If faster than light travel were possible, how badly would it break physics?

One for science buffs - if you could dumb down the answer for a …dumb person it’d be appreciated.

So let’s say that one day a race of intergalactic supertrolls land in front of the White House and let us know that they find it just adorable that humans think c is the speed limit. They say “Watch this” and a half-second later come back with their ship and dump Voyager 1 on the lawn. With a cackle they set off.

How much of our physics goes into the trash? Would it be the equivalent of mathematics finding out that it was possible to divide by zero?

You would hear the loud pops of every scientists brains simultaneously stroking out and bleeding from their noses and ears.

Well, something would have to give. Either Special Relativity is wrong and the Universe really does contain privileged reference frames, or time travel is possible. Special relativity is extremely thoroughly tested, and has so far passed all of its tests with flying colors, so it would take quite a lot to convince us that it was wrong. With time travel, meanwhile, the objections are not so much physical as logical, with things like the Grandfather Paradox.

I would expect a mix of three outcomes: Physicists who conclude that time travel is possible, mathematicians who conclude that SR is wrong, and many of both who consider it a fluke or a hoax and don’t give it any further thought until we can replicate the experiment (at which point it would depend on the results of further experiments).

We would immediately understand that there is far more to reality than we have immediately access to. If not given access to their technology to study how it works, we might never find out, but even if we could only verify their superluminal travel without directly viewing it, whatever means they had of providing evidence there might be clues as to the nature of the phenomenon that would allow it. Most theoretical physicists would probably find grant money quite easy to come by.

I vaguely remember a sci-fi short story I read long ago (don’t remember the author) that revolved around a bunch of civilian scientists gathered together by the government in some secret facility and shown, under great secrecy, some seemingly-impossible feat that was filmed by government, supposedly done by aliens (don’t remember exactly what it was - maybe anti-gravity?). And asked if they can figure out what they did.

And in the end they do. Of course they find out later that there were no aliens, the film was fake, and it was all done to make them break through the barriers they had in their heads about it being impossible.

I think I read somewhere that FTL travel is not only possible but likely, and some of its dynamics would be inverse (e.g., it takes less energy to go faster). The problem is, to accelerate to the speed of light, the amount of energy needed to go faster increases on a curve that is parabolic: it is simply not possible, as far as we know, to provide enough additional energy to make it that last step from really, really, blindingly, absurdly fast and c. Similarly, the braking energy to get to c if you are going faster would be unattainable. The speed of light itself seems to be the really difficult barrier.


I believe you’re thinking about tachyons which only go faster than light and i does require energy to slow them down. I don’t think most physicists believe them likely to exist.

The harder you throw a ball, the faster it goes. That isn’t a theory, it’s built in to how the world works and you don’t need to understand anything about physics to understand that there’s no way to get around this deal. The harder you throw, the faster it goes.

So what if an alien threw a ball harder and it went slower? You’d want to know how, because there is no way to explain it. In the end, if you couldn’t replicate it, it would be a “curiosity” and wouldn’t change our understanding of physics a bit.

It depends on what you mean by “exist”. Even if they do in some sense exist, they can’t interact with tardyons (ordinary familiar particles like we’re familiar with), and if something can’t interact with us, what does it even mean to say that it exists at all?

“Dude, you brought that with you.”

I believe it would break all of relativistic physics, which is not just about speed but about the nature of space and time and everything in it – i.e.- matter and energy. It would all have to go, and be replaced by some unimaginable completely different theory. It would also have to account for time paradoxes, or these would in some unknowable way have to not occur. Or, IOW, ain’t gonna happen – or at least, vanishingly improbable.

If an object is 100 light-years away, it only has a meaningful existence in terms of classical simultaneity the way that we see it, that is, 100 years in the past. In relativistic terms “now” on this other planet is therefore 100 years in the future, the time it would take us to get there at c. If we somehow went faster we would arrive in the past, and we could affect causality. For example if we traveled twice the speed of light and arrived in 50 years earth time, and changed the luminance that was being observed from earth, we would paradoxically somehow be affecting light rays that had already left the planet 50 years earlier! The paradox is particularly apparent if we returned to earth at a similar superliminal speed. We might arrive before we were born, causing all manner of interesting impossibilities.

