how come our space travelling speed is limited?

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To sum up, I understand fully that fuel is an issue, I’m talking theoretical.

In space there is little to no resistance because of how few particles floating about. But if you were always running and engine, pushing matter out behind you, youd have a constant acceleration. So it seems like we should be able to travel near speed of light (combining this constant acceleration with abusing gravity for slingshot maneuvers). Yet people always say nothing can travel the speed of light.

In: Physics

9 Answers

Anonymous 0 Comments

There are 2 problems at work here. One is based on thr rockets themselves, and one on the fundamentals of physics.

Chemical rockets can only ever go so fast. This limit is the speed of the exhaust gasses going the other way. This is one of the reasons you might want a hydrogen based rocket, since it is very light and thus its exhaust can reach a very high speed.

The second problem is somrthing called the gamma factor. As we go faster, the energy we need to accelerate increases by an amount equal to the gamma factor. However, as we get closer to the speed of light thr gamma factor increases exponentially, and even becomes infinite at the speed of light. And there isn’t infinite energy in the universe, so we won’t be able to get anything to move that fast.
(This is also where we get the term relativistic mass from. You multiply mass with the gamma factor (it only becomes relevant at high relativistic speeds) and hey presto, it is relativistic mass. Keep in mind though that the mass doesn’t actually change. It is shorthand to make it easier on students.)

Anonymous 0 Comments

In short? A constant thrust force produced by an engine won’t accelerate you past the speed of light because as you approach that speed, the mass of the ship approaches infinity.

Fast things are more massive. This is the result of Einstein’s famous equation, as fast things have more energy, and increasing the energy of an object necessarily means increasing its mass. The speed of light is the speed at which the mass of an object made of matter would be infinite. As a result, getting there through traditional means would require infinite energy. Photons get to travel at that speed because they have zero mass. In fact, all things with energy and without mass must necessarily move at the speed of light.

Because of this infinite energy problem, all hypothesized methods of moving faster than light involve fucking around with space itself rather than moving through it faster. A wormhole, for instance, could theoretically have pretty normal space in the middle, but connect two points an arbitrary distance away through a shorter path. If this works, it would be because we managed to bend space itself so two distant points touched. Alcubierre drives (also known as warp drives) work by surrounding the ship in a bubble of space, then accelerating the bubble through normal space at faster than light. This works because there’s no rule stating space itself can’t travel faster than light, just objects in it. So-called hyperdrives would work by shoving the ship out of 3 dimensional space into higher dimensions where all points in 3d space can be accessed without technically moving.

All of these drives have serious issues. The most plausible one, which requires the least exotic physics thought to be likely impossible, is the Alcubierre drive. Unfortunately, warping space in the required way requires negative energy, which we do not know how to generate in sufficient quantities. We think negative energy is a thing, in the form of virtual quantum particles, but actually producing the stuff may well be impossible.

Anonymous 0 Comments

Isnt the way around this is to create a gravitational vacuum in front of the object? I.e. a gravitational ripple that the object can ride like a wave.

Anonymous 0 Comments

The faster you want to go, the more fuel you need. Going from 9800 to 9900km/h requires more energy expenditure than going from 0 to 100km/h.

Now not only do you need extra fuel for that extra burn, but you need extra fuel for holding onto that extra fuel all that time. The extra weight keeps adding more and more the more you want to accelerate.

Anonymous 0 Comments

An object’s mass will increase the closer it gets to the speed of light, and so the energy required to accelerate it does too. As an object reaches the speed of light it’s mass becomes infinite, and so does the energy required to accelerate it.

Or something like that, it’s been a while before I read this kind of stuff.

Anonymous 0 Comments

If you have shitload of fuel and shitload of times you could get to near speed of light. But as you go faster in relativistic speeds, it requires MORE and more fuel to move just a little bit faster.

Anonymous 0 Comments

As you get closer to the speed of light, the energy required to maintain constant acceleration increases exponentially… by the time you reached the edge of light speed you would be using up an infinite amount of energy – i.e. more fuel than is in the entire universe, not just in your rocket boosters.

Anonymous 0 Comments

Inertial mass increases with speed, though; so the faster an object moves, the greater its increase in mass. If you could find a theoretical way around that, then you’d be in Nobel prize territory.

Anonymous 0 Comments

Theoretically, given unlimited energy resources in propulsion, you can get as close to speed of light as you would like.

Realistically, even bridging Earth’s gravity well is very expensive, much less accelerate a probe to near light speed. It’s also an interesting consideration of what value is it to send a probe fast: communicating with Voyager (which is still very close to the solar system) is quite difficult, so sending any high-fidelity data would be a challenge on its own.