Double the Earth's radius. That's all we're doing. Take the planet from 6,371 kilometres across to 12,742. Same rocky composition, same density, just... bigger.
Sounds manageable. It is not.
A planet with twice the radius and the same density has eight times the mass. Eight. And surface gravity scales with mass divided by the square of the radius, which means gravity on this new Earth is roughly twice what you're used to. You now weigh what two of you would weigh. Standing up feels like giving someone a permanent piggyback ride.
Your body notices immediately
The human skeleton can handle temporary loads of about 5g before things start snapping. So 2g won't kill you outright. But "won't kill you outright" is doing a lot of work in that sentence.
Your heart, designed to pump blood against 1g, now has to push twice as hard to get blood to your brain. Blood pools in your legs. Your cardiovascular system, already the leading cause of death in the developed world, becomes an even more unreliable piece of kit. Heart attacks at 30 would be normal.
Your knees and lower back take the worst of it. The cartilage in a human knee joint compresses under load, and at twice the gravity it's being asked to do double duty every second of every day. Arthritis wouldn't be a condition of old age. It would be a condition of being alive.
Running would look different. Slower, lower to the ground, more of a controlled forward lean than the bouncy stride we're used to. Jumping? You'd get half the height. A basketball hoop at 3.05 metres might as well be on the roof.
The atmosphere is a problem
More mass means stronger gravitational pull on atmospheric gases. This bigger Earth would hold a much thicker atmosphere. Not just a bit thicker. The air column above you would be significantly denser, and atmospheric pressure at sea level could be somewhere between two and four times what we experience now.
Thick atmosphere means thick weather. Storms would hit harder because wind carries more energy when the air is denser. A "gentle breeze" at 15 km/h would push you around like a stiff gust does today. Hurricanes would be genuinely terrifying, though the increased surface gravity might suppress some of the vertical convection that drives the worst of them. It's a complicated trade-off.
Then there's the light.
A thicker atmosphere scatters more sunlight. Sunrise wouldn't be the sharp event you know. It would be a long, slow brightening that takes the better part of an hour, the sky shifting through deep reds and oranges for ages before the sun actually clears the horizon. Sunset the same. Photographers would love it. Everyone trying to sleep with blackout curtains would hate it.
Aviation is grounded
Commercial aircraft fly because of a careful balance between thrust, weight, lift, and drag. Double the gravity and you've doubled the weight of the aircraft. Double the atmospheric density and you've changed the lift and drag equations completely.
More air density actually helps with lift at low speeds, which sounds like good news until you realise it also dramatically increases drag. A Boeing 737, which has a maximum takeoff weight of about 79,000 kg at 1g, now effectively weighs 158,000 kg. The engines don't produce twice the thrust. You're not getting off the ground.
You'd need to completely redesign every aircraft. Shorter, stubbier wings. Massively more powerful engines. Lower cruising speeds. The economics of air travel would change beyond recognition. Flights would burn far more fuel per kilometre, ticket prices would be astronomical, and the range of aircraft would shrink. Forget nonstop London to Sydney. You'd be lucky to make London to Rome.
Rockets have it even worse. The escape velocity of this planet would be about 15.8 km/s, compared to Earth's 11.2 km/s. Getting to orbit requires exponentially more fuel because of the tyranny of the rocket equation (the fuel needed to lift the fuel needed to lift the fuel...). Space programmes might never get off the ground. Literally.
Everything we've built is wrong
Every bridge, building, dam, and road on Earth is engineered for 1g. At 2g, the dead load on every structure doubles. Suspension bridges sag. Tall buildings compress their lower floors beyond design tolerances. The Burj Khalifa, if you somehow transported it to this planet, would crush itself.
Construction on double-Earth would favour squat, wide buildings. No skyscrapers, no soaring cathedrals, no dramatic cantilevers. Everything would be low and heavy. Architecture would look more like brutalism had a baby with a bunker. Roman arches would make a comeback because they're one of the most efficient ways to distribute compressive load, and compression is what this planet does to everything.
Water infrastructure changes too. Pumping water uphill against twice the gravity requires twice the energy. The pressure in deep-water pipes increases. Dams holding back reservoirs experience double the hydrostatic pressure at every depth. Many existing dam designs would fail.
Evolution would have gone differently
If Earth had always been this size, life would have evolved for it. Animals would be shorter, stockier, with thicker bones and more powerful hearts. The square-cube law already limits how large animals can get (it's why elephants have such thick legs relative to their body), and at 2g those limits shrink considerably.
No giraffes. Their circulatory system already pushes blood two metres straight up against normal gravity, requiring blood pressure roughly twice that of humans. Double the gravity and that trick becomes physiologically impossible.
Trees would be shorter. Wood has a compressive strength limit, and at some height the trunk can't support its own weight. Redwoods reaching 115 metres in our world might max out at 50 or 60 metres here. Forests would be dense and low, a thick canopy close to the ground.
Insects, oddly, might do alright. Their exoskeletons are strong relative to their mass, and their small size means the square-cube law works in their favour. A beetle doesn't care much about 2g. A beetle is already living in a world where surface tension and air resistance matter more than gravity.
The geology
More mass means more internal heat from gravitational compression and radioactive decay. Plate tectonics would be more active. Volcanoes erupt more frequently. Earthquakes hit harder, and they already hit hard enough, thank you.
Mountains would be lower. On our Earth, the Himalayas are being pushed up by tectonic collision and worn down by erosion, reaching an equilibrium around 8,849 metres at Everest's peak. On double-Earth, the rock itself can't support as much height before the base compresses. Mountains might top out at four or five thousand metres. The "roof of the world" would be more of a loft conversion.
The ocean basins would be shallower for similar reasons, but the total volume of water wouldn't change. More water in shallower basins means more surface coverage. This Earth might be 80-85% ocean instead of our 71%. Less land. Less farmable land. More pressure on every square kilometre of dry ground.
Would we still be us?
If humans had evolved on this planet, we wouldn't look like us. We'd be shorter, probably 1.2 to 1.5 metres tall on average, with denser bones and barrel chests to house oversized hearts and lungs. Our legs would be thicker relative to our height. We'd tire faster, move slower, and probably never have invented the high jump.
But we'd be adapted. We'd know nothing else. Our sports would be different, our architecture, our sense of what counts as "tall" or "far." And if one of us were somehow transported to our actual, smaller Earth, they'd feel like a superhero. Leaping higher than anyone, running faster, lifting more.
There's a version of this thought experiment that explains Superman, actually. Krypton was supposed to have had much stronger gravity than Earth. Kal-El wasn't magic. He was just overbuilt for the planet he landed on.