Kilocalories (th)/hour to Petawatt

In microgravity, muscles never work against their own weight — even walking requires zero effort. ISS astronauts exercise ~2.5 hours/day burning 400–600 kcal/h on resistive machines and treadmills with bungee harnesses, yet still lose 1–2% muscle mass per month. The problem is not total energy expenditure but the absence of constant low-level gravitational loading that Earth provides 24/7. Ground-based standing and walking burn only 80–120 kcal/h but provide continuous mechanical stimulus that exercise bursts cannot fully replace.

Most machines use crude formulas based only on speed/resistance and assume a 70–80 kg user. They often report gross calories (including resting metabolic rate you'd burn anyway) rather than net additional calories from exercise. Studies show treadmills overestimate by 15–20%, ellipticals by 25–40%, and stationary bikes by 10–15%. The machines have an incentive to flatter you — higher numbers keep you coming back. Always discount the displayed number by at least 20%.

Surprisingly little extra. The brain uses about 20% of resting metabolic energy (~15–20 kcal/h) regardless of what you are thinking. Intense mental work — chess tournaments, exams, complex coding — increases brain glucose consumption by only 5–10%, adding roughly 1–2 kcal/h. Chess grandmasters who lose weight during tournaments are not burning it with their brains — they lose it through stress hormones elevating heart rate, skipping meals, and disrupted sleep. The brain is always "on" at nearly full power; thinking harder barely moves the needle.

Almost linearly for weight-bearing exercise: a 100 kg person burns roughly 60–70% more kcal/h than a 60 kg person walking or running at the same speed. For cycling and swimming (where body weight is supported), the difference is smaller — maybe 20–30%. This is why heavier people find it "easier" to create a caloric deficit through exercise, and why lightweight people need to exercise longer for the same caloric burn. It's simple physics: moving more mass requires more energy.

Basal Metabolic Rate for adults is typically 55–85 kcal/h (1,300–2,000 kcal/day), depending on age, sex, weight, and muscle mass. It accounts for 60–75% of total daily energy expenditure — far more than exercise for most people. This is why crash diets backfire: severe calorie restriction can drop BMR by 10–20% (metabolic adaptation), reducing your burn by 200–400 kcal/day. Your body literally becomes more efficient, fighting your weight loss efforts.

It's a time trick. A petawatt laser concentrates a modest amount of energy (maybe 100–500 joules) into a pulse lasting 10–100 femtoseconds. Dividing a few hundred joules by 10⁻¹⁴ seconds gives you 10¹⁵–10¹⁶ watts — surpassing the Sun's 3.8 × 10²⁶ W is still far off, but these lasers do exceed total human power consumption by 100,000×. The catch: the total energy delivered is only enough to heat a cup of coffee.

Primarily for nuclear fusion research (compressing fuel pellets), particle acceleration (laser wakefield acceleration can produce electron beams rivalling billion-dollar synchrotrons), medical isotope production, and probing extreme states of matter found in stellar cores. The ELI (Extreme Light Infrastructure) project in Europe uses petawatt lasers to recreate conditions found in supernovae, helping astrophysicists study cosmic explosions in a lab.

Solar flares can briefly release 10–100 PW of electromagnetic radiation. The Chicxulub asteroid impact (the one that killed the dinosaurs) delivered roughly 4 × 10²³ watts during the few seconds of impact — about 100 million petawatts. Gamma-ray bursts top everything at 10²⁵–10²⁶ PW, briefly outshining the entire observable universe. Even supernovae "only" sustain about 10³⁶ PW for a few seconds at peak.

Building one costs $50–500 million. Operating costs are surprisingly modest per shot — each pulse uses only a few hundred joules (less than lifting an apple one meter), but the capacitor banks and cooling systems draw megawatts of continuous power. The NIF facility costs about $350 million per year to operate. Individual shots are "cheap" in energy terms but the infrastructure to achieve them is staggering.

In theory yes, but in practice current petawatt lasers are terrible weapons. They fire one pulse every few minutes to hours, require warehouse-sized buildings of equipment, and deliver total energy equivalent to a firecracker. Military-grade laser weapons focus on sustained power (100–300 kW continuous beams), not ultrashort pulses. A petawatt laser is a precision scientific scalpel, not a blunt instrument — brilliant for physics, useless for destruction.