*Written by OC Wanderer, author of Destiny Among the Stars, a LitRPG sci-fi series set in the real stellar neighborhood. Published August 31, 2026.*
[A shadow](https://ocwanderer.com/blog/is-darkness-faster-than-light) can outrun light because its moving edge is a pattern of separate events. Waves make the question more tempting: here are crests traveling through a real medium, with a speed we can measure. Could one of them carry something useful across the stars?
Suppose a crew could ride a crest that outruns light, hailing home before the light from their ship arrived. I would love that shortcut. The trouble begins when we ask what the crest would actually deliver, and it gets stranger when a laboratory pulse seems to leave a gas cell before its peak has even entered. There is plenty here to excite a science-fiction author, even if the message home has to take the usual slow route.
## An endless wave carries rhythm without news
Phase velocity is the speed at which one point of a wave's cycle, such as a crest, moves along. In some materials that speed is above *c*, the speed of light in a vacuum, and X-rays in most glasses are an example.<sup class="footnote-ref" data-footnote="true" id="fnref-1"><a class="footnote-link" href="#fn-1">1</a></sup> Picture a radio tone that has played forever and will play forever: an endless hum across the stars. Every crest looks like the one before it. A receiver can hear the rhythm, but which crest would announce that the crew had arrived? There is no distinctive moment in that perfectly repeating wave to mark the news.
To send news, the crew has to break that rhythm: switch the wave on, switch it off, alter it at a moment they choose. Such a disturbance is built from waves of different frequencies added together, and in a material they can travel at different crest speeds. The resulting pulse has its own pace, called the group velocity.<sup class="footnote-ref" data-footnote="true" id="fnref-2"><a class="footnote-link" href="#fn-2">2</a></sup> Following that pulse is a better way to look for a message, though, as the laboratory experiments show, even its peak can lead us astray.
In Book 1, Chapter 26, Luca's log for Day 40 records that Danny confirmed the 2.3-second signal near Proxima "isn't natural. It has structure." Structure in a wave means changes over time, and changes are what a message is made of.
## Satellite navigation delivers both speeds in one signal, and they differ
The odd thing is that we already live with these two speeds. A GPS satellite transmits a radio carrier wave with a timing code riding on it. Before reaching your receiver, the signal crosses the ionosphere, the layer of the upper atmosphere full of free electrons. That layer advances the carrier's crests and delays the code by the same amount, to first order: the phase travels faster than light, while the timing information falls behind.<sup class="footnote-ref" data-footnote="true" id="fnref-3"><a class="footnote-link" href="#fn-3">3</a></sup>
A receiver can track both. At the frequency of the GPS L1 signal, every 10¹⁶ electrons per square meter along the path shifts the measurement by about 16 centimeters, in opposite directions for the carrier and the code.<sup class="footnote-ref" data-footnote="true" id="fnref-4"><a class="footnote-link" href="#fn-4">4</a></sup> I love how practical this distinction becomes: the crests can race ahead, but the basic position fix still comes from the delayed code. Our everyday navigation already separates a fast-moving pattern from the news we need.
## A laboratory gas can make a whole pulse peak early
Now imagine watching a pulse peak leave a gas cell before its incoming peak has entered. That sounds like a wonderful problem for a ship's communications officer. In a 2000 experiment, a team sent a laser pulse 3.7 microseconds long through cesium vapor prepared to have a strongly reversed response across a narrow band of colors. The pulse's group velocity, the speed of its peak, came out negative, about −*c*/310. The peak left the cell 62 nanoseconds earlier than a pulse crossing the same distance in a vacuum would have. Light needs only about 0.2 nanoseconds to cross the cell, so the peak came out before the incoming peak had gone in.<sup class="footnote-ref" data-footnote="true" id="fnref-5"><a class="footnote-link" href="#fn-5">5</a></sup> The 62 nanoseconds are under 2 percent of the pulse's length: the smooth shape moved forward by a small slice of itself, enough to produce that startling order of events.
The authors explain the effect through classical interference between the pulse's different frequency components.<sup class="footnote-ref" data-footnote="true" id="fnref-6"><a class="footnote-link" href="#fn-6">6</a></sup> I picture the gradual rise of that smooth pulse giving the vapor a head start: light that has already arrived supplies the material from which an early peak can be rebuilt. A peak is different from a crest, but both tempt us to mistake an identifiable shape for a new message. What happens if the sender introduces a surprise the smooth rise could not have announced?
