Stephen Hawking - Right Now You May Be Creating Your Past
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Love and Light.
OUGHTN'T WE DIG UP GALILEO AND BURN THE CORPSE? (via AOG) Exploring Stephen Hawking's Flexiverse (Amanda Gefter, 20 April 2006, New Scientist)
Hawking and Hartle's original work on the quantum properties of the cosmos suggested that imaginary time, which seemed like a mathematical curiosity in the sum-over-histories approach, held the answer to understanding the origin of the universe.
Add up the histories of the universe in imaginary time, and time is transformed into space. The result is that, when the universe was small enough to be governed by quantum mechanics, it had four spatial dimensions and no dimension of time: where time would usually come to an end at a singularity, a new dimension of space appears, and, poof! The singularity vanishes.
In terms of the universe's history, that means there is no point A. Like the surface of a sphere, the universe is finite but has no definable starting point, or "boundary". Hence the idea's name: the no-boundary proposal.
This has led Hawking to define a new kind of cosmology. The traditional approach, which Hawking calls "bottom-up" cosmology, tries to specify the initial state of the universe and work from there. This is doomed to fail, Hawking says, because we know nothing about the starting conditions. Instead, he suggests, we should use the no-boundary proposal to do "top-down" cosmology, where the only input into our models of the universe comes from what we observe now - together with the idea that our universe has no boundary in the past.
The result of this process, he says, solves a long-standing problem of cosmology: fine-tuning. Most cosmologists think, for example, that the universe went through an early burst of rapid expansion, or "inflation". There is some evidence to support the claim, but there's also a problem. Standard inflationary models require a very improbable initial state, one that must have "finely tuned" values that cause inflation to start, then stop in a certain way after a certain time: a complicated prescription whose only justification is to produce a flat universe without any strange topology, and so on - a universe like ours.
Such a prescriptive method makes hard and unsatisfying work of producing the universe we see today. While a cosmologist can put these values into the equations "by hand", it is not exactly a satisfactory way to develop our model of how the universe works. In the no-boundary theory, however, there simply is no defined initial state. "In the usual approach it is difficult to explain how inflation began," says Hawking. "But it occurs naturally in top-down with the no-boundary condition. It doesn't need fine tuning."
To do top-down cosmology, Hawking and Hertog first take a whole raft of possible histories, all of which would result in a universe with features familiar to us. "We then calculate the probability for other features of the universe, given the constraints," Hertog says. Specify a universe that is three-dimensional and flat, for instance, and you can have histories that involve inflation and histories that don't. "Top-down cosmology does not predict that all possible universes have to begin with a period of inflation, but that inflation occurs naturally within a certain subclass of universes," Hertog says. The process creates a probability for each scenario, and so Hertog can see which kind of history is most likely. "What we find is that the inflating histories generally have the largest probability."
In many ways, top-down cosmology is an unsettling idea. Usually, science demands that our observations come out as output - we certainly don't expect them to be the input. That, after all, denies us the chance to see if the theory matches up with observations. What's more, the sum over histories is formed by calculating the various probabilities for a universe like ours to arise out of literally nothing: that means we can never know anything for certain about how our universe got to be as it is.
We shouldn't be surprised, Hertog says: quantum theory has long shown us that it is impossible for us to know everything about the world around us. In "classical" physics, we can predict both the exact momentum and position of a particle at any time, but quantum mechanics doesn't allow it. No one suggests that quantum mechanics is wrong because of this, Hertog points out - and experiments have shown that it is not. What quantum theory has given us now, Hertog says, is some indication about the nature of inflation, where before we had none. "Before, we had no prediction at all - and indeed no notion of likeliness - on this issue."
For many, it remains a difficult argument to swallow. Science since Copernicus has aimed to model a universe in which we are mere by-products, but top-down cosmology turns that on its head, rendering the history of the universe a by-product of our observations. All in all, it is very like the "anthropic landscape" argument that is causing controversy among string theorists (see "Putting the you into universe").
Princeton University physicist Paul Steinhardt is certainly unimpressed by Hawking and Hertog's scheme. "It's kind of giving up on the problem," he says. "We've all been hoping to calculate things from first principles. Stephen doesn't think that's possible, but I'm not convinced of that. They might be right, but it's much too early to take this approach; it looks to me like throwing in the towel."
Stanford University's Andrei Linde is similarly unconvinced. There are a number of technical assumptions that make him sceptical. "I don't buy it," he says.
The past is out there
The merits of Hawking and Hertog's new approach to cosmology might be decided by experiment. The theory predicts specific kinds of fluctuations in two cosmological phenomena: the cosmic microwave background radiation produced just after the big bang, and the spectrum of primordial gravitational waves. These fluctuations arise from applying the uncertainty principle of quantum mechanics to Hawking and Hertog's scheme: in this scenario, the universe's shape is never precisely determined, but is influenced by other histories with similar geometries.
If Hawking and Hertog are right, quantum uncertainty will manifest as slight differences from what standard inflationary theory predicts for the CMB. The top-down predictions only differ from the standard cosmological model at a level of precision that has not yet been reached in observations, however. The top-down signature in the gravitational wave spectrum should be easier to differentiate, but since we haven't yet detected any gravitational waves, we'll have to wait for that proof too.
For Hawking and Hertog, there's simply no doubt that top-down cosmology is the only answer. It's simple: if you can't know the initial state of the universe, you can't work forwards from the beginning: the top-down approach is the only one that works.
Hartle agrees. Hawking and Hertog's scheme may seem strange, but it is the only way forward because we are part of the experiment we are trying to observe. "It's a different viewpoint, but it's sort of inevitable," he says. "Colsmologists certainly should be paying attention to this work."
The trouble, of course, is that if they are right, we're involved in the making of that history. In that case, we have a new set of instructions for building a universe. Step one: look around you. Step two: find the set of all possible histories that end up as a universe like the one you see. Step three: add them together and create a history for yourself.
It's entirely predictable that physics is collapsing towards a homocentric view of the Universe, but Hawking is, of course, quite wrong about there not being an Observer all along.
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