from Stargate to interstellar wormholes have long been one of our favorite methods for traveling across fictional universes but they've also been a very serious field of study for some of the greatest minds over the last century so what is the holdup when do we get to wormhole ourselves out of here in 1915 Carl's fart shields discovered a solution to Einstein's brand-new equations of general relativity a solution that we now understand describes a black hole but before black holes were ever taken seriously thus far sealed solution revealed the possibility of the wormhole in 1916 Ludwig flam realized that in certain coordinate systems the gravitational hole described by the sparse field solution was not a dead end rather it was a two-sided funnel but leading where as we saw in our recent episode thus far sealed solution describes two symmetric regions of space-time and the funnel itself is the wormhole connecting them in 1935 Albert Einstein along with Nathan Rosen expanded on flam's idea but not into a theory of wormholes instead as a theory of particles einstein-rosen imagine two regions described by this farcical solution not as parallel universes but rather as overlapping layers of the same universe it's easier to visualize in 2d where each layer is a sheet in this picture the funnels connecting the layers would behave like particles that could move around and interact with each other and if you thread the funnels with electromagnetic field lines then they act like charged particles it turns out that this is almost certainly not what particles are but the einstein-rosen paper inspired others to take the wormhole seriously and inspired the name for these swaths shield wormholes the einstein-rosen bridge the wormhole a mostly dormant for another 20 years before it was resurrected this time not to build particles but in an attempt to break causality John Archibald wheeler along with his ex student Bob fuller realized that Einstein and Rosen's bridge do not necessarily have to connect parallel layers of reality it could instead connect distant regions of our universe fuller and wheeler realized that such a multiply connected space-time could allow near instantaneous travel across the universe because the wormhole throat would remain the same very short length no matter how far apart its ends were and they also realize that this would allow time trouble for example if one end of the wormhole were accelerated into a circular path that near the speed of light its clock would freeze compared to the other end by entering the non-moving end of the wormhole you would always travel backwards to the instant when the other end was first accelerated and frozen in time fuller and wheeler knew that any traversable wormhole could violate causality so to save causality they dill deeper into the swath shield wormhole than anyone had before we are going to try to follow them we've been showing this classic depiction of the wormhole described by this fast yield solution it represents the fabric of only two dimensions of space at a single instant in time the technical name is an embedding diagram a 2d spatial sheet sliced out of 4d space-time let's keep time frozen for the moment and add back in the third special dimension the ring of the wormhole is actually a sphere the size of the sparse field event horizon if we could pass through that event horizon we would not find ourselves in a black hole we'd be instantly transported to the region beyond but if we unfreeze the wormhole if we add back the dimension of time that journey becomes impossible full and wheeler proved that this vast field wormhole can is on itself so fast that nothing not even light can make it through to see that we need to turn back to a representation of the black hole that we saw in our parallel universe episode this is the crew scale Zakaria's diagram now I don't go into the complexities of this right now check out the previous episode for more details but the TLDR is that moving up means moving into the future and moving left means moving towards the black hole but space and time coordinates are blended and mutated so that time doesn't freeze of the event horizon which itself becomes this 45-degree path sublight speed paths are always at a steeper angle the strange extra layer that Einstein and Rosen discovered is represented as a mirror reflection to our universe the black hole also has a mirror reflection that we call the white hole the black hole represented here is a so called eternal black hole no matter where you are in the universe if you travel to the left of any possible speed you'll find the event horizon and enter the interior of the black hole but what if you could travel at impossible speeds then you could potentially reach this strange point the nexus between the white and the black hole the Chris Kyle Sakura's diagram shows that faster than light travel is needed to cross the wormhole but to really understand this we need to see how the wormhole evolves the embedding diagram we saw was a particular time slice of this fast filled space-time if we instead take a slice a little further in the future we see the wormhole throat has narrowed in width and passing through means crossing to event horizons and the region in between is the inside of a black hole when that time slice hits the singularity the wormhole has closed if we include time slices in the past we see the full evolution of the wormhole it opens it widens it shrinks and then pinches off again the savate shield solution really represents two things it accurately describes a non-rotating black hole if you just consider its future state where the wormhole is pinched off but if you consider its entire time line it describes a wormhole that opens and then shuts again fuller and wheeler showed that even at the speed of light nothing can get through such a wormhole so this fast shield wormhole is not reversible but that did not deter science fiction writers from adopting the wormhole as they go to intergalactic travel mode one of the first examples was in Carl Sagan's novel contact to make sure he got the physics right Sagan called up his friend Kip Thorne Sagan's initial idea was to make the wormhole a two sided black hole of course we all know that such as fast shield wormhole cannot be traversed Thorne knew that too and so quickly derived the equations describing an actual traversable wormhole as the story goes this happens scribbling in the passenger seat as his wife drove the family on their vacation now if Kipps help Carl got the wormhole right in the final version of contact it's a good thing they didn't send a poet so just how do you design a traversable wormhole well the equations of general relativity permit any smoothly varying shape for the fabric of space-time and any topology the only limitation is the nature of the matter and energy that space-time contains in fact by defining the geometry you want eg a wormhole the required manner distribution is automatically given Kip Thorne and his student Michael Morris figured out a range of mass distributions that would keep the throat of the wormhole open but all of their solutions required a type of matter that simply may not exist they needed a substance to resist the collapse of the wormhole and we call such a substance exotic matter a common way to think