The hypothetical that occurs to me is: if one were to establish a pair of entangled particles and shoot one of them directly into a black hole, what could the behavior of the retained particle tell us about the black hole’s interior?
Yet, when viewed through the lens of general relativity, from our frame of reference, no matter we can observe falling into a black hole actually ever crosses the event horizon, due to time dilation, so a hypothetical entangled particle pair could reveal information about the environment at or near the event horizon.
In fact, from our perspective, it would seem that the concept of the singularity could be a myth. The entire mass of a black hole occupies the region of the event horizon, where time reaches a standstill. In terms of basic gravitational mechanics, it makes sense: if you could dig a hole to the center of a planet, the gravitational balance of the planet’s mass extending in all directions would render you practically weightless, pulled equally in every direction.
Which is to say that forces generally tend to balance each other. Why would a black hole not simply be a basketball, a tremendous empty shell of matter accumulated at the event horizon, with inverted balancing gravity pushing everything outward?
No more than sending one part of the pair to alpha centuri would tell us about that star system.
It is only when you are able to compare the results of the entangled particles that anything interesting arises.
We don’t see it actually cross, but that doesn’t mean that we see it just hover there either. It appears to get further and further away, and accelerate away from us at greater and greater velocity, until it is red-shifted into invisibility, effectively no longer existing on the outside of the event horizon.
This does not mean, however, that the time of a test particle dropped into the black hole stops, it just does so in relation to the outside observer. If you were to fall into a black hole (a big enough one that you are not destroyed by tidal forces before you get there), then you would not notice anything in particular as you passed the point of no return.
You are doomed, however, where your future now inevitably intersects with the “singularity” or whatever is the ultimate center of the black hole.
The event horizon is where space itself is moving away from the rest of the universe at the speed of light. Anything beyond is moving away from the rest of the universe at greater than the speed of light, and is therefore unreachable.
The singularity is a myth. It is where general relativity breaks down and no longer gives us useful answers. There is no point of infinite density, but we also do not actually know what does lie within.
One of the reasons for a search for quantum gravity is to tell where relativity breaks down, and the reason that black holes are so interesting in this search is because it has to break down somewhere between the event horizon and the “singularity”.
That is similar to a fairly popular hypothesis, that the entirety of the black hole is contained on the event horizon, in one shape or another. The holographic principle states this explicitly, that all the information in the black hole is contained on the event horizon.
However, the firewall hypothesis, that there is some actual physical barrier to run into at the event horizon is less supported.
It may be, but it’s a bit hard to check out. More likely, there is something going on inside of the black hole that we don’t quite understand yet. There is no known mechanism that would support a shell of mass in that way.
Every time that relativity has been tested, at greater and greater precisions with more and more accuracy, it has been shown to be absolutely correct.
However, it also says that there is a singularity in black holes, and that the universe started with a singularity.
A singularity is not a thing, it is a breakdown in math. 1/0 is a singularity, that doesn’t mean that it is a tangible thing. It’s not where all the numbers become infinitely dense, it’s just where math doesn’t give a sensible answer anymore.
So, there is something about relativity that is not complete, we just don’t know what it is yet. Hopefully, a theory of quantum gravity could solve that.
This reminds me of a docudrama I saw recently. It started out as: “What would happen if aliens invaded?” But the show quickly turned into “what kind of damage could a sentient quantum computer do?”
One of the ways the aliens figured out to communicate across the vast distances is to use quantum entangled particles.
They take half the entangled particles and put them in one device and they take the other half and put them in the other device. Each device goes into a different ship. As long as they have those devices, the two ships theoretically can talk to each other no matter how far apart in real time.
Well, I’m sure I mangled what the show was saying but this wasn’t a “sci fi” show. It was a documentary meant to educate lay people on current scientific theories.
Complete with interviews from scientists pushing said theories.
Entanglement is “spooky”, and it implies that there is some sort of non local FTL effect.
But it is also certain that it cannot be used to actually communicate FTL. No information is transferred. Until you are able to compare the results that you got from both particles, you cannot even tell that anything has happened.
If you have an entangled particle. There is one thing you can do with it. Measure it. And when you measure it, it will be either up or down (or whatever entangled state you are using.) That will be all the information you get out of it.
You will know that, if someone does the same measurement on the other particle, that they will get the answer you did not.
That’s it. You cannot tell if they have measured it yet, or if they used a measurement setup different than your own. None of that information is available until it is sent to you by light or slower means.
There is no way to use entangled particles to transmit information faster than light.
As already pointed out by @k9bfriender and @Riemann, you can’t use (only) entangled particles to send information between two systems—this is the ‘no communication theorem’ of quantum mechanics. The reason why that’s the case is similar to classically sharing two envelopes, one with a red, the other with a blue card in it: once I open mine, I know that yours must contain the other color, but since I can’t ‘force’ mine to a specific color, I can’t send you a message thereby. The difference is just that in quantum mechanics, you have different ways of measuring, with different, mutually exclusive possible outcomes—say, different ways of opening the envelope, such that you’ll either find a card that’s red or blue, or a card that’s green or yellow, for example. Still, however, you can’t use such a thing for signalling.
