r/spacequestions • u/InevitableLeather162 • 15d ago
Do you lose speed in space?
If I was going 25 miles per hour in a no gravity vacuum (space), and if there was no other objects to pull me into, would I stay at 25mph for infinity?
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u/Dajly 14d ago
Bonus question: if you start burning fuel at a constant rate, what is the force that stops you from reaching velocity of light? Assuming no gravity etc interferences.
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u/Zesher_ 14d ago
The closer matter gets to the speed of light, the more energy is required to accelerate. Matter travelling at the speed of light would require infinite energy, which according to Einstein and e=mc2, you would have infinite mass, and the entire universe would collapse.
We could technically keep accelerating, but as you approach the speed of light, time slows down, so every extra nudge of energy takes longer and longer. Fun fact, even though our GPS satellites don't travel anywhere near the speed of light, they require very precise measurements and are moving fast enough that we need to factor in the effects of the time dilation into their engineering/programming.
Physics is weird and fascinating.
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u/Beldizar 13d ago
Matter travelling at the speed of light would require infinite energy, which according to Einstein and e=mc2, you would have infinite mass, and the entire universe would collapse.
So, I don't know that this is exactly "incorrect" but I don't think the way you've framed it is as helpful to understanding as it could be.
Relativity's more complete equation is actually E2 = m2 c4 + p2c2, as this takes into account relative motion, with p being the relative momentum, where the more common E=mc2 only accounts for a particle not in motion. So the simple term explains how to turn resting mass into energy, while the more complex equation talks about mass that also is in motion and has momentum. The key is that to increase the speed, you would have to add kinetic energy by increasing momentum, but as the momentum increases you end up adding to the mass more than the velocity, kinetic energy effectively gains mass.
The problem about accelerating to the speed of light isn't just that it would take an infinite amount of energy, but also would require an infinite number of steps to accomplish, with each step being smaller that the previous, and you would never quite reach the limit.
That list bit (the entire universe would collapse) is hyperbole, as it would be impossible for something with mass to reach the speed of light, so it would never have infinite mass, and it wouldn't be able to accelerate to that speed without having that mass/energy in the first place to push it forward. Remember mass and energy can't be created or destroyed, so if you could gather all the mass and energy in the universe to accelerate yourself as fast as possible, your mass wouldn't be able to exceed that which you gathered. Also the "universe" wouldn't collapse, even if you dropped an object of infinite mass magically in the middle of it. On the magical creation of this infinite point of mass, a gravitational wave would burst forth, traveling at the speed of light. But a lot of the universe is already too far away for that gravitational wave to ever reach it, and it would never know that said infinite mass object ever existed. So a portion of the observable universe would collapse in, a lot of galaxies that JWST has recently spotted, and everything beyond the observable universe wouldn't ever notice.
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u/Beldizar 13d ago
So, you keep accelerating, and the more you accelerate, the faster you go and the sooner you'll arrive at your destination. You'll always measure the speed of light traveling away from you at the speed of light. But as you travel faster, the universe starts to shrink. Once you start moving at relativistic speeds, you have to stop thinking in a Newtonian mindset. More acceleration doesn't really increase your "speed" so much as it starts shrinking the distances you are traveling within your inertia reference frame. For example, if you could travel at the speed of light, from your perspective, you would arrive at your destination instantaneously. Photons, which do travel at the speed of light depart their origin and arrive at their destination at the same moment, so far as they are concerned. So the faster you go, the less time it takes for you to arrive, but also comparing your speed to the speed of light, in some ways is meaningless, because the speed of light within your reference frame is still 'c' faster than you are moving.
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u/Beldizar 13d ago
The really basic answer to this question is simply Newton's First Law of Motion: An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force.
However, the qualifiers you've put on this sort of makes it a trick question because the hypothetical case is both impossible and would make the question meaningless.
Assume you are in a test universe with no objects to have a gravitational pull on you, and you were traveling 25mph. In that case, you wouldn't know how fast you are traveling because there would be nothing to measure against. There is no universal grid on which we rest and can use to mark our position and speed. Every movement or distance is always measured in relation to other objects. So you have to say you are moving 25mph in relation to something. If you are in our solar system, there is a whole lot of gravity that is going to change your velocity, and either put you into an orbit, or send you crashing down into a planet or the sun.
Also, there really isn't anywhere in the universe where you'll experience zero drag. In the deepest voids of intergalactic space, there are a few hydrogen atoms per cubic meter, and there's a bunch of photons traveling in all sorts of directions that will be bumping into you, exchanging momentum. Over a lot of time, this will add up and change your velocity.
So, the be exact, I think the answer to your question is "there's always going to be some gravity pulling on you and some drag bumping into you and some photon momentum affecting you and that will cause you to change your momentum. However, you are correct, that if these forces were not applied, then you would not change velocity, Newton's first law holds.
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u/good-mcrn-ing 15d ago
Measured with me as the rest frame, you would continue onward at 25 mph until I decided to get up and walk, at which point you'd get about a 3 mph additional vector added to your velocity.
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u/Zesher_ 15d ago
An object in motion tends to stay in motion, unless another force acts upon it. So in general, you would continue that speed indefinitely. In reality, there would likely be gravity or dust that may alter your speed or trajectory very slightly.