This captures the spirit of the dilemma:

*There was a young lady named Bright
Who could travel much faster than light.

She departed one day
In a relative way
And arrived on the previous night.*

Note of course that time travel itself lets you travel faster than light. You just get in your spaceship and head for Alpha Centauri at 50% of c. Eight years later you arrive. Then you set the time controls for 7 years and 364 days in the past, and now it took you only a day to travel 4 light years.

Or you can set the clock for 8 years and 1 day, and now it took you -1 days to travel 4 light years.

I don’t think it would ‘break physics’ so much as it would cause us to examine the laws of FTL travel that, up until now, we’ve had no reason to even bother with other than on a theoretical level. My first WAG is that E=MC^2 would still hold up for Less than FTL travel, since it has for so far. So unless their vehicle went to infinite mass, we’d have to find a different equation for FTL travel, either that or we’d find a new and/or improved version of FTL travel. And and there’d probably be a lot of money poured into relativity study.

Keep in mind, if it happened, physics isn’t broken, we just need to figure out how to explain it. For a simpler explanation, if I suddenly figured out how to walk through a plate of glass, I didn’t break physics, but physicists would have to take another look at impenetrability since it’s clearly not a given anymore.
Perhaps maybe E=MC^2 up to C and then something changes after C that we need to take a look at since up until now we’ve had no way to experiment with.

I just heard NdGT speaking about FTL travel and saying that, if someone traveled that fast, that they would/could technically exist everywhere at once since for the people observing it time would stand still. That is, this ‘alien’ with this technology could travel to anywhere in the, well anywhere, and you (the observer) wouldn’t even know the ship moved.
To my mind, that’s the part, if any, that would ‘break physics’. If we could magically* adopt FTL travel, what happens with all these vehicles that can suddenly exist everywhere at once. Yes, I know, they don’t actually exist every where at once, but an area of relativity that was theoretical is suddenly reality. IMO, that’s where the real fall out is going to be. (But I could be way off base, this is based mostly on listening to Star Talk on a semi-regular basis).

However, while the predictions of special relativity (and with somewhat less precision, general relativity) have been validated in their own scale of application, they break down when approaching the level of quantum interactions; it takes a certain degree of not-quite-handwaving to get special relativity to work with electrodynamics field theory, and of course we have no accepted workable quantum theory of gravitation which functions within the provisions of general relativity, which suggests that such theories are only approximations of a deeper and more fundamental theory that unifies all interactions in a coherent and symmetric fashion, albeit one that has to function pretty close to the observations we’ve currently made that validate QED and SR.

Causality (and the assumption that arbitrary travel along the time dimension), on the other hand, is purely an assumption that lets us sleep at night, and there are all sorts of ways to violate causality within the framework of special relativity, albeit not without some problems in creating the sufficient conditions in a stable universe that doesn’t contain negative mass or fields of negative energy density. Within quantum mechanics, however, causality in the colloquial sense can’t even rigorously defined without assuming a series of other postulates that are even more distressing from a relativity standpoint.

Practical, demonstrable superluminal travel would certainly require a recasting of what we think we know about fundamental gravitational physics today, but it wouldn’t “break” physics any more than the discovery that there was no luminiferous aether to convey electrical waves through a vacuum stopped wireless radios from transmitting or the sun from conveying its warming radiations through ninety-three million miles of space to our pleasant corner of the universe. The exact scope and mode of retooling would depend upon the specific mechanism by which this faster-than-light traverse was conducted, but as long as it doesn’t violate some previously experimentally observed limitation, e.g. mass-energy isn’t literally being accelerated to or exceeding c via transfer of local momentum, then it just becomes a new phenomenon that we have to figure out modifies or exists below our current approximation of fundamental physics.

