Shadows can travel faster than light, but cannot carry matter or information
Is superluminal flight possible?
Sections in this article have subheadings and you can refer to each section separately.
Simple examples of FTL travel
1. Cherenkov effect
When we talk about superluminal motion, we mean the speed of light in a vacuum. c(299 792 458 m/s). Therefore, the Cherenkov effect cannot be considered as an example of superluminal motion.
2. Third observer
If the rocket A flies away from me with speed 0.6c to the west, and the rocket B flies away from me with speed 0.6c east, then I see that the distance between A And B increases with speed 1.2c. Watching the missiles fly A And B from the outside, the third observer sees that the total removal velocity of the missiles is greater than c .
but relative speed is not equal to the sum of the speeds. rocket speed A regarding the rocket B is the rate at which the distance to the rocket increases A, which is seen by an observer flying on a rocket B. Relative velocity must be calculated using the relativistic velocity addition formula. (See How do You Add Velocities in Special Relativity?) In this example, the relative velocity is approximately 0.88c. So in this example we didn't get FTL.
3. Light and shadow
Think about how fast the shadow can move. If the lamp is close, then the shadow of your finger on the far wall moves much faster than the finger moves. When moving the finger parallel to the wall, the speed of the shadow in D/d times greater than the speed of a finger. Here d is the distance from the lamp to the finger, and D- from the lamp to the wall. The speed will be even greater if the wall is at an angle. If the wall is very far away, then the movement of the shadow will lag behind the movement of the finger, since the light takes time to reach the wall, but the speed of the shadow moving along the wall will increase even more. The speed of a shadow is not limited by the speed of light.
Another object that can travel faster than light is a spot of light from a laser aimed at the moon. The distance to the Moon is 385,000 km. You can calculate the speed of movement of the light spot on the surface of the Moon by yourself with small fluctuations of the laser pointer in your hand. You might also like the example of a wave hitting a straight line of beach at a slight angle. With what speed can the point of intersection of the wave and the shore move along the beach?
All these things can happen in nature. For example, a beam of light from a pulsar can run along a dust cloud. A powerful explosion can create spherical waves of light or radiation. When these waves intersect with a surface, circles of light appear on that surface and expand faster than light. Such a phenomenon is observed, for example, when an electromagnetic pulse from a lightning flash passes through the upper atmosphere.
4. Solid body
If you have a long, rigid rod and you hit one end of the rod, doesn't the other end immediately move? Is this not a way of superluminal transmission of information?
That would be right if there were perfectly rigid bodies. In practice, the impact is transmitted along the rod at the speed of sound, which depends on the elasticity and density of the rod material. In addition, the theory of relativity limits the possible speeds of sound in a material by the value c .
The same principle applies if you hold a string or rod vertically, release it, and it begins to fall under the influence of gravity. The top end you let go starts to fall immediately, but the bottom end will only start moving after a while, as the loss of the holding force is transmitted down the rod at the speed of sound in the material.
The formulation of the relativistic theory of elasticity is rather complicated, but the general idea can be illustrated using Newtonian mechanics. The equation of longitudinal motion of an ideally elastic body can be derived from Hooke's law. Denote the linear density of the rod ρ , Young's modulus Y. Longitudinal offset X satisfies the wave equation
ρ d 2 X/dt 2 - Y d 2 X/dx 2 = 0
Plane wave solution travels at the speed of sound s, which is determined from the formula s 2 = Y/ρ. The wave equation does not allow the perturbations of the medium to move faster than with the speed s. In addition, the theory of relativity gives a limit to the amount of elasticity: Y< ρc 2 . In practice, no known material approaches this limit. Note also that even if the speed of sound is close to c, then the matter itself does not necessarily move with relativistic speed.
Although there are no solid bodies in nature, there is motion of rigid bodies, which can be used to overcome the speed of light. This topic belongs to the already described section of shadows and light spots. (See The Superluminal Scissors, The Rigid Rotating Disk in Relativity).