## Put an unexpected message on the beam
That surprise is the useful test for a crew trying to call home. A 2003 team started from the light-cone rule I used in the [shadow post](https://ocwanderer.com/blog/is-darkness-faster-than-light): no signal can cause an effect at a place that light from its source could not yet have reached, because breaking the rule would let an effect precede its cause.<sup class="footnote-ref" data-footnote="true" id="fnref-7"><a class="footnote-link" href="#fn-7">7</a></sup> They prepared potassium vapor in which pulses had a group velocity above *c*, encoded a "1" or a "0" on the beam, and compared its arrival with light crossing an equal distance in a vacuum.<sup class="footnote-ref" data-footnote="true" id="fnref-8"><a class="footnote-link" href="#fn-8">8</a></sup>
The smooth pulse did run ahead, by about a tenth of a pulse width. The instant when the 1s and 0s begin to differ did not seem to speed up. Detecting a new bit through the medium took slightly longer than detecting it through a vacuum, even though the medium's group velocity vastly exceeded *c*.<sup class="footnote-ref" data-footnote="true" id="fnref-9"><a class="footnote-link" href="#fn-9">9</a></sup> That is the distinction I would want the crew to notice before trusting their miraculous radio: the pulse peak gets a head start, but their choice of message does not. A sudden switch from "0" to "1" cannot be predicted from the smooth wave before it. A calculation from 1914 puts the very first edge of a pulse, the front, at *c*;<sup class="footnote-ref" data-footnote="true" id="fnref-10"><a class="footnote-link" href="#fn-10">10</a></sup> later experimental work supports the role of abrupt changes as carriers of new information.<sup class="footnote-ref" data-footnote="true" id="fnref-11"><a class="footnote-link" href="#fn-11">11</a></sup>
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In Book 1, Chapter 15, Luca reports a hijack attempt to his father on Genesis Platform, and Athan answers after a pause the text calls "the light-lag stretching the silence." I can imagine wanting to grab one of those fast crests and hurry an answer along. The urgency of a message makes the difference between a repeating rhythm and fresh news matter all the more.
## The crest gets ahead; the crew still has to wait
A crest marks a repeating position in a wave, while a message needs a change. That leaves the crew I imagined with a tantalizing fast-moving pattern and the familiar wait for a reply. What fascinates me is how much strangeness survives that limit: even a pulse emerging early can be real physics, without giving us a shortcut for the news we most want to send.
With shadows and crests already on the list, I wonder what other things physics calls faster than light, and whether any come closer to something a ship could use: [what are all the things that can "move" faster than light without breaking physics?](https://ocwanderer.com/blog/what-are-all-the-things-that-can-move-faster-than-light-without-breaking-physics)
> [!lore] If you want to follow a crew talking to home across the light-lag, [start *Destiny Among the Stars*](https://ocwanderer.com/storytime/story/destiny-among-the-stars).
## Endnotes
1. <span id="fn-1"></span> Wikipedia, [“Faster-than-light”](https://en.wikipedia.org/wiki/Faster-than-light), section on phase velocity above *c*. **Context.** The phase velocity of an electromagnetic wave in a medium can exceed *c*, for example in most glasses at X-ray frequencies, and a phase velocity above *c* does not imply signals above *c* because a purely monochromatic component cannot convey information. <a class="footnote-backref" href="#fnref-1" aria-label="Back to reference 1">↩</a>
2. <span id="fn-2"></span> Wikipedia, [“Phase velocity”](https://en.wikipedia.org/wiki/Phase_velocity). **Context.** Phase velocity is ω/k; in a dispersive medium it depends on frequency, and group velocity follows v_g = c / (n + ω∂n/∂ω). That a message-carrying change is built from many frequencies is the standard Fourier account and is the author's wording. <a class="footnote-backref" href="#fnref-2" aria-label="Back to reference 2">↩</a>
3. <span id="fn-3"></span> European Space Agency, GNSS Science Support Centre, [“Ionospheric Delay”](https://gssc.esa.int/navipedia/index.php/Ionospheric_Delay), equations 7 and 8: n_ph = 1 − (40.3/f²)N_e and n_gr = 1 + (40.3/f²)N_e. An index below 1 means a phase velocity above *c*. The page states that the phase travels faster than the speed of light but no information is carried, so relativity is not violated. <a class="footnote-backref" href="#fnref-3" aria-label="Back to reference 3">↩</a>
