In tunneling physics, a decades-old paradox is resolved

Jul 26, 2006

As if the concept of quantum tunneling—where atoms pass through barriers—isn't confusing enough, one of the vexing questions within that area of physics is why particles seem to travel faster than the speed of light when passing through a barrier, but not when they travel through empty space.

Also puzzling is why the time spent by the particle in the barrier does not seem to increase as the barrier is made longer and longer.

This paradox has stirred debate in the physics community since 1932, but Herbert Winful, a professor at the University of Michigan's College of Engineering, believes he's put an end to these questions. Winful says his theoretical results show that what's being calculated and measured isn't the time it takes the particle to go from A to B (passing through a barrier in between) "but the time it takes to empty the barrier of energy already stored in the barrier." The technical term for this time is the "group delay."

Winful worked out his theory mathematically, using photonic band gap structures. Such structures filter, or "tune" out, certain wavelengths of light and let certain others pass through. He then calculated the delay for electromagnetic waves that made it through the band gap and found that the result was exactly equal to the time it takes energy to escape from the barrier through both ends of the barrier.

Here is how group delay works in quantum tunneling: imagine two tour buses, one with 100 passengers and the other with 10 passengers. The buses are heading toward the same restaurant across town. They arrive together, but the bus with 10 people empties more quickly so those diners get to eat first. If you define the arrival time as the average time at which a passenger arrives at the dinner table, then this time is shorter for the bus with fewer passengers. This also explains why the so-called group delay is the same no matter the distance traveled.

In quantum tunneling most of the particles (people on the bus) bounce off the barrier and only a tiny fraction makes it through. The presence of the barrier reduces the amount of energy that can be stored compared to the amount stored in a barrier-free region. The delay time measured is directly proportional to the stored energy and is the time it takes to release this stored energy.

The time doesn't change when the barrier is widened because the barrier has a certain energy storage capacity, which does not increase with length, just as the bus has a fixed capacity regardless of the distance traveled, Winful said.

Winful presented his results in an invited paper July 25 at the Slow and Fast Light Conference in Washington, DC.

"This is an important question from a fundamental physics viewpoint, but it's also important because it can tell you the ultimate speed tunneling devices can operate," said Winful. "My result is actually in a way is a bit of a downer, because it shows that we can't do that (travel faster than light)." But, he said, it's comforting to know that Einstein was right. Einstein's theory of relativity tells us that nothing can travel faster than light, about 186,171 miles per second.

Quantum tunneling is used in scanning tunneling microscopes, which make observations at the atomic scale possible, and certain electronic devices, such as tunneling diodes and Josephson junctions.

Source: University of Michigan

Explore further: Ultrafast imaging of complex systems in 3D at near atomic resolution nears

add to favorites email to friend print save as pdf

Related Stories

A coating that protects against heat and oxidation

Nov 04, 2014

Researchers have developed a coating technique that they plan to use to protect tur- bine engine and waste incinerator components against heat and oxidation. A topcoat from micro-scaled hollow aluminium oxide ...

Partnerships drive new transportation solutions

Oct 27, 2014

Hybrid car sales have taken off in recent years, with a fuel-sipping combination of electric- and gas-powered technologies that simultaneously deliver energy efficiency, low emissions, and strong performance. ...

Australia PM rebuffs Booker Prize winner criticism

Oct 16, 2014

Prime Minister Tony Abbott on Thursday brushed off criticism about Australia's environmental policies by newly crowned Booker Prize winner Richard Flanagan, saying the country had "a very, very strong" record.

Recommended for you

What's next for the Large Hadron Collider?

Dec 17, 2014

The world's most powerful particle collider is waking up from a well-earned rest. After roughly two years of heavy maintenance, scientists have nearly doubled the power of the Large Hadron Collider (LHC) ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.