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Comment Re:With all due respect to Mr. Hawking and us... (Score 1) 279

This is known already. Because, you know, physics.

Physics ain't done, son. Lots of stuff is deemed impossible before the next theoretical breakthrough.

When we have a fully-working model of the universe, then we can declare it impossible. Until then, avoid being too certain. The history is science is littered with fools who made certain declarations based on current, incomplete theory.

Based on what we know to date, FTL travel appears to be impossible.

Comment Re:"they'd be back if it happened again" (Score 1) 237

"The police told me they'd be back if it happened again." For what crime? Is it normal for police in Canada to threaten to invade an innocent couple's home for doing something legal?

Tor is a thorn in the side of despotic regimes. They will harass anybody who runs an exit node. Best case, they break down a door and find some pot in an ash tray, then lock this couple up for a few years. It's good for the police union, good for the prison industry, and good for the black ops programs funding their budget with drug smuggling.

Win-win-win (unless you're a subject of the regime).

Comment Re:Totally. (Score 1) 122

his country is full of extremely stupid, gullible, and ridiculously-overarmed people, and a small subset of whom probably thinks it would be a good thing to bring harm to the First Lady.

Meanwhile, Jefferson often complained about the never-ending parade of people who walked into his office at all hours of the day to complain.

But he didn't have a Department of Education. Or bombing campaigns in sixteen countries (the Barbary Pirates not withstanding).

Comment Re:With all due respect to Mr. Hawking and us... (Score 1) 279

There's no possibility that aliens capable of FTL would find us remotely interesting. Once you get to that technology, energy and resource problems either have been solved, or become very easily solvable.

And if they care at all about things like us, they already have had probes in our system for eons, by all averages. It would be absurd to think they can't build self-replicating probes at our level of technology plus a few hundred years as a minimum. Once you have that, if you care about the galaxy, you map it.

There's nothing we can tell them that they don't already know. They haven't destroyed us, so they won't.

Comment Re:I Think this article might be a bit misleading. (Score 1) 189

Sorry, I'm hopeless at explaining hence why I could never teach.

There aren't two identical "messages" in quantum teleportation. That would violate the no-cloning theorem. Instead there's one message that originally exists at point A and later exists at point B instead.

But - and this is the bit where it involves QM weirdness - there is no way to "read" a complete quantum state and record it classically. Think of it as a two bit word in a computer where every time you read one bit the reading circuit randomly disturbs the other bit.

QM teleportation lets us move that word from one place to another without changing either bit (but note that the original word (letter) is destroyed in the process.)

Comment Re:I Think this article might be a bit misleading. (Score 1) 189

I don't know what else you saw but basically the entanglement leaves the target (teleportation) end in a superposition of four states, only one of which is the one you want the others are complementary states.

The sender makes a measurement at their end to determine which one of the four states is the correct one and then transmits it to the receiver. The receiver can then isolate the correct state from the others that would otherwise cancel out all knowledge of the original state.

Comment Re:Can Anyone Explain This To Me? (Score 2) 189

Oh well. I tried to write a comment with a diagram but hit submit instead of preview :-(

Consider four directions on a plane. x axis (we'll call that |+>), y axis (we'll call that |->) y=-x (we'll call that |0>) and y=x (we'll call that |1>)

Modulo some constant factors, I hope it's obvious that you can build up some of those vectors from others:

|1> = |+> + |->
|-> = |0> + |1>
etc.

These are the directions of a plane polarized photon.

We setup some photons that are polarized in the |1> direction and then pass them through a polarization filter.

If the filter points along the |1> direction then all of them pass. If the filter passes along the |0> direction then none of them pass.

Now we put the filter along the |-> direction. What happens.

|1> = s|+> + s|-> (s is 1/sqrt(2) - which can be deduced from standard trig - the lines must be the same length)

When we measure along the |-> direction the s|-> part will pass the filter but the s|+> part wont.

