What do you do when giant alien monsters attack your planet? No, you don't inspect the aliens for weaknesses. Nope, you don't attack them with the resources that you already have. Of course, you build giant, highly inefficient robots to counter them while using up unnecessary materials. Seriously, why wouldn't you just build a massive mobile weapon instead of robots?
Jokes aside, even with its scientific inaccuracies, Pacific Rim managed to pique our interest till the end. But could we build a full-sized Jaeger in the real world? Let's find out. There are lots of assumptions in the calculations, but as Hermann Gottlieb said in the first movie:
Numbers do not lie. Politics, poetry, promises, these are lies. Numbers are the closest we get to the handwriting of god.
1.
The Jaegers look incredible when they're throwing punches against a kaiju, but just lifting one of those colossal arms would take an incredible amount of power. Based on human proportions:
Assuming the Jaeger to be 80 m tall,
Arm length ≈ 40% of height = 80 m x 0.4 = 32 m.
Taking the Jaeger's weight to be 7,080 tons,
Arm weight ≈ 10% of total body weight = 7,080 tons x 0.1 = 708 tons
Assuming the arm has a uniform mass, the torque required to hold the arm up is equal to the weight acting at the center, multiplied by the distance to the center. From here, let's say Torque=T. Here's how the equation looks given what we've just figured out.
\({T={\frac {32} {2}*708,000}}\) = 11,328,000 kgm = 111,089,731 Nm
That is a LOT of torque. You would require 88,461 Bugattis just to hold the robot arm straight out at the shoulder from its starting position.
2.
The average ocean depth is 4.27 kilometers (14,000 feet), so the Jaegers would have to stay near shore or be fully submersible. Engines need air and fuel to run, so if it's running engines to power the hydraulics for 'muscles' it can't go underwater unless it has massive air and gasoline/diesel storage tanks. Also consider the corrosion affects of saltwater, though there are some processes that treat metal to be corrosion resistant for 500 hours.
Anyone who has stepped in mud can also confirm that it doesn't support much weight, so we're pretty sure these things would sink into the bottom of the ocean and crumble the concrete they stand on. Jumping and running or walking increases the pressure too.
That last point about the ocean floor is especially pertinent if you factor in the steep drop-offs near some coasts. For instance, the Golden Gate Bridge is seen being obliterated in the the movie. Near the pillars of the bridge, the depth is probably only about 10 or 15 feet depending on the tide, but if you pass one you'll quickly find yourself in the deepest part of the San Francisco Bay, with the floor up to 377 feet below the surface.
If a Jaeger found itself on one of these inclines, the soft soil under the water, which is constantly being churned by the strong currents that run through the inlet would give way, and the Jaeger would have an exceedingly difficult time just standing up, let alone fighting.
3.
The square-cube law states that, as a shape grows in size, its volume grows faster than its surface area. When applied to the real world this principle helps explain phenomena like why large mammals like elephants have a harder time cooling themselves than small ones like mice.
Jaegers are hot. With all those diesel engines powering their movements, it's a wonder their steel bodies haven't melted right off. Take for example the arm of a Jaeger. It requires about 90,000 diesel engines to move. Diesel engines are a good choice, since they have a high thermal efficiency and produce a lot of torque.
Even in diesel engines, though, 65% of the heat energy is unused and gets dissipated into the atmosphere. This heat energy is the reason that car engines need so much air to cool down. The heat released by 90,000 of these engines will likely exceed the melting point of steel, leading the Jaeger to become a steaming pile of goop on the street.
The same goes for the Jaegers powered by nuclear reactors, Gypsy Danger and Cherno Alpha. The heat released by a nuclear reactor would probably kill the pilots. That is, if the radiation doesn't kill them first.
Just like you would expect, the real-world physics of a functioning Jaeger don't really hold up; there are tons of other problems that we couldn't even begin to cover in this post. But don't see these as reasons why one couldn't exist - see these as reasons for the next generation of kids to head to college and overcome the problems of extreme tensile stress, limited mobile power supplies, and how exactly you make a 7,080 ton robot walk in what would basically be quicksand to them. Then, maybe, our grandkids' grandkids will have the privilege of seeing one of these man-made monsters level a city block.
So hopefully if Earth is attacked by giant monsters it will be after we've figured out how to create a power supply and an engine capable of generating way more power than our current technology can provide. Otherwise, the battle might be over before it begins.