# Why can't we go faster than light? Can't we technically do so?

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Why is there a "speed limit" in the universe? According to special relativity, nothing can move faster than light, but Newton's first law states that unless an external force affects it, an object's speed will always stay the same. So, if we find a large space of perfect vacuum, with no force affecting any part of it, and if we had enough fuel and thrust, we could technically go at the speed of light and faster since there would be no external force affecting us, there would be nothing to slow us down.

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That's not quite how special relativity works. The speed of light isn't a cosmic speed limit, at least not the way we think of speed limits (like on roads). Rather, it represents a fixed velocity-the speed of light will always seem as fast as it is, regardless of how fast you're moving. That is to say, the speed of light will be the same relative to you, regardless of whether you're moving (from the perspective of an outside observer) at 1% of the speed of light or 99%. What happens instead as you approach the speed of light, relative to the resting reference frame, is that time begins to pass slower for you than it does for an observer (among other things), and that your rate of acceleration relative to the resting reference frame decreases.

In other words, if you start from 0 m/s and accelerate by 2.8x10^8 m/s, light will still appear to move away from you at 2.998*10^8 m/s. And, to everyone looking in on your spacecraft, time will be passing slower for you than it is for them.

Make sense?

Edited by IncongruousGoat
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Relativity superseded Newtonian physics in our understanding of the universe. Newtonian physics is useful and fairly accurate for certain applications, but this is not one of them.

Basically, you're asking the wrong question. Being unable to go faster than light is not what relativity states, rather, it implies this, since light has the same speed in all reference frames (in vacuum....). It partially comes from observations. We get the same answer for the speed of light in a vacuum whether we're on a jet plane going mach 3 or "stationary" on the ground, or orbiting at many kilometers per second. The answer comes out the same. Since this is true, light must have the same speed. Not only that, but for Maxwell's equations to work, the speed of light must be constant. Essentially, in a region with no charge or currents (as in a vacuum), the equations reduce down to having c and some other already known fundamental constants. Essentially, if c was not a constant number, these constants wouldn't be constant either, and of course that would wreak havoc on known physics. So, since c must be a constant, then it must be the same in all reference frames, otherwise it is not constant for charges (which have different frames depending on what you're calculating). So, light speed must be a constant in all reference frames. This implies that it is impossible to go faster than light.

(If there's anyone more knowledgeable on this subject on these forums, please listen to them instead)

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2 hours ago, Grand Ship Builder said:

Why is there a "speed limit" in the universe?

Why is there a universe?

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Apparently interactions between two quantum entangled particles are instantaneous over great distances. So, logically - there should be something that moves faster than photons. Tachyons?

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2 hours ago, IncongruousGoat said:

That's not quite how special relativity works. The speed of light isn't a cosmic speed limit, at least not the way we think of speed limits (like on roads). Rather, it represents a fixed velocity-the speed of light will always seem as fast as it is, regardless of how fast you're moving. That is to say, the speed of light will be the same relative to you, regardless of whether you're moving (from the perspective of an outside observer) at 1% of the speed of light or 99%. What happens instead as you approach the speed of light, relative to the resting reference frame, is that time begins to pass slower for you than it does for an observer (among other things), and that your rate of acceleration relative to the resting reference frame decreases.

In other words, if you start from 0 m/s and accelerate by 2.8x10^8 m/s, light will still appear to move away from you at 2.998*10^8 m/s. And, to everyone looking in on your spacecraft, time will be passing slower for you than it is for them.

Make sense?

1 hour ago, Bill Phil said:

Relativity superseded Newtonian physics in our understanding of the universe. Newtonian physics is useful and fairly accurate for certain applications, but this is not one of them.

