What if we could travel at the speed of light?

Light is super fast. Flick on a lamp and the light is there at once!

What if a spaceship could go as fast as light? Where could we go?

Light is the fastest thing in the universe. NASA says nothing travels faster than light, which moves about 186,000 miles, or about 300,000 kilometers, every second.

So what if we could build a spaceship that traveled at the speed of light? How long would trips to the Moon or the stars take? And could we really ever do it? This is a thought experiment: choose a trip and find out what the numbers say, and what physics says about the limit.

Light in a vacuum travels about 186,000 miles (about 300,000 km) per second, and NASA notes that nothing in the universe travels faster. Astronomers use it as a ruler: a light-day is about 25.9 billion km, and a light-year about 9 trillion km.

This thought experiment imagines light-speed travel to the Moon and to the nearest stars, compares it with real spacecraft, then asks why, according to our current understanding of physics, an object with mass cannot reach that speed at all.

Real spaceships are very fast. But light is much, much faster!

Astronauts took about three days to reach the Moon. Light gets there in about one second.

NASA's Artemis II astronauts took about three days to travel to the Moon. Light covers the same distance, about 384,400 kilometers, in just 1.3 seconds.

The fastest spacecraft ever, NASA's Parker Solar Probe, reached about 430,000 miles per hour near the Sun in December 2024: faster than any human-made object has ever moved. That sounds incredible, but light is still about 1,560 times faster.

NASA gives the average Earth–Moon distance as 384,400 km, which light crosses in 1.3 s; Artemis II's crew took about three days. Sunlight needs about 8 minutes 20 seconds to reach Earth.

The record for human-made speed belongs to Parker Solar Probe, which passed 3.8 million miles above the Sun's surface on 24 December 2024 at about 430,000 mph (about 192 km/s). Dividing light's 299,792 km/s by that gives a factor of about 1,560. Voyager 1, cruising at nearly 61,000 km/h, is far slower.

At light speed, you could zip to the Moon in about one second!

But the stars are very far away. Even at light speed, the next star would take more than four years.

At light speed, the Moon would be 1.3 seconds away, and the Sun about 8 minutes 20 seconds. NASA says Mars is between 3 and 22 minutes away by light, depending on where the planets are.

The stars are a different story. NASA Space Place says the next-closest star after the Sun is about 4.3 light-years away, so even light takes about 4.3 years to get there. At Parker Solar Probe's record speed, the same trip would take about 6,700 years.

At c the light-times are: Moon 1.3 s, Sun about 8.3 minutes, Mars 3 to 22 minutes, Neptune about 4 hours 10 minutes (NASA). The next-nearest star system is about 4.3 light-years away (NASA Space Place).

At Parker Solar Probe's 192 km/s, 4.3 light-years would take about 6,700 years; at Voyager 1's roughly 61,000 km/h (about 17 km/s), about 76,000 years. NASA calculated that Voyager 1 would reach one light-day from Earth on 18 November 2026, 49 years after its 1977 launch: a measure of how small even a light-day is next to the gaps between stars.

The closer something gets to light speed, the harder it is to make it go faster.

To reach light speed, a spaceship would need endless energy. Nothing has that much!

Scientists' best understanding of physics, from Einstein's theory of relativity, says that objects with mass cannot even reach the speed of light. As something gets closer and closer to light speed, it takes more and more energy to speed it up, and reaching light speed would take an infinite amount.

Scientists have pushed tiny particles very close: NASA's Cosmicopia says the fastest electrons from particle accelerators reach about 0.999999 times light speed. But never all the way.

NASA's Cosmicopia summarizes the current understanding: nothing can go faster than light, and objects with mass cannot reach it, because it would require an infinite amount of energy. The fastest accelerator electrons reach about 0.999999 c; Cosmicopia adds that no electron has ever been made to go faster than light.

Relativity also changes time. Cosmicopia gives the time-dilation relation: time on Earth = traveler's time × 1/√(1 − v²/c²). At v = c this becomes infinite, which is one more sign that light speed is out of reach for anything with mass. At 99% of c the factor is about 7.1.

Scientists use light to measure space. A light-year is how far light goes in a whole year.

When we look at a star, we see light that left it long ago.

Because light covers huge distances so quickly, astronomers use it to measure space. NASA Space Place explains that a light-year, the distance light travels in a year, is about 9 trillion kilometers.

That also means we see the past. Sunlight is about 8.3 minutes old when it reaches us. Light from the next-closest star left it about 4.3 years ago. And the Andromeda galaxy, 2.5 million light-years away, looks the way it did 2.5 million years ago.

A light-year is about 9 trillion km (6 trillion miles), and NASA notes that looking out means looking back: we see the Sun as it was about 8.3 minutes ago, the next-nearest star as it was 4.3 years ago and Andromeda as it was 2.5 million years ago.

Light-time also governs spaceflight. Radio signals travel at light speed, so commands to a Mars rover take 3 to 22 minutes each way, and from 18 November 2026, NASA calculated, a signal to Voyager 1 needs a full day, so a command and its reply take about two days.