It’s a staple of science fiction that to get to interstellar destinations as quick smart as possible, you need to travel as close to light speed as possible because 1) near light speed is pretty fast, and 2) time slows down as you approach light speed (and stands still should you ever achieve the speed of light – which of course you can’t do, even in theory). Related to that time distortion is the fact that the faster and faster you go, your mass increases and your length decreases (and therefore your volume decreases and therefore your density increases). While that’s not noticeable or really even measurable as you press down on your automobile’s accelerator pedal, once you start going at appreciable fractions of light speed, it is noticeable and measurable. The upshot is what happens to you (and your presumably interstellar spaceship) as you really rev it up? I suspect that ultimately, as you come close to light speed, your mass and density will increase to such a degree that you’d turn into a Black Hole, with yummy singularity at the center)! Does this then put the kibosh on close to light speed interstellar journeys?
Well, since I wrote the above, I’ve spotted a physics FAQ page which addressed this very issue. The prime reason apparently is that the formation of Black Holes is still poorly understood. Their answer was ‘no’. However, I remain unconvinced, for reasons below – arguments by analogy!
There are two frames of reference here, that of the person in the interstellar spaceship, and that of an external observer (in say an identical spaceship that’s not moving, say relative to Planet Earth). From the point of view (POV) of the traveler piloting the moving interstellar ship, nothing is apparently happening. It’s not apparent that your mass is increasing, your time is slowing down, your length is contracting, because all measuring instruments, scales, clocks, rulers are equally affected. But the external observer would notice the differences (assuming they had sophisticated measuring instruments).
Of course the traveling interstellar pilot would always experience or feel the constantly ever increasing acceleration (probably interpreted as a source of artificial gravity according to Einstein’s Principle of Equivalence between acceleration and gravity), just like (even with your eyes closed) you know acceleration is happening when the car you’re in turns a corner because you are pushed to one side of the vehicle, or when you are pushed into the back of your seat when the aircraft starts its takeoff run. You could interpret the pushing as a form of artificial gravity. That’s in contrast to feeling no extra forces when the car or plane is moving at constant velocity in a straight line. If your eyes are closed, you might swear you’re not moving at all. Additionally, you (the pilot), if you are carrying an onboard fuel supply, would notice that your fuel supply was decreasing at an ever increasing rate as increasing fuel units are needed to keep up the extra and ever increasing acceleration to push the ever increasing massiveness of you and your ship towards light speed. If it takes one unit of fuel to accelerate you one unit, and then it takes two units, then three, etc. you could probably deduce that your mass was increasing.
Analogy #1 – Does an electron ‘feel’ or ‘notice’ becoming more and more massive as it is being ever accelerated in a particle accelerator? Probably not. But, from the POV of the experimenter who’s counting on it, and from the administrator who has to pay the ever accelerating energy bill (it takes ever increasing amounts of energy to keep pushing the electron faster and faster because the electron is becoming heavier and heavier the faster and faster it goes), the effect is real enough.
Analogy #2 – Say you want to commit suicide by diving into a massive Black Hole, which, of course, has high gravity. And say you are observed attempting this by someone who’s a safe distance away, where there isn’t high gravity. Now clocks run slower in high gravitational fields and faster in low gravitational fields. So, from the POV of our suicide victim, unaware or unable to measure the slowing time rate (again, all measuring devices in the same vicinity are equally affected), a minute is still a minute is still a minute, and her demise from her POV is in quick-smart (her normal) time. To the external observer, a minute is still a minute is still a minute, but his watch is running much faster relative to that of the victim’ watch. So, he sees her demise (by observing her watch) dragged out and on and on and on and on in molasses time. Or put another way, she see him living life in the fast lane (assuming she can see his watch); he sees her living life in the slow lane. However, in both cases, the suicide diver via Black Hole is observed. She enters the Black Hole and becomes spaghetti-fied (if she goes in head first, the gravity at her head is stronger than the gravity at her feet and she becomes stretched out like a noodle as down she goes). Time may tick over at different rates for the external male and the female victim, but it doesn’t ever reach the point of standing still. Why? No macro object, like our wannabe suicide person, can travel at or above light speed (roughly 300,000 kilometers per second). Thus, our victim can only approach the Black Hole and enter it at less than the speed of light. Time slows down more and more (according to an external observer) for the person going ever and ever faster. But, while time would stop at light speed, the victim never gets to light speed (it’s impossible) and thus he sees her enter the event horizon – she then vanishes from sight. Thus, spaghetti-fication inevitably occurs – fast from the POV of the victim; long and dragged out according to the outside observer. If time stood still for the victim as witnessed by the outside observer, yet proceeded normally as perceived from the victim’s POV, then we’d have a case of something that both is, and is not, which is absurd.
The same applies; methinks, in the case of approaching light speed and turning into a Black Hole. The outside observer would notice it, just like the experimenter notices the mass increase in the accelerating electron, even if the person (or particle) doing the accelerating doesn’t. But again, you can’t have a case of something that both occurs and does not occur. Perhaps in the case of the unfortunate interstellar traveler who notices nothing amiss, it’s another case of what you don’t know or can’t measure can still hurt you.
In any event, it will be many generations yet before this approaching light speed interstellar journey becomes a factual scenario – it’s currently just another thought experiment. No doubt when the time comes, if the time ever comes, state of the art physics will know the answer in advance.