How Time Travel is Possible through a Black Hole...to the PAST!

time travel is nothing special your time traveling right now into the future one tick of the clock at a time but relativity theory shows that time that ticks for you is not absolute the flow of time is relative and it can be molded and changed it turns out that higher gravity and higher speed can slow time down when compared to a reference frame that has lower gravity and lower speed this really does happen and has been proven in many experiments so for example when scientists placed one atomic clock at sea level and another synchronized atomic clock on the top of a mountain they found that the clock at sea level takes slightly slower than the one on the mountain similarly atomic clocks placed on a fast-moving jet were found to take slightly slower than synchronized clocks at rest on the ground so the idea of traveling to the future is not exotic at all you're doing that right now and if you could speed yourself up to a significant fraction of the speed of light or put yourself near an extreme gravitational source you could slow down time enough to travel decades into the future earth in a matter of months or weeks or less this was depicted in the movie interstellar for example but the question is how would you come back to the time you started with it's no fun going to the future if you can never come back but to do this you would have to travel back in time this is not so straightforward is it theoretically possible let's take a look coming up right now you might ask where do i get inspiration for these videos one source for me is magellan tv today's sponsor a video that inspired me is called faster than light the dream of interstellar flight it takes a scientific look at how we could potentially travel to planets in other solar systems at extreme speeds using technology that's on the drawing board right now magellan is a new type of streaming documentary service founded by the filmmakers themselves who bring you high quality documentary content featured subjects include history nature science and technology you can watch it on any of your devices as well as your tv anytime without ads magellan has a new offer right now for arvin ashvier's you can get 30 off your annual membership that's an entire year for less than 350 a month and this is valid for prior subscribers too i highly recommend magellan tv but be sure to click the link in the description in order to better understand time travel it's important to understand the concept of light cones we discussed this in a prior video but let's do a quick review space time consists of four dimensions three dimensions of space and one dimension of time but in order to visualize it in our three-dimensional brain we eliminate one of the spatial dimensions and create a 3d looking graph the horizontal axis represent movement in space and the vertical axis represents movement and time in order to have consistent units on all the axes we convert the time axis to length by multiplying time by the speed of light c times t this results in representing time in terms of lengths for example one meter of time is about one three hundred millionth of a second the time it takes light to travel one meter all points on this graph are called events so using this graph an object not moving spatially will create a timeline going straight up because it will be moving in time only this is called the object's world line an object moving in one or more of the spatial dimensions will create a diagonal line because it will be moving both in time and space if we flash a light this point will be an event we can call this event a and it will create a cone because the light will propagate in both spatial dimensions and the time dimension the volume of the cone represents all future events that can be causally connected to event a event a also has a past light cone which represents all events in the past that are causally connected to event a points outside the cone are events that are causally disconnected from the event a in other words they can't have an effect on event a the coordinates are made so that anything moving at the speed of light will always create a 45 degree angle from vertical so the whirl line of any moving object will always be somewhere within or on the cone any world line moving inside the light cone is called time like a particle moving at the speed of light would move at a 45 degree angle between the time and space component a world line like this is called light like because it is how light moves now what happens if an object has a whirl line close to the space component we get what's called a space-like world line since this is outside the light cone it would mean that the object is traveling faster than the speed of light in principle there isn't any problem with such world lines as long as no information is transferred because this would ruin causality as discussed in the previous video but as we've discussed before moving anything to the speed of light would require an infinite amount of energy to accelerate and this is not possible in addition moving an object close to the speed of light will alter time in that it will be slower compared to a stationary object but if we want to do really complex stuff like traveling back in time then we would have to not only slow time down but make it go in the opposite direction how can we do that is it even possible as we showed in the prior video going beyond the speed of light can create scenarios that allow you to travel back in time but since this is not physically possible we need to figure out some other clever manipulation of space-time and anything that involves manipulations of space-time is going to be described by einstein's equations of general relativity the simplest space time is a flat space time the equation for such a space time looks like this where ds describes how a line moves or evolves in this space time and all the terms with d and some component squared tells us what happens to these components of space time at different coordinates dt describes the time component and x y and z describes the three spatial components now this is expressed in cartesian coordinates but the exact same equation can be expressed in spherical coordinates like this now we have a metric in terms of the radius from the center r and the two angles theta and phi as shown in this graph but to have any hope of traveling back in time we need to come up with something much more exotic than just flat space-time like we have here it turns out that the first solution ever presented to einstein's field equations was done by carl schwarzschild essentially he formulated a simple non-flat spacetime and it happened to describe a black hole at a time when no one had ever even heard of it the first dt-squared component describes what happens at different times the next dr squared component describes what happens at different