The Science Of Back To The Future: Where’s My Mr. Fusion?

Forbes Carmen Drahl Oct. 15, 2015

Today, the future is now— for fans of the Back to the Future trilogy, at least. October 21, 2015 marks the day when Marty McFly and Emmett “Doc” Brown touched down in their DeLorean time machine, flying in from 30 years in the past.

“Back to the Future”‘s DeLorean time machine, with Mr. Fusion positioned atop the rear. Wikimedia Commons/Andre30c

It wouldn’t be a proper celebration of Back to the Future Day without tie-in merchandise, or speculation about whether the producers will correctly predict the Cubs winning the World Series. Or, yes, talk of hoverboards.

Today, though, I’m interested in another gizmo from Back to the Future‘s vision of 2015, the one that made the sequels’ time travel possible: Mr. Fusion. Recall that when Doc came back from the future, he’d replaced the plutonium fueling the DeLorean’s time-travel circuits with a conveniently travel-sized fusion reactor. Alas, though 2015 has brought wearable-tech glasses similar to those featured in the film, fusion power remains out of reach.

What’s taking us so long? Today is Back to the Future Day. Where’s my Mr. Fusion?

To get some answers, I spoke with Egemen Kolemen. He’s a specialist in the control of fusion plasmas at Princeton University, where he is an assistant professor with joint appointments at the Andlinger Center for Energy and the Environment, the department of mechanical and aerospace engineering, and the Princeton Plasma Physics Laboratory.

Egemen Kolemen, Princeton University (Credit: Barbara Buckinx)

Our Q&A is below, and is lightly edited for clarity.

Carmen Drahl: Thanks for talking futuristic fusion with me, Dr. Kolemen! In Back to the Future, the central time travel device was originally powered by plutonium, but this was replaced by the Mr. Fusion reactor. What’s the difference between how radioactive plutonium might generate electricity and how a fusion reactor would?

Egemen Kolemen: Radioactive plutonium produces energy by splitting up to smaller elements, which is known as the fission reaction. In a fusion reactor, we combine small nuclei to make energy. Soon enough, we hope to move from fission to fusion technology just as Back to the Future II predicted. This would help Doc Brown avoid troubles with the terrorists in the original movie— we use water as fusion fuel and you cannot make an atomic bomb with water!

CD: Mr. Fusion was able to generate power from household garbage— a banana peel, Miller beer, and even a beer can! What are the typical starting materials that are used in fusion reactors today?

EK: We use isotopes of the element hydrogen (known as deuterium and tritium) in current fusion reactors. There is enough heavy hydrogen fuel in sea water, H2O, to fuel the world’s energy needs for billions of years.

CD: In theory, could Mr. Fusion have been generating power by fusing together several different atoms or isotopes contained inside the garbage? Or could the reactor only work by fusing deuterium and tritium in the garbage? In other words— could you generate energy from fusing many different nuclei?

EK: Theoretically, fusing elements lighter than iron releases nuclear energy. If a clever physicist could overcome the engineering problems, a machine like Mr. Fusion could physically make energy by fusing banana or beer which mostly consists of carbon, oxygen and hydrogen. (It would even work for a beer can made of aluminum!). This is the process that happens in the core of large stars which is the birthplace of all the heavy elements in the universe.

However, as the elements get heavier, the fusion process gets harder and requires more energy to initialize and becomes less economical. That is why we use the isotopes of hydrogen, the lightest element in the universe.

There is no physics reason why you could not fuse many different nuclei. Stars fuse many different elements with each other. However, just as cars are designed to run with only one specific type of fuel (gasoline, diesel, etc.) to make them more efficient and economical, fusion reactors that work on preset fuels would be cheaper and an easier engineering challenge.

CD: Mr. Fusion was the size of a coffee maker. That’s pretty tiny compared to the typical fusion reactor— the experimental reactor being built in France will be over 5000 tons! What are the challenges to making fusion reactors small?

EK: The fusion process happens when two particles hit each other each other at high velocity inside the reactor. So, the energy production in a fusion reactor grows roughly in proportion to the volume (or the number of particles inside) of the reactor. At the same time, the energy loss occurs mostly due to the drifting particles out from the surface of the reactor. As the reactor size gets bigger, volume which grows roughly as the cube of the radius increases much faster than the surface which grows as the square of the radius. As a fusion reactor gets bigger the ratio of energy production to losses increases making it easier to produce net energy.

We use very strong magnetic forces to be able to confine the deuterium, tritium and electrons inside the reactor. If one can build superconducting coils that can produce much stronger magnetic fields (i.e. a stronger trap) than the ones available today, a smaller machine might be possible. This is an active research topic!

Interior of the National Spherical Torus Experiment, a magnetic fusion device constructed by the Princeton Plasma Physics Laboratory, with the Oak Ridge National Laboratory, Columbia University, and the University of Washington at Seattle. (Credit: Elle Starkman, PPPL Office of Communications)

CD: We’ve all seen car engines malfunction. Could Mr. Fusion explode in a mushroom cloud destroying everything in a 250 mile radius if something went wrong?

EK: Fusion is a delicate process that needs constant control. Unlike fission reactors, fusion cannot have a runaway chain reaction. If the “engine” malfunctions, the fusion process would stop immediately. So no explosion is possible. That is one of the many advantages of fusion energy.

CD: Mr. Fusion would have to be doing hot fusion, because cold fusion is a myth, right?

EK: There is no scientifically known net energy-producing cold fusion reaction.

CD: So since you do have to heat your starting materials to extremely high temperatures to achieve fusion, how cool can we make the outside of a fusion reactor—cool enough to safely attach to a vehicle?

EK: The wall inside the fusion vessel would be hot just as the inside of the car engine. However, the reactor would have a cooling system, similar to the radiator in a car, making the outside of the reactor at room temperature. The reactor would be as safe as the car engine.

CD: We still haven’t passed the break-even energy point in nuclear fusion— where we get more energy out of fusion than we had to put in. What gives you reason to think we’ll get there?

EK: We have been running experiments on many different fusion reactors all around the world and comparing the experimental data we obtain to computed simulations of the physics process. We have been upgrading our simulation capabilities using new numerical methods and tools and checking against the experimental data. At this point, we can reproduce and predict experimental outcomes in existing fusion reactors. We believe that our understanding of the physics and capability to simulate processes are advanced enough to roughly predict how a reactor will behave. Based on this understanding, we designed the ITER (International Thermonuclear Experimental Reactor) nuclear fusion reactor. ITER is under construction under an international collaboration, and it is expected to come online in a decade. We predict that we will obtain 10 times more fusion energy output as the power we put in. As with any research, we can only know the exact answer after running the machine but we try to do our best to predict the reactor behavior.

CD: Thank you for your time, Dr. Kolemen!

People work on a nuclear reactor at the construction site of the International Thermonuclear Experimental Reactor (ITER) on July11, 2014 in Saint-Paul-les-Durance, southern France. (BORIS HORVAT/AFP/Getty Images)