5. Phase velocity
wave equation
d 2 u/dt 2 - c 2 d 2 u/dx 2 + w 2 u = 0
has a solution in the form
u \u003d A cos (ax - bt), c 2 a 2 - b 2 + w 2 \u003d 0
These are sinusoidal waves propagating at a speed v
v = b/a = sqrt(c 2 + w 2 /a 2)
But it's more than c. Maybe this is the equation for tachyons? (see section below). No, this is the usual relativistic equation for a particle with mass.
To eliminate the paradox, you need to distinguish between "phase velocity" v ph , and "group velocity" v gr , and
v ph v gr = c 2
The solution in the form of a wave may have dispersion in frequency. In this case, the wave packet moves with a group velocity that is less than c. Using a wave packet, information can only be transmitted at the group velocity. Waves in a wave packet move with phase velocity. Phase velocity is another example of FTL motion that cannot be used to communicate.
6. Superluminal galaxies
7. Relativistic rocket
Let an observer on Earth see a spacecraft moving away at a speed 0.8c According to the theory of relativity, he will see that the clock on the spacecraft is running 5/3 times slower. If we divide the distance to the ship by the time of flight according to the onboard clock, we get the speed 4/3c. The observer concludes that, using his on-board clock, the pilot of the ship will also determine that he is flying at a superluminal speed. From the pilot's point of view, his clock is running normally, and interstellar space has shrunk by a factor of 5/3. Therefore, it flies the known distances between the stars faster, at a speed 4/3c .
Time dilation is a real effect that could in principle be used in space travel to cover great distances in a short amount of time from the point of view of astronauts. At a constant acceleration of 1g, astronauts will not only have a comfortable artificial gravity, but will also be able to traverse the galaxy in just 12 years of proper time. During the journey, they will age by 12 years.
But it's still not superluminal flight. You can't calculate speed using distance and time defined in different frames of reference.
8. Gravity speed
Some insist that the speed of gravity is much faster c or even infinite. See Does Gravity Travel at the Speed of Light? and What is Gravitational Radiation? Gravitational perturbations and gravitational waves propagate at a speed c .
9. EPR paradox
10. Virtual photons
11. Quantum tunnel effect
In quantum mechanics, the tunnel effect allows a particle to overcome a barrier, even if its energy is not enough for this. It is possible to calculate the tunneling time through such a barrier. And it may turn out to be less than what is required for light to overcome the same distance at a speed c. Can it be used to send messages faster than light?
Quantum electrodynamics says "No!" Nevertheless, an experiment was carried out that demonstrated the superluminal transmission of information using the tunnel effect. Through a barrier 11.4 cm wide at a speed of 4.7 c Mozart's Fortieth Symphony was presented. The explanation for this experiment is very controversial. Most physicists believe that with the help of the tunnel effect it is impossible to transmit information faster than light. If it were possible, then why not send a signal to the past by placing the equipment in a rapidly moving frame of reference.
17. Quantum field theory
With the exception of gravity, all observed physical phenomena correspond to the "Standard Model". The Standard Model is a relativistic quantum field theory that explains the electromagnetic and nuclear forces and all known particles. In this theory, any pair of operators corresponding to physical observables separated by a spacelike interval of events "commutes" (that is, one can change the order of these operators). In principle, this implies that in the Standard Model the force cannot travel faster than light, and this can be considered the quantum field equivalent of the infinite energy argument.
However, there are no impeccably rigorous proofs in the quantum field theory of the Standard Model. No one has yet even proven that this theory is internally consistent. Most likely, it is not. In any case, there is no guarantee that there are no yet undiscovered particles or forces that do not obey the ban on superluminal movement. There is also no generalization of this theory, including gravity and general relativity. Many physicists working in the field of quantum gravity doubt that the simple concepts of causality and locality will be generalized. There is no guarantee that in a future more complete theory the speed of light will retain the meaning of the limiting speed.
18. Grandpa Paradox
In special relativity, a particle traveling faster than light in one frame of reference moves back in time in another frame of reference. FTL travel or information transmission would make it possible to travel or send a message to the past. If such time travel were possible, then you could go back in time and change the course of history by killing your grandfather.