Basically, you're asking the wrong question. Being unable to go faster than light is not what relativity states, rather, it implies this, since light has the same speed in all reference frames (in vacuum....). It partially comes from observations. We get the same answer for the speed of light in a vacuum whether we're on a jet plane going mach 3 or "stationary" on the ground, or orbiting at many kilometers per second. The answer comes out the same. Since this is true, light must have the same speed. Not only that, but for Maxwell's equations to work, the speed of light must be constant. Essentially, in a region with no charge or currents (as in a vacuum), the equations reduce down to having c and some other already known fundamental constants. Essentially, if c was not a constant number, these constants wouldn't be constant either, and of course that would wreak havoc on known physics. So, since c must be a constant, then it must be the same in all reference frames, otherwise it is not constant for charges (which have different frames depending on what you're calculating). So, light speed must be a constant in all reference frames. This implies that it is impossible to go faster than light.

(If there's anyone more knowledgeable on this subject on these forums, please listen to them instead)

While i'm not exactly read up on the math that goes into this, the issue I have with this whole idea is that it doesn't actually take into consideration all actual reference frames where time dilation doesn't cause the same effects. If you step out of the universe and look at a race between light and an object at 99%c, it should read like it would at normal speeds. Light would win the race by that 1%. Presuming the object can actually continue to exceed c, then it would start winning once it overcomes the head start light has.

Unless the universe just doesn't work in a logical way I can't see any way this wouldn't be what would happen. This is also why I don't think FTL breaks causality. With a limited reference frame supposedly you're breaking it, but it doesn't take into account that there's more frames of reference than just space and time, and that ultimately the object doing the travelling (be it energy, light, or physical matter) still has to actually travel a distance as c is not instantaneous. If we try sending signals into our past we might technically do it, but all we'd see in the past is just garbled noise if anything at all.

Now, that doesn't handwave the problem of FTL.  Time dilation is still a problem obviously for in-universe observers as well as the travelling object. But this doesn't make exceeding FTL impossible, just means doing so is ineffective as a means of travel; time dilation says that much, and the only way I can think of that might solve that is if we could instantaneously accelerate to superluminal speeds well in excess of c. IE, speeds so fast its indistinguishable from teleportation even to the observer outside of the universe.

And this is why the best odds we have for "FTL" travel really aren't FTL travel at all, but rather a way to just get around having to do it that way.  Alcubierre (lets remember this is all theoretical here. I think we all know the issues with this) warp, wormhole generation, generational ships, etc.  Why engineer some fantastical way to exceed the speed of light and avoid time dilation when we can engineer some less fantastical ways to abuse spacetime or just do it the low tech way and let a 5th generation of descendants be the ones to arrive at the destination?

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This is the clearest answer I could find:

Quote

All the particles that move at the speed of light (e.g. photons) have zero rest mass. As a particle with mass approaches the speed of light, its energy increases and becomes infinite at the speed of light, which is the reason why it can never be accelerated to reach that speed.

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2 hours ago, Scotius said:

Apparently interactions between two quantum entangled particles are instantaneous over great distances. So, logically - there should be something that moves faster than photons. Tachyons?

In theory, it is not possible to transmit any information using quantum entanglement. Since no information is transmitted, there is no need to posit any particle moving faster than light.

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2 hours ago, Scotius said:

Apparently interactions between two quantum entangled particles are instantaneous over great distances. So, logically - there should be something that moves faster than photons. Tachyons?

The key word here is "apparently", which means you have to observe something else and convey that it is what it is. Communicate with photons or any other conventional things. So... no.

The other way of getting faster than light is simply moving away from something so far that the universe do it's job of expanding it's parts inbetween such that you move faster than lightspeed wrt that thing.

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3 hours ago, G'th said:

While i'm not exactly read up on the math that goes into this, the issue I have with this whole idea is that it does.n't actually take into consideration all actual reference frames where time dilation doesn't cause the same effects.

It does consider all frames of reference.

That is the very hypothesis of the theory of relativity: the speed of light is the same for all observators. Given that after a century all the attempts to falsify that simple statement failed we end up with no choice but accept it's consequences*.

