if a space craft can travel with no air resistance, why cant it continue accelerating to the speed of light?

just a thoeretical question, i was thinking at some point in future if we could power a craft by solar energy maybe to run a ion drive engine

Because the faster the ship goes the more the mass increases, kind of like the Gravitron ride at the circus.

The more the mass increases the more energy is needed to keep the spaceship from slowing down.

Good luck finding enough energy to power a spaceship to the speed of light.

In the absence of a force it will travel at a steady speed.

In the presence of a force, the ENERGY will increase indefinitely, but (from Einstein's relativity) although the speed will continue to increase, it will be by smaller and smaller amounts, and can never reach the speed of light itself.

1. Acceleration needs energy.
2. The closer to the speed of light, the more energy is required.
3. The velocity will approach the speed of light eventually, but only after a very long time, and an immense expenditure of energy.

4. The space craft will never actually reach the speed of light. To do so would require infinite energy.

As Einstein said in his infamous theory of Relativity that

"Nothing can move at the speed of light besides light itself. "


If an object is travelling near the speed of light, weird things happen in space and time preventing the object to reach the speed of light.

Because the fastest propulsion in existence is light, so if you can somehow hitch a ride on LIGHT beams and use it as your propulsion then you can reach the speed of light.

Because the speed of light is constant in all reference frames. No matter how fast the spaceship travels, the pilot observes light rays still passing it by at c. No matter how long he accelerates, he can never catch up to those light rays, or even make the least headway. Since he never observes himself to catch up to any light rays, neither can anyone else.

When we discover that the speed of light is constant, we must admit that there is something wrong with Newtonian physics. F does not equal ma, if we insist on a being change of coordinate speed with time. It turns out that F=ma is wrong. The corrections to Newton's Laws of motion due to constancy of c are called "special relativity."

In SR, constant force results not in constantly increasing speed, but in constantly increasing *rapidity*. For speeds much slower than c, rapidity and speed are the same thing, so nobody noticed that the definition of acceleration was all wrong. No amount of force can accelerate you to c, because there are no reference frames in which you can travel along with a light ray. Light rays all travel past you at c, no matter your speed relative to other bodies.

It has nothing to do with mass increasing. Your mass does not increase near c. What happens is that the definition of momentum as being mv is wrong. The correct definition of momentum is (gamma)mv. Some high school textbooks declare "gamma*m" to be the mass to make SR equations look kind of like Newton's Laws, but that concept irredeemably leads to vast conceptual errors.

What about the speed of Dark, has that been forgotten ?

A couple of reasons....

First, to accelerate, a vehicle requires it to carry some sort of fuel. That fuel adds mass to the vehicle. That extra mass requires even more force to accelerate it: larger engines, more fuel, and more structure to support those larger engines and fuel. You pretty quickly end up at a point of diminishing returns where the best engines currently available simply can't accelerate the bulk of all that fuel.

Even if you were able to overcome the engineering problems, you'd still have to deal with the effects of Relativity. Since mass and energy are related, as you accelerate an object (i.e. adding kinetic energy to it), it's mass increases. As the mass increases, it requires ever more energy to accelerate the object. This in turn translates to an even greater increase in mass... and so on. Eventually you reach a point where the ship either collapses under its own weight or the engines of the ship simply can't push the extra mass.

Because each increment of speed takes more and more energy, even without any friction. If it takes X amount of energy to get to 10% the speed of light, then it takes 1.0155 X to get to 20%, 1.0856 X to get to 40%, and 1.3933 to get to 70%.

To get to 99% takes 7.053 X, and to get to 99.99% takes 70.358 X.
Then 99.999999% takes 7036 X. And so on ... you never have enough energy to get all the way to light speed.