in last week's episode we looked at an extremely powerful tool for understanding the strange space-time both in and around black holes this is the Penrose diagram it compactify is our representation of the dimensions of space and time allowing us to fit onto the one diagram the infinitely stretched space-time in the vicinity of a black hole's event horizon the Penrose diagram allows us to easily understand the limits of our access to this universe that limit is defined by where something traveling at the speed of light can get to the speed of light is always at a 45 degree angle on this diagram today we're going to use the Penrose diagram to look at the difference between the idealized theoretical black hole that we discussed in the previous episode versus the real Astrophysical black hole's that actually dwell out there in the universe and then we'll use this knowledge to address a very serious although questionably plausible scenario involving an alien black hole attack the Penrose diagram we looked at represents a SWAT shield black hole so no electric charge and no rotation but also an eternal black hole it always existed never growing never leaking of course there is no such thing at the very least even a swath shield black hole must have formed at some point a black hole forms when the core of a very massive star collapses under its own gravity at the end of the star's life so what does this look like on a Penrose diagram let's start with a nice empty universe empty except for a single giant star when the core of this star has fused all of its elements into iron it'll start to collapse under its own weight there's a specific size that represents the point of no return for this collapse this is the swath shield radius the larger the mass of the collapsing object the larger this radius if the Stars core collapses to a size smaller than its own swats shield radius then the event horizon forms engulfing what's left of the star below that horizon but above the still shrinking surface of the star space-time takes on the mad properties of the black hole interior space and time switch places and the singularity soon forms with all space within the black hole flowing towards it faster than the speed of light outside the black hole the event horizon becomes the new edge of the universe on our Penrose diagram the shape of space-time outside the horizon warps to make this diagonal line a line of constant radius the radius of the new black hole but here is something really weird there are regions in this otherwise normal space where everything is doomed to fall into the singularity even though the black hole has not finished forming there's a region where all forward light cones only include the singularity even before the true event horizon forms for anything in this region there isn't enough time to clear the impending event horizon even traveling at the speed of light on the Penrose diagram we should extend our effective event horizon backwards to include that space this invisible horizon of doom grows as the star shrinks and finally merges with the true event horizon any observers within this extended event horizon are cut off from any future causal connection with the rest of the universe or are they okay in the case of the collapsing star that's still a core is going to be an insanely hot dense place and not great for observers but we can imagine a scenario in which the black hole is about to form around you and yet you are perfectly comfortable all the way up to and even briefly after that happens scenario a super advanced alien civilization decides to build a giant black hole that will engulf the planet Earth it doesn't matter why I don't know they're light-years away maybe they just saw the first Star Wars prequel so these guys plan to destroy the earth with a kugelblitz a black hole formed entirely from light their spaceships form a sphere around the solar system and blast a pulse of light inwards this gigantic shell of lightly sensor on the earth and will reach us in about a day now light has energy and so has a gravitational effect this blast has a mass energy equivalence of a hundred thousand suns the swash shield radius of a black hole with this mass is about one light-second so the kugelblitz event horizon forms just after the shell of light passes the moon at that point it will look exactly like a black hole from the outside from the inside earth has one second in which it notices absolutely nothing wrong before it's consumed in the singularity why because Newton's shell theorem tells us that the entire inside of a spherically symmetric shell of mass or energy fills no gravitational force from that shell space inside the collapsing kugelblitz would remain comfortably flat until the collapsing shell overtakes it even after the event horizon forms so here's my challenge question our own homeworld security agents discovered this plot so there's a chance we can do something two competing plans are put forward plan a is to build an infinitely strong Dyson Sphere surrounding the earth just outside the moon's orbital radius it would completely absorb the incoming pulse storing it as a ridiculous amount of electrical energy and the accompanying increase in mass maybe that energy can then power a super weapon to fight the aliens plan B is to launch a vast satellite network that can produce a perfectly reflective spherical force shield about halfway between the earth and the moon this shield will reflect all light outwards and uses an impossible emdrive technology that allows it to ignore conservation of momentum so the satellites don't get ricocheted back to earth maybe the outgoing light wave will destroy the alien ships let's assume that these plans can be executed but only exactly as described and humanity has to choose one and only one you receive a conference call from various world leaders they've heard that you watch PBS space time and so they want your opinion on the plans which do you recommend as slightly less hopeless operation phoenix eggs Dyson Sphere or Operation disco balls reflective shield the president is especially upset so draw a nice Penrose diagram of the situation to show why your preferred plan has the best shot submit your carefully explained and diagram dancer to PBS space time at gmail.com within two weeks of release of this episode for a chance to win a space time t-shirt and obviously to save the world so assuming there's no kugelblitz apocalypse I'll see you next week for a new episode of space time