The bigger question then becomes a somewhat philosophical one; how do we live in a world where causality at a macroscopic level can no longer be assured. You may think that Back to the Future was a bit of a mess with all the Tannens and McFlys shooting around hither and zither, but that is nothing compared to the mess we’re going to have when anyone with a spaceship and a superlumobilizer can go back to last week and bet on the winning horse at the pari-mutuel track, or knock off your grandfather in order to eliminate your affections for his would-be paramour. There is a reason that time travel stories comprise a substantial sub-genre of science fiction; because, in a very real sense, the lack of causality and a rigorous arrow of time is kind of the ultimate in existential terror.


You really should buy a TV.

For Beauty is nothing but the beginning of terror, and we adore it so because it serenely refuses to destroy us…

That was Rilke. But Leo asks: OP has macroscopic, but why you in your response about the real world? Bizarro-ness is already freaky in quantum levels. Could FTL not exist, somehow (given your equanimity, pace [a big pace]) the aforementioned terror, while still letting us macroscopic types to go on our day to day as assuredly as we do now, expecting and working quite well with our yes/no reality of things and not particularly mindful of a statistical result determining that my grandfather exists?

(FTR, he’s dead, so he doesn’t.)

ETA: assuming no FTL.

wolfpup, while it is true that special relativity states that FTL implies time travel, the explanation you give is not the reason why. There are two definitions of “now” that one might use in relativity, and in neither of them is “now” for an object 100 ly away 100 years ago. In the first definition, “now” means “at this time and in this place”, and so a distant planet doesn’t even have a “now”. In the second definition, “now” just means “at this time”, but “this time” depends on your frame of reference, and depending on your frame, “now” for that distant planet could be anything up to (but not equal to) 100 years in the past or future.


I thought I was making a funny, having read “Star Trek” instead of Star Talk.

I really should buy a brain.

I’m glad you clarified that because I was confused. OTOH, that discussion (on Star Talk) came from a question about Warp Drive on Star Trek. Having never watched ST, I hesitate to repeat the question (and NdGT’s answer) word for word for fear of botching it since it’s not all that the vernacular isn’t all that familiar to me.

Although this site says the following:

IIRC, Neil said, Warp 10 or Warp 1 (C) is basically the same WRT that statement. At which point whichever comedian he had on that day made a joke about them being sold a feature they really didn’t need since any FTL travel was good enough and being being able to travel ‘infinitely fast’ was just as good as being able to travel at C.
(But again, the terms aren’t that familiar to me, so I may have them wrong.

In other words, in relation to any particular reference frame, you are always moving at a constant rate, c, through space-time, but if you are stationary or with a very slow change of distance to the reference frame, most of that rate is through time. When you are moving at a high rate of speed, the motion with respect to time (as measured by an observer who is stationary to the reference frame) is slowed. The ability to travel through time at an arbitrary rate separate from your velocity with respect to a specified reference frame would require either non-local connections between points in space-time or a fundamental retooling of the basic premise being special relativity (i.e. that some reference frames are “privileged”, allowing objects to move at different rates through space-time than c). There is no particular reason that either of these exceptions couldn’t be true except that they would invalidate special relativity and we’ve seen absolutely no signs of this in the last century and change that we’ve been testing them. Quantum electrodynamics (QED), the relativistic theory of electrodynamics, is the most precisely tested theory in all of science, and despite a bit of wonkiness required to make it work theoretically has defined the parameters of special relativity at the fundamental level to an extent that any alternative theory would have to match those observations while falsifying the other assumptions of relativity. It’s not impossible, and in fact, many consider it highly likely that it is only an approximation of a more fundamental mechanism, but in order to allow for “time travel” or “faster than light travel” (which are, as others have pointed out, the same thing) at a macroscopic level there has to be some special set of conditions that are not seen in normal experience.