# If an immortal, fabricated particle could travel FTL, would it exist in all times, or only the past relative to its present?

This seemed too scifi or hypothetical for physics, so here goes.

Suppose a manipulated quark with 0 mass could travel faster than the speed of light. It's known that the faster you go, the slower time goes for you relative to an outside observer. At 1c, time essentially "stops" and everything outside that the particle sees seems to be standing still in time, while it continues to travel according to its own normal internal time. At 2c, outer time can't really get any slower than 0, so my reasoning is that it must start going backwards.

I am not asserting that my particle can accelerate from 1c to 2c, but rather trying to rationalize what will at > 1c in the first place, which this particle constantly travels at. In that case, wouldn't its vector through spacetime be negative, meaning it is bound to travel ever-backwards in time? Due to the expansion of the universe over time, would the universe would be shrinking for the particle, and all scattered masses of the universe actively unraveling and falling towards a single point in 4 dimensional space? As space compresses, its relative velocity would become slower and slower, until it arrives at the CMB, or if we add its negative time into the equation, the actual beginning of the universe, where negative time starts slowing back towards absolute 0?

If we observed this quark for a planck instant in our world, would we know without a doubt this quark has come from the future, and could we use this knowledge to acquire information from the quark, much like in the movie Interstellar, except these quarks come explicitly from the future rather than morse code from a timeless existence? Or would we not be able to detect its existence at all as it passes through our spacetime?

EDIT: reading the answers and comments so far, I'd like to point out that all or most of you seem to know a lot more about this than I do. To the point that now I am second guessing the validity of my question. In terms of what I deem to be a "good answer," everyone seems to be doing a great job just by providing their thoughts on the matter, even if in some cases your answers are in conflict with one another. I said quark just to give it some validity, as quark is a generic term for subatomic particles, which are the only things I can reasonably imagine to travel FTL in the first place. Tachyons and antiparticles seem to describe this proposed "quark" in more cohesive terms. I'm not up on my spin and colour game so I can't speak for that sort of technical jargon, but it makes sense that they would be inverse if they were travelling backwards in time. I said the particle has 0 mass, because I thought it was a necessary prerequisite for FTL in the first place. It's not important that the particle has 0 mass, it actually helps if it could have mass, because then there is logical recourse for the particle to begin to make observations of its surroundings.

In the spirit of clarifying concrete questions, are tachyons/antiparticles proposed to travel only backwards in time, by some informed schools of thought? Also, how might we go about identifying them and possibly extracting info from them? If you answered these already, of course disregard. Bonus question, do you feel that my interpretation is accurate, where the particle eventually arrives at the beginning of the universe, where its relative velocity, and the progression of time itself approaches absolute 0? If so, then I'm imagining the beginning of the universe, where there is rapid outward expansion of spacetime, energy, and particles, while at the same time, negative spacetime, negative energy, and antiparticles are all rushing towards point 0 to be consumed by a state of timelessness and non-existence.

• Have you read up on Tachyons? They are the real-life physics version of what you describe. (Well, they're the theoretical real-life physics version, with all the rigor we apply to real particles, except nobody has ever found a reason to believe they actually exist yet) Commented Mar 16, 2017 at 21:27
• Don't get me started with tachyons. Commented Mar 17, 2017 at 6:32
• All very interesting questions. However I'm not sure what sort of answer you expect. Any answer relying on current physics would end up handwaving at Tachyons or telling you it couldn't happen. One danger of lies in specifying this particle is a quark - Did you see this as important? (Are you interested in the colour charge? Typically anti-matter is treated as moving backwards in time but it isn't actually believed this is happening. Would your particle have anti-colour? It would most likely break conservation laws if it did but wasn't an anti-quark). What do you imagine a good answer to be? Commented Mar 17, 2017 at 10:23
• A particle with zero mass would travel at c. It could only travel faster than c if it had mass (and then it would be a tachyon) Commented Mar 17, 2017 at 16:36
• I've edited new info into the bottom of my answer. Thank you all for the comments and critiques. Commented Mar 17, 2017 at 21:43

Travelling backward in time is a common description of anti-particles. It would appear to have the handedness and all charges reversed. In fact, this is necessary due to quantum mechanics needing to exist outside the light cone, and spacelike separated events do not have a universal past/future ordering. So is that an electron moving one way or a positron moving the other? Am I emitting a virtual W+ or absorbing a W−? It must be such that either view is right.