This is a very strong argument against the possibility of FTL travel. True, there remains an almost improbable possibility that some limited superluminal travel is possible, which does not allow a return to the past. Or maybe time travel is possible, but causality is violated in some consistent way. All this is very implausible, but if we are discussing FTL, it is better to be ready for new ideas.
The reverse is also true. If we could travel back in time, we could overcome the speed of light. It is possible to go back in time, fly somewhere at low speed, and arrive there before the light sent in the usual way arrives. See Time Travel for details on this topic.
Open questions of FTL travel
In this last section, I will describe some serious ideas about possible faster-than-light travel. These topics are not often included in the FAQ, because they are more like a lot of new questions than answers. They are included here to show that serious research is being done in this direction. Only a short introduction to the topic is given. Details can be found on the Internet. As with everything on the Internet, be critical of them.
19. Tachyons
Tachyons are hypothetical particles that travel faster than light locally. To do this, they must have an imaginary mass value. In this case, the energy and momentum of the tachyon are real quantities. There is no reason to believe that superluminal particles cannot be detected. Shadows and highlights can travel faster than light and can be detected.
So far, tachyons have not been found, and physicists doubt their existence. There were claims that in experiments to measure the mass of neutrinos produced by the beta decay of tritium, neutrinos were tachyons. This is doubtful, but has not yet been definitively refuted.
There are problems in the theory of tachyons. In addition to possibly violating causality, tachyons also make the vacuum unstable. It may be possible to circumvent these difficulties, but even then we will not be able to use tachyons for superluminal transmission of messages.
Most physicists believe that the appearance of tachyons in a theory is a sign of some problems with this theory. The idea of tachyons is so popular with the public simply because they are often mentioned in fantasy literature. See Tachyons.
20. Wormholes
The most famous method of global FTL travel is the use of "wormholes". A wormhole is a slit in space-time from one point in the universe to another, which allows you to get from one end of the hole to the other faster than the usual path. Wormholes are described by the general theory of relativity. To create them, you need to change the topology of space-time. Maybe this will become possible within the framework of the quantum theory of gravity.
To keep a wormhole open, you need areas of space with negative energies. C.W.Misner and K.S.Thorne proposed to use the Casimir effect on a large scale to create negative energy. Visser suggested using cosmic strings for this. These are very speculative ideas and may not be possible. Maybe the required form of exotic matter with negative energy does not exist.
Dedicated to direct measurement of the speed of neutrinos. The results sound sensational: the speed of the neutrino turned out to be slightly - but statistically significant! - more than the speed of light. The collaboration article contains an analysis of various sources of errors and uncertainties, however, the reaction of the vast majority of physicists remains very skeptical, primarily because such a result does not agree with other experimental data on the properties of neutrinos.
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Experiment Details
The idea of the experiment (see OPERA experiment) is very simple. The neutrino beam is born at CERN, flies through the Earth to the Italian laboratory Gran Sasso and passes through a special OPERA neutrino detector there. Neutrinos interact very weakly with matter, but due to the fact that their flux from CERN is very large, some neutrinos still collide with atoms inside the detector. There they generate a cascade of charged particles and thus leave their signal in the detector. Neutrinos at CERN are not born continuously, but in "bursts", and if we know the moment of birth of a neutrino and the moment of its absorption in the detector, as well as the distance between the two laboratories, we can calculate the speed of the neutrino.
The distance between the source and the detector in a straight line is about 730 km and it was measured with an accuracy of 20 cm (the exact distance between the reference points is 730534.61 ± 0.20 meters). True, the process leading to the birth of a neutrino is not at all localized with such accuracy. At CERN, a beam of high-energy protons flies out of the SPS accelerator, is dropped onto a graphite target and generates secondary particles in it, including mesons. They continue to fly forward at near-light speed and decay into muons on the fly with the emission of neutrinos. Muons also decay and give rise to additional neutrinos. Then all particles, except for neutrinos, are absorbed in the thickness of the substance, and they freely reach the place of detection. The general scheme of this part of the experiment is shown in fig. one.
