No immediate plausibility issues with this picture, since the speedometer says 0.8c. Getting it past 1.0c is where it gets tricky.
No immediate plausibility issues with this picture, since the speedometer says 0.8c. Getting it past 1.0c is where it gets tricky.

Cosmology, Physics

Astronomy Without A Telescope – Apparent Superluminal Motion

1 Jan , 2011 by

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The recent list of Universe Today’s Top 10 Stories of 2010 included the story Faster than Light Pulsars Discovered – which on further reading made it clear that the phenomenon being studied wasn’t exactly moving faster than light.

Anyhow, this prompted me to look up different ways in which apparent superluminal motion might be generated, partly to reassure myself that the bottom hadn’t fallen out of relativity physics and partly to see if these things could be adequately explained in plain English. Here goes…

1) Cause and effect illusions
The faster than light pulsar story is essentially about hypothetical light booms – which are a bit like a sonic booms, where it’s not the sonic boom, but the sound source, that exceeds the speed of sound – so that individual sound pulses merge to form a single shock wave moving at the speed of sound.

Now, whether anything like this really happens with light from pulsars remains a point of debate, but one of the model’s proponents has demonstrated the effect in a laboratory – see this Scientific American blog post.

What you do is to arrange a line of light bulbs which are independently triggered. It’s easy enough to make them fire off in sequence – first 1, then 2, then 3 etc – and you can keep reducing the time delay between each one firing until you have a situation where bulb 2 fires off after bulb 1 in less time than light would need to travel the distance between bulbs 1 and 2. It’s just a trick really – there is no causal connection between the bulbs firing – but it looks as though a sequence of actions (first 1, then 2, then 3 etc) moved faster than light across the row of bulbs. This illusion is an example of apparent superluminal motion.

There are a range of possible scenarios as to why a superluminal Mexican wave of synchrotron radiation might emanate from different point sources around a rapidly rotating neutron star within an intense magnetic field. As long as the emanations from these point sources are not causally connected, this outcome does not violate relativity physics.

2) Making light faster than light
You can produce an apparent superluminal motion of light itself by manipulating its wavelength. If we consider a photon as a wave packet, that wave packet can be stretched linearly so that the leading edge of the wave arrives at its destination faster, since it is pushed ahead of the remainder of the wave – meaning that it travels faster than light.

However, the physical nature of ‘the leading edge of a wave packet’ is not clear. The whole wave packet is equivalent to one photon – and the leading edge of the stretched out wave packet cannot carry any significant information. Indeed, by being stretched out and attenuated, it may become indistinguishable from background noise.

Also this trick requires the light to be moving through a refractive medium, not a vacuum. If you are keen on the technical details, you can make phase velocity or group velocity faster than c (the speed of light in a vacuum) – but not signal velocity. In any case, since information (or the photon as a complete unit) is not moving faster than light, relativity physics is not violated.

3) Getting a kick out of gain media
You can mimic more dramatic superluminal motion through a gain medium where the leading edge of a light pulse stimulates the emission of a new pulse at the far end of the gain medium – as though a light pulse hits one end of a Newton’s Cradle and new pulse is projected out from the other end. If you want to see a laboratory set-up, try here. Although light appears to jump the gap superluminally, in fact it’s a new light pulse emerging at the other end – and still just moving at standard light speed.

Light faster than light. Left: Stretching the waveform of light can make the leading edge of the wave seem to move faster than light. Right: Gain media can act like a Newton's Cradle, making light seem to jump the gap superluminally.

4) The relativistic jet illusion
If an active galaxy, like M87, is pushing out a jet of superheated plasma moving at close to the speed of light – and the jet is roughly aligned with your line of sight from Earth – you can be fooled into thinking its contents are moving faster than light.

If that jet is 5,000 light years long, it should take at least 5,000 years for anything in it to cross that distance of 5,000 light years. A photon emitted by a particle of jet material at point A near the start of the jet really will take 5,000 years to reach you. But meanwhile, the particle of jet material continues moving towards you nearly as fast as that photon. So when the particle emits another photon at point B, a point near the tip of the jet – that second photon will reach your eye in much less than 5,000 years after the first photon, from point A. This will give you the impression that the particle crossed 5,000 light years from points A to B in much less than 5,000 years. But it is just an optical illusion – relativity physics remains unsullied.

5) Unknowable superluminal motion
It is entirely possible that objects beyond the horizon of the observable universe are moving away from our position faster than the speed of light – as a consequence of the universe’s cumulative expansion, which makes distant galaxies appear to move away faster than close galaxies. But since light from hypothetical objects beyond the observable horizon will never reach Earth, their existence is unknowable by direct observation from Earth – and does not represent a violation of relativity physics.

