Is faster-than-light travel coming? Don't worry, listen to what the experts say

The theory of relativity requires that "the speed of any particle is less than the speed of light". The speed here refers specifically to the speed measured at the time and place of the particle, that is, the local speed. As long as it is not equivalent to this speed definition, superluminal speed does not necessarily violate the theory of relativity.

——Li Li, Associate Researcher, Institute of Theoretical Physics, Chinese Academy of Sciences

The protagonists in science fiction works often use a tool called a warp drive to travel faster than the speed of light. In the fictional universe of Star Trek, the warp drive is a superluminal propulsion device. Inspired by this, physicist Miguel Alcubierre proposed the concept of a warp drive in a scientific sense in 1994 based on the theory of general relativity.

Recently, the internationally renowned journal European Physical Journal C reported that Harold White, a physicist at the US Defense Advanced Research Projects Agency (DARPA), led a team to discover a warp bubble in the real world. Some people believe that White's nanoscale warp bubble is expected to open a door to the manufacture of warp engines and lead mankind into the superluminal era.

In this regard, Li Li, an associate researcher at the Institute of Theoretical Physics of the Chinese Academy of Sciences, told reporters in mid-February that White's team had done some numerical simulations and predicted that a negative energy density distribution could be obtained under a certain microstructure, which is somewhat similar to the negative energy required to maintain the Alcubierre space-time structure (warp bubble). However, whether it can be linked to the warp bubble or the warp engine remains to be studied.

Negative energy is essential for faster-than-light travel

Theoretical physicist Michio Kaku pointed out in "Unbelievable Physics" that according to general relativity, there are two ways to move faster than the speed of light: stretching space and warping space. The best example of the former is the warp drive, and the latter is the wormhole.

Li Li introduced that the warp engine technology is to create an artificial bubble of normal space-time outside the spacecraft, which is called a warp bubble. The space-time near the warp bubble is distorted so much that the space in front of the spacecraft is compressed and the space behind is stretched, so that the spacecraft will be carried away by the warp bubble. To people who are very far away from the spacecraft, the speed of the spacecraft can exceed the speed of light, while the crew members in the spacecraft will think they are stationary. "The entire bubble moves with the spacecraft, which is similar to a small boat drifting in a river carried by the current," he said.

The specific idea of ​​distorting space-time proposed by Alcubierre in 1994 can achieve the effect of flying at any "speed", but this space-time structure (referring to the warp bubble) requires exotic matter with negative energy to maintain.

"Usually, physicists first use positive energy to propel a spacecraft, which always moves slower than the speed of light. In order to exceed the speed of light, the fuel must be replaced." Michio Kaku wrote in the book that negative energy is irreplaceable for superluminal travel, and perhaps negative energy does exist. In this regard, Li Li explained that in order to propel a spacecraft at superluminal speed, it is necessary to find matter with negative energy. Scientists' efforts to find negative matter in nature have so far been fruitless, and it has not yet been proven to exist, which is the same as the situation encountered with wormholes.

Li Li said that the energy that people know of that is closest to negative energy is generated through the so-called Casimir effect, which refers to the attraction between two close metal plates. This is actually a quantum effect that causes the energy density in the area between the metal plates to be lower than that outside the metal plates. It is generally believed that the energy density outside the metal plates is zero, so the area between the metal plates has a negative energy density. Li Li said that this is also the source of the negative energy density reported by White's team.

The superluminal movement of warp bubbles does not violate the theory of relativity

The speed of light is a limiting speed, and the speed of any object cannot exceed the speed of light. This concept in the special theory of relativity has been deeply rooted in people's minds. If an object moves close to the speed of light, it will bring about significant length contraction effect, clock slowing effect and mass increase effect.

So, does the warp bubble's superluminal movement violate the theory of relativity?

Li Li emphasized that the "superluminal speed" here is only an apparent superluminal speed and does not violate the theory of relativity. He explained that the theory of relativity requires that "the speed (speed magnitude) of any particle is less than the speed of light", and the speed here specifically refers to the speed (local speed) obtained by measuring the particle at that time and place. As long as it is not equivalent to this speed definition, superluminal speed does not necessarily violate the theory of relativity.

"Although the theory of relativity imposes limits on the speed of an object's movement, it does not impose any limits on the speed at which space itself expands." Li Li gave an example, the current universe is expanding at an accelerating rate, and the rate at which two galaxies move away from each other increases with the distance. For galaxies that are far enough away, the recession rate will be greater than the speed of light, but this is not because of the movement of the galaxies (clusters) themselves, but because space itself is expanding. At this time, "superluminal speed" does not violate the theory of relativity.

