Captain America: Brave New World opens in theaters globally on Valentine’s Day 2025, bringing with it a popular expansion to Hulk-lore, the Red Hulk.
Like all Hulks across various forms of media, there is a good chance that the origins of the Red Hulk in the latest installment of the Marvel Cinematic Universe (MCU) are tied to a form of high-energy radiation called “gamma-rays.”
Gamma rays originate from a variety of sources in both the Marvel Universe and the real world. Primary among non-fictional cosmic gamma-ray sources, are supernovas, neutron stars, and gamma-ray bursts, the most powerful explosions since the Big Bang, the cause of which is still shrouded in mystery.
While Space.com can’t make you as well-versed in gamma science as Bruce Banner (probably a good thing; look how that worked for him), we can give you a quick refresher and bring you up to speed before you watch the movie.
Speaking of which, check out our How to Watch Captain America: Brave New World guide to find out how and when you can catch the latest Marvel blockbuster for yourselves.
Seeing Red: What do we know about Red Hulk so far?
From the Captain America: Brave New World trailer, eagle-eyed (or Falcon-eyed) MCU fans know a few things about the Red Hulk already. The character will be the rage-induced form of Thaddeus “Thunderbolt” Ross, who has appeared in several other Marvel movies. Ross will look a little different than he did in Captain America: Civil War even before his transformation. The character is now played by Harrison Ford after the death of the role’s prior incumbent, William Hurt, in 2022.
Also, fans can see from the trailer that Red Hulk is clearly going to be involved in a throwdown with Captain America, Sam Wilson, played by Anthony Mackie.
Other than that, there isn’t a lot else we know about how Ross became a red-hued rage monster or how he went from a five-star general to the newly elected President of the United States, for that matter! But one thing is almost certain: as is the case for all Hulks (there are more of them than you think), it will involve gamma rays.
Comic fans have noticed some clues, other clues concerning the origin of the Red Hulk and connecting it to gamma radiation, too. The Captain America: Brave New World trailer features a “blink and you’ll miss him” appearance by Samuell Sterns, played by Tim Blake Nelson, better known as “the Leader.”
Movie fans last saw Sterns getting exposed to gamma-irradiated blood way back in The Incredible Hulk in 2008, right at the dawn of the MCU. It’s taken a while, but it now looks like Stern has followed the career progression of his comic book counterpart, getting a massive brain boost from the gamma exposure and becoming a diabolical criminal mastermind.
Comic book fans who’ve followed the Leader’s exploits since his first appearance way back in Tales to Astonish issue 62, published in 1964, he usually brings with him two things: misery for Bruce Banner and gamma-spawned monstrosities.
Clearly, gamma-rays have woven their way through the lore of the Marvel Universe in comics, movies, cartoons, and video games, but when Stan Lee and Jack Kirby created the Hulk back in 1962’s The Incredible Hulk issue 1, they didn’t invent gamma-rays.
As Lee and Kirby did with cosmic rays for the origin of the Fantastic Four, the legendary creators plucked the term gamma rays from real-world science. But Marvel’s gamma-rays and those of science can be radically different.
Space Hulk! Gamma rays in the real world
Let’s face it: most of us aren’t going to be playing around with our own gamma bombs like Bruce Banner or with vials of gamma-infected blood like Samuel Sterns, so how likely are we to be exposed to gamma rays?
Earth is naturally radioactive, so we are all constantly exposed to a small amount of background radiation. Additionally, gamma rays are used in medicine, including in radiotherapy-based cancer treatments. Highly focused beams of radiation can form “gamma knives” for extremely precise radiosurgery. Small amounts of gamma-ray exposure can arise from nuclear weapons and even from lightning strikes.
Space is a major source of gamma-rays, with these high energy photons created in various powerful cosmic events like the supernova death of massive stars and objects like the disks of gas and dust that surround feeding supermassive black holes and rapidly spinning neutron stars called pulsars.
Fortunately, Earth’s atmosphere protects us from major exposure to gamma rays originating from beyond the Earth.
However, nothing would protect us, or the planet, from the most powerful source of gamma rays in the known universe, gamma-ray bursts.
Though gamma-ray bursts only last from a few milliseconds to a couple of minutes, they can be hundreds of times brighter than an average supernova. That means a gamma-ray burst can put out more energy than the sun would emit over many trillions of years, if our star was even capable of living longer than around 10 billion years.
Gamma-ray bursts are observed occurring all across the sky at a rate of about one per day, though their true number is probably much larger. If one were to occur close to Earth, say in the Milky Way, and its beam was directed toward Earth, our entire planet could be sterilized.
