If a spacecraft could travel through space at the speed of light in vacuum, how long would it take to cover a distance of 4 light years?

Introduction / Context:
The term light year is widely used in astronomy to describe large distances between stars and galaxies. Many learners confuse it with a unit of time because of the word year. In fact, it is a unit of distance defined in terms of how far light travels in one year. This question tests whether you can correctly relate distance measured in light years to travel time when moving at the speed of light.

Given Data / Assumptions:

• One light year is the distance that light travels in vacuum in one year.
• The spacecraft is assumed to travel at the speed of light in vacuum.
• The distance to be covered is 4 light years.
• Relativistic complications are ignored; we use the simple definition.

Concept / Approach:
By definition, if an object moves at the speed of light, it covers one light year in one year of time. Therefore, to cover 4 light years at the same speed, it would take four times as long, that is, four years. The important idea is that a light year is a measure of distance, not time, and that distance divided by speed gives the travel time.

Step-by-Step Solution:
Step 1: Recall the definition: distance of 1 light year = distance travelled by light in 1 year at speed c in vacuum. Step 2: If a spacecraft also travels with speed equal to c, it will cover 1 light year in 1 year of travel time. Step 3: The required distance is 4 light years, which is four times the distance for 1 light year. Step 4: Time taken = distance / speed. When distance is quadrupled and speed is unchanged, time is also quadrupled, so time = 4 years.

Verification / Alternative check:
You can think of this like everyday motion with simpler units. If a car covers 1 kilometre every minute, then to cover 4 kilometres at the same speed, it needs 4 minutes. The same proportional reasoning applies for light years and years, just with much larger scales. The math is simple scaling with distance proportional to time for constant speed.

Why Other Options Are Wrong:
4 seconds: This would mean that light travels 1 light year in 1 second, which contradicts the definition. 4 minutes: Similar reasoning; the unit year in light year would lose its meaning. 40 years: This would correspond to travelling at one tenth of light speed, not at the full speed of light, so it does not match the given condition.

Common Pitfalls:
Students often treat light year as a time interval rather than a distance. Another confusion arises when they ignore the definition and try to overcomplicate the problem. The key is to remember that if you move at the same speed as the reference (light) and cover a multiple of the light year distance, the time in years scales in the same way.

Final Answer:
At the speed of light, it would take 4 years to travel 4 light years.