Second, you are thinking of space-time diagrams. Look at some of my FTL posts for more on the subject. Now draw your FTL transit track as you described.

In your case, you are wondering how it would be experienced by other things (world lines) if this was an actual trajectory or world line of a particle.

Here is one of the drawings I made for FTL discussion: The green line would be the world line of a FTL object.

Well, you can see that any normal world line will not stay close to it over time, so the particle will be moving fast. Draw in light pulses (as I do in orange) and return echos, to see how you will “see” this motion.

What is experienced by the particle? As I allude to here (“time inside the ship”) the proper time simply does not compute. The reason why light speed is a limit is far deeper than the “reason” you note; Minkowski spacetime simply won’t let things move through it that way. So you might make up your own nature of how time passes to this object. Once you decide that, you can look at where it crosses other world lines and how its own radar would work, and build of narratives on what it experiences in the situation, and compare that with what normal world lines are saying.

• common description? Any references for that? All my years studying physics, especially particle physics, and I've never seen it. Commented Mar 17, 2017 at 6:28
• How about this as a starting point? Or wikipedia, or the Messenger Lectures that I linked to, and every popular book/article (e.g.) showing how Feynman diagrams can be interpreted different ways as you rotate them. Commented Mar 17, 2017 at 6:47

There is no difference between time moving forwards and backwards. If a particle moving backwards in time, it will be observed as a antiparticle which is interesting but no one will immediately jump to the conclusion that the object is coming from the future. Observations at this point will not help, but, if the observers know that the particle is moving 2c, they might deduce that it moves backwards in time.

However, I think that it will not be possible to interact with the object for it to carry information back in time. After all any manipulation mechanism will at most move at the speed of light and will not be able to reach the object. Head on collisions might never occur or they might end up being spectacular light shows. Either way you will not be modifying the object in a meaningful way. Only way I could think of is to embed information before speeding the object back in time, which might not be possible as the object probably will be created moving backwards in time similar to photons created moving at 1c.

No.

In the relativity, time is not a variable, it is a parameter. Time moving backwards doesn't work, or you had to step over the limits of the Special Relativity (this is about the effects of the speeds nearing $c$).

Anything moving any fast, including moving particles, are moving forwardly in the time and never back.

In the General Relativity (it is about black holes, wormholes and gravitational things), it is not directly forbidden, but always requires something which probably can't exist. For example, it would require matter with negative or imaginary mass.

On the math of the SR, FTL particles aren't directly forbidden. What is forbidden, is to step over the $c$ in any direction.

There are (were) hyphothetised tachyons, FTL particles. On the math of the SR, they had to have imaginary mass (consider a ball whose mass a $\sqrt{-1} kg$, very surreal).

Besides their "interesting" properties, their kinetical energy grows as they decelerate, they can't ever decelerate below $c$, and their mass grows (and their time slows, etc) as they decelerate. Thus, they behave exactly oppositely, as the ordinary particles, the bradyons.

If these particles can interact with our world, it causes that we can send a message to ourself into the past. Yes, we can see that as some like a "time arrow pointing back", but it may be better to interpret it as a logical contradiction.

Later there were quantummechanical descriptions of the tachyons where also negative mass was eligible and they didn't had to require the more surreal imaginary mass.

