Reaching the speed of light is not possible for objects with mass according to our current understanding of physics. As an object with mass approaches the speed of light, its relativistic mass increases, and the amount of energy required to accelerate it further also increases. In addition, the closer an object with mass gets to the speed of light, the more its mass increases, and it would require an infinite amount of energy to accelerate a massive object to the speed of light.
That being said, let's consider a scenario where you want to accelerate a spaceship to a significant fraction of the speed of light, such as 90% of the speed of light. In this case, the acceleration process needs to be carefully managed to avoid subjecting the human inside to lethal levels of g-force.
The human body can tolerate different levels of g-force depending on the direction of acceleration and the duration of exposure. Typically, humans can handle around 5g (five times the acceleration due to gravity) for brief periods without severe consequences. However, if the acceleration is sustained for an extended period, even lower levels of g-force can become dangerous.
To minimize the risk to the human inside, the acceleration process would need to be gradual and prolonged. The exact duration would depend on the maximum g-force that the human can withstand and the rate of acceleration achievable by the spaceship. By gradually increasing the acceleration over an extended period, the human inside could potentially endure the acceleration without fatal consequences.
It's important to note that the maximum g-force a human can withstand varies from person to person and is affected by factors such as age, physical condition, and individual tolerance. Therefore, it's challenging to provide an exact time frame for reaching a specific speed without endangering the human occupant.
In summary, to reach a significant fraction of the speed of light while minimizing the risk to the human occupant, the acceleration process would need to be gradual and prolonged. The exact duration would depend on the maximum g-force the human can tolerate and the rate of acceleration achievable by the spaceship.