Hyperdrive, warp drive, Holtzman drive we have a lot of names for faster than light speed or
FTL space travel in popular science fiction to boldly go where no man has gone before
but sadly in reality traveling to our nearest neighboring star system is a distant dream and
would need us nearly 6,000 years to get there so how come we would ever be able to achieve such
a technology that can make us zoom through the entire galaxy in one lifetime or less
maybe never because we are not supposed to but why welcome the Factnomenal and
let's find out how FTL travel can create time travel and thus paradoxes
Space is big well to be honest that is quite an understatement it's ginormous so when an author
decides to write about a futuristic swashbuckling adventure in space they are bound to come up with
some sort of faster-than-light communication and travel help me Obi-wan Kenobi you're my only hope
if they don't do so planet hopping would become downright impossible as their protagonists
would be long dead before they can answer the call of distress on the nearest star system
however faster than like communication of travel breaks something very fundamental about physics
something that is often ignored by sci-fi and difficult for non-physicists to understand if you
allow faster than light travel and communication then you break causality and when causality breaks
you are allowing time travel in a nutshell the universe is asking us to choose two of these three
relativity causality and or ftl universe has made its choice already for us by picking relativity
and causality this is why it is on us to find a way to travel faster than light but will we ever
succeed Dr Stephen Hawking believed that we won't but before we talk about why dr hawking was such a
Debbie downer let's try to understand the hand the universe has dealt us the universe is largely made
up of space and time or space-time which according to Albert Einstein is a four-dimensional model if
we try to draw these four dimensions in a diagram well well we can't it's a two-dimensional screen
we can't even draw three dimensions on it but that shouldn't stop us from understanding space time
so let's just suppress all space dimensions and draw the entirety of space as a line
so if we are stationary at a point in the space line time will tick forward or orthogonal to the
space directions around us for others time will also move similarly no matter where they are
on the space line because of relativity now the special thing about relativity is that
everyone measures the speed of light to be the same it can be shown in a space-time diagram
by saying that every space-time diagram has light traveling at 45 degrees relative to the time axis
light travels on lines that are called null for example on this diagram let's say that two
null lines of light are emitted from an event that happened now and let's denote now as t equals zero
to not get confused for the rest of the video even though space is a line in this diagram it still
contains all of the space dimensions and these rays of light are really emanating out in a sphere
around us because light travels at 45 degrees anything traveling slower than light from this
t equals zero event is closer to the time axis than the light rays and anything faster than
light is further away from the time axis i know diagrams are hard to follow but let's keep this in
mind while we move to an example of FTL actually happening let's say we on earth have built an FTL
communication device that lets us talk to the inhabitants of the planet Proxima Centauri b
4.25 light years away as you remember where we are on the space line and where Proxima
Centauri residents would be is relative so let's make another point for them different from ours
this is because Proxima Centauri is moving at essentially the same
velocity as earth the differences are small compared to the speed of light
thus there are no big relativistic events between our counting of time and the proximal Centaurians
now let's imagine that some event occurs away from earth oriented in such a way
that the light from the event hits us before it reaches Proxima Centauri the space time diagram
for that would look like this first we see the light then the light reaches Proxima Centauri
notice that the light rays from the event are traveling at 45 degrees to the earth's time axis
after all it is light and light travels at 45 degrees on space-time diagrams so now let's add
in FTL communication we see the event we get on the FTL phone and we tell the proximal centurions
they get the phone call immediately and now have years to prepare for the arrival of the
light from whatever the event is we are already flirting with the boundaries of science fiction so
let's say it's a giant laser beam traveling at the speed of light obliterating everything in its
path and by coincidence it's on course to strike Proxima Centaurian's home planet now the diagram
with FTL communication added to the equation would look something like this do you see the problem
no well we all agree that the event happened first then earth calls Proxima Centauri then
the light reaches Proxima Centauri everything looks sugar and sweet there even though the
proximal centurions hear about the event early no causality has been violated after all we all