Note, the General Relativity doesn't forbid FTL travel. Something falling into a black hole, if it is below the event horizon, we can interpret it that it can't leave because it falls already FTL inwardly. Or: we don't really know, how big is the Universe, because anything far away as around 30-40 billion light years are moving away with FTL speeds from us.

What also the GR still forbids, is that there is still no FTL travel in local intertial systems (i.e. without gravitation).

If you want any non-conventional result with GR (alcubierre drive, wormhole, time travel, anything), first the things will be very complex and contradictory, and second, you need always negative matter, imaginary matter or some similar (and, of course, masses like the Sun as energy, concentrated into human-sized devices).

There are quantummechanical results which suggest that wormholes, time travel (backwardly) would cause such quantum fluctuations which destory their reason. I.e. if you have a "time machine" or a wormhole into the past, you will have also such quantummechanical effects which destroy their cause of existence. I can google for that a little bit if you ask.

There was a young lady named Bright Who's speed was faster than light.
She started one day In a relative way and returned the previous night.

A.H. Reginald Buller

Just because it's a fun question, I'll give this a go. You wrote:

wouldn't its vector through spacetime be negative, meaning it is bound to travel ever-backwards in time? Due to the expansion of the universe over time, would the universe would be shrinking for the particle, and all scattered masses of the universe actively unraveling and falling towards a single point in 4 dimensional space? As space compresses, its relative velocity would become slower and slower, until it arrives at the CMB, or if we add its negative time into the equation, the actual beginning of the universe, where negative time starts slowing back towards absolute 0?

You're covering a lot of ground here so this won't be short.

First, it's entirely hypothetical so lets just make that extra clear. Second, that it would actually travel back in time relative to the rest of the universe is a popular idea, but not the only possibility. A 2nd axis of imaginary time also works mathematically.

Take the basic relativistic principal that light is always observed at c, but an object traveling faster than c, light from behind it would never catch up to it. If it was created, lets say the geniuses at CERN create an FTL particle by turning their dial up to 11 and the FTL flew on it's merry way it couldn't see the scientists staring at it's path in astonishment cause the light from their astonished faces would never reach the particle. It could see the scientists past, but never their astonishment. IN fact, as it flew by planet after planet, alien after alien would see this unique FTL and think, wow, that's cool, but they could never catch it to study it, and it would never see the alien's reaction to it as that light would never reach it. In a sense, it would carry an event horizon in it's wake, unable to see any future of anything it flew past.

An FTL particle, would see what's behind it as it caught up to that light and what's ahead of it, as that light passed it. It would see both what's ahead of it and what's behind it on the same plane in the direction it travels, what's behind it, red shifted and moving backwards in time, what's ahead of it, blue shifted and moving forward in time. Now if you flip it's time dilation, that probably flips too, but the whole thing gives me a headache trying to work it out. At relativistic speeds, observations of time get complicated. Kind of like, if you fall into a black hole you wouldn't see the universe "speed up" as your time slowed down because the light and objects you're falling in with have the same time dilation as you. Traveling FTL, the particle catches up to the past even without negative time dilation. Observed time and real time get tricky at really high speed.

Also, since you specifically mentioned a particle, particles can't "see". Particles pass by stuff without seeing it or they interact, essentially bumping into something. It's the "you can't observe a quantum particle without changing it problem".

To "see", you need a method of photon resolution and capture. In other words, a lens and a photon receptor with a surface area that's designed to catch many many photons and produce a picture, like a camera or our eye. A particle, if it bumps into a single photon it's momentum changes. Particles only "see" what they hit, like a blind person driving a car.

So, lets say your particle is carrying a camera, now we can talk about what it "sees" and, what it sees is weird both running into the light ahead of it and catching up to light from the past from what's behind it, both appearing infront of it on the same screen.

That said, with many photos and careful measurements, it could calculate that the observable universe is a sphere heading towards a big crunch, but that would take some number-crunching. It probably wouldn't be obvious or readily apparant.

