"A team at the Hong Kong University of Science and Technology, led by Professor Shengwang Du, has concluded that single photons cannot travel faster than the speed of light in a vacuum," PC Magazine reported Tuesday.[1]  --  "[O]ur results bring a closure to the debate on the true speed of information carried by a single photon," Du said.  --  "According to Du, the scientific community got all excited about time travel several years ago with the discovery of 'superluminal propagation of optical pulses,' which basically said that a group of optical pulses could move faster than the speed of the light.  Du, however, said this was only a visual effect and could not actually be used to transmit real information."  --  But a BBC report on the experiment said that "Time travel by other means, however, is not entirely ruled out.  Einstein's theory of general relativity, in which space and time are two intertwined aspects of the same medium, would permit the bending of the medium to join two different times - a situation popularized as creating a 'wormhole.'"[2]  --  The BBC story elicited 600 comments, one of which read:  "I am currently visiting your time from 2385, so I would like to point out that there is still a basic flaw in your understanding.  Keep banging the rocks together guys, you will get there." ...

1.

SORRY DOC, SCIENTISTS SAY TIME TRAVEL IS IMPOSSIBLE

By Chloe Albanesius

PC Magazine

July 26, 2011

http://www.pcmag.com/article2/0,2817,2389132,00.asp


Dashing the hopes of "Back to the Future" and "Bill and Ted" fans alike, a group of Hong Kong scientists claims that recent research proves that time travel is impossible.

A team at the Hong Kong University of Science and Technology, led by Professor Shengwang Du, has concluded that single photons cannot travel faster than the speed of light in a vacuum.  Unfortunately for time travel buffs, photons apparently obey the laws of physics, regardless of whether you have a magic phone booth or can get that DeLorean up to 88 miles per hour.

"The results add to our understanding of how a single photon moves.  They also confirm the upper bound on how fast information travels with light," Professor Du said in a statement.  "By showing that single photons cannot travel faster than the speed of light, our results bring a closure to the debate on the true speed of information carried by a single photon.  Our findings will also likely have potential applications by giving scientists a better picture on the transmission of quantum information."

According to Du, the scientific community got all excited about time travel several years ago with the discovery of "superluminal propagation of optical pulses," which basically said that a group of optical pulses could move faster than the speed of the light. Du, however, said this was only a visual effect and could not actually be used to transmit real information. People then focused on a single photon moving faster than the speed of light, but  "because of lack of experimental evidence of single photon velocity, this is also an open debate among physicists," Du said.

As a result, Du's team measured the maximum speed of a single proton, which showed that it obeys the universe's speed limit and "confirms Einstein's causality; that is, an effect cannot occur before its cause," researchers said.

In their tests, researchers managed to separate the optical precursor, a wave-like structure at the front of an optical pulse, from the rest of the photon wave packet.  To accomplish this, Du's team created a pair of photons and passed one of them through a group of laser-cooled rubidium atoms, which allowed them to observe optical precursors for the first time.

"The team found that, as the fastest part of a single photon, the precursor wave front always travels at the speed of light in vacuum," researchers said.  "The main wave packet of the single photon travels no faster than the speed of light in vacuum in any dispersive medium, and can be delayed up to 500 nanoseconds in a slow light medium.  Even in a superluminal medium where the group velocity (of an optical pulse peak) is faster than the speed of light in vacuum, the main part of the single photon has no possibility to travel faster than its precursor."

So, until someone came come up with something that travels faster than the speed of light, it looks like time travel will be confined to movies and TV.

2.

Science & environment

TIME TRAVEL: LIGHT SPEED RESULTS CAST FRESH DOUBTS


BBC News
July 26, 2011

http://www.bbc.co.uk/news/science-environment-14289114


Physicists have confirmed the ultimate speed limit for the packets of light called photons -- making time travel even less likely than thought.

The speed of light in vacuum is the Universe's ultimate speed limit, but experiments in recent years suggested that single photons might beat it.

If they could, theory allows for the prospect of time travel.

Now, a paper in Physical Review Letters shows that individual photons too are limited to the vacuum speed limit.

That means that photons maintain the principle of causality laid out in Einstein's theory of special relativity -- that is, an event's effect cannot precede its cause by travelling faster than light.  It is violation of this causality that would, in principle, permit time travel.

While the limit in vacuum is a fixed number -- some 300,000 km per second -- the speed of light can vary widely in different materials.

These differences explain everything from why a straw looks bent in a glass of water to experiments in cold gases of atoms in which light's speed is actively manipulated.

Some of those experiments showed "superluminal" behavior, in which photons travelled faster than the speed of light in a given medium.

It remained, however, to determine whether or not individual photons could exceed the vacuum limit.

ALL RELATIVE


Now, Shengwang Du and colleagues at the Hong Kong University of Science and Technology have measured what is known as an optical precursor.

Like the wind that moves ahead of a speeding train, optical precursors are the waves that precede photons in a material; before now, such optical precursors have never been directly observed for single photons.

By passing pairs of photons through a vapor of atoms held at just 100 millionths of a degree above absolute zero -- the Universe's ultimate low-temperature limit -- the team showed that the optical precursor and the photon that caused it are indeed limited to the vacuum speed of light.

"By showing that single photons cannot travel faster than the speed of light, our results bring a closure to the debate on the true speed of information carried by a single photon," said Professor Du.

Thus, photons cannot time travel, and moving information around at faster-than-light speeds is impossible.

But the work has more prosaic implications.

"Our findings will also likely have potential applications by giving scientists a better picture on the transmission of quantum information," said Professor Du.

Time travel by other means, however, is not entirely ruled out.

Einstein's theory of general relativity, in which space and time are two intertwined aspects of the same medium, would permit the bending of the medium to join two different times -- a situation popularized as creating a "wormhole."