4. <span id="fn-4"></span> ESA, [“Ionospheric Delay”](https://gssc.esa.int/navipedia/index.php/Ionospheric_Delay): 1 TECU = 10¹⁶ electrons/m²; 0.1 TECU is about 1.6 cm of delay on GPS L1 (1575.42 MHz), so 1 TECU is about 16 cm; phase measurements suffer advancement (a negative delay) and code measurements a positive delay; because the effect depends on the inverse square of the frequency, a receiver using two frequencies can remove it up to more than 99.9 percent. <a class="footnote-backref" href="#fnref-4" aria-label="Back to reference 4">↩</a>
5. <span id="fn-5"></span> L. J. Wang, A. Kuzmich, and A. Dogariu, [“Gain-assisted superluminal light propagation,”](https://doi.org/10.1038/35018520) *Nature* 406 (2000): 277–279. **Primary research.** Gain-assisted linear anomalous dispersion in cesium gas; negative group velocity of −*c*/310. The 3.7 microsecond pulse duration, the 62 ns advance and the 0.2 ns vacuum transit are from the press account at [The NESS, “Speed of Light Repealed?”](https://theness.com/index.php/speed-of-light-repealed/). Author calculation: 62 ns ÷ 3,700 ns = 1.7 percent. <a class="footnote-backref" href="#fnref-5" aria-label="Back to reference 5">↩</a>
6. <span id="fn-6"></span> The authors' statement, quoted in [The NESS](https://theness.com/index.php/speed-of-light-repealed/): the observed superluminal light pulse propagation is not at odds with causality, being a direct consequence of classical interference between its different frequency components. The reading that the early peak is produced from the pulse's rise is the author's own reasoning, as is the reading of the 0-to-1 switch in the Stenner section; the light-crossing inference (0.2 ns against 62 ns) is arithmetic on the reported figures. <a class="footnote-backref" href="#fnref-6" aria-label="Back to reference 6">↩</a>
7. <span id="fn-7"></span> M. D. Stenner, D. J. Gauthier, and M. A. Neifeld, [“The speed of information in a ‘fast-light’ optical medium,”](https://doi.org/10.1038/nature02016) *Nature* 425 (2003): 695–698. **Primary research.** From the abstract: no signal can cause an effect outside the source light cone, and violating that principle leads to an effect preceding its cause. <a class="footnote-backref" href="#fnref-7" aria-label="Back to reference 7">↩</a>
8. <span id="fn-8"></span> Physics World, [“Clear message for causality”](https://physicsworld.com/a/clear-message-for-causality/): a realistic signal representing a "1" or a "0" encoded on the light beams, in a sample of potassium vapour. <a class="footnote-backref" href="#fnref-8" aria-label="Back to reference 8">↩</a>
9. <span id="fn-9"></span> Stenner, Gauthier, and Neifeld (2003), abstract: the time to detect information through a fast-light medium is slightly longer than through a vacuum, even though the group velocity vastly exceeds *c*. Physics World (same page): the smooth pulse advance of about a tenth of a pulse width, and the instant the 1s and 0s begin to differ does not seem to be accelerated. <a class="footnote-backref" href="#fnref-9" aria-label="Back to reference 9">↩</a>
10. <span id="fn-10"></span> A. Sommerfeld, “Über die Fortpflanzung des Lichtes in dispergierenden Medien,” *Annalen der Physik* 44 (1914): 177–202; L. Brillouin, same title, *Annalen der Physik* 44 (1914): 203–240. Front velocity: [Wikipedia, “Front velocity”](https://en.wikipedia.org/wiki/Front_velocity) (the earliest appearance of an electromagnetic disturbance travels at *c*); the 1914 attribution and page numbers are from a search result and [Springer, “On the nature of the Sommerfeld–Brillouin forerunners”](https://link.springer.com/article/10.1007/s40509-019-00210-9). <a class="footnote-backref" href="#fnref-10" aria-label="Back to reference 10">↩</a>
11. <span id="fn-11"></span> “Causality and information transfer in simultaneously slow- and fast-light media,” [arXiv:1709.09244](https://arxiv.org/abs/1709.09244) (2017). Abstract: four-wave mixing in warm potassium vapor; the information velocity is constrained to be at or less than *c*; the results support the concept that points of non-analyticity on optical pulses correspond to carriers of new information. <a class="footnote-backref" href="#fnref-11" aria-label="Back to reference 11">↩</a>