Go Back   Physics Help Forum > High School and Pre-University Physics Help > Light and Optics

Light and Optics Light and Optics Physics Help Forum

Like Tree1Likes
  • 1 Post By susanwen
Reply
 
LinkBack Thread Tools Display Modes
Old Mar 28th 2014, 08:43 PM   #1
Senior Member
 
Join Date: Nov 2013
Location: New Zealand
Posts: 534
Can light travel faster than the speed of light?

Can someone tell me if there is any truth to this? Someone on another forum claims that light can sometimes travel faster than the speed of light and sometimes arrive before it left. In order not to misquote I have his response in its entirety.

they broke that [light speed] in the year 2000 ..
In the experiment, NEC scientists measured the time taken by a pulse of light to pass through a 6cm-long specially prepared chamber containing cesium gas*2. The 3-microsecond long pulse of light would normally take only 0.2 nanoseconds to pass through the chamber in a vacuum. But when passed through the specially prepared chamber, light emerged 62 nanoseconds earlier than it would have had it passed through the chamber in a vacuum. This unusual phenomenon is the result of "anomalous dispersion", an effect not seen in nature in transparent materials and is created by the non-natural thermal state of the cesium gas used in the chamber.

so it arrived 61.8 nanoseconds before sent ..
.2 nanoseconds is subtracted as what the normal speed of light is ..


A pulse of light consists of many components, each at a different wavelength as can be seen when sunlight passes through a prism in "normal dispersion" and is broken down into its constituent colors. The effect of anomalous dispersion on the wavelengths of the components of light, however, is to modify them. Anomalous dispersion causes components with a shorter wavelength in a vacuum to have a longer wavelength in the chamber and conversely, components with a longer wavelength in a vacuum have a shorter wavelength in the chamber. Unlike with normal dispersion, anomalous dispersion has the extraordinary effect of enabling a light pulse to appear again at a distant point along its direction of propagation and produce the exact shape of the light pulse that entered the chamber. A light pulse can thus traverse the distance between two points faster than its vacuum speed.

"Our experiment shows that the generally held misconception that nothing can move faster than the speed of light, is wrong. Einstein's Theory of Relativity still stands, however, because it is still correct to say that information cannot be transmitted faster than the vacuum speed of light," said Dr. Lijun Wang. "We will continue to study the nature of light and hopefully it will provide us with a better insight about the natural world and further stimulate new thinking towards peaceful applications that will benefit all humanity."

NOTES:
1 - Refers to the speed of light in a vacuum which is defined as 299,792.4580 km/sec.
2 - Natural Cesium can exist in 16 possible quantum mechanical states, called "hyperfine ground state magnetic sublevels". NEC scientists drove almost all cesium atoms to only one of the 16 possible quantum mechanical states which corresponds to almost absolutely zero degree temperature in the Kelvin scale (-273.15 degrees Centigrade) and not a naturally occurring temperature on earth. This was achieved through "optical pumping" using lasers, themselves not a naturally occurring phenomenon.

Anomalous refractive index:
To achieve their peculiar effect, Dr Wang's group fired laser beams through a trap of caesium atoms. By adjusting the frequency of the laser beams to match those of the energy levels in the atoms, the researchers were able to achieve an effect called "anomalous refractive index." This boosts the pulses' so-called "group velocity" to a speed faster than what we understand to be the speed of light - just short of 300 million metres per second. The group velocity of a light pulse depends upon the mixture of frequencies within the pulse and the medium through which it travels. It need not be the speed of the pulse itself. The important thing, however, is that whilst the group velocity can be manipulated to be faster than the speed of light, it is not possible to use this effect to send information faster than the speed of light. Because of the fast group velocity, the leading edge of the pulse appears to leave the caesium-filled chamber 62 billionths of a second before it arrives.
kiwiheretic is offline   Reply With Quote
Old Mar 29th 2014, 08:40 AM   #2
Physics Team
 
ChipB's Avatar
 
Join Date: Jun 2010
Location: Morristown, NJ USA
Posts: 2,320
Originally Posted by kiwiheretic View Post
Can someone tell me if there is any truth to this? Someone on another forum claims that light can sometimes travel faster than the speed of light and sometimes arrive before it left. In order not to misquote I have his response in its entirety.
Here's a link to an article from 2000 that talks about this experiment:
http://physicsworld.com/cws/article/...t-speed-record

Note that the person you quoted has intentionally misstated what occurred. The light did not reach the end of the tube before being sent - what happened was the width of the pulse was shortened, so that the back end of the pulse caught up with the main pulse, and the entire pulse exited faster than one would anticipate. This is a consequence of the wave nature of light, and the fact that a "pulse" actually contains many wavelengths all superimposed, and the refractive properties of the cesium gas causes the various wavelengths to travel at different speeds. Needless to say, Maciej's followup posts have absolutely nothing to do with this.

