Tractor beams: How scientists are turning science fiction into reality

Tractor beams have long been a staple of science fiction, but recent advances in physics and technology have made them a reality using laser or sound waves. Although not ready for space battles yet, these tractor beams have a lot of potential applications in things like drug discovery and precision manufacturing. As research into tractor beams continues, they could have significant implications for the future of physics, technology, and how we approach complex problems.

Today we will learn all there is to know about tractor beams' past, present, and future.

What is a tractor beam?

Tractor beams use laser or sound waves to manipulate particles, molecules, or larger objects from a distance. The term "tractor beam" was first coined by science fiction author E. E. Smith in 1931, and since then, they have become a popular trope in science fiction. They are frequently shown in television and film as a beam of energy that can grasp and move items from a distance. For example, in Start Trek, starships can manipulate and move objects in space using tractor beams.

But, recent advances in science and technology have moved the idea of tractor beams from science fiction to reality. In 2013, scientists created a tractor beam that could pull objects on a microscopic level. Since then, several studies have successfully created such small-scale tractor beams. For instance, in 2014, scientists created a reversible tractor beam that could transfer gold-coated hollow glass spheres against the power flow of a laser over tens of centimeters. In 2016, a study used tractor beams to move bacterial cells. More recently, in 2019, another study used tractor beams to assemble nanoscale materials in a process called "photonic nanosoldering." 

Although these developments might seem small, they have the potential to revolutionize many industries like healthcare, where they could be used to perform precise and non-invasive surgery. 

So let's now take a look at the physics behind how they actually work. 

The science behind tractor beams

Tractor beams either use lasers or sound waves to move objects. This means there are two possible mechanisms by which a tractor beam can manipulate an object.

The conceptual problem in explaining tractor beams is counter-intuitively linked with momentum. Photons carry momentum, which can be transferred to any object in the photon's path. This means the object will be 'pushed' away from the laser beam. This explains how an object can be moved away using tractor beams. But what about being attracted to the tractor beam?

The foundational work came from Lee and colleagues in a paper published in 2010. In this study, they proposed using optical solenoid beams, which, unlike other light beams, had the unique property of exerting forces on illuminated objects in a direction opposite to the direction of light propagation, i.e., an attractive force. 

Thanks to scientists, tractor beams are now a reality. Source: Abel M'Vada via GIPHY

This research was further confirmed in 2011 by Chen and colleagues in a paper published in Nature Photonics. They mathematically established the condition for the existence of an optical pulling force, which causes objects to be moved toward the beam. Chen and his team showed that the scattering force of photons and certain conditions on the incident wave was responsible for the optical pulling. The levitation achieved using a laser is known as optical levitation. 

Further experimental confirmation of optical transport using a tractor beam came three years later by a team led by Brzobohaty. They presented a geometry that could be used to produce a tractor beam and use it to manipulate microparticles of various sizes. 

It wasn't until January 2023 that optical laser pulling was demonstrated on a macroscopic object. Lei Wang, from QingDao University of Science and Technology in China, and his team attributed the optical pulling to the Knudsen force, which is a force that arises when there is a difference in pressure on opposite sides of a small gap. The difference in pressures can exert a force on an object in the vicinity of the gap.

The researchers used a specially-designed graphene-SiO2 composite. When irradiated with a laser, gas molecules on the back side of the composite received more energy, which pushed the object toward the laser. By combining this with the low air pressure of a rarified gas environment, the researchers were able to move objects large enough to be visible to the naked eye. 

However, all of these studies used light and not sound waves. 

The use of acoustic or sound waves to manipulate small particles was first studied as early as 1982. Since then, single beams of sound waves, or acoustic tweezers, have been used to manipulate small objects, particularly biological objects. This method is generally safe for biological applications, making it a prime candidate for biomedical use. The use of sound waves as tractor beams for manipulating larger objects or other manipulations wasn't seen until much later.

In 2015, Asier Marzo of the Public University of Navarre, Spain, and the University of Bristol, and his team, experimentally demonstrated the use of tractor beams that use ultrasound to lift, rotate, and manipulate particles. Sound waves, particularly ultrasonic waves, can radiate high pressure and create areas of high pressure, known as acoustic traps. Particles and objects can become trapped in these areas, where they can be manipulated. This is known as acoustic levitation.

Applications of tractor beams

Since tractor beams are still in the early stages of research, there are no current applications for them. However, since the science behind them is relatively clear, they could have a number of potential future applications. 

• Precision manufacturing: Tractor beams could be used with exceptional precision to assemble and manipulate small components, enabling efficient and accurate manufacturing of complex products.

• Biomedical applications: Tractor beams could be used in medical applications to move small materials non-invasively, enabling more precise drug delivery and targeted therapies.

• Materials science: Tractor beams also could be used to manipulate and study the properties of materials at the nano and micro-scale, potentially leading to the development of new materials and applications.

• Space exploration: If more powerful tractor beams were developed, objects like asteroids or space debris could be caught and manipulated using tractor beams to avoid accidents, capture and manipulate useful materials, and allow the safer exploration of space.

• Robotics: Robots could use Tractor beams to manipulate and control very small objects, allowing for more precise and efficient movements and actions of robots.

• Environmental cleanup: Tractor beams could be used to collect and remove small particles or contaminants from the environment, such as microplastics in the ocean or pollutants in the air.

• Security and defense: Tractor beams could one day be used to manipulate and control small objects in security and defense applications, such as disarming explosives or disabling drones.

Challenges and limitations

While tractor beams hold significant promise, a number of challenges and limitations must be considered.

There are several issues with light scattering in the context of tractor beams that use lasers. Light scatters in all directions, meaning that the laser beam must be highly directional to apply a focused force on the object. Additionally, depending on an object's characteristics, such as mass or size, a different type of laser may be needed for different objects.

The magnitude of the optical pulling force must increase significantly for macroscopic objects. This means that the size and complexity (and cost) of the equipment required to generate the beam may also become a limiting factor. Therefore, parameter choice is highly sensitive for both the equipment and the object being manipulated.

Another limitation of using lasers for tractor beams is overheating. There are studies showing that the high intensity of light required by optical tweezers can damage live biological specimens. 

The environment in which the tractor beam is used can also present challenges. If an object is in a liquid or gas, the laser may be scattered or absorbed by the medium, reducing its effectiveness. Furthermore, if an object is moving, the beam may need to be adjusted in real-time to keep up with its movements.

Despite these challenges and limitations, tractor beams hold significant promise for various applications. With continued research and development, it is possible that many of these limitations can be overcome, paving the way for new and exciting applications of this technology.

Future of tractor beams

The development of tractor beams has come a long way since they were first introduced as a concept in science fiction. Scientists and researchers have made significant strides in turning this technology into a reality, and its potential applications are vast.

The future of tractor beams looks incredibly promising. From medical applications to manufacturing and even space exploration, this technology holds the potential to revolutionize the way we interact with the world around us. Imagine a future where we can manipulate objects without any physical contact, allowing us to work with dangerous or sensitive materials without the risk of harm.

However, much work still needs to be done before we see the widespread adoption of tractor beams. The challenges and limitations surrounding this technology must be addressed and overcome, including issues such as light scattering and overheating of the laser, precise control of the beam, and environmental challenges. Continued research and development are crucial to overcome these challenges and pave the way for new and exciting applications of this technology.

Source: Paramount+ via GIPHY