Lightspeed

[jgravatt]

Its been a couple of years since I took Cosmology at UVa, so I'm not really on top of things anymore (if I ever was). Anyways, my roommate tells me that tests have shown that faster than light travel is possible for even particles w/ mass. Is this true? Also, is the speed of light increasing as he also says?

Back when I was taught, anything w/ mass could not attain the speed of light, which was also a constant. Whats going on?

[Laser Jock]

Hi jgravatt, welcome to the BABB!

I think I can say without any reservation that you are right and your roommate is wrong. Any particle that has mass cannot be accelerated to the speed of light (much less faster) without infinite energy.

I seem to recall some speculation that the speed of light may be changing, but there is no real evidence for it. So I think that you can insist that it's a constant with great confidence.

[parejkoj]

Welcome to the BABB! Hopefully I can shed a little light on this.... sorry. ops:

Anyways, my roommate tells me that tests have shown that faster than light travel is possible for even particles w/ mass. Is this true?

Somewhat true, but it depends mostly on semantics. It is possible for a particle to move faster than the speed of light in a certain medium. There are two possibilities: Cerenkov radiation, which is well documented and perfectly understood,
http://www.physlink.com/Education/AskExperts/ae219.cfm

or what can be called anomalous dispersion, as discussed in one of Phil's Bad News articles,
http://www.badastronomy.com/bad/news/ftl.html

I just saw a talk on this second type a few weeks ago and from what I gathered, there may be something slightly more complicated going on (involving the potential well filling at up at specific wavelengths as the beam enters, for those who care), but there still isn't anything travelling faster than light, it just seems that way. Short explanation, I know. I could give more if someone asked, but I don't have time at the moment.

Also, is the speed of light increasing as he also says?

I've heard talk of this as well, but as far as I know, it is only theoretical speculation for the moment. From what we can currently tell, the speed of light is a constant. There is some small evidence that physical constansts -- 'c' included, for you tired light fans out there! -- may change on universal timescales, but the jury is still out on that one. From what we currently know, 'c' has always been what it is today.

[tracer]

Anyways, my roommate tells me that tests have shown that faster than light travel is possible for even particles w/ mass. Is this true?

Somewhat true, but it depends mostly on semantics. It is possible for a particle to move faster than the speed of light in a certain medium.

It is also possible for certain hypothetical particles with mass to travel faster than c. Such hypothetical particles are called tachyons, and would have to always travel faster than c, because their relativistic mass would increase toward infinity as they slowed down approaching c.

Actually, when I say these hypothetical particles would "have mass," it's a little trickier than that. The equation for the relativistic mass of a particle is (rest mass) divided by the square root of (1 - v^2/c^2). If v > c, the thing inside the square root sign is negative, so the square root evaluates to an imaginary number. When you divide a positive number (such as rest mass) by a positive imaginary number, you end up with a negative imaginary number. So, if a tachyon had a positive real "rest mass", you would measure a negative imaginary mass for it as it went zipping by faster-than-light. You'd measure a positive real mass for the tachyon only if its "rest mass" were negative and imaginary.

[parejkoj]

It is also possible for certain hypothetical particles with mass to travel faster than c. Such hypothetical particles are called tachyons, and would have to always travel faster than c, because their relativistic mass would increase toward infinity as they slowed down approaching c.

Phooey. Yes, there are those pesky tachyons, but they haven't been observed, nor as far as I know, have any of their expected effects been seen. Still purely theoretical constructs. Unless there has been some new development that I missed... has there?

[jgravatt]

what about claims that scientists have managed to stop a beam of light, aka light saber? I imagine that if this was done it involved magnetic fields or something similar.

[Cougar]

...is the speed of light increasing as he also says?

Courtesy of Steve Carlip on the sci.astro faq....

Subject: Have physical constants changed with time?
Author: Steve Carlip <carlip@dirac.ucdavis.edu>

The fundamental laws of physics, as we presently understand them, depend on about 25 parameters, such as Planck's constant h, the gravitational constant G, and the mass and charge of the electron. It is natural to ask whether these parameters are really constants, or whether they vary in space or time.

Interest in this question was spurred by Dirac's large number
hypothesis. The "large number" in question is the ratio of the
electric and the gravitational force between two electrons, which is
about 10^40; there is no obvious explanation of why such a huge number
should appear in physics. Dirac pointed out that this number is
nearly the same as the age of the Universe in atomic units, and
suggested in 1937 that this coincidence could be understood if
fundamental constants---in particular, G---varied as the Universe
aged. The ratio of electromagnetic and gravitational interactions
would then be large simply because the Universe is old. Such a
variation lies outside ordinary general relativity, but can be
incorporated by a fairly simple modification of the theory. Other
models, including the Brans-Dicke theory of gravity and some versions
of superstring theory, also predict physical "constants" that vary.

Over the past few decades, there have been extensive searches for
evidence of variation of fundamental "constants." Among the methods
used have been astrophysical observations of the spectra of distant
stars, searches for variations of planetary radii and moments of
inertia, investigations of orbital evolution, searches for anomalous
luminosities of faint stars, studies of abundance ratios of radioactive
nuclides, and (for current variations) direct laboratory measurements.

