# Entanglement spin swaps are instantaneous?

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How can we be so sure? I know it's fast ..but, what if it swaps by the speed of light instead? I know there are some experiments that try to rule this out, however, I don't think they take into consideration that the signal to swap may be initiated by the action that observes one of the particles ..not the particle itself choosing a spin. I say this because of the weird timing of the delayed choice experiments.

## Answers and Replies

DrChinese
Gold Member
How can we be so sure?
This reference shows it must occur (if there is a speed involved) at least 10,000 times the speed of light:

https://arxiv.org/abs/1303.0614
In the well-known EPR paper, Einstein et al. called the nonlocal correlation in quantum entanglement as spooky action at a distance'. If the spooky action does exist, what is its speed? All previous experiments along this direction have locality loopholes and thus can be explained without having to invoke any spooky action' at all. Here, we strictly closed the locality loopholes by observing a 12-hour continuous violation of Bell inequality and concluded that the lower bound speed of `spooky action' was four orders of magnitude of the speed of light if the Earth's speed in any inertial reference frame was less than 10^(-3) times of the speed of light.

This is the paper I was talking about when I said "some experiments that try to rule this out". The detectors were within 15.3km, hardly a distance that can prove speed of light isn't involved ..especially when you consider my statement about they don't know when to start the clock if considering the delayed choice experiment.

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Nugatory
Mentor
The detectors were within 15.3km, hardly a distance that can prove speed of light isn't involved
Why not? That's 50 microseconds of light travel time. 50 usecs is short in terms of human perception, but an eternity for modern lab equipment where we routinely measure time intervals a million times shorter. The computer I'm using to compose this post wouldn't work at all if we couldn't properly handle propagation delays of a few picoseconds (there are a lot of gates involved in a single cycle of a 3GHz processor)... and 50 picoseconds is to 50 microseconds