## Has there been an experiment that measured speed faster than the speed of light in vacuum? (Advanced)

Is that true that in some experiment was measured a speed, faster than the speed-of-light in the vacuum?

Yes.

If the answer is YES, what kind of particle was used for that experiment: a photon, a neutrino or other particle?

Photon.

According to Einstein it is not possible to send a signal from one body to other body at speeds faster than light? The value measured is a new limit?

No. It is still not possible to send information faster than the speed of light, and causality is not violated. Further, the authors of the experiment say that no object with a finite rest mass can travel faster than the speed of light.

If you have the technical knowledge, the best place to read about it is the Nature article. The issue is 20 July, 2000 (vol 406), page 243.

The experiment created a pulse whose group velocity had a negative value of -c/310 (c=speed of light). Note that this is in a medium and the speed of light in vacuum is still c. The difference in transit times between traveling in vacuum and in the medium = L/v - L/c where L is the length of the cavity, v is the speed of light in the medium. Now, if v is sufficiently negative, then the difference in transit times can become sufficiently negative so that the peak of the pulse emerges from the medium at an instant earlier than when the peak of the pulse enters.

So what is happening? As the pulse has finite duration, one needs an infinite number of waves of different frequencies to be added together to give the required pulse. The shorter the desired pulse, the larger the bandwidth of frequencies that must be used. All light pulses are therefore formed by a packet of waves of different frequency, each of which has a different amplitude and phase. The speed of individual waves is the phase velocity and the velocity at which the peak of the wavepacket is known as the group velocity. It is the group velocity that determines how fast information is transferred.

A negative group velocity results when the phases of the different frequency components are shifted by the medium through which they travel, so that the wavepacket they form at the exit is brought forward in time compared with the same pulse travelling through vacuum. The peak of the pulse at the output results from the forward rising edge of the input pulse, which occurs far earlier in time, making it consistent with causality. An abdrupt feature in the light pulse would not be able to travel faster than c.

For more details of the experiment, please refer to the Nature paper.

According to that experiment, will it be possible for any kind of particle to travel faster than 300,000 km/sec?

No. Now, I have to make a distinction here. A theoretical particle CAN travel faster than the speed of light, but then it will ALWAYS travel faster than the speed of light. One such class of particles is called tachyons. When we say that no particle can travel faster than the speed of light, what we mean is that a particle with finite rest mass cannot cross the speed of light limit; in other words, it cannot speed up to faster than speed of light starting from a speed less than the speed of light.

Space-time is filled with all kinds of radiation. Will that mean that, in a fraction of time, in the same place, there where photons traveling at different speeds?

No. All radiation travel at c unless they pass through matter.

Which phenomena (from the Sun, the Earth and other sources) that can affect us in a significant way will be detected first if there is a change in the speed of light?

It will not be possible to detect a change in the speed of light (in vacuum)! It is only possible to detect a change in dimensionless numbers that are ratios of other constants that have dimensions (like c). I refer you to this article.