How long would it take to accelerate a spacecraft to 99 per cent light speed without major injury to astronauts within it? (continued)
Bryn Glover
Kirkby Malzeard, North Yorkshire, UK
Advertisement
Reading between the lines of the question about humans travelling at near-light speed, one might assume that the possibilities for human interstellar flight were on the questioner鈥檚 mind. As if the three published answers were not depressing enough for that ambition, may I point out that each of them omitted a vital component for star travel: presumably, once the 99-per-centers approached their destination, they would need to stop to land and explore.
If deceleration is to be as non-traumatic as acceleration, then it will require the same amount of time and fuel as was consumed for the start of the journey, and the monstrous extra mass of that to be carried will clearly affect the calculations for acceleration.
I haven鈥檛 done any careful calculations; when the message is so blunt, accuracy is immaterial. In an earlier issue, New Scientist reported on the discovery of a planet that may support life, and at 125 light years away, may well be the nearest such planet. To travel such a distance using sensible and possible technology would, I reckon, take rather longer than the existence of the Homo genus on Earth. It seems as though the dreams of Earth billionaires to colonise space will need to wait until Star Trek鈥�s James T. Kirk comes along after all.
Mel Earp
Macclesfield, Cheshire, UK
The key number in many relativistic calculations is the Lorentz factor. It is often thought that this number becomes extremely large extremely quickly as you approach the speed of light. But this doesn鈥檛 happen as fast as you might imagine; it certainly doesn鈥檛 need to invoke the words 鈥渋nfinite鈥� to stress the point.
In the question, the suggested final speed is 99 per cent the speed of light, or 99c. At this speed, the Lorentz factor is only slightly over 7. This would certainly be noticeable to a stationary observer in terms of length contraction and time dilation. What is also affected is the amount of energy needed to get that next bit of acceleration, seven-fold at this speed. Hence the need to carry lots of fuel, which merely adds to the problem. The added energy requirement makes the spacecraft look more massive to the observer and is known as the Lorentz mass, in this case, seven times the spacecraft鈥檚 rest mass.
Presumably, once the 99-per-centers approached their destination, they would need to stop to land and explore
By way of comparison, the Large Hadron Collider (LHC) accelerates protons to about 99.999999 per cent the speed of light, or 0.99999999c. At this speed, the Lorentz factor is about 7000. Again, this isn鈥檛 indescribably big. It does affect the energy required and is why the LHC consumes so much 鈥� along with its magnets, of course. The LHC does this in a time frame measured in tens of minutes, but at this acceleration rate, it wouldn鈥檛 be comfortable for a passenger on one of the protons!
In both cases, there are no black holes and no fatal drawing-in of neighbouring masses to create one.
Guy Cox
Sydney, Australia
Some years ago, I was writing a science fiction story about a team colonising a planet 10 light years away. To keep the colonists healthy, they would accelerate at 1g for half the journey and decelerate at 1g for the second half, as to experience Earth鈥檚 gravity the whole way. Their maximum speed would be 98 per cent of the speed of light. So, it would take 20 years to an outside observer, or six years from the passengers鈥� perspective, to reach 98 per cent light speed at 1g acceleration.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
New Scientist Ltd retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.