There’s some interesting arguments, insights and history connected to this, however. Suppose, for the moment, that you could make a copy of your envelope, without looking into it: then, all of a sudden, I could send you a message. The reason for that is that once I look into my envelope, and see, for example, a red card, if you look into your envelope in the way such as to either reveal red or blue, you’ll always find a blue one, and when you look so as to either find a green or yellow card, you’ll get random results. Thus, with many copies of your envelope, you could measure half of them in the red vs. blue-way, and the other half in the yellow vs. green-way; the first half will then always show blue, and the second half random results. Thus, you know that I must’ve looked into my envelope using the red vs. blue-discriminating way—and we’ve just successfully exchanged one bit of information.
So, is that a way to transmit information after all? In fact, such a device was proposed by Nick Herbert, and famously made it to publication despite everybody being certain that it can’t possibly work—because nobody, at the time, could figure out why. The reason is that, perhaps surprisingly, you can’t actually copy quantum states—this is the content of the quantum no-cloning theorem. So, if you have an unknown quantum state, there’s no general procedure that leaves you with two copies of that state, for every possible state.
This also entails that quantum measurement, in general, must change the state measured—as otherwise, you could trivially clone: you perform a measurement, then re-prepare the state your device tells you you have. The other direction works, too: if you can clone, you can always do perfect measurement by simply creating many copies of the state, and measuring them in all the relevant ways (doing something called quantum tomography), and then you’ll eventually know all there is to know about it.
So actually, in this sense, the impossibility of faster-than-light communication means quantum mechanics must have a measurement problem! Or, as things are, vice versa.
Also related to this is the principle known as monogamy of entanglement: in its simplest sense, if you have two particles (maximally) entangled with each other, you can’t entangle them with a third one. If you could clone, you could easily violate that, by just cloning one half of an entangled pair. Now here we come back to tossing entangled particles into black holes: the firewall argument, already alluded to by @k9bfriender, takes such a situation and proceeds to show that in black hole spacetimes, you can apparently have a particle correlated with more than one partner. The reasoning is a bit complex, but the gist is: a black hole that evaporates via Hawking radiation, in a simplified picture, does so essentially by creating entangled particle pairs, one of which falls into the black hole, the other of which escapes. But additionally, once a black hole has more than half-way evaporated, all of the information coming out after that point must be entangled with particles that have evaporated at an earlier time. But then, these particles must be entangled both with early Hawking-radiation and with modes behind the horizon: contradicting monogamy.
So it seems that the combination of entanglement and black holes does spell some trouble for otherwise well-established notions, like monogamy and no-cloning. Moreover, it’s usually thought that our current theories should be valid up to close to the black hole singularity, where we’ll need quantum gravity. But the argument actually paints a problem for physics at the horizon, which generally (according to the equivalence principle of general relativity) isn’t a special place—indeed, you wouldn’t necessarily know it if you crossed one. Something has to give, here: one possibility is that entanglement is ‘broken’ at the horizon. This would entail that the black hole horizon can’t be ordinary spacetime (as two adjacent regions of ordinary spacetime are highly entangled), and must, instead, be a seething mass of highly energetic modes—a ‘firewall’.
But ultimately, what exactly happens, nobody knows yet.
Now, how are you going to get their particle to spin clockwise or counterclockwise?
That’s the thing. The whole point of entanglement is that you don’t know what either of them is doing. From a quantum standpoint, both are in superposition, and both are spinning both clockwise and counterclockwise, or rather, up and down for simplicity.
When you measure your particle, you have no way of influencing it. It is what it is. So, if you get a spin up, he gets a spin down; if you get a spin down, he gets a spin up. What information has been transmitted?
It’s not a perfect thought experiment, because it wouldn’t violate Bell’s inequality as real quantum superpositions do, but a way to think about it is that you have two boxes.
You put a 1 in one box, a 0 in the other. You close the boxes up and mix them up. Now, you send them into opposite rooms, each with a person ready to open the box.
Alice opens her box, and gets a 1. Bob opens his, and gets a 0. What information did Alice send Bob?
Lets say that we do this 100 times, so Alice and Bob both have 100 boxes, we’ll even say that they are labeled 1-100. How does Alice use this arrangement to send Bob a message?
Hawking radiation does not “leave” the black hole. It is a particle/anti-particle pair that forms outside the event horizon, with one of the particles falling in while the other flies away.
I am not a mathematician, so that much is beyond me. As far as I can tell, renormalization is along the lines of the basic limit trick used in differential calculus, where you would be dividing by zero, but you just rephrase it to get at what you want.
The problem is, it looks like all of spacetime, the only reality we know how to work with, gets squeezed down past recognition. The other side of the event horizon is simply not something we can do more than blindly stab at. We could extrapolate/interpolate from what we know, but it is really nothing more than guesswork.
I mean, we have the imaginary realm, and we can do useful stuff with complex numbers, so going off the reservation is not entirely crazy or worthless. But the “singularity” itself is just a hypothesis, based on extrapolation of traditional math. We have no idea what is actually in there.
What actually happens is that it gains negative energy. No mass is ever lost.
A close analysis shows that the exponential redshifting effect of extreme gravity very close to the event horizon almost tears the escaping photon apart, and in addition very slightly amplifies it. The amplification gives rise to a “partner wave”, which carries negative energy and passes through the event horizon, where it remains trapped, reducing the total energy of the black hole. The escaping photon adds an equal amount of positive energy to the wider universe outside the black hole. In this way, no matter or energy ever actually leaves the black hole itself.