The entire cascade leading to the appearance of a neutrino beam can stretch for hundreds of meters. However, since all the particles in this bunch fly forward at near-light speed, for the detection time there is practically no difference whether the neutrino was born immediately or after a kilometer of the way (however, it is of great importance when exactly the original proton that led to the birth of this neutrino flew out of the accelerator). As a result, the produced neutrinos by and large simply repeat the profile of the original proton beam. Therefore, the key parameter here is precisely the time profile of the proton beam emitted from the accelerator, in particular, the exact position of its leading and trailing edges, and this profile is measured with good time s m resolution (see Fig. 2).
Each session of dropping a proton beam onto a target (in English such a session is called spill, "splash") lasts about 10 microseconds and leads to the birth of a huge number of neutrinos. However, almost all of them fly through the Earth (and the detector) without interaction. In the same rare cases when the detector does register a neutrino, it is impossible to say at what exact moment during the 10-microsecond interval it was emitted. The analysis can be carried out only statistically, that is, to accumulate many cases of neutrino detection and construct their time distribution relative to the starting point for each session. In the detector, the point of time is taken as the origin when the conditional signal moving at the speed of light and emitted exactly at the moment of the leading edge of the proton beam reaches the detector. Accurate measurement of this moment was made possible by synchronizing the clocks in the two laboratories to within a few nanoseconds.
On fig. 3 shows an example of such a distribution. The black dots are real neutrino data recorded by the detector and summed over a large number of sessions. The red curve shows a conventional "reference" signal that would move at the speed of light. You can see that the data starts at about 1048.5 ns. before reference signal. This, however, does not yet mean that the neutrino is actually ahead of the light by a microsecond, but is only a reason to carefully measure all cable lengths, equipment response speeds, electronics delay times, and so on. This recheck was done and found to shift the "reference" moment by 988 ns. Thus, it turns out that the neutrino signal actually outruns the reference one, but only by about 60 nanoseconds. In terms of the neutrino speed, this corresponds to an excess of the speed of light by about 0.0025%.
The error of this measurement was estimated by the authors of the analysis at 10 nanoseconds, which includes both statistical and systematic errors. Thus, the authors claim that they "see" the superluminal movement of neutrinos at a statistical confidence level of six standard deviations.
The difference between the results and expectations by six standard deviations is already quite large and is called in elementary particle physics the loud word "discovery". However, this number must be understood correctly: it only means that the probability statistical fluctuations in the data is very small, but does not indicate how reliable the data processing technique is and how well physicists have taken into account all instrumental errors. After all, there are many examples in elementary particle physics where unusual signals with exceptionally high statistical certainty have not been confirmed by other experiments.
What do superluminal neutrinos contradict?
Contrary to popular belief, special relativity does not in itself prohibit the existence of particles moving at superluminal speeds. However, for such particles (they are generally called "tachyons"), the speed of light is also a limit, but only from below - they cannot move slower than it. In this case, the dependence of the energy of particles on the speed turns out to be inverse: the greater the energy, the closer the speed of tachyons to the speed of light.
Much more serious problems begin in quantum field theory. This theory replaces quantum mechanics when it comes to quantum particles with high energies. In this theory, particles are not points, but, relatively speaking, clumps of the material field, and they cannot be considered separately from the field. It turns out that tachyons lower the energy of the field, which means they make the vacuum unstable. It is then more profitable for the void to spontaneously break up into a huge number of these particles, and therefore it is simply meaningless to consider the movement of one tachyon in ordinary empty space. We can say that a tachyon is not a particle, but an instability of the vacuum.
In the case of tachyon-fermions, the situation is somewhat more complicated, but even there, comparable difficulties arise that hinder the creation of a self-consistent tachyon quantum field theory, which includes the usual theory of relativity.