Imagine if you're traveling 2c and you're holding a flashlight. If you hold the flashlight ahead of you, no matter which way you point it, you see the light shining in your eyes, because you're catching up and passing it's light no matter which way it points, but it would change color as you twirled it.

If you hold the flashlight behind you, and turn it on, you'd never see the light because it would never catch up to you. You couldn't tell when it was turned on, unless it was turned on when it was ahead of you. But even in that weirdness, calculate the big crunch if you understood dopler and had accurate enough telescopes would be possible. It takes very accurate measurement of type 1a supernovas to even detect and measure dark energy. It's quite subtle.

Now if your FTL could see other FTL objects and not just rely on light traveling at c, then the universe might appear somewhat more like we're used to.

Now, as to your question "would it go slower", no. Dark energy doesn't slow down objects. It spreads out space between objects. As a result, your FTL might see distant objects slow down over time some over long periods of time. Near by objects, the dark energy effect shrinks to zero, so it would always have it's FTL relative velocity. All it would notice is very distant objects slowing down gradually. As to what it would see at the big bang/crunch - take this with a grain of salt, but I see no reason why it would slow down, unless it collides with something, but since FTL particles don't exist, all this is very hypothetical.

You wrote:

If we observed this quark for a planck instant in our world, would we know without a doubt this quark has come from the future, and could we use this knowledge to acquire information from the quark, much like in the movie Interstellar, except these quarks come explicitly from the future rather than morse code from a timeless existence? Or would we not be able to detect its existence at all as it passes through our spacetime?

There's a few issues here.

We observe particles when they crash into each other or by their effects. An electron has never been directly observed for example, but it's effects have been. We have to assume our FTL particle could be observed and measured. But measuremnet doesn't prove time. We'd need to know a lot more. Decaying particles, for example, are good at demonstrating time dilation because a partcle that decays in a nanosecond that travels at close to the speed of light, travels quite a bit further than 1 light-nonosecond (roughly 1 foot). It's theorized that observation of a tackyon, you'd see it move in two directinos away from itself. like the picture below. That would be a good example of FTL, not necessarily an example of traveling backwards in time (unless I'm missing something).

Evidence of an object traveling backwards in time is tricky. You'd need to observe either information from the future, or, perhaps, a revere decay. (again, I'm not an expert). #JDługosz gives a nice explanation of that here. Needless to say it gets pretty technical.

Now, people sometimes say antiparticles are particles traveling backward through time. And that kind of works on on paper, but antiparticles don't travel faster than c and they can't bring messages from the future - so we need to approach the "travel back in time" with some hesitation. It's not straight forward and what I've read, traveling backwards in time is only possible in quantum physics if there's a loop. No going back and changing history, only, it already happened, so it can, or it must, go back in time to do what it already did. No grandfather paradoxes in quantum physics. In other words, it doesn't matter if antiparticles travel backwards in time. They don't travel backwards in time in a way that we can ever use. We can't send messages into the past using antiparticles.

The tachyonic antitelephone is a clever idea, but 1) tachyons probably don't exist and 2) even if they did, there's no guarantee they'd be able to carry messages into the past, so . . . there's issues with real time travel, even, sending mesages into the past time travel. I don't want to say it's impossible, but there are big uncertainties.

So, to summarize, if there was an FTL particle, it might be posible to detect it but detecting it and using it to send morese code to the past are two different things. It's also worth noting that detecting a particle isn't a guarantee. Many particles were very difficult to detect. The Neutrino and Higgs Boson come to mind. Dark matter too was only detected by gravity. If FTL particles exist, there's no guarantee we could detect them.

You wrote:

In the spirit of clarifying concrete questions, are tachyons/antiparticles proposed to travel only backwards in time, by some informed schools of thought? Also, how might we go about identifying them and possibly extracting info from them? If you answered these already, of course disregard. Bonus question, do you feel that my interpretation is accurate, where the particle eventually arrives at the beginning of the universe, where its relative velocity, and the progression of time itself approaches absolute 0? If so, then I'm imagining the beginning of the universe, where there is rapid outward expansion of spacetime, energy, and particles, while at the same time, negative spacetime, negative energy, and antiparticles are all rushing towards point 0 to be consumed by a state of timelessness and non-existence.