One powerful approach has been to study the "Oklo Phenomenon," a uranium
deposit in Gabon that became a natural nuclear reactor about 1.8 billion
years ago; the isotopic composition of fission products has permitted a
detailed investigation of possible changes in nuclear interactions.
Another has been to examine ratios of spectral lines of distant quasars
coming from different types of atomic transitions (resonant, fine
structure, and hyperfine). The resulting frequencies have different
dependences on the electron charge and mass, the speed of light, and
Planck's constant, and can be used to compare these parameters to their
present values on Earth. Solar eclipses provide another sensitive test
of variations of the gravitational constant. If G had varied, the
eclipse track would have been different from the one we calculate today,
so the mere fact that a total eclipse occurred at a particular location
provides a powerful constraint, even if the date is poorly known.

So far, these investigations have found no evidence of variation of
fundamental "constants." The current observational limits for most
constants are on the order of one part in 10^10 to one part in 10^11 per
year. So to the best of our current ability to observe, the
fundamental constants really are constant.


For a good short introduction to the large number hypothesis and the
constancy of G, see:

C.M. Will, _Was Einstein Right?_ (Basic Books, 1986)

For more technical analyses of a variety of measurements, see:

L. L. Cowie & A. Songaila, Astrophysical Journal (1995) v. 453,
p. 596 also available online at http://adsabs.harvard.edu/cgi-bin/np...pJ...453..596C

P. Sisterna & H. Vucetich, Physical Review D41 (1990) 1034 and
Physical Review D44 (1991) 3096

E.R. Cohen, in _Gravitational Measurements, Fundamental Metrology and
Constants_, V. De Sabbata & V.N. Melnikov, editors (Kluwer
Academic Publishers, 1988)

"The Constants of Physics," Philosophical Transactions of the Royal
Society of London A310 (1983) 209--363

[Laser Jock]

what about claims that scientists have managed to stop a beam of light, aka light saber? I imagine that if this was done it involved magnetic fields or something similar.

I'm a little more familiar with the slow and stopped light research. The stopped light research is really nothing like a light saber. The scientist have managed to "store" a pulse of light in a dilute atomic gas. Some time later (when they feel like it) they turn on a second laser and it "releases" the pulse to continue on it's orginal path.

Here is a link talking about it.

http://www.aip.org/enews/physnews/2001/split/521-1.html

Magnetic fields may be present in the experiment, but they are not important in producing the effect.

[eburacum45]

There is also a relatively respected scientist, Joao Magueijo of Imperial College, London, who suggested the the speed of light was different in the early stages of the universe;

he does seem to think it is constant nowadays though.
http://theory.ic.ac.uk/~magueijo/vsl.html

[Ian Goddard]

Professor Moffat may be the or a source of the or a variable c hypothesis:

Speed Of Light May Not Be Constant, Physicist Suggests...
http://www.sciencedaily.com/releases...1005114024.htm
http://www.spacedaily.com/news/cosmology-03f.html
http://arxiv.org/abs/hep-th/0208122

[tracer]

The "large number" in question is the ratio of the electric and the gravitational force between two electrons, which is about 10^40; there is no obvious explanation of why such a huge number should appear in physics. Dirac pointed out that this number is nearly the same as the age of the Universe in atomic units,

I've heard of an atomic mass unit, but what in Sam Hill is an atomic time unit?!

[Eroica]

my roommate tells me that tests have shown that faster than light travel is possible for even particles w/ mass.

Nothing that has mass can travel through space at or faster than the speed of light, but it can appear to do so if the space itself is moving. Due to the expansion of the universe, there are galaxies that are so remote from us they are receding from us at faster than the speed of light. They're not travelling through space at superluminal speeds: it's just the expansion of space itself that creates the effect. From their point of view, we're receding at superluminal velocities.

Also, rapidly rotating stars like black holes can drag space around with them, creating a vortex of spinning space on their event horizons.

I remember seeing a TV documentary recently about some guy who believes he can send messages into the past by accelerating electrons beyond the speed of light using a similar trick. By making a laser rotate, he thinks he can create some sort of vortex in which space itself will be dragged along. If he can accelerate the electrons in this vortex close to the speed of light, they will be travelling faster than light from our perspective and will then start travelling back in time!

I don't agree with the bit about them going back in time, but the rest sounds pretty plausible.

[AstroSmurf]

I've heard of an atomic mass unit, but what in Sam Hill is an atomic time unit?!

Some quick googling gave me that it's a unit defined by Dirac himself, and equals e²/mc³. I have no info on what "e" and "m" is, but it's apparently an attempt to find a convenient time-scale for particle interactions.

[swansont]

Courtesy of Steve Carlip on the sci.astro faq....

Subject: Have physical constants changed with time?
Author: Steve Carlip <carlip@dirac.ucdavis.edu>
(snip)
So far, these investigations have found no evidence of variation of
fundamental "constants." The current observational limits for most
constants are on the order of one part in 10^10 to one part in 10^11 per
year. So to the best of our current ability to observe, the
fundamental constants really are constant.

Just to note that recent measurements comparing Rb and Cs atomic fountain clocks show that the fine structure constant is currenly changing at a rate less than about a part in 10^16/year (and consistent with zero).

[Stylesjl]

I hear that matter can travel faster than the speed of light but not at the speed the of light

So in order to travel faster we would have to skip the actual speed of light

Though my dad told me this so i can't use evidence to support this