But an individual photon can't get dimmer therefore it must either pass or not. Half the photons do pass and half don't (and it's random whether any one photon gets through the detector)

The ones that do get through are now in state |-> which is also |0>+|1> (again with factors of sqrt 2)

If we now measure along the |1> direction again we now lose half the photons again (due to that |0> component)

Quantum teleportation involves taking a photon in state a|0> + b|1> (for unknown values of a and b) and taking very careful measurements that don't destroy a and b but instead transfer them to another photon without us actually knowing what they are.

Comment Re:Coincidence circuit again, i.e. filtering (Score 3, Informative) 189

You got the first paragraph right. But then got sidetracked by tennis balls.

There are *two* complementary quantum states that you can measure. Measuring one destroys all knowledge of the other.

There is no classical system that behaves like this, therefore any analogy that doesn't invoke some magic artificial property of a classical object won't represent what happens in QM.

In your example you need tennis balls that randomly change colour when you measure their spin and can magically reverse spin when you look what colour they are.

Comment Re:Confused (Score 1) 189

Quantum teleportation requires the use of a classical channel. The entangled particles can be exchanged in advance (provided they can be stored without breaking the entanglement which is difficult in practice but trivial in theory)

The classical data can only be transferred at the time the teleportation is done - hence that limits the speed of the teleportation to the speed of light.

Comment Re:I Think this article might be a bit misleading. (Score 5, Informative) 189

It's effectively** equivalent to having two identical letters containing a random message

No. you're describing entanglement.

Teleportation is subtly different.

Teleportation consists of transferring the quantum state of one particle to another particle via the use of entangled particles (and a classical channel)

The beauty of this is that the entangled state can be set up in advance. You then give me a particle that you might or might not know something about its quantum state (but importantly, I do not know what you know about it so cannot measure that quantum state in advance). I can transfer the state of that particle to another particle that Bob has via some entangled particles we exchanged earlier *plus* some standard classical information that goes over classical channels (it's this classical information that limits the teleportation to the speed of light)

The particle that Bob ends up with is in an identical state the the one you gave me (and which I still have).

N.B. This is quantum teleportation, not quantum cloning which is not possible. The act of getting the quantum state to Bob affects my particle in a way that means I cannot also extract any information from it about the original state of your particle.

Comment Re:Coincidence circuit again, i.e. filtering (Score 2) 189

I think you misunderstand.

The experiments that are discarded are where the two end points don't measure the same quantum variable.

For photons, for example, you can measure whether linear polarization is up-down/left-right or diagonal-left-up diagonal-right-down/diagonal-right-up diagonal-left down.

If both ends measure the up-down/left-right state then one will get up-down, one left-right. If both measure the diagonal polarization then again they will get complementary results. But if one measures up-down and the other measures diagonal then we cannot tell anything useful any more than trying to compare two sweets where one person says what shape it is and the other says what flavour it is so those results get discarded.

There is additional statistical analysis - due to the fact that these experiments are done on single photons and sometimes detectors fire when there is no photon and sometimes they don't fire when there is so we cannot expect 100% correlation - but that's nothing to do with discarding some of the results.

Comment Re:Nobody knows yet (Score 1) 165

Basically, they have promised to veto any Article 50 agreement that doesn't continue to allow free travel (with ID) for their citizens to the UK, as is currently the case. Any Article 50 agreement requires a unanamous vote in favour - all 27 remaining countries

This isn't quite correct. An article 50 agreement requires, iirc at least 50% of member states representing at least 66% of the EU population.

However, I think disconnecting access to the free market from freedom of travel does require unanimity. That's an independent rule of the EU IIUC unrelated to article 50.

What isn't obvious at this point is whether the UK can negotiate some (acceptable) restrictions in trade in return for some (acceptable) restrictions in movement.

It may well end up with WTO rules due to the 2 year negotiating period expiring. Britain will then, of course, lose it's passporting rights to the financial markets. Whether the finance industry will have relocated in time is debatable - it probably depends on how early on failure of negotiations becomes apparent.

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