In his opinion, the "superluminal" movement of the warp engine can also be attributed to space expansion. "The spacecraft itself does not move relative to the warp bubble in the warp bubble, thus avoiding the mass increase effect and time dilation effect (clock slow effect) proposed in the theory of relativity. At the same time, the space-time near the warp bubble is distorted very much, driving the entire warp bubble and the spacecraft inside the bubble forward." Li Li said.

Faster-than-light travel is not theoretically possible

In 1905, Einstein published On the Dynamics of Moving Bodies and established the special theory of relativity. The special theory of relativity is based on two basic principles: the principle of the constancy of the speed of light and the principle of special relativity. More importantly, Einstein unified time and space into a whole "space-time".

Li Li introduced that in the special theory of relativity, two events are simultaneous from the perspective of one reference frame, but they are no longer simultaneous from the perspective of another reference frame in relative motion. "It is this destruction of Newton's absolute view of space and time that leads to many seemingly abnormal phenomena, the three most typical of which are the length contraction effect, the clock slowing effect, and the mass increase effect," he pointed out.

The most important point is the relativity of "simultaneity". For example, to measure the length of a ruler, you need to measure the coordinates of both ends of the ruler at the same time, and the length of the ruler is obtained based on the difference between the two. This is easy to do for a stationary ruler, but it becomes complicated when the ruler is moving. Since simultaneity is relative, the simultaneous measurement in a moving reference frame is different from that in a stationary frame. This length contraction effect is not caused by physical mechanisms such as elasticity, but is a completely kinematic effect. There is only one physical ruler, but different inertial frames have different "simultaneous surfaces", resulting in different measured lengths. The same kinematic effect is the clock slowdown effect, that is, for a moving object, its changes will slow down when measured by the clock of a stationary observer. Nothing really shrinks in the length contraction effect, and no clock's rate really decreases in the clock slowdown effect.

More specifically, both the length contraction and clock dilation effects come from the requirement in special relativity that the form of physical laws remain unchanged under Lorentz changes (Lorentz covariance), and the mass increase effect is also a direct result of this situation.

Li Li said that in the extreme case where the speed of a moving object is close to the speed of light, the scale contraction ratio will become infinite, and the clock will become like it has stopped. In addition, the mass (energy) of an object will also tend to diverge when it approaches the speed of light. This means that infinite energy is required to make the speed of an object reach the speed of light. From this, we can see that the speed of light is a limiting speed, and no object can move beyond the speed of light. When the speed is much less than the speed of light, the relativistic effect cannot be reflected, and Newtonian mechanics can be used as a good approximation of special relativity at low speeds.

"These effects in the special theory of relativity have been verified by a large number of experiments. Even the satellite navigation systems that we often use in our daily lives must consider the influence of the clock slow effect." Li Li added that the quantum field theory produced by the combination of special relativity and quantum mechanics has achieved great success in describing subatomic particles, and established a standard model of particle physics that uniformly describes the three basic interactions of electromagnetic force, strong force and weak force. Amazingly, relativity is also very important in chemistry. The average speed of the inner electrons of heavy element atoms can be as high as two-thirds of the speed of light, so the relativistic effect must be considered, which leads to the shiny golden color of gold and the low melting point of mercury that we see in daily life.

As a hypothetical superluminal theory, there is no evidence for its existence

Neither general relativity nor quantum field theory allows the local velocity to exceed the speed of light. If observations reveal that the local velocity exceeds the speed of light, it will pose a major challenge to fundamental physics.

In 2011, researchers from the OPERA experiment team at the Gran Sasso National Laboratory in Italy discovered the "neutrino superluminal" phenomenon during an experiment, which caused a huge sensation. However, it was eventually discovered that this was an erroneous result caused by problems with the experimental setup.

People have also proposed some superluminal theories based on various motivations. Tachyons are a theoretically hypothetical superluminal particle that always moves at superluminal speeds. In theory, there may be a universe composed of tachyons, but people have never found direct or indirect evidence of the existence of tachyons. In addition, there are some modified gravitational theories that also have superluminal propagation phenomena, such as some gravitational theories with high-order derivative corrections and dual-metric theories.

"Objectively speaking, these theories all have some problems to a greater or lesser extent, and no credible experimental signs of superluminal speed have been found so far. However, the development of science tells us that it is necessary to keep an open mind," said Li Li.

Source: Science and Technology Daily

Science and Technology Daily reporter Tang Fang

Original title: Have warp bubbles been discovered in the real world? Don’t worry, it’s too early to travel faster than light