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The speed of light is the fastest speed at which energy or information can travel through space. It is equal to 186,282 miles per second (299,792 kilometres per second).
According to Einstein's theory of relativity, it is impossible for a ship with mass to reach the speed of light. However, it is theoretically possible for a ship to travel at a speed extremely close to the speed of light.
If a ship were to travel at a speed close to the speed of light, time would pass more slowly for the ship's passengers than for an observer on Earth. For example, if a traveller could reach 99% of the speed of light, they would experience the trip from Earth to the centre of the Milky Way in about 20 years, while nearly 30,000 years would have passed on Earth.
Communication between two ships travelling at a speed close to the speed of light would be possible, as the relative speed between the two ships would be zero. However, communication with Earth would be challenging due to time dilation and the fact that signals would need to be sped up or slowed down to be intelligible.
| Characteristics | Values |
|---|---|
| Can a ship travel at the speed of light? | No |
| What would the passengers experience? | Time would pass normally for them. They would think, breathe, love, cry, and steer the ship as if they were merely crossing the Atlantic. |
| What would an observer on Earth experience? | A light-speed traveler experiences less time passing than an observer on Earth. |
| Acceleration required to reach 99% the speed of light | 32.2 ft/s2 (9.8 m/s2) |
| Time taken to reach 99% the speed of light with the above acceleration | 2.65 years |
| Time taken to travel to the centre of the Milky Way (30,000 light-years) at 99% the speed of light with the above acceleration | 20 years |
| Time taken to travel to the centre of the Milky Way (30,000 light-years) at the speed of light | Instantaneous |
| Time taken to travel to the centre of the Milky Way (30,000 light-years) at the speed of light from the perspective of an observer on Earth | 30,000 years |
| Time taken to complete a round trip to the centre of the Milky Way (30,000 light-years) at the speed of light from the perspective of an observer on Earth | 60,000 years |
| Is communication between two ships travelling at the speed of light possible? | No |
What You'll Learn
The speed of light is the upper limit
According to Einstein's theory of relativity, nothing with mass can travel at the speed of light. However, massless particles like photons can reach the speed of light.
If a ship were to travel at the speed of light, it would experience no time passing. The journey would be instantaneous. As a result, there would be no time for the passengers to send or receive any messages, or to do anything else, during the journey.
If a ship were to travel at 99.99999% of the speed of light, it would be a different story. Time would pass more slowly for the ship's passengers than for an observer on Earth. For example, if a traveller could reach 99% of the speed of light by accelerating at 32.2 ft/s2, they would experience the trip from Earth to the centre of the Milky Way in about 20 years. In contrast, nearly 30,0000 years would have passed on Earth by the time they reached the centre of the galaxy.
If a ship were to travel at a speed slightly lower than the speed of light, it could communicate with another ship travelling at the same speed. This is because the speed of light is the same for all observers. Therefore, if one ship were to shine a light beam at the other, they would see exactly what they would see if they were stationary.
However, if a ship were to travel at a speed slightly lower than the speed of light and wanted to communicate with a ship travelling in front of it, it would be impossible. This is because the light signal must travel faster than the ship itself through the same medium (vacuum). In other words, the radio signal must travel faster than light, which is impossible.
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Time dilation
Special relativity, one part of Einstein's theory, states that two observers moving at a constant speed relative to each other will measure different times between the same events. This effect becomes more noticeable as velocities approach the speed of light. For example, a spaceship travelling at 95% of the speed of light to a planet 9.5 light-years away would take 3.12 years for the crew, but 10 years for a stationary observer on Earth.
The time dilation effect has been confirmed through observations of unstable elementary particles travelling at nearly the speed of light, as well as experiments comparing atomic clocks on Earth and in airplanes. These experiments have also confirmed the contribution of gravitational effects to time dilation.
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Light is invariant
In physics, the speed of light is often denoted as "c". Light is invariant, meaning that in the vacuum of space, it will always propagate at c, away from or towards you. This is the fundamental premise of Einstein's theory of special relativity.
The speed of light is the only speed that massless objects are allowed to travel at. As the speed of an object with mass approaches c, its mass increases without bound, meaning that only massless particles can reach the speed of light.
The speed of light is also the speed at which time travels. If the sun were to explode, the event would not be visible from Earth until the light from the explosion reached our planet.
The speed of light is a speed that can be thought of as instant travel. A photon takes no time to arrive at its destination, no matter the distance.
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Massless objects travel at the speed of light
Massless particles, such as photons, are the only particles that can travel at the speed of light. This is because, as a particle's mass increases, it needs more energy to accelerate, with infinite energy required to reach the speed of light.
Massless particles must travel at the speed of light, and particles with mass cannot. This is a result of special relativity, which describes the relationship between energy, mass, and momentum.
In special relativity, the velocity of a particle is given by:
> v = p * c^2 / E
Where:
- V is velocity
- P is momentum
- C is the speed of light
- E is energy
The energy and momentum of a particle are related by:
> E^2 = (mc^2)^2 + (pc)^2
Where:
M is the mass of the particle
This gives:
> v = p / sqrt((mc)^2 + (p)^2) * c
From this equation, we can see that there is no real momentum, *p, that can give a velocity *v* higher than *c*, and only infinite momentum gives *v=c* when *m≠0*.
Photons, which are massless particles, require no energy to move at the speed of light. This is because their energy and momentum are related by the equation:
> E = pc
Where:
- E is energy
- P is momentum
- C is the speed of light
Since m=0 for a massless particle, this equation simplifies to E=pc. So, massless particles can have energy and momentum.
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Energy requirements
According to Einstein's theory of relativity, the speed of light is a universal speed limit. Nothing with mass can travel at the speed of light because, as an object approaches the speed of light, its mass increases without bound, and it would take an infinite amount of energy to accelerate it to the speed of light.
The energy required to accelerate an object to a certain velocity approaches infinity as that velocity approaches the speed of light. This means that the amount of energy needed to accelerate an object to light speed is infinite.
Therefore, it is impossible to travel at the speed of light because it would require an infinite amount of energy, which is not feasible.
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Frequently asked questions
No. Only massless particles can travel at the speed of light.
Time would pass normally for the people on the ship, but a lot more time would have passed on Earth.
Yes, they can communicate normally.
No, because their view will develop tunnel vision, eventually resulting in everything being squished into a singularity in front of them.