The universe don't care if you have issues with that. Accordingly with the best info we have, that ia how it works.

*Answering OP's question, one of the consequences predicted by relativity is the distortion of the spacetime. For a photon (which always move at speed of light) the space is so compressed that in the photon's frame of refence there is no here and  there, the distance is 0 and the photon can transverse it in no time.

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I thought of this idea too. Untill i realized photos aren't like marbles.

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10 hours ago, Grand Ship Builder said:

Why is there a "speed limit" in the universe? According to special relativity, nothing can move faster than light, but Newton's first law states that unless an external force affects it, an object's speed will always stay the same. So, if we find a large space of perfect vacuum, with no force affecting any part of it, and if we had enough fuel and thrust, we could technically go at the speed of light and faster since there would be no external force affecting us, there would be nothing to slow us down.

Short answer, because matter embedded in spacetime is subject to the properties of that space-time. Space-time is not just literal empty space with literally nothing in it and no possible detail. It is actually, when you go right down into the weeds, a rather complex thing. At high relative speeds (in space, speed can only be measured "relative" to another object, there is no "fixed point" to measure against) the relative flow of time stats to alter, yes this is for real (eg: GPS satellites need a very accurate clock to work. But there are orbiting the EArth at several thousnad miles per hour, this has a measureable effect on the flow of time for the satellite relative to the EArth's surface and must be corrected for.)

One should also note that many of our most fundamental scientific principles are only our best approximations. Such as, particles like electrons and protons are not actually little spherical balls, and have properties which, say billiard balls, do not and under certain circumstances they do strange things. Light also, is not a particle, although sometimes it acts like it, an "wave" is a very vague term for the propagation of electric and magnetic fields, although someimtes light does not even act like a wave.

So the short answer to "Why cant we go faster than the speed of light" is that it is due to the fundamental properties of the universe that we live in. The way that time, distance and velocity interact only resembles the "classical" "billiard ball" model at certain scales - which happen to be the scale most familiar with us (scale on the order of a metre, give or take a few order of magnitude) - just like how a predictive model only works within a certain set of conditions and if you go beyond them the answer they spit out will not be correct, like if you hacked a flight simulator to take you to space, the simulator has no data on how an aircraft would operate in space so now it give you wrong answers.

So essentially the answer is because of the interplay between relative speed, distance and time, which are all defined by the fundamental properties of our universe.

Or TL;DR, the answer is literally, "because it just is ok?"

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5 minutes ago, p1t1o said:

(eg: GPS satellites need a very accurate clock to work. But there are orbiting the EArth at several thousnad miles per hour, this has a measureable effect on the flow of time for the satellite relative to the EArth's surface and must be corrected for.

They're not as deep into Earth's gravity field as we are, that has a much bigger impact on time dilation than the velocity they're moving at. But that is also a relativistic effect, and the result is the same: those clocks experience time at a different rate than we do.

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1 hour ago, NSEP said:

Untill i realized photos aren't like marbles.

I fail to notice where  it was implied otherwise.

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It comes down to two reasons.  First F=dp/dt  which traditionally looks like ma, but thanks to the Lorentz transformation goes asymptotically to the speed of light.  No known means of accelerating mass will reach the speed of light.

The second is that much of Einstein's reasoning behind the "absolute speed limit" involves casuality (effects only happening after the cause).  While the "twins paradox" may *appear* to be a paradox, "shooting your father before you were born" would be a real paradox.  Assuming that Einstein is correct and that a photon effectively travels over zero time, exceeding the speed of a photon would change the "past".  All attempts to get quantum effects to transmit information faster than light have failed (something to point out when people claim to always look to quantum mechanics when it disagrees with general relativity), making this appear to be true.

I don't think Star Trek intentionally included "traveling in time and changing the past" because it would be possible in a universe that allows warp drive, but it does balance that bit out (although in reality they would be causing "grasshopper wing" effects *all* the time).