And you thought the Hulk was capable of causing widespread destruction! It turns out gamma-ray bursts put even gamma-ray beasts to shame, no matter what color those Hulks are. Speaking of which…
Green, Grey, Red or Blue? What color are gamma-rays?
Spend a bit of time in the Marvel comic book universe, and you will discover that not only are there many Hulks, but they come in a veritable rainbow of colors.
Despite Banner’s OG Hulk being most familiar to us as a green-hued behemoth, he was actually grey during his first appearance, returning to this muted color scheme in a critically acclaimed run in the 1980s by legendary writer Peter David. Arguably, the second most famous Hulk, Jennifer Walters, or “She-Hulk,” has stuck chiefly to standard green since her introduction back in The Savage She-Hulk issue 1 (1979), but she too has briefly flirted with being grey.
The most radical diversion from green Hulks came in 2008 when writer Jeph Loeb introduced Red Hulk, sometimes affectionately known as “Rulk,” to comic book canon. Since then, we had an orange Hulk (powered by solar energy) in 2011, a purple Hulk in 2012 (who was a Marvel character also more associated with green, Norman Osborn the Green Goblin!), and Bruce Banner’s long-time pal Rick Jones became an armored blue gamma-monster in 2008.
How does all this rainbow tom-foolery relate to gamma radiation itself, if at all?
Gamma rays are a form of electromagnetic radiation, and so is the visible light that our eyes have evolved to see. All electromagnetic radiation has a wavelength and a frequency. We can see just a relatively narrow band of the electromagnetic spectrum (390 to 700 nanometers).
On one side of this band, there is electromagnetic radiation that is too long wavelength and low frequency for our eyes to see. Examples of this are radio waves and infrared radiation.
On the other side of the band is high-frequency short-wavelength radiation or ultraviolet light. This includes X-rays and the highest frequency electromagnetic radiation, gamma rays, which have wavelengths that are billions of times shorter than visible light.
Gamma rays don’t have a color because our eyes can’t see this form of radiation. So they definitely aren’t green as often depicted on the comic book page. But, would they cause you to go green, red, blue… or turn you into an over-muscled rage monster at all even?
What would gamma-ray exposure do to you?
Being exposed to gamma rays has clearly been no walk in the park for Bruce Banner.
One minute, he’s trying to open a jar of peanut butter; the next, he finds he’s destroyed his favorite purple pants (and a good portion of the Eastern Seaboard). Plus, he’s constantly got a whole host of gamma-irradiated creeps, like the Leader and the Abomination, coming after him. THEN, he discovers his father-in-law, who has been chasing him with the U.S. Army for decades, turns himself into a snazzy red facsimile of himself!
In the real world, we would be spared Banner’s frustrations, but exposure to excess gamma rays wouldn’t be much fun either. That’s because the wavelength and frequency of electromagnetic radiation are related to its energy. The shorter the wavelength, the higher the frequency and the higher the energy.
This is significant because of a process called ionization. Atoms have nuclei composed of positively charged protons and neutral neutrons around which negatively charged electrons buzz.
The charge of the electrons usually balances the charge of protons, meaning atoms are neutral, but electrons can be stripped away from atoms, and these atoms become charged ions.
For electrons to break free of atoms, they need to absorb photons, the particles of electromagnetic radiation, with a certain amount of energy, the “ionization energy.” This energy is different depending on what “shell” around an atom the electron occupies. Electrons on the outer shell are the easiest to liberate, requiring the least ionization energy.
This process isn’t like filling a bucket with water until a rubber duck floats over the rim. Absorbing an abundance of lower-energy photons won’t do the trick for ionization. The electron won’t budge until it absorbs a photon with the ionization energy.
Just one photon with the necessary ionization energy can liberate an electron (though it isn’t guaranteed as this is a quantum process and thus completely random). That’s why photons with high energy, like gamma-ray photons, are referred to as “ionizing radiation.”
Here’s the problem that makes real-world gamma rays so dangerous. The number of electrons in the outer shell of an atom determines how that element interacts with other elements. Changing these interactions can lead to different chemical reactions and malfunctioning cells in our bodies.
Gamma rays are a particularly damaging form of ionizing radiation because their high energy means they can penetrate a sheet of lead, and thus, they can easily pass through our bodies, affecting internal organs and tissues.
Low doses of gamma radiation over time can lead to an increased risk of cancer due to DNA damage. High doses of gamma radiation can trigger symptoms like nausea, vomiting and diarrhea, hair loss, and even death.
A far less grim interpretation of the dangers of gamma radiation can be seen in Captain America: Brave New World when it opens in cinemas on Feb. 14.
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