Just because we can't conceive it today, it does not mean it is impossible! This is so very true because one hundred years ago a man flying in a plane was an unthinkable thought. Scientific fiction has a habit of always becoming reality as science is advancing the limits of technology are bringing Science fiction closer to reality more than ever!

Darth Vader's Death Star, Captain Ozlo's Skylark and Captain Kirk's Starship Enterprise
all have a tractor beam, which is essentially an audible light beam gadget used by spaceships to grab and lift objects, as well as capturing enemy space ships. If we could actually build a real life working tractor beam, how would a real life tractor beam work both in space and on planet earth?
To my knowledge, air and oxygen molecules are almost non-existent in outer space plus space is a vacuum and we know sound does not travel in a vacuum. The main purpose of building a tractor beam would be to pull objects without having to reach out for them. The basic concept behind building a real life Star Trek style tractor beam is to have an energy force hit an object and either pull it or push it. The energy force could be a gravity force, an electric force, a magnetic force, sound energy and a light field of force, or a combination of all those forces. We know these energy forces would certainly work on earth, but would these energy forces work in space?

Photon beam
Laser really sounds like a great candidate for building a tractor beam since it is based on the stimulated emission of electromagnetic radiation. Because lasers retain their beam quality for such long distances, a tractor beam using laser makes sense. In fact, a laser tractor beam has already been proven to work for nanotechnology manipulation by a couple of scientists (Dr Vladlen Shvedov and Dr Cyril Hnatovsky) who used a  hollow laser beam to move and repel nano particles a distance of several centimeters. The way the scientists were able to attract and repel the nano particles was through the energy created by the laser heating up the air around the nano particles.

A laser tractor beam would work excellently on earth but not in space since a laser tractor beam works by heating the air molecules around the object, and in space, there are hardly any air molecules.

Sound beam
We know sound waves can be used to move physical objects, though traditionally this has only been able to be accomplished through acoustic levitation or similar. However, scientists from the Universities of Bristol and Sussex have been able to move objects outside of an enclosed acoustic area without the use of acoustic levitation. They invented an 'acoustic hologram' tractor beam that uses sound to lift, pull, and rotate objects in the air by adjusting the frequency of sound.

To make the sound tractor beam work, the scientists had to arrange several speakers and orchestrate each and every sound produced by each speaker in a synchronized manner to produce a force field pocket where the object would sit. This technique of tweaking sound waves allowed the scientists to be able to move objects with sub-milliliter precision, making this technique excellent for Acoustic tweezing cytometry.
Electron beam (Cathode ray)
Using a superconducting magnetic field is another plausible way to build a tractor beam powerful enough to attract and repel objects. An electron tractor beam would create a magnetic field similar to that of an electrified wire fence. The object would be  repelled by simply blasting the beam into an object to charge it, and then using the opposite charge to attract it. I can really see this working for a tractor beam in space since NASA already uses this technology for spacecraft propulsion to create thrust by accelerating ions (Ion thruster).

Light beam (biaxial birefringent media)
Light manipulation is another way to bring to life a fictional tractor beam. We know it is the pressure of light which pushes comet tails away from the sun so, by using the principle of the pressure of light we could build a light tractor beam capable of repelling objects. But what about attracting them? That's exactly what the research team led by Dr. Tomas Cizmar at the University of St. Andrews in Scotland have successfully accomplished. They have figured out a way to turn light pressure into a negative force in order to attract objects. They were able to accomplish this by shooting a VERDI V5 laser beam through a lens and a polarizeable electrical insulator suspended between two microscope coverslips (one of them a half-silvered mirror). The action of entering and reflecting beams interfering with one another, produces a standing wave which pushes in the wrong direction (similar to an eddy current force).
Gravity beam
We know gravity can be manipulated but gravity cannot be created so, a gravity tractor beam sounds like the most viable option to building a real life space tractor beam. A gravity tractor beam could be accomplished by creating a gravity shield. The good news is that gravity shields are proven technology. The bad news is that the scientist behind it never actually released details of how he created a gravity shield. Ning Li actually theorized the Gravity shield and it was later proven by Evgeny Podkletnov in the late 1990's. Essentially, by spinning a superconducting disk very fast, he was able create a very strong magnetic field that would reduce the effect of gravity, making objects lighter within the gravity shield.

While building a real life tractor beam UFO style maybe a long way off, one thing we know for sure is that tractor beam technology is no longer Sci-Fi! Ongoing advances in tractor beam technology will bring many useful uses in the field of nanotechnology manipulation, electronics industry, biomedical sciences, surgery and even hoverboards (like seen in Back-to-the-Future-II)