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16 hours ago, Grand Ship Builder said:

[...], but Newton's first law states [...]

95% of what you are taught in high school physics, up to and including Newton's laws, is wrong. Not wrong in the sense that it is completely incorrect, but wrong in the sense that it has since been superseded by modern theories that are more correct. It's important to understand that all scientific models of the universe are descriptive approximations of how we think the universe works based on the observations that have been made, and that new observations can subsequently overturn previous models that fail to account for them. So Newtonian physics is a model that explains the observations that were available to Newton at the time, but failed to adequately explain later observations (most importantly, its inconsistency with Maxwell's equations of electromagnetism). Special relativity was formulated as a new model to replace Newtonian mechanics (and was itself later superseded by general relativity, which extended relativity to include gravitation). But the current theories (general relativity, quantum chromodynamics, etc...) aren't taught in high schools because they require mathematics that, for various reasons (most of them stupid), also aren't taught in high school. So the older theories are taught instead, because they're considered "close enough" for the needs of most laymen. (And indeed, even in physics, we still use Newton's laws as a first-approximation for non-relativistic motion.)

The take away from all of this is that you can't apply Newton's laws in a relativistic frame work. It don't work like that.

As for an actual answer to your question, it specifically comes down to the fact that at relativistic speeds, F does not equal ma. More specifically, because of mass-energy equivalence (the ever-so-famous E = mc2), as an object with mass gains velocity, it also gains (relativistic) mass from its kinetic energy, which means that you need more force to accelerate it the same amount. The faster you get, the more mass you gain, the more force needed to accelerate, increasing asymptotically as you approach the speed of light. What this means is that you can approach the speed of light, but never reach it.

Photons can travel at c because they have no rest mass. (They do, however, still have energy, and thus momentum.) (Also, photons don't accelerate. They're always traveling at light speed.)

Other than the impossibility of accelerating an object with mass past the speed of light though, there's nothing specifically preventing you from traveling faster than light. Although if you did, it would violate causality, but to explain why I'd need some more time to prepare diagrams. And also probably some calculus.

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Non-technical explanation: nitpickers, please don't bother complaining that I'm mis-using the word "heavy," that photons actually behave as if they have a tiny bit of mass, and that sort of thing. I am trying to keep this as simple as possible.

To make an object go faster, you have to push it. The object now has more energy, but something we do not encounter in the everyday world is that energy itself has a tiny bit of bulk to it, so that when you add energy to the object, it actually behaves as if it is heavier.  We do not notice it because the effect is so tiny, but when you put A LOT of energy into the motion of an object, the effect becomes apparent. What happens when you push a heavier object? It doesn't move as much, right? So as you keep making the object go faster and faster, it behaves as if it is heavier and heavier. You may start off pushing a grain of dust to reach light speed, but as it gets closer to that speed, it starts to respond as if it's as heavy as a baseball, then as heavy as a bowling ball, then like a car, a building, a mountain, etc. You can just keep pushing it and making it go faster, but it responds less every time. Due to some very complex math which, to be honest, I do not entirely understand, the result is that ALL THE ENERGY IN UNIVERSE would still not be enough to make that original grain of dust reach light speed. Only things which had no mass to start with, like photons, can be pushed hard enough to reach light speed, because if they're mass is zero, anything multiplying that zero mass still results in zero mass. And if you can't make something reach light speed, you certainly can't make something go beyond it, either.

Science fiction stories come up with all kinds of ways to get around this rule, but they're based on speculation rather than knowledge, and nobody knows of any way to do it for real.

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2 hours ago, wumpus said:

The second is that much of Einstein's reasoning behind the "absolute speed limit" involves casuality (effects only happening after the cause)

Einstein reasoning behind the constant value of the speed of light was "Maxwell's Equations says so, every other result of those equations is consistent to observed reality" .