Last edited by ChipB; Mar 31st 2014 at 07:07 AM.
ChipB is offline   Reply With Quote
Old Mar 30th 2014, 06:44 PM   #3
Senior Member
 
Join Date: Nov 2013
Location: New Zealand
Posts: 534
Yep, thanks for that. Might be interesting if I read up on actual experiment.

Originally Posted by ChipB View Post
Here's a link to an article from 2000 that talks about this experiment:
http://physicsworld.com/cws/article/...t-speed-record

Note that the person you quoted has intentionally misstated what occurred. The light did not reach the end of the tube before being sent - what happened was the width of the pulse was shortened, so that the back end of the pulse caught up with the main pulse, and the entire pulse exited faster than one would anticipate. This is a consequence of the wave nature of light, and the fact that a "pulse" actually contains many wavelengths all superimposed, and the refractive properties of the cesium gas causes the various wavelengths to travel at different speeds. Needless to say, your other followup posts have absolutely nothing to do with this.
kiwiheretic is offline   Reply With Quote
Old Apr 2nd 2014, 07:55 AM   #4
Junior Member
 
Join Date: Apr 2014
Posts: 2
no of coures else you do an explosion bigger than big bang to get what you ask ...
photonics is offline   Reply With Quote
Old Apr 2nd 2014, 11:52 AM   #5
Senior Member
 
Join Date: Nov 2013
Location: New Zealand
Posts: 534
Originally Posted by photonics View Post
no of coures else you do an explosion bigger than big bang to get what you ask ...
Right, we don't want to cause that. @MaciejMarianMarosz , please don't blow up the universe.
kiwiheretic is offline   Reply With Quote
Old Yesterday, 04:33 PM   #6
Junior Member
 
Join Date: Nov 2018
Posts: 1
This experiment is huge in its implications, either way. If scientists have duplicated this experiment and were successful in showing speeds propagated faster than light, we would have heard of it. They couldn’t keep this a secret. On the other hand, if the experiment “succeeded” in showing something wrong with the experiment itself, or other intervening effects not taken into account, this too is a scientific success, bringing us closer to whatever configuration, whatever model “works” to produce the effects reported, that too is scientific progress; though it would be more fun to have the speed of light to play around with.
topsquark likes this.
susanwen is offline   Reply With Quote
Old Yesterday, 05:22 PM   #7
Forum Admin
 
topsquark's Avatar
 
Join Date: Apr 2008
Location: On the dance floor, baby!
Posts: 2,466
I agree that its big. However it is also very specific in its experimental design. As another example it is possible for the speed of an electron to be greater than that of light. But... This happens in a nuclear reactor in which the source of the electrons is bathed in water. The speed of light in the reactor is smaller than the speed of light in a vacuum so it is indeed within the laws of Physics to happen. (Look up Cherenkov radiation.) But it's not likely to happen in any other manner.

I really wish that the news services would get their facts straight before they reported this stuff.

-Dan
__________________
Do not meddle in the affairs of dragons for you are crunchy and taste good with ketchup.

See the forum rules here.
topsquark is offline   Reply With Quote
Reply

  Physics Help Forum > High School and Pre-University Physics Help > Light and Optics

Tags
faster, light, speed, travel



Thread Tools
Display Modes


Similar Physics Forum Discussions
Thread Thread Starter Forum Replies Last Post
Travel at speed of light? oem7110 Special and General Relativity 3 Jan 7th 2017 12:09 PM
Faster than the speed of light? Jamesbolt Theoretical Physics 5 Nov 25th 2013 08:31 AM
Faster-than-light Germanlongstrike General Physics 26 Oct 10th 2013 09:04 AM
How come photons travel at the speed of light? urooj177 Special and General Relativity 5 Jan 5th 2013 07:30 PM


Facebook Twitter Google+ RSS Feed