radii the third component does not change compared to flat space time because a black hole is spherically symmetrical the rs term represents the smallest possible radius of an object where light can still escape it's called the source shield radius it's a point beyond which nothing can escape the black hole because in order to escape you would have to go faster than the speed of light which you can't do it's like an information barrier between the inside and outside of the black hole the important thing to note in this equation is that when r is equal to rs in the dr squared term we get a zero in the denominator this makes the term undefinable its physical representation and meaning is the event horizon the zero in the denominator signifies the point of no escape let's now look at what happens to the light cone of an object falling into a black hole if the object is far away then it's not falling anywhere and it's just like an upright static light cone as it starts falling into the black hole we would see the light cone starting to tilt the reason it tilts is because gravity is a warping of space-time the strong gravitation of the black hole attracts light and other objects towards it future whirl lines point increasingly towards the black hole only whirl lines moving away from the black hole at higher and higher speeds will escape it as objects get closer to it and exactly at the event horizon the light cone lies tilted between the time and space axes at 45 degrees all future events point to inside of the event horizon this means that just prior to entering the event horizon a particle could escape away by traveling close to the speed of light however once it enters the event horizon there's no timeline which leads to the outside of the event rise there is no escape even at the speed of light the light cone of the object will no longer have any future whirl line to the outside and it thus loses connection to anything outside the black hole the light cone will continue to tilt inside the black hole as it falls towards the singularity the singularity at the center of the black hole represents a point of infinite mass density but a finite mass eventually the light cone will point completely towards the singularity of the center this means that all future events will lie the singularity there's no escaping it the singularity is a future moment in time rather than a point in space all future world lines point to the singularity of the center it turns out that the space time inside the black hole has the potential to allow travel back in time i'll show how in a minute but the important thing to remember is that even if we go back in time inside the black hole the event horizon prevents us from escaping the black hole so even if we can travel back in time what good is it if we're trapped inside the black hole not being able to escape into our time in the real universe so we have to figure out a way to get rid of the event horizon and it turns out there is a way in 1965 the current newman metric was described by ezra newman this metric is even more complicated because it describes a rotating black hole we can write the metric of this space time as shown here the angles theta and phi are the same as in the source yield metric a is the angular momentum rho is a function of the radius angular momentum and the angle theta the delta is a function of the radius the source shield radius angular momentum and newton's constant of gravity q is the charge black holes can have a charge there are ways we can remove the event horizon in this metric what we have to do is to make sure that the delta on the dr squared term does not equal zero similar to what it does in the schwarzschild metric signifying the event horizon in other words if we can avoid getting a zero in the dr squared term there will be no event horizon it turns out when you do the math that if the angular momentum divided by the mass of the black hole times the speed of light is greater than half the source yield radius then delta can never equal zero in other words if the black hole is spinning fast enough the event horizon disappears it is then no longer a black hole but a naked singularity a naked singularity is just a singularity with no event horizon now why is this important well the reason this is important is because when you don't have an event horizon you can go near the singularity in the center but come right back out there is no event horizon that otherwise prevents you from coming back out of the black hole and what we can do by going near a singularity and coming back out is theoretically traveled back in time how does this happen well we can construct what's called a closed time-like curve which allows world lines from the future cone to loop around into the past light come let's look more closely at how that happens if you get close enough to the singularity the spinning singularity warps the space time around it such that we can have world lines that loop around the singularity in such a way that the future world lines end up in the past light cone looked another way we can loop our light cone around the singularity such that our future light cone ends up in the past light cone of where you started and now since we're not bound inside the black hole by the boundary of the event horizon we can come out of the space time back to about where we started but at a time before we started we went back in time this is a real solution to einstein's field equations so now you might think this is too good to be true what's the catch well there are a few problems first of all just because something works in the theory of general relativity doesn't mean it works in reality there's good reason to believe that black holes can't physically rotate fast enough for the event horizon to disappear so scientists don't think naked singularities can exist in addition the solution shown here just considers a test particle with little to no gravitation and not a human if you tried the mathematics with the human the higher mass and gravity does not seem to work the same way so the concept of going close to a singularity and traveling back in time may not be possible but i'll have you know that no concrete proof of this has been made so what am i saying i'm saying there might still be a chance i'll see you in the next video my friend [Applause] [Music] [Applause] [Music] you

Channel: Arvin Ash

Views: 187,876

Keywords: time travel, is time travel possible, time machine, black hole, is time travel real, travel back in time, time travel theory, time travel real, time traveling, time travel explained, closed timelike curve, closed timelike loop, naked singularity, kerr-newman black hole, black hole time travelling, schwarzschild radius, schwarzschild metric, rotating black hole, what is an event horizon, can we time travel back in time

Id: 1JnCArN0zIk

Length: 14min 46sec (886 seconds)

Published: Fri Jul 09 2021