So no, the reasoning don't involve casualty at all. As @GreenWolf explained scientific models are not The Truth but just our current best approach to explain what we observe.  In that context the individual ideas of the scientist is of little (to no) relevance,  the value of or model is the ability to make precise predictions/descriptions of what happens in reality.
In the case of relativity,  many of those predictions stands just short of breaking casuality and this is an apparent 'weak point' since we always observed effects after causes. But while relativity comes close to the edge, no one got it crossing the line.

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1 hour ago, GreenWolf said:

Other than the impossibility of accelerating an object with mass past the speed of light though, there's nothing specifically preventing you from traveling faster than light. Although if you did, it would violate causality, but to explain why I'd need some more time to prepare diagrams. And also probably some calculus.

And while FTL travel is not necessarily impossible,  we also have never observed it. So any model can be 'consistent' with the observations, equally 'correct' and equally irrelevant. As Vanamonde pointed we have no knowledge about it.

1 hour ago, GreenWolf said:

Other than the impossibility of accelerating an object with mass past the speed of light though, there's nothing specifically preventing you from traveling faster than light. Although if you did, it would violate causality, but to explain why I'd need some more time to prepare diagrams. And also probably some calculus.

And while FTL travel is not necessarily impossible,  we also have never observed it. So any model can be 'consistent' with the observations, equally 'correct' and equally irrelevant. As Vanamonde pointed we have no knowledge about it.

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Here's an answer from a guy at Fermilab framed around the unity of spacetime. I think he uses "speed" in a confusing way towards the end, but otherwise it's an interesting way to think about it.

Spoiler

Alright, got it.

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21 hours ago, Spricigo said:

It does consider all frames of reference.

That is the very hypothesis of the theory of relativity: the speed of light is the same for all observators. Given that after a century all the attempts to falsify that simple statement failed we end up with no choice but accept it's consequences*.

The universe don't care if you have issues with that. Accordingly with the best info we have, that ia how it works.

*Answering OP's question, one of the consequences predicted by relativity is the distortion of the spacetime. For a photon (which always move at speed of light) the space is so compressed that in the photon's frame of refence there is no here and  there, the distance is 0 and the photon can transverse it in no time.

I'd be interested to see how these scientists were able to step out of the universe in order to confirm this.

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58 minutes ago, G'th said:

I'd be interested to see how these scientists were able to step out of the universe in order to confirm this.

8 hours ago, Spricigo said:

...never observed it. So any model can be 'consistent' with the observations, equally 'correct' and equally irrelevant.

Anyway if you really think Relativity is 'wrong' figure out how to observe something that make it evident.  That is the scientific method and it fits very well in scientific discussions.

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I have wished to say this for a long time but so far I could not hold anybody's attention long enough to completely be comprehended.

Star Trek gets around going faster than the speed of light based on the mathematics of Energy = Space (operand) (unknown components and variables). The complete formula is supposed to be the work of Zephram Cochrane. Albert Einstein simple formula of E=mc2 is supposed to be the guide to the fictional character of Cochrane's E=S(space) plus some unknown operands and variables. The concept is to use energy to create space and encircle a starship with it (the fictional name is subspace), the second thing to do is to draw space tightly in then suddenly release it behind the ship. Space will then expand back to its original position and the ship rides the wave. A wave is not a particle and as such space is drawn in tight and suddenly released against the artificial space (subspace). This subspace does not exist in normal space and so is not subject to normal space physical laws. The key to Star Trek's mathematic lies in the unknown formula that energy can create space if all the ingredients are there. In the real-world we don't know if this is possible. I notice some talk about Relativity, I'll have to read up on that again to enter into that discussion. One thing I wanted to clarify was this unknown mathematical formula that Star Trek uses to justify there faster-than-light starships. In theory it might work. The warpfield word used in Star Trek sounds like in means both the subspace field surround the starship and the compressing and expanding of space to create the wave.

I've been wanting to say this for a long time.

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