However, this is also not the last word in theory. Just as experimenters measure everything that can be measured, theorists also test all possible hypothetical models that do not contradict the available data. In particular, there are theories in which a slight, not yet noticed deviation from the postulates of the theory of relativity is allowed - for example, the speed of light itself can be a variable. Such theories do not yet have direct experimental support, but they have not yet been closed.
This brief sketch of the theoretical possibilities can be summed up as follows: despite the fact that in some theoretical models the movement with superluminal speed is possible, they remain only hypothetical constructions. All currently available experimental data are described by standard theories without superluminal motion. Therefore, if it were reliably confirmed for at least some particles, quantum field theory would have to be radically redone.
Is it worth considering the result of OPERA in this sense as the "first sign"? Not yet. Perhaps the most important reason for skepticism is the fact that the OPERA result does not agree with other experimental data on neutrinos.
First, during the famous supernova SN1987A, neutrinos were also registered, which arrived a few hours before the light pulse. This does not mean that neutrinos traveled faster than light, but only reflects the fact that neutrinos are emitted at an earlier stage of the collapse of the nucleus during a supernova explosion than light. However, since neutrinos and light, having spent 170,000 years on the road, did not separate for more than a few hours, it means that their speeds are very close and differ by no more than billionths. The OPERA experiment shows a thousand times stronger discrepancy.
Here, of course, we can say that neutrinos produced during supernova explosions and CERN neutrinos differ greatly in energy (several tens of MeV in supernovae and 10–40 GeV in the described experiment), and the neutrino velocity varies depending on energy. But this change in this case works in the “wrong” direction: after all, the higher the energy of tachyons, the closer their speed should be to the speed of light. Of course, even here one can come up with some kind of modification of the tachyon theory, in which this dependence would be completely different, but in this case it will be necessary to discuss the “double-hypothetical” model.
Further, from the multitude of experimental data on neutrino oscillations obtained in recent years, it follows that the masses of all neutrinos differ from each other only by fractions of an electronvolt. If the result of OPERA is perceived as a manifestation of the superluminal motion of a neutrino, then the value of the square of the mass of at least one neutrino will be of the order of –(100 MeV) 2 (the negative square of the mass is the mathematical manifestation of the fact that the particle is considered a tachyon). Then you have to admit that all varieties of neutrinos are tachyons and have approximately the same mass. On the other hand, direct measurement of the neutrino mass in the beta decay of tritium nuclei shows that the neutrino mass (modulo) should not exceed 2 electron volts. In other words, it will not be possible to reconcile all these data with each other.
The conclusion from this can be drawn as follows: the declared result of the OPERA collaboration is difficult to fit into any, even the most exotic, theoretical models.
What's next?
In all large collaborations in elementary particle physics, it is normal practice for each specific analysis to be performed by a small group of participants, and only then the results be submitted for general discussion. In this case, apparently, this stage was too short, as a result of which not all the participants in the collaboration agreed to put their signature under the article (the full list includes 216 participants in the experiment, and the preprint has only 174 authors). Therefore, in the near future, most likely, many additional checks will be carried out within the collaboration, and only after that the article will be sent to print.
Of course, now one can also expect a stream of theoretical papers with various exotic explanations of this result. However, until the claimed result is reliably rechecked, it cannot be considered a full-fledged discovery.
A team of scientists from the OPERA experiment, in collaboration with the European Organization for Nuclear Research (CERN), has published sensational results from an experiment to overcome the speed of light. The results of the experiment refute Albert Einstein's special theory of relativity, on which all modern physics is based. The theory says that the speed of light is 299,792,458 m/s, and elementary particles cannot move faster than the speed of light.
Nevertheless, scientists recorded its excess by a neutrino beam by 60 nanoseconds when overcoming 732 km. This happened on September 22 during an experiment conducted by an international group of nuclear physicists from Italy, France, Russia, Korea, Japan and other countries.
The experiment proceeded as follows: a proton beam was accelerated in a special accelerator and hit with it at the center of a special target. This is how mesons were born - particles consisting of quarks.
