“Superluminal” implies “faster than light” speeds. This, according to physics, does not exist.
However, light packets can travel faster than the speed of light without violating Einstein’s holy theories.
Combine that with an idea proposed by Lord Rayleigh, and it becomes possible for something to look like it moves backwards in time.
It helps to briefly look at sound instead of light, which is what Lord Rayleigh did.
If a loud object moves towards an observer, it sounds like the pitch increases (and vice versa- an object moving away would sound lower). Here is an example.
This is known as the Doppler effect, and it is due to the source of the sound moving quickly enough such that the waves it emits are compressed. And because the distance between the wave crests is what determines pitch, compressing these waves increases the pitch.
The higher the frequency, the higher the pitch.
Lord Rayleigh suggested that something moving even faster than the speed of sound would outpace the sound waves, such that a stationary observer would hear the sound backwards.
Basically, if a particularly loud individual riding a rocket approached you at exactly twice the speed of sound screaming “HEEELP”, it would sound like they were screaming “PLEEEH” at exactly the same pitch and volume.
Since light behaves like a wave, the same logic should apply. If a television was strapped to a rocket moving towards you at twice the speed of light and it played a clip of someone walking, it would look like they were walking backwards.
The sound example can be tested because there are many examples of objects moving twice the speed of sound. Difficult, yes- imagine trying to hear Beethoven being played from a supersonic jet- but at least within the realm of possibility.
According to Einstein, however, nothing can move faster than the speed of light.
So why does the term “superluminal” even exist?
Under certain conditions, it is possible for a packet of light to travel faster than the speed of light without violating special relativity.
To illustrate, imagine a bathtub with a rubber duck and an angled screen.
Artist’s rendering of an overhead view of a bathtub
If a wave were created at the bottom of the screen, the duck would ride the wave, be pushed against the screen, and move in a diagonal line alongside it.
The duck will move in a diagonal line against the screen.
The speed of the duck is actually greater than the speed of the wave, because it is moving in a diagonal rather than a straight line.
The experimental setup used in the research of interest was very similar- involving a light source, a mirror, and a camera.
The green arrow represents the motion of the light wave
The corner of the wave as it hits the mirror, produces a spot on the mirror that is picked up by the camera. Since the wave itself moves at the speed of light, the spot will travel faster than the speed of light.
However, if the spot is moving fast enough, then it will appear to be moving backwards.
Normally, the camera will pick up the spot moving left to right. The sharper the angle of the mirror, the faster the spot will move.
Based on the mechanics of waves, if the angle of the mirror is sharp enough, then the spot will be moving twice the speed of light.
And if it does this, it will appear to move from right to left.
In fact, this is exactly what was observed.
On the left series (from top to bottom), the splotch moves from left to right. On the right series, with a sharper mirror angle, the splotch moves from right to left. Each image is a snapshot taken by the camera
Now, if one were to curve the mirror (such that the angle starts out steep and becomes shallower, and the spot starts out moving faster than the speed of light and ends up moving slower), you get an interesting effect.
A pair of images appears, with two spots starting at both the left and the right, which move together and annihilate each other.
With a concave mirror, two spots will be created at opposite ends, which will converge. With a convex mirror, the spots will appear in the middle and diverge.
But in actuality there is only one spot- it just travels at different speeds. What we are witnessing is two events that occur at different moments in time, at the same time.
We are either predicting the future, observing the past, or both.
Physics is weird.
Sources:
Clerici et al. (2016). Observation of image pair creation and annihilation from superluminal scattering sources. Science Advances: Vol 2:4, DOI: 10.1126/sciadv.1501691
Image of Doppler effect retrieved from http://www-inst.eecs.berkeley.edu/~eegsa/or/images/music1.png
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