# What would be measured, under current technology, of a particle moving faster than light? (Somewhat duplicate)

Sorry, since the question has been posed, but I insist that others posed not from this position.

Can any listener deduce whether we could measure, by analyzing electromagnetic disturbance, a particle$^1$ traveling faster than light? (or current measuring device, for the sake of assuming for questioning)

Studying to understand cosmic rays, inductance, and capture technologies by the artifacts of Tesla.. The name most studied by us uneducated electrical folk.

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$^1$ For the 2011 OPERA neutrino experiment, see this Phys.SE post.

• Assuming you're thinking of neutrinos rather than neutrons, there was a big dust-up about this a while back. – rob May 31 '14 at 4:31
• the opera link asnwers that there exist methods of measuring faster than light motion experimentally. In the Opera case it just happened to be an instrumentation error that gave a non zero value, but given a good system one can do it. – anna v May 31 '14 at 7:54
• Back in the 70s there was a flurry of activity around calculating the properties of tachyons though it all petered out quickly, because it never came to anything concrete. There is an excellent popular level summary in Time Travel and Warp Drives by Everett and Roman chapter 6. – John Rennie May 31 '14 at 10:09

First, just to get this out of the way, a neutron has no charge, and could not be directly detected using an electromagnetic disturbance (since it does not produce an EM disturbance).

Next, a particle with a "real" mass cannot travel faster than the speed of light. To briefly describe why, I present the full form of the popular Einstein mass energy equivalence principle (which is typically notated as $E = m c^2$):

$$E^2 = (mc^2)^2 + (pc)^2$$

In the case of a particle with sufficiently low momentum, the second term on the right vanishes, and the equation reduces to its famous form. However, a superluminal velocity requires $p>E/c$, which is only possible if $(mc^2)^2 < 0$. Since $c$ is a positive, fundamental constant of the universe (unchanging in this argument), this requires that $m^2<0$, which implies that the mass is imaginary.

There are, however, hypothetical particles (since they have never been observed to exist) called Tachyons, which have an imaginary mass. Note also that, just as a particle with a real mass is limited to travel below the speed of light, so too is a Tachyon restricted to speeds greater than $c$.

Now, to answer you question: Tachyons, should they exist, will otherwise behave like other particles, such as neutrosn, in the way they interact with our more traditional types of matter. In this way, a tachyon can sill scatter off of ordinary particles, and can, in theory, be detected with ordinary neutron detectors (something which I won't delve into here). However, as a final note, the momentum conservation gets a bit strangle, since the mass is no longer real, so a bit of reformulation of some of the relevant equations is necessary, but the principles are all essentially the same.

• Thank you, I already understand the tie between mass and energy. But, if these particles lack real mass, how could they impart force on real mass to create disturbance? Sorry if this is ignorant, believe me, it's oblivion. – David May 31 '14 at 1:24
• It will all depend on the specific theory, that has to be a quantum field theory. In qft there exist vertices that allow the probability of interaction with the electromagnetic, weak and strong fields. It is not necessary to have a positive mass, as long as energy and momentum is conserved overall. The vertex operators will assure the interaction with matter which will enable the detection of the passage of a particle. – anna v May 31 '14 at 8:01