As we know from Einstein's theory of relativity, no mass can accelerate to or faster than the speed of light (c), but how is this true, and why? It is not a clear-cut answer since Tsiolkovky's famous rocket equation tells us that an object can approach or exceed the speed of light with sufficiently high exhaust velocity or mass ratio. Although, in reality, this is in fact not at all the case. Why is this?
This is because according to Einstein's theory of special relativity, an object's mass increases as it approaches the speed of light. But how can we correct Tsiolkovky's equation for relativistic velocities?
It can be corrected by taking the mass of the rocket as a function of velocity.
First, we start with Tsiolkovsky's classical equation, where v _ e is exhaust velocity, m _ i is initial mass, and m _ f is final mass:
We know from Newtonian physics that change in velocity (dV) simply equals acceleration multiplied by time,
Substituting for dV,
Solving for time,
This equation will be named proper time as it is the time span in which acceleration takes place. We can then apply special relativity by taking the hyperbolic motion equation [2],
Then substituting the second equation for a*t, we get the following final equation:
This equation is the relativistic rocket equation, which is essentially Tsiolkovsky's equation corrected for Einstein's theory of special relativity. The function will predict a rocket's maximum speed according to Einstein's principles and as such, with increasing mass ratio, the function asymptotically approaches but never exceeds the speed of light. This is the case because in order to reach the speed of light, you need an infinitely high mass ratio and infinite energy. This is shown in the following graph.
This figure assumes an exhaust velocity of 0.6c (specific impulse of approx. 18.3 million seconds), which is about on par for a high-grade matter-antimatter annihilation rocket. Typical of single-staged interstellar vehicles are mass ratios anywhere between 5 to 20, so according to this graph, the antimatter rocket would achieve a phenomenal maximum dV of 0.946 c. However, in reality, this exhaust velocity reduces to as low as 0.58c [4] when losses to gamma rays, non-perfect exhaust jet collimation, and pion reflection are considered. So that becomes a maximum dV of 0.940c for the rocket.
So with all this in mind, I would like to open the comments up for discussion. What are your thoughts on the tyranny of the relativistic rocket equation? Will humanity find ways to exploit this reality for benefit? Or will we simply engineer around it? Will we avoid conventional propulsion altogether? Laser sail highways amongst the stars perhaps?
Sources:
[1] Forward, Robert L. (1995), A Transparent Derivation of the Relativistic Rocket Equation, AIAA 95-3060, Retrieved from: http://www.relativitycalculator.com/images/rocket_equations/AIAA.pdf
[2] Misner, Charles W., Thorne, Kip. S., Wheeler, John A. (1973), Gravitation, W. H. Freeman, Chapter 6, ISBN 0-7167-0344-0
Further Reading:
[3] https://en.wikipedia.org/wiki/Tsiolkovsky _ rocket _ equation
[4] https://en.wikipedia.org/wiki/Relativistic _ rocket
[5] https://en.wikipedia.org/wiki/Antimatter _ rocket
[6] https://en.wikipedia.org/wiki/Hyperbolic _ motion _ (relativity)
I hope you got yourself an A on this term paper, quite creative and unique in my limited knowledge. Well this would have you findable in a google search real quick. Hope that is a wanted side effect