewing it:

I would agree with [Shermer] in betting against aliens being bipedal primates, and I think the point is worth making, but I think he greatly overestimates the odds against. [University of Cambridge paleontologist] Simon Conway Morris, whose authority is not to be dismissed, thinks it positively likely that aliens would be, in effect, bipedal primates. [Harvard University biologist] Ed Wilson gave at least some time to the speculation that, if it had not been for the end-Cretaceous catastrophe, dinosaurs might have produced something like the attached [referring to paleontologist Dale A. Russell’s illustrated evolutionary projection of how a bipedal dinosaur might have evolved into a reptilian humanoid].

I replied to Dawkins that if something like a smart, technological, bipedal humanoid has a certain level of inevitability because of how evolution unfolds, then it would have happened more than once here. In his 2001 book Nonzero: The Logic of Human Destiny, Robert Wright argues that our existence precludes other terrestrial intelligences of our level from arising. But Neanderthals were as close as one can get to a counterfactual experiment: they had hundreds of thousands of years to themselves in Europe without our interference and showed nothing like the technological and cultural progress of the modern humans who displaced them. Dawkins’s rejoinder to me is enlightening:

But you are leaping from one extreme to the other. In the film vignette, you implied a quite staggering rarity, so rare that you don’t expect two humanoid life-forms in the entire universe. Now you are ... pointing out, correctly, that a certain inevitability would predict that humanoids should have evolved more than once on Earth! So, yes, we can say that humanoids are fairly improbable, but not necessarily all that improbable! Anything approaching “a certain inevitability” would mean millions or even billions of humanoid life-forms in the universe, simply because the number of available planets is so huge. Now, my guess is intermediate between your two extremes ... I suspect that humanoids are not so very rare as to justify the statistical superlatives that you permitted yourself in the vignette.

Good point. But of the 60 to 80 phyla of animals, only one, the chordates, led to intelligence, and only the vertebrates actually developed it. Of all the vertebrates, only mammals evolved brains big enough for higher intelligence. And of the 24 orders of mammals only one—ours, the primates—has technological intelligence. As the late Harvard evolutionary biologist Ernst Mayr concluded: “Nothing demonstrates the improbability of the origin of high intelligence better than the millions of phyletic lineages that failed to achieve it.” In fact, Mayr calculated that even though there have evolved perhaps as many as 50 billion species on Earth, “only one of these achieved the kind of intelligence needed to establish a civilization.”

The late astronomer Carl Sagan, in a Planetary Society debate with Mayr (Bioastronomy News, Vol. 7, No. 4, 1995), noted that technologically communicating species “may live on the land or in the sea or air. They may have unimaginable chemistries, shapes, sizes, colors, appendages and opinions. We are not requiring that they follow the particular route that led to the evolution of humans. There may be many different evolutionary pathways, each unlikely, but the sum of the number of pathways to intelligence may nevertheless be quite substantial.Thus, the probability of intelligent life evolving elsewhere in the cosmos may be very high even while the odds of it being humanoid may be very low. I strongly suspect that we are blinded by Protagoras’ bias (“Man is the measure of all things”) when we project ourselves into the alien Other