of exotic matter is anything with negative energy density something with negative mass could produce the required negative gravity but actually exotic matter is a broader class of weirdness than that thorne and Morris describe a type of matter exerting an outward pressure capable of holding open the wormhole but without the enormous mass or energy density there would normally accompany such a substance and that would counter the outlet push in general exotic matter violates the so called energy conditions of general relativity these are constraints that are placed on allowable distributions of mass and energy in Einstein's equation in order for those equations to make physical sense but these energy conditions are more guidelines than rules and have been observed to be violated in some cases for example in dark energy and in the Casimir effect the Casimir effect is by far the most popular speculative mechanism for opening a wormhole in it two conducting plates that are bought very close together will block components of the quantum vacuum from existing in that region the gap between the plates will then have a negative energy density relative to the surroundings unfortunately the Casimir effect is very very weak and the negative energy density it produces is probably always countered by the positive energy density of the mass of the plates themselves for any physically possible material but if you could create a Casimir negative energy wall in the center of a wormhole that should keep it open unfortunately anyone traveling through the wormhole would also have to pass through this region of extremely high negative energy density which probably wouldn't be healthy Matt Visser who literally wrote the textbook on wormholes fix this last problem by constructing a number of wormhole geometries that kept the exotic matter out of the way of travelers for example he devised a sort of cubic wormhole wires of exotic matter defined sharp edges while space is relatively flat and safer travel on the sides it turns out there are lots of ways to make wormholes if exotic matter exists at all although physics has not yet ruled out its existence exotic matter threatens horrible causal paradoxes many physicists believe it can't exist if for no other reason than our very strong sense that the universe has to make sense at a fundamental level Stephen Hawking expressed this in his chronology protection conjecture there can be no closed timelike curves paths back to your own past except in useless circumstances like on quantum scales or if they're hidden by an event horizon combined with Roger Penrose as cosmic censorship conjecture which ensures little singularities are surrounded by an event horizon our greatest minds seem convinced that traversable wormholes are impossible there is one example of a potentially traversable wormhole that may not need exotic matter and that's the wormhole through a rotating or charged black hole now I promised a more in-depth episode on rotating black hole interiors so I'll save the details for then but for now let me just say that they're probably also hopelessly unstable and at any rate there's no known way to determine where one leads to and that's a big killer for wormholes in general perhaps as big as the requirement of exotic matter to create a shortcut between two regions of space the topology of that space needs to be altered which is probably genuinely impossible the best option may be to find an existing wormhole and alter it to fit your needs and there is one place natural wormholes might exist in the quantum vacuum this was another idea of John wheelers he suggested that on the smallest possible scales the geometry and even the topology of the fabric of space-time may fluctuate wildly its form shifting due to Heisenberg's uncertainty principle the result is what we call the space-time foam and it may imply that tiny wormholes form and vanish in a fraction of a second so perhaps an extremely advanced civilization may have learned to pluck one of these wormholes from the space-time foam and amplify it to macroscopic scales stabilizing it with exotic matter even if they are possible we are a long long way off being able to create a traversable wormhole for the first many millennia of our star fearing future we'll have to take the long way around the universe but even if we can't use them as intergalactic shortcuts wormholes are granting new insights into reality for example Leonard Susskind and one malva Senna have speculated that the einstein-rosen bridge may explain quantum entanglement itself first described by Einstein and Rosen along with Boris Podolsky the ER equals EPR conjecture links wormholes with entanglement and may provide a passage not to distant parts of space-time but to a deeper understanding of its nature which we will of course discuss in an upcoming episode of space-time next week we won't be releasing a regular episode instead we're going to do the very first spacetime livestream it'll be an AMA ask me anything style at least you can ask me pretty much anything and also just a chance to hang out seriously guys I really need people to hang out with and there's no one I'd rather have with the new lot the live stream will be next Tuesday the 28th 5 p.

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eastern US time I hope to see you there ok so last week we talked about these bizarre giant gamma-ray structures that seemed to have erupted from near the Milky Way central black hole the Fermi bubbles let's get your comments leo willen berg asks whether we've observed Fermi bubbles in other galaxies well actually yes google image search for super bubbles a lot of what you'll see is the galaxy NGC 3079 which has very similar structures to the Fermi bubbles in this case observed with the Chandra x-ray satellites Leo also asks whether these bubbles might be caused by dark matter interactions with the black hole well I'm afraid I don't know of any theory of dark matter that would do that but who knows dark matter is weird but probably don't marwene Bell Hodge us about the interstellar medium isn't the space between the stars and galaxies an empty void well actually no particularly within a galaxy the space between the stars is full of gas radiation cosmic rays and dust and when I say full I mean it's everywhere but it's very diffuse most of the space is filled with ionized hydrogen protons stripped of their electrons with densities between one particle per cubic centimeter and one particle per cubic meter its vacuum enough to kill you fast but there's enough of the stuff to produce for example the Fermi bubbles Maffeo corrected me for good reason I said that here in my apartment I'm not floating off in space somewhere but morphia points out that I really am floating off in space we all are along with the entire planet way to make me feel better about being locked in my apartment which I now see as a cool spaceship and Leo Schultz was already on to that apparently I reside in a golden pyramid floating above the galaxy well now I do in my dreams at least Andrew Ballard criticized my use of the expression global pandemic stating that obviously all pandemics are global so far Andrew so far as soon as we have our first Galactic pandemic everyone will be very grateful that I was specific in this case you.