The Math Forum

Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Math Forum » Discussions » sci.math.* » sci.math

Notice: We are no longer accepting new posts, but the forums will continue to be readable.

Replies: 1   Last Post: Mar 18, 2013 3:42 AM

Advanced Search

Back to Topic List Back to Topic List Jump to Tree View Jump to Tree View   Messages: [ Previous | Next ]
Pentcho Valev

Posts: 6,212
Registered: 12/13/04
Posted: Mar 17, 2013 7:40 PM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply

Experimentalists send a signal, detect its arrival and determine its speed: the speed is (e.g. four times) greater than c=300000 km/s. A world sensation of course but then experimentalists have to devise some reason why Divine Albert's Divine Theory remains unaffected, e.g. "Divine Albert's Divine Theory is unaffected because crocodiles cannot fly":
"Electric signals can be transmitted at least four times faster than the speed of light using only basic equipment that would be found in virtually any college science department. Scientists have sent light signals at faster-than-light speeds over the distances of a few metres for the last two decades - but only with the aid of complicated, expensive equipment. Now physicists at Middle Tennessee State University have broken that speed limit over distances of nearly 120 metres, using off-the-shelf equipment costing just $500. (...) While the peak moves faster than light speed, the total energy of the pulse does not. This means Einstein's relativity is preserved, so do not expect super-fast starships or time machines anytime soon."
"This is exactly what the EPFL team has demonstrated. Using their Stimulated Brillouin Scattering (SBS) method, the group was able to slow a light signal down by a factor of 3.6, creating a sort of temporary "optical memory." They were also able to create extreme conditions in which the light signal travelled faster than 300 million meters a second. And even though this seems to violate all sorts of cherished physical assumptions, Einstein needn't move over - relativity isn't called into question, because only a portion of the signal is affected."
"The technique developed at NIST is called four-wave mixing, and it works by altering some parts of each individual light pulse. This makes the light move forward faster than it normally would when traveling through a vacuum. (...) The physicists explain that the new research does not violate Albert Einstein's theory on general relativity - which states that the speed of light in a vacuum is the fastest achievable in the Universe. They say that a sort of loophole exists in this theory. By careful tuning of the light source and advanced calculations, it is possible to nudge portions of the light pulses so that they arrive at their destination ahead or behind the main pulse. (...) With four-wave mixing, the NIST investigators produced laser pulses that arrived at their destination a full 50 nanoseconds faster than photons traveling through a vacuum."
"A nano-sized bar of glass encased in silver allows visible light to pass through at near infinite speed. The technique may spur advances in optical computing. (...) In a vacuum the refractive index is 1, and the speed of light cannot break Einstein's universal limit of 300,000 kilometres per second. Normal materials have positive indexes, and they transmit at the speed of light in a vacuum divided by their refractive index. Ordinary glass, for instance, has an index of about 1.5, so light moves through it at about 200,000 kilometres per second. The new material contains a nano-scale structure that guides light waves through the metal-coated glass. It is the first with a refractive index below 0.1, which means that light passes through it at almost infinite speed, says Albert Polman at the FOM Institute AMOLF in Amsterdam, the Netherlands. But the speed of light has not, technically, been broken. The wave is moving quickly, but its "group velocity" ? the speed at which information is travelling ? is near zero."

Do the numerous experiments reporting the existence of superluminal signals refute Einstein's relativity in a straightforward way? Yes they do. In the example below, if the deactivation device is sending a superluminal signal to the bomb, then special relativity ends up in a contradiction: the bomb explodes in the frame of the train but does not in the frame of the tunnel:
pp. 41-42: "11.6. Train in a tunnel. A train and a tunnel both have proper lengths L. The train moves toward the tunnel at speed v. A bomb is located at the front of the train. The bomb is designed to explode when the front of the train passes the far end of the tunnel. A deactivation sensor is located at the back of the train. When the back of the train passes the near end of the tunnel, the sensor tells the bomb to disarm itself. Does the bomb explode?"

The solution to the train in a tunnel problem is on p. 57 in David Morin's text:

p. 57: "Yes, the bomb explodes. This is clear in the frame of the train... (...) We can, however, also look at things in the frame of the tunnel... (...) Therefore, the deactivation device gets triggered before the front of the train passes the far end of the tunnel, so you might think that the bomb does not explode. We appear to have a paradox. The resolution to this paradox is that the deactivation device cannot instantaneously tell the bomb to deactivate itself. It takes a finite time for the signal to travel the length of the train from the sensor to the bomb. And it turns out that this transmission time makes it impossible for the deactivation signal to get to the bomb before the bomb gets to the far end of the tunnel, no matter how fast the train is moving. Let's show this. The signal has the best chance of winning this "race" if it has speed c, so let's assume this is the case..."

Pentcho Valev

Point your RSS reader here for a feed of the latest messages in this topic.

[Privacy Policy] [Terms of Use]

© The Math Forum at NCTM 1994-2018. All Rights Reserved.