During the decay of mesons, neutrinos are born, - Academician of the Russian Academy of Sciences Valery Rubakov, chief researcher at the Institute for Nuclear Research of the Russian Academy of Sciences, explained to Izvestia. - The beam is positioned so that the neutrino flies 732 km and hits the Italian underground laboratory in Gran Sasso. It has a special detector that records the speed of the neutrino beam.
The results of the study split the scientific world. Some scientists refuse to believe the results.
What they did at CERN is impossible from the modern standpoint of physics, - Academician of the Russian Academy of Sciences Spartak Belyaev, scientific director of the Institute of General and Nuclear Physics, told Izvestia. - It is necessary to check this experiment and its results - perhaps they were simply mistaken. All the experiments carried out before that fit into the existing theory, and because of one once conducted experiment, it is not worth raising a panic.
At the same time, Academician Belyaev admits that if it is possible to prove that neutrinos can move faster than the speed of light, this will be a revolution.
We then have to break all the physics, he said.
If the results are confirmed, this is a revolution, Academician Rubakov agrees. - It is difficult to say how it will turn out for the townsfolk. In general, of course, it is possible to change the special theory of relativity, but it is extremely difficult to do this, and it is not entirely clear which theory will crystallize as a result.
Rubakov drew attention to the fact that the report states that over the three years of the experiment, 15,000 events were recorded and measured.
The statistics are very good, and an international group of reputable scientists participated in the experiment,” Rubakov sums up.
Academicians stressed that the world is regularly attempting to experimentally refute the special theory of relativity. However, none of them has given positive results so far.
In September 2011, physicist Antonio Ereditato shocked the world. His statement could have turned our understanding of the universe upside down. If the data collected by the 160 scientists of the OPERA project were correct, the unbelievable was observed. The particles - in this case neutrinos - were moving faster than light. According to Einstein's theory of relativity, this is impossible. And the consequences of such an observation would be incredible. Perhaps the very foundations of physics would have to be revised.
Although Ereditato said that he and his team were "extremely confident" in their results, they did not say that the data was completely accurate. Instead, they asked other scientists to help them figure out what was going on.
In the end, it turned out that OPERA's results were wrong. A badly connected cable caused a synchronization problem and the signals from the GPS satellites were inaccurate. There was an unexpected delay in the signal. As a result, measurements of the time it took the neutrino to overcome a certain distance showed an extra 73 nanoseconds: it seemed that the neutrinos flew faster than light.
Despite months of careful checking before the start of the experiment and rechecking the data afterwards, the scientists were seriously mistaken. Ereditato resigned, despite the remarks of many that such errors always occurred due to the extreme complexity of particle accelerators.
Why would the suggestion - the mere suggestion - that something could travel faster than light cause such a fuss? How sure are we that nothing can overcome this barrier?
Let's deal with the second of these questions first. The speed of light in a vacuum is 299,792.458 kilometers per second - for convenience, this number is rounded up to 300,000 kilometers per second. It's quite fast. The Sun is 150 million kilometers from the Earth, and the light from it reaches the Earth in just eight minutes and twenty seconds.
Can any of our creations compete in the race with light? One of the fastest man-made objects ever built, the New Horizons space probe whizzed past Pluto and Charon in July 2015. He reached a speed relative to the Earth of 16 km / s. Much less than 300,000 km/s.
However, we had tiny particles that were moving quite fast. In the early 1960s, William Bertozzi at MIT experimented with accelerating electrons to even higher speeds.
Since electrons have a negative charge, they can be accelerated—more specifically, repelled—by applying the same negative charge to a material. The more energy is applied, the faster the electrons accelerate.
One would think that one would simply need to increase the applied energy in order to accelerate to a speed of 300,000 km/s. But it turns out that electrons just can't move that fast. Bertozzi's experiments showed that the use of more energy does not lead to a directly proportional increase in the speed of the electrons.
Instead, huge amounts of additional energy had to be applied to change the speed of the electrons even slightly. It got closer and closer to the speed of light, but never reached it.