Basically no. How particles experience time in and of itself is a bit unclear, but while defining an anti-Proton or an anti-electrion (positron) as a particle traveling backwards in time - from a certain perspectdive on paper, it works, but that doesn't mean it's actually traveling backwards through time and the big bang becomes a big crunch and the "end" of the universe. Covered in my detail here, but the first answer says that most physicists don't believe that. I don't htink there's much harm in it if you want to look at it that way, or use that as a though experiment, but I think it's fairly unconventional. Things can be looked at from a certain perspective, but that doesn't make it so.

To my thinking, for example, a Proton and an anti-Proton are almost the same thing they have 3 similar quarks with similar mass, just oposite spin. To my thinking, a Proton and an anti-Proton both move through time in the same direction, just one is right handed th eother is left handed. I think the move backwards through time is a clever metaphor, but not an accurate picture. - in my opinion.

Does a clock run backwards when you watch it in a mirror? Not really. But take the numbers away and it appears to.

The anti-particle question ad the FTL question are quite different. FTL may well be impossible and if it's possible, it might operate by imaginary time, not reverse time. imaginary time is fun, it opens up a whole nother series of questions. Distance dilation, as you approach the speed of light, the distance to objects grows smaller. Imaginary time, there's a 2nd axis of time, where, if you travel at 2c or 4c, at least by the equations, the distance to the person in the FTL ship grows larger, so, to the person on Earth, the trip to Alpha Centauri takes less time, to the person in the ship, the fastest trip is very close to c, as the ship travels faster than c, due to imaginary time dilation, the trip takes longer and longer cause the star moves further and further away. This imaginary time approach works better in many ways than the backwards time approach in general relativity. That said, until FTL is actually discovered, it's all just silliness.

Hope that wasn't too long and that it makes at least a little bit of sense.

If energy = mass * the speed of light squared, then a zero mass quark also has zero energy and thus zero existence since what we perceive as reality is vibrating waves that have at least some amount of energy.

Assuming the mass is instead an infinitessimally small number approaching zero, we may proceed.

I don't adhere to the postulation that going much faster than speed of light reverses the effects of time. Just because people are moving too slowly for you to perceive change isn't a ceiling. It is simply the limitation of your ability to perceive any difference, but if you had another person in your frame of reference going the speed of light then you would be able to better perceive how much faster you are going at greater than 1c.

I also don't believe that this tiny and unique particle would be discoverable. It took 50 years to find the Higgs Boson which we knew to be looking for, and that was a repeatable process.

• Your first sentence. Aren't photons massless with energy? Commented Mar 17, 2017 at 6:26
• @Mołot Yes, but zero rest mass results that the particle can't go below c, and its mass (with c) is determined by different formulas (for examle, in the case of light, it is $E=h\nu$, i.e. its frequency determines its energy and not its rest mass and speed). Commented Mar 17, 2017 at 19:52
• Erik, quarks don't have zero rest mass, their rest mass is 4 or 8 MeV, which is 10-20 times bigger as the electron. Although zero rest mass quarks probably wouldn't behave very differently (they are interacting with the gluon field of the atom and it would give them still some mass). Probably some isotopes would be stable or unstable, but the atoms would look nearly similar. Although it could have interesting side effects (for example, a little bit different deuteron mass would result that only a very few atoms would exist with a higher mass as helium, and thus probably we wouldn't exist). Commented Mar 17, 2017 at 19:54
• E=mc² only describes objects that have mass but aren't moving. The full equation, used for bodies in motion, is E²=(mc²)²+(pc)² Source: gizmodo.com/5955723/… Commented Mar 17, 2017 at 22:29