And lastly, not so much unknowable as theoretical is the notion of early cosmic inflation, which also involves an expansion of space-time rather than movement within space-time – so no violation there either.

Other stuff…
I’m not sure that the above is an exhaustive list and I have deliberately left out other theoretical proposals such as quantum entanglement and the Alcubierre warp drive. Either of these, if real, would arguably violate relativity physics – so perhaps need to be considered with a higher level of skepticism.

By  -      
Steve Nerlich is a very amateur Australian astronomer, publisher of the Cheap Astronomy website and the weekly Cheap Astronomy Podcasts and one of the team of volunteer explainers at Canberra Deep Space Communications Complex - part of NASA's Deep Space Network.



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Lawrence B. Crowell
Member
Lawrence B. Crowell
January 1, 2011 5:43 PM
Quantum entanglement does not communicate information. The no signalling theorem tells us that violations of the Bell inequalities are due to noncausal processes. There is no underlying classical process which sets up quantum strangeness and nonlocality. The motion of distant galaxies with z > 1 is faster than light. In fact the CMB has a z ~ 1000, which means this region of the universe is moving away at a thousand times the speed of light. We might get alarmed at this prospect, but in fact these galaxies are not moving with a velocity in particular, but are being frame dragged by the expansion of the universe. An observer who falls into a black hole travels faster than… Read more »
Super Earth
Member
Super Earth
January 2, 2011 3:41 PM

In this post:

http://www.universetoday.com/16698/warp-drive-and-cloaking-devices-not-just-science-fiction-anymore/

Was based on a paper:

“Warp Drive: A New Approach”
http://arxiv.org/abs/0712.1649

That proposes using vacuum energy originated from the casimir effect on circular extra dimensions. There seems not be disasters here from negative energy.

If extra dimensions are indeed large as ADD and RS models (to this model to work, extra dimension must be far larger than the Planck lenght), it is really an exciting possibility for future engineers!

By the way, vacuum energy (a.k.a. cosmological constant) doesn’t convert the entire universe (and any warp drive that exploits it) in a perpetual motion machine of the first kind (that is, that creates energy from nothing)?

damian
Member
January 1, 2011 6:27 PM
If sentience persists above our brief physical lives then you might not need FTL travel. A hypothetical eternal being has all the time in the universe. Humans have invented the notion as a kind of mental surrogate, its real value is not in the physical world, but in our capacity to imagine ourselves traveling at FTL. Certainly makes for good sci-fi The truth is that we will (probably) never be able to travel as biological entities through the vastness of the universe, however perhaps the undefined dark stuff we suspect exists could be a hint of a higher order of existence where such constraints are less stringent. One can imagine such things with as much impunity as FTL… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 2, 2011 10:30 AM
To Steve’s proposals I would add Oort cloud jumping. By using the local resources of stars Oort cloud comets we could eventually populate the entire galaxy. Hollowed out comets could be stationary or (preferably) movable. In fact, modulo that we need to learn the actual mining and habitation (likely including more fully close artificial biospheres, still only at some ~ 70 % material closure), I have been told that we have the necessary technology already. If you do the numbers apparently we wouldn’t have to wait for fusion reactors for energy, the amount of fissionable material in comets would suffice. Unless you have a specific show stopper (and some hundred meters of hydrogen stuffed ices is a good… Read more »
Greg
Member
Greg
January 1, 2011 8:42 PM
This is a pretty good simplification of a complex topic. I have made this argument before and not heard any satisfactory response yet, so here goes: If I wanted to set up a faster than light communicion system using phase velocity or group velocity componets of photons, it should be possible. All I would need is a detector set up to detect a phase shifted signal. The signal itself can be used as the message. The signal need not carry any information within itself. Using a codebook of pre-arranged sequences a series of phase shifted pulses could be sent in a certain order. The order of the signals would convey the message, there need not be any message… Read more »
Manu
Member
Manu
January 2, 2011 5:12 AM

No way.
Phase or group velocities only have meaning within an established wave.
You first need to create that wave, the front of which (your signal) doesn’t go FTL.

Throw a stone in a pond: the disturbance propagates outwards at signal speed, but wavelets within it ‘go faster’: they are born at the back of the wave, overtake it and die at the wavefront. Their speed cannot make any signal go faster than the disturbance reaches the opposite shore.