Imagine moving towards the door in small steps, each one covering half the distance from your current position to the door. Strictly speaking, you will never reach the door, because after each step you take, you will have a distance to overcome. Bertozzi faced a similar problem when dealing with his electrons.
But light is made up of particles called photons. Why can these particles move at the speed of light, but electrons cannot?
"As objects go faster and faster, they get heavier - the heavier they get, the harder it is for them to accelerate, so you never reach the speed of light," says Roger Russoul, a physicist at the University of Melbourne in Australia. “A photon has no mass. If it had mass, it couldn't move at the speed of light."
Photons are special. They not only lack mass, which provides them with complete freedom of movement in the vacuum of space, they also do not need to accelerate. The natural energy they have at their disposal moves in waves, just like them, so at the time of their creation, they already have maximum speed. In some ways, it's easier to think of light as energy rather than as a stream of particles, although, in truth, light is both.
However, light travels much more slowly than we might expect. While internet techs like to talk about communications that run "at the speed of light" in fiber, light travels 40% slower in fiber glass than it does in a vacuum.
In reality, the photons travel at 300,000 km/s, but they encounter a certain amount of interference caused by other photons that are emitted by the glass atoms as the main light wave passes. It may not be easy to understand, but at least we tried.
In the same way, within the framework of special experiments with individual photons, it was possible to slow them down quite impressively. But for most cases, 300,000 will be true. We have not seen or created anything that could move as fast or even faster. There are special points, but before we touch on them, let's touch on our other issue. Why is it so important that the speed of light rule be followed strictly?
The answer has to do with the person named, as is often the case in physics. His special theory of relativity explores the many implications of his universal speed limits. One of the most important elements of the theory is the idea that the speed of light is constant. No matter where you are or how fast you are moving, light always travels at the same speed.
But this raises several conceptual problems.
Imagine the light that falls from a flashlight onto a mirror on the ceiling of a stationary spacecraft. The light goes up, reflects off the mirror, and falls on the floor of the spacecraft. Let's say he covers a distance of 10 meters.
Now imagine that this spacecraft starts moving at an enormous speed of many thousands of kilometers per second. When you turn on the flashlight, the light behaves as before: it shines upward, hits the mirror, and reflects on the floor. But to do this, the light has to travel diagonally, not vertically. After all, the mirror is now rapidly moving along with the spacecraft.
Accordingly, the distance that the light overcomes increases. Let's say 5 meters. It turns out 15 meters in general, not 10.
And despite this, although the distance has increased, Einstein's theories state that light will still travel at the same speed. Since speed is distance divided by time, since the speed has remained the same and the distance has increased, the time must also increase. Yes, time itself must stretch. And although it sounds strange, it has been experimentally confirmed.
This phenomenon is called time dilation. Time moves more slowly for people who move in fast moving vehicles relative to those who are stationary.
For example, time is 0.007 seconds slower for astronauts on the International Space Station, which travels at 7.66 km/s relative to the Earth, compared to humans on the planet. Even more interesting is the situation with particles like the aforementioned electrons, which can move close to the speed of light. In the case of these particles, the degree of deceleration will be enormous.
Stephen Kolthammer, an experimental physicist at the University of Oxford in the UK, points to the example of particles called muons.
Muons are unstable: they quickly decay into simpler particles. So fast that most of the muons leaving the Sun must have decayed by the time they reach the Earth. But in reality, muons arrive on Earth from the Sun in colossal volumes. Physicists have long tried to understand why.
“The answer to this puzzle is that muons are generated with such energy that they move at a speed close to the speed of light,” says Kolthammer. “Their sense of time, so to speak, their internal clock is slow.”
Muons "stay alive" longer than expected relative to us, thanks to a true, natural time warp. When objects move quickly relative to other objects, their length also decreases, shrinks. These consequences, time dilation and length reduction, are examples of how space-time changes depending on the movement of things - me, you or a spacecraft - that have mass.
What is important, as Einstein said, light is not affected because it has no mass. That is why these principles go hand in hand. If objects could move faster than light, they would obey the fundamental laws that describe how the universe works. These are the key principles. Now we can talk about a few exceptions and digressions.
