Posts Tagged ‘space-time

Entangled toy universe shows time may be an illusion

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How can so many clocks be wrong? (Image: Peter Dazely/Getty)

How can so many clocks be wrong? (Image: Peter Dazely/Getty)

by Jacob Aron
Time is an illusion – at least in a toy model of the universe made of two particles of light. The experiment shows that what we perceive as the passage of time might emerge from the strange property of quantum entanglementMovie Camera. The finding could assist in solving the long-standing problem of how to unify modern physics.

Physicists have two ways of describing reality, quantum mechanics for the small world of particles and general relativity for the larger world of planets and black holes. But the two theories do not get along: attempts to combine their equations into a unified theory produce seemingly nonsensical answers. One early attempt in the 1960s was the Wheeler-DeWitt equation, which managed to quantise general relativity – by leaving out time altogether.

“It means that the universe should not evolve. But of course we see evolution,” says Marco Genovese at the National Institute of Metrological Research in Torino, Italy.

In 1983 theorists Don Page and William Wootters suggested that quantum entanglement might provide a solution to the Wheeler-DeWitt “problem of time”Movie Camera. When quantum objects are entangled, measuring the properties of one changes those of the other. Mathematically, they showed that a clock entangled with the rest of the universe would appear to tick when viewed by an observer within that universe. But if a hypothetical observer existed outside the universe, when they looked in, everything would appear stationary.

Photon clock

For the first time, Genovese and colleagues have demonstrated this effect in a physical system, albeit in a “universe” that contains only two photons. The team started by sending a pair of entangled photons along two separate paths. The photons start out polarised, or orientated, either horizontally or vertically, and the polarisation rotates as both photons pass though a quartz plate and on to a series of detectors.

The entangled photons exist in a superposition of both horizontal and vertical states simultaneously until they are observed. But the thicker the plate, the longer it takes the photons to pass through and the more their polarisation evolves, affecting the probability that either one will take a particular value.

In one mode of the experiment, one of the photons is treated like a clock with a tick that can alternate between horizontal and vertical polarisation. Because of entanglement, reading this clock will affect the polarisation value of the second photon. That means an observer that reads the clock influences the photons’ universe and becomes part of it. The observer is then able to gauge the polarisation value of the other photon based on quantum probabilities.

Since photons passing through a thicker quartz plate experience a different degree of change, repeating the experiment with plates of different thicknesses confirms that the second photon’s polarisation varies with time.

In another mode, the experimenter is a “super-observer” that exists outside of the universe, and so measures the quantum state of the system as a whole. From that vantage point, the state of both photons taken together is always the same, giving the appearance of a static universe.

Quantum cosmos?

“It’s very nice these people have done an experiment to illustrate this effect and show how in practice it can occur,” says Page, who is now at the University of Alberta in Edmonton, Canada.

But not everyone thinks the Wheeler-DeWitt equation is the correct route to unification of the quantum and classical worlds, says Lee Smolin at the Perimeter Institute in Waterloo, Ontario, Canada. “They have verified in the context of a laboratory system that quantum mechanics is working correctly,” he says. But Smolin argues that any correct description of the universe must include time.

Genovese acknowledges that the result does not cinch the issue. Instead, he sees the work as a hint that quantum equations can in some ways mesh with general relativity, offering hope for a unified theory. The next step will be moving beyond the toy universe and seeing whether a similar effect scales up to explain what we see on a cosmic level.

“It’s a visualisation of the phenomenon, it’s not a proof,” Genovese says of the experiment. “You should look to the universe itself for that.”

Journal reference:


Written by physicsgg

October 28, 2013 at 11:46 pm

Experimental observation of the “end of time event”

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Experimental observation of the “end of time event” in a plasmonic hyperbolic metamaterial illuminated with 488 nm light.

Extraordinary rays in a hyperbolic metamaterial behave as particle world lines in a three dimensional (2+1) Minkowski spacetime. We analyze electromagnetic field behavior at the boundaries of this effective spacetime depending on the boundary orientation. If the boundary is perpendicular to the space-like direction in the metamaterial, an effective Rindler horizon may be observed which produces Hawking radiation. On the other hand, if the boundary is perpendicular to the time-likedirection an unusual physics situation is created, which can be called “the end of time”. It appears that in the lossless approximation electromagnetic field diverges at the interface in both situations. Experimental observations of the “end of time” using plasmonic metamaterials confirm this conclusion….Read more:

Written by physicsgg

July 26, 2011 at 8:48 am


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Some People Talk About Space-Time Invisibility Cloaks. At Cornell, They Built One

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Demonstrating the world’s first device that creates a hole in time

A Temporal 'Time Cloak' Envisioned

We’ve written previously about the theoretical possibility of “event cloaks”–metamaterial space-time devices that could theoretically conceal an entire event in time from the view of an outsider. Well, while some bright minds were just talking about bending space-time to their whims, a team at Cornell was doing it. And it works. For 110 nanoseconds.
There’s a more thorough explanation of this notion in our previous coverage, but briefly this is the idea: basically, you need two time-lenses–lenses that can compress and decompress light in time. This is actually possible to do using an electro-optic modulator (what, you don’t have one?). Basically, using two of these modulators you would slow down or compress the light traveling through the first lens, and then set up a second lens downrange from the first that would decompress, or accelerate, the incoming photons from the first lens.
Got that? Refer to this handy gif, courtesy of some blokes working on a similar idea at Imperial College London:

Think of the photons like steadily flowing traffic on a highway. If you slow the traffic at a point upstream, you create a gap. You can cross the highway through the gap and then accelerate that traffic to catch up to the traffic ahead, closing the gap. To someone further downstream, the gap is not there–to that observer, the gap might as well have never existed because there’s no evidence of it.

During that gap, whatever occurs goes unrecorded. But, as we noted above, you’d have to be pretty quick were you to use such a device to pull some kind of shenanigans. The current device the Cornell gents have built creates a 110 nanosecond event gap, and they concede that the best it could achieve is 120 microseconds. But, as KFC notes at Technology Review, rarely is anything final in cutting edge theoretical physics.

Details at arXiv.

Written by physicsgg

July 19, 2011 at 5:02 pm


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