Impossible assumes we know everything there is to know about, well, everything. There was a time when everything mad knew said that traveling faster than the speed of sound was impossible. These same geniuses said escape velocity was impossible. I'm sure before that people said powered flight was impossible. We know of theoretical particles that could travel faster than light if we could prove their existence, like tachyons, so can't we say that it's improbable, instead? If space, not our universe, is really infinite, which is the only logical conclusion I can come to, it seems to me our laws of physics would only apply to us, in our universe. They might not even apply here if we "did the math". Like remember when scientists said nothing was smaller than an atom, that it was impossible to have any particles smaller than that? How'd that work out? Every absolute in science seems to be broken at some point.

We are a VERY young species in the greater scheme of things and it always amazes me how humans now, in the year 2012 think they have it all figured out. These are the same folks who say the fact that since aliens haven't visited us, they can't exist, or don't have the technology to travel here. Which presumes they want to. Or that they're simply not as advanced. And then you see those UFO videos the Navy has. I'm not one for conspiracy theories, but objects traveling faster than the speed of sound with no sonic boom: explain that. Or any of the other alleged details. Like Mach speed to 0 and hovering in less than a second. Nothing on Earth we know of does that, so maybe they are here? I have no idea. I really don't care if they are, to be honest. I'm more interested in the physics.

I realize the amount of energy it takes to travel at light speed, but that's what we know of light speed. And of gravity. What if FTL doesn't utilize the same physics? What if you never need to go light speed to go to FTL speeds? What if in the future someone figures it out by tapping into dark matter or dark energy? If one or both exist everywhere in the universe, and, presumably, beyond, and you only need to tap into it, not actually "use it up" like a fuel, there's your energy source.

Or maybe man figures out how to cheat and uses a stable wormhole if they're ever proven to be real, or figures out how to create one (Event Horizon). I know 100% for a fact it wouldn't happen in our lifetimes, but how can anyone say 2 Billion years from now if man still exists they won't be able to travel faster than light to an absolute certainty? Maybe technology will be created to remove the mass of an object in space, I don't know, it seems to me if that were possible light speed would be easy at that point. Most of what we know about the physics of the universe are theories. Like with black holes, which violates the laws of physics if information is destroyed, but since we can't really test that theory, we just go with it.

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    $\begingroup$ Hello ac smith. I don't think this is actually about astronomy, rather it is about physical theory, and should be asked on physics stack exchange, where it already has an answer. (You are right, new physics could open the possiblity of ftl travel, But it would be really surprising because it would allow us to change the past physicsmatt.com/blog/2016/8/25/why-ftl-implies-time-travel) $\endgroup$
    – James K
    Commented Mar 27, 2021 at 6:36
  • $\begingroup$ An Alcubierre Drive yould theoretically allow FTL travel without actually going faster than light. $\endgroup$
    – jng224
    Commented Mar 27, 2021 at 10:36
  • $\begingroup$ There's no need to travel faster than light. If you had a ship that could accelerate at a constant 1 g, you could cross the galaxy in about 12 years, ship time. See math.ucr.edu/home/baez/physics/Relativity/SR/Rocket/rocket.html $\endgroup$
    – PM 2Ring
    Commented Mar 27, 2021 at 10:54
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    $\begingroup$ Can you add to this a link or a citation or a quote from some authoritative source that says this is "impossible" and then does not go on to explain why? Just saying "People say this is impossible; everyone's saying it!" without evidence that people with knowledge of topic really do say this doesn't work as the premise of a Stack Exchange question. I think you'll find out that knowledgable people generally explain why it's currently thought to be impossible. Maybe the person down the street or on some nonstandard YouTube channel says it, but maybe those are not the right folks to quote. $\endgroup$
    – uhoh
    Commented Mar 27, 2021 at 12:01
  • $\begingroup$ If you google "faster than light" (no quotes), you'll see dozens of pages that claim faster than light travel is impossible and dozens of pages that claim it's possible. One of my favorites is the disagreement Isaac Asimov and Arthur C Clarke had on the issue: coopertoons.com/caricatures/… $\endgroup$
    – Guest
    Commented Mar 31, 2021 at 2:32

3 Answers 3


It’s not people who say we cannot travel at or faster than the speed of light, it’s the theoretical models that say so.

Those models, in particular, general relativity, are the best theories available so far to explain everything we can observe and nothing we observe is in contradiction to those theories. In the frame of those models we must come to the conclusion that travel at or faster than the speed of light is impossible.

Of course we already know, at least there is the assumption, that general relativity in its current form cannot be the complete picture. One example: the underlying mathematical model allows for singularities (I.e. infinite values for some physical properties). The assumption is that when Quantum Mechanics and relativity theory are combined, a proper model without singularities will emerge. Than this new model may possibly also tell a new story concerning travel at the speed of light.


The strongest argument against FTL travel-- or communication-- is that it would allow causality violations just as traveling backwards in time would. Given FTL travel and different relativistic frames of reference, you could change things in the past. And no, it doesn't mean things would just appear to go back in time; you would have frames of reference in which effect actually would precede cause. This link provides a thorough explanation.

So the same aspect of reality that seems to forbid going back in time forbids FTL travel.


First, there is no "...amount of energy it takes to travel at light speed." That is the issue. The amount of energy required to approach the speed of light increases asymptotically, reaching infinity at the speed of light itself. This is the issue. There is no conventional way to accelerate mass to the speed of light threshold. That said, Relativity does not prevent objects from traveling faster than the speed of light, only from being accelerated from below to above that threshold.

1:10-2:20 is the relevant section.

SpaceTime, on the other hand, is essentially unlimited in its expansive velocities. The commonly accepted Inflation Theory posits that the entire Universe expanded many times the speed of light for a brief period just after the Big Bang. Expansion continues to accelerate galaxies beyond the speed of light from our reference frame, with z ∼ 1.46 being the cutoff point. Our galaxy, from their perspective, has also been so accelerated.


Theorized warp drives, such as the Alcubierre Drive, hope to get around the acceleration issue by harnessing the velocity potential of SpaceTime to accelerate the effected SpaceTime region within the warp bubble, relative to which the Drive itself is essentially motionless, thus avoiding the light speed barrier altogether.


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