On the one hand, although we have not seen anything that travels faster than light, this does not mean that this speed limit cannot be theoretically beaten under very specific conditions. Take, for example, the expansion of the universe itself. Galaxies in the universe are moving away from each other at a speed much faster than the speed of light.
Another interesting situation concerns particles that share the same properties at the same time, no matter how far apart they are. This is the so-called "quantum entanglement". A photon will spin up and down randomly choosing from two possible states, but the choice of direction of rotation will accurately reflect on another photon anywhere else if they are entangled.
Two scientists, each studying their own photon, will get the same result at the same time, faster than the speed of light could allow.
However, in both of these examples, it is important to note that no information travels faster than the speed of light between two objects. We can calculate the expansion of the universe, but we cannot observe faster-than-light objects in it: they have disappeared from view.
As for the two scientists with their photons, although they could get the same result at the same time, they could not let each other know about it faster than the light travels between them.
“That doesn't create any problems for us, because if you can send faster-than-light signals, you get bizarre paradoxes where information can somehow go back in time,” says Kolthammer.
There is another possible way to make faster-than-light travel technically possible: rifts in space-time that allow the traveler to escape the rules of conventional travel.
Gerald Cleaver of Baylor University in Texas believes that one day we will be able to build a spacecraft that travels faster than light. Which moves through the wormhole. Wormholes are loops in space-time that fit nicely into Einstein's theories. They could allow an astronaut to jump from one end of the universe to the other using an anomaly in space-time, some form of cosmic shortcut.
An object traveling through a wormhole would not exceed the speed of light, but could theoretically reach its destination faster than light that follows the "normal" path. But wormholes may not be accessible to space travel at all. Could there be another way to actively warp spacetime to go faster than 300,000 km/s relative to anyone else?
Cleaver also explored the idea of an "Alcubierre engine", in 1994. It describes a situation in which space-time contracts in front of the spacecraft, pushing it forward, and expands behind it, also pushing it forward. “But then,” says Cleaver, “there were problems: how to do it and how much energy would be needed.”
In 2008, he and his graduate student Richard Obousi calculated how much energy would be needed.
"We imagined a 10m x 10m x 10m ship - 1,000 cubic meters - and calculated that the amount of energy needed to start the process would be equivalent to the mass of an entire Jupiter."
After that, the energy must be constantly "poured" so that the process does not end. No one knows if this will ever be possible, or what the required technologies will look like. “I don’t want to be quoted for centuries afterwards as if I predicted something that will never happen,” Cleaver says, “but so far I don’t see solutions.”
So, faster-than-light travel remains a fantasy for now. So far, the only way is to plunge into deep suspended animation. And yet, not everything is so bad. In most cases, we talked about visible light. But in reality, light is so much more. From radio waves and microwaves to visible light, ultraviolet radiation, X-rays and gamma rays emitted by atoms as they decay, these beautiful rays are all made up of the same thing: photons.
The difference is in energy, and therefore in wavelength. Together, these rays make up the electromagnetic spectrum. The fact that radio waves travel at the speed of light, for example, is incredibly useful for communications.
In his research, Kolthammer creates a circuit that uses photons to send signals from one part of the circuit to another, so it deserves the right to comment on the usefulness of the incredible speed of light.
"The very fact that we've built the infrastructure of the Internet, for example, and before that light-based radio, has to do with the ease with which we can transmit it," he notes. And he adds that light acts as a communication force of the Universe. When the electrons in the cell phone start to jitter, photons fly out and cause the electrons in the other cell phone to jitter too. This is how a phone call is born. The shivering of electrons in the Sun also emits photons - in huge numbers - which, of course, form the light that gives life on Earth warmth and, ahem, light.
Light is the universal language of the universe. Its speed - 299,792.458 km/s - remains constant. Meanwhile, space and time are malleable. Perhaps we should think not about how to move faster than light, but how to move faster through this space and this time? Ripe to the root, so to speak?