agree on what happened first don't we no effect precedes its cause but this diagram only depicts
the causality and ftl remember earlier in this video we mentioned how the laws of the universe
force us to choose two of three principles relativity causality and FTL and clearly
according to this diagram we chose causality nftl but the universe has already made the
choice to stick with causality and relativity so something's amiss we need to fix this the
timeline in the diagram only works when everyone is moving in the same frame of reference like us
and Proxima Centauri so to see the problem let's add a new observer moving at high speeds relative
to earth and Proxima Centauri say a spaceship moving at a speed faster than light as well
here's where the relativistic effects start coming into play relativity tells us that everyone moving
with constant velocity is totally justified in saying they are stationary so for a moment if
we ignore the whole earth revolving around the sun and the sun moving with the rest of the galaxy we
can say the earth is stationary and similarly Proxima Centauri is stationary but neither we nor
Proxima Centauri is moving at the speed of light the aforementioned spaceship is however for them
they are at rest too if they drew their own space diagram with their own axis their time
axis will be aligned with the ship's trajectory in addition they have a space axis just like we do
relativity mixes up space and time so their space axis would be slanted for us just like their time
axis is skewed it turns out that the space axis is flipped across the 45 degree null line this weird
mixture of space and time of observers we perceive as moving is a necessary part of relativity so
what happens now let's ask when the spaceship sees the various events in this diagram
to do that we need to know the lines of constant time for the ship that's not too
hard lines of constant time for us are lines in the space-time diagram parallel to the space axis
so it is for the spaceship their lines of constant time look like this now let's recall the events
we discussed earlier we see the space laser beam pass by us on its course to hit Proxima Centauri
we call Proxima Centauri on the ftl phone the Proximal Centaurians do whatever they
do in response to that call and then they see the light of the event but on the ship things will be
different according to the diagram they would see the phone call received on Proxima Centaurithen
they would witness the phone call placed from earth effect precedes cause thus causality
is broken now before you argue that light takes a finite amount of time to travel and a spaceship
can see the events happening immediately well you are forgetting that the light from the
phone call reception arrives well before the light from the placing of the phone call the
causality would be violated nonetheless basically that spaceship by traveling faster than light
is traveling in time and therefore has the power to cause time paradoxes
under relativity whether certain events occur simultaneously is no longer an absolute thing
but a relative one in this fascinating animation if b is stationary then events a b and c all
happen simultaneously however if b is moving towards c b's plane of simultaneity slopes upward
leaving c in its past on the other hand if b is moving toward a c is now its future this is why
the spaceship in our little hypothesis witnesses the event of Proxima Centaurians receiving the
warning call before the earth makes the call now they have information that can be used to
alter the course of events they can intercept the call when earth makes it or interrupt earth from
making the call in the first place this will not only break time but may break the entire universe
this is why the universe has put a safety measure a speed limit in the fundamental of physics that
we can breach even if future societies have strict taboos against interfering with the past
the idea that such taboos would hold for all societies until the end of time seems
unsustainable since ftl is also time travel the same observation would seem to rule out most forms
of it as we know every law is broken by someone it doesn't seem realistic that laws of time and
causality are an exception to this if means to break them are possible at any given time in the
future stargates or wormholes where a destination version has to be built might be the only ones
that avoid this issue but wormholes would have their own problem the wormholes in fiction always
connect distant points together but wormholes are connections between two points in space-time
there's no particular reason it would be limited to some arbitrary version of now
indeed a natural wormhole like the one in star trek deep space 9 would be more likely to open
to some distant point in the future long after the heat death of the universe than somewhere
along the badgering plane of simultaneity however to build a wormhole we would require an incredible
amount of mass and energy and then two they might not be stable it's a sad reality but it might be
true that we are bound to our star system well at least for a long long future maybe we should
start taking good care of our planet shouldn't we tell us in the comments how you think interstellar
travel can be achieved without breaking time and as always thanks for watching Factnomenal