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 2, 2011 11:39 AM
Manu has the right of it. When you decompose an ideal “light on” intensity step function into Fourier components you see that you need an infinite number of them. Therefore, in reality you can’t see the leading edge go FTL for the same reason that the post already mentioned, and as Manu so vividly explains. The wiki links provided in the post makes a pretty good job covering all the caveats, so I hope they will provide your wished for satisfactory response. If not, perhaps you can work something better out with the wiki articles authors. Also, but this is now technical, fully using a predated code book isn’t actual information transfer of Shannon information at that moment.… Read more »
Prime
Member
Prime
January 2, 2011 8:55 AM

Centrifugal force, which is exponential in nature, when applied for propulsion, can overcome accelerative gravity, and the light speed barrier, according to the Russians.
http://community.webtv.net/hotmail.com/prime137/Exponential

Prime

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 2, 2011 10:20 AM
Wow, that was a fast presentation! A little too fast, since some technical errors crept in there. 1) The physical nature of the leading edge of a wave packet can be unclear in some cases (and the text tries to convey that), in other cases they can encode information like OAM modes. Or at least, that is AFAIU what some colleagues are trying to tell me. 2) The inflation discussion seems a bit confused. Whether or not inflation locally exceeds speed of light, summed over cosmical distances it does (or we wouldn’t have a cosmological horizon). None of these inflation effects are purely theoretical or worse, “unknowable” which IMO mean “not amenable to observation or theory”. They are… Read more »
Torbjorn Larsson OM
Member
Torbjorn Larsson OM
January 2, 2011 11:19 AM

Oh. I actually didn’t parse the claim on quantum entanglement correctly, since it is such a ludicrous (sorry!) claim even if it is based on a simple misunderstanding. We know that it exists and we know that it results in correlations (as described by Bell test experiments) and doesn’t involve causal signaling.

Anyway, it didn’t make my initial list but now I have commented above instead.

JohnHKMurrell
Member
JohnHKMurrell
January 2, 2011 12:00 PM
The article states ‘What you do is to arrange a line of light bulbs which are independently triggered. It’s easy enough to make them fire off in sequence – first 1, then 2, then 3 etc – and you can keep reducing the time delay between each one firing until you have a situation where bulb 2 fires off after bulb 1 in less time than light would need to travel the distance between bulbs 1 and 2.’ So how do you acheive this you can’t trigger them from the previous bulb as this information will only travel to you at the speed of light as a result you can’t reduce the time delay to below the speed… Read more »
Manu
Member
Manu
January 2, 2011 4:06 PM

“you then have problems with relativity as the clocks having been accelerated will all show different times”
In this thought experiment, nothing moves: the clocks would have no problems.
Easier yet, just connect all the bulbs to a central switch with equal length wires.

Manu
Member
Manu
January 2, 2011 4:25 PM
The main thing to understand about all this FTL business is that the ‘speed of light’ isn’t a ‘barrier’ such as the sound barrier. There is no hope of anything truly FTL because velocity > c is not impossible, it is meaningless. Speed as we understand it does not exist. Velocities don’t add. If you walk at 1 km/h in a 100 km/h train, you _won’t_ travel at 101 (nor 99) km/h. True, the difference is infinitesimal at those speeds, no more so at large fractions of c. The main basis for Special relativity is this: c is _constant_ for all observers, whatever their motion. Shoot a laser beam towards that Klingon vessel moving at 0.9c: all observers… Read more »
Greg
Member
Greg
January 2, 2011 8:27 PM
I do not believe my question should be so easily dismissed. This is a link to a review of the now classic Princeton Univ. experiment that I had in mind: http://www.iitk.ac.in/infocell/Archive/dirjuly3/science_light.html After reviewing the material, I will will provide a more succinct scenario for what I was thinking: A future military force is interested in protecting an important base from a laser attack, let’s call it the White House. They install a communications network around the area connected to light detectors serving as an early warning system. Let’s then say that a lossless anomalous dispersive medium is used (similar to that used in the Princeton experiment) within the cable network of the detection/warning system that is connected to… Read more »
Lawrence B. Crowell
Member
Lawrence B. Crowell
January 3, 2011 10:38 AM

There is a lot of confusion here. A linear wave will exhibit a dispersion that is itself linear. The example with water waves is a case of the Kortewig-deVreis-Boussenesque system of differential equations which are nonlinear. A light wave in vacuum is linear, and the signal it carries is strictly the speed of light. If there are nonlinear media the EM wave propagates through you can get some of these effect, but with a phase velocity that is less than the speed of light in vacuum.

LC

mick
Member
mick
January 4, 2011 5:41 AM

AFAIU, simply put, one should confuse with light speed (depending on the medium) and c (absolute light speed in a vacuum). Phase shift can be faster than the wave it travel within but not faster than c.

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