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Title: General Relativistic Shock-Waves Propagating at the Speed of Light
Authors: Michael Brian Scott

13th August 2002

We investigate shock-wave solutions of the Einstein equations in the case when the speed of propagation is equal to the speed of light. The work extends the shock matching theory of Smoller and Temple to the lightlike case. After a brief introduction to general relativity, we introduce a previously known generalisation of the second fundamental form by Barrabes and Israel. Then we use this to develop an extension of a shock matching theory, which characterises solutions of the Einstein equations when the spacetime metric is only Lipschitz continuous across a hypersurface, to include the case when the hypersurface is lightlike. The theory also demonstrates an unexpected result that the matching of the generalised second fundamental form alone is not a sufficient condition for conservation conditions to hold across the interface. Using this theory we then construct a new exact solution of the Einstein equations that can be interpreted as an outgoing spherical shock wave that propagates at the speed of light. This is done by matching a Friedman Robertson Walker (FRW) metric, which is a geometric model for the universe, to a Tolman Oppenheimer Volkoff (TOV) metric, which models a static isothermal spacetime. Then our theory is used to show that the matched FRW, TOV metric is a solution. The pressure and density are nite on each side of the shock throughout the solution, the sound speeds, on each side of the shock, are constant and subluminous. Moreover, the pressure and density are smaller at the leading edge of the shock which is consistent with the Lax entropy condition in classical gas dynamics. However, the shock speed is greater than all the characteristic speeds. The solution also yields a surprising result in that the solution is not equal to the limit of previously known subluminous solutions as they tend to the speed of light.

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Doubt on faster-than-light neutrino result
 
Subatomic particles called neutrinos cannot move faster than the speed of light, according to a new report.
The findings challenge a result reported in September that, if true, would undermine a century of physics.
The team at the INFN-Gran Sasso laboratory in Italy said they had measured faster-than-light speeds in neutrinos sent from Cern, 730km away.
Now a different team at the lab reports findings that, they say, cast doubt on that surprising result.

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Title: A search for the analogue to Cherenkov radiation by high energy neutrinos at superluminal speeds in ICARUS
Authors: ICARUS Collaboration: M. Antonello (1), P. Aprili (1), B. Baibussinov (2), M. Baldo Ceolin (2), P. Benetti (3), E. Calligarich (3), N. Canci (1), F. Carbonara (4), S. Centro (2), A. Cesana (6), K. Cieslik (7), D. B. Cline (8), A. G. Cocco (4), A. Dabrowska (7), D. Dequal (2), A. Dermenev (9), R. Dolfini (3), C. Farnese (2), A. Fava (2), A. Ferrari (10), G. Fiorillo (4), D. Gibin (2), A. Gigli Berzolari (3), S. Gninenko (9), T. Golan (11), A. Guglielmi (2), M. Haranczyk (7), J. Holeczek (12), M. Kirsanov (9), J. Kisiel (12), I. Kochanek (12), J. Lagoda (13), S. Mania (12), G. Mannocchi (14), A. Menegolli (3), G. Meng (2), C. Montanari (3), S. Otwinowski (8), T. J. Palczewski (13), L. Periale (14), A. Piazzoli (3), P. Picchi (14), F. Pietropaolo (2), P. Plonski (15), P. Przewlocki (13), A. Rappoldi (3), et al. (22 additional authors not shown)
(Version v2)

The OPERA collaboration has claimed evidence of superluminal {\nu}{_\mu} propagation between CERN and the LNGS. Cohen and Glashow argued that such neutrinos should lose energy by producing photons and e+e- pairs, through Z0 mediated processes analogous to Cherenkov radiation. In terms of the parameter {\delta}=(v^2_nu-v^2_c)/v^2_c, the OPERA result implies {\delta} = 5 x 10-5. For this value of {\delta} a very significant deformation of the neutrino energy spectrum and an abundant production of photons and e+e- pairs should be observed at LNGS. We present an analysis based on the 2010 data set from the ICARUS experiment, located at Gran Sasso and using the same neutrino beam from CERN. We find that the neutrino energy distribution of the ICARUS events in LAr agrees with the expectations for an unperturbed spectrum of the CERN neutrino beam. Our results therefore refute a superluminal interpretation of the OPERA result according to the Cohen and Glashow prediction for a weak currents analog to Cherenkov radiation. In particular no superluminal Cherenkov like e+e- pair or {\gamma} event has been directly observed inside the fiducial volume of the "bubble chamber like" ICARUS TPC-LAr detector, setting much stricter limits to the value of {\delta}, comparable with the one due to the observations from the SN1987A.

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Neutrino experiment repeat at Cern finds same result

The team behind the finding in September that neutrinos may travel faster than light has carried out an improved version of their experiment - and found the same result.
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CERN excludes 1 error in faster-than-light finding

Scientists at the world's largest physics lab say they have ruled out one possible error that could have distorted measurements they took that appear to show particles travelling faster than light.
The European Organisation for Nuclear Research, or CERN, says more precise testing has confirmed the accuracy of at least one part of the experiment.

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Evidence of faster-than-light particles puts IU theoretical physicist in spotlight

Whether physicists in Europe last week recorded a timing error in the billionths-of-a-second range or actually evidenced particles travelling faster than light, the phone is still ringing off the hook in the office of Alan Kostelecky, Indiana University Bloomington distinguished professor of physics.
Kostelecky's research investigates the possibility that a unified theory tying together quantum physics and gravity could lead to tiny but observable deviations from Einstein's Theory of Relativity. As part of this effort more than 25 years ago, Kostelecky and colleagues theorised in the paper "The Neutrino as a Tachyon" that neutrinos might travel faster than light.

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Ed ~ There are obvious possible sources of errors in the MINOS detector results that need to be checked; the timing of the production, or recording, of the neutrinos, and the distance from source to detector may have changed or is inaccurate.



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Speed-of-light experiments yield baffling result at LHC

Puzzling results from Cern, home of the LHC, have confounded physicists - because it appears subatomic particles have exceeded the speed of light.
Neutrinos sent through the ground from Cern toward the Gran Sasso laboratory 732km away seemed to show up a few billionths of a second early.
The results will soon be online to draw closer scrutiny to a result that, if true, would upend a century of physics.

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Cosmic currents may move faster than light

While nothing with mass can move faster than the speed of light, scientists now think some weird, faster-than-light currents may be the powerhouse for fast-spinning stars.
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Title: Apparent Faster-Than-Light Pulse Propagation in Interstellar Space: A new probe of the Interstellar Medium
Authors: F. A. Jenet, D. Fleckenstein, A. Ford, A. Garcia, R. Miller, J. Rivera, K. Stovall
(Version v3)

Radio pulsars emit regular bursts of radio radiation that propagate through the interstellar medium (ISM), the tenuous gas and plasma between the stars. Previously known dispersive properties of the ISM cause low frequency pulses to be delayed in time with respect to high frequency ones. This effect can be explained by the presence of free electrons in the medium. The ISM also contains neutral hydrogen which has a well known resonance at 1420.4 MHz. Electro-magnetic theory predicts that at such a resonance, the induced dispersive effects will be drastically different from those of the free electrons. Pulses travelling through a cloud of neutral hydrogen should undergo "anomalous dispersion", which causes the group velocity of the medium to be larger than the speed of light in vacuum. This superluminal group velocity causes pulses containing frequencies near the resonance to arrive earlier in time with respect to other pulses. Hence, these pulses appear to travel faster than light. This phenomenon is caused by an interplay between the time scales present in the pulse and the time scales present in the medium. Although counter-intuitive, it does not violate the laws of special relativity. Here, we present Arecibo observations of the radio pulsar PSR B1937+21 that show clear evidence of anomalous dispersion. Though this effect is known in laboratory physics, this is the first time it has been directly observed in an astrophysical context, and it has the potential to be a useful tool for studying the properties of neutral hydrogen in the Galaxy.

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Is time travel possible?
Theoretical physicist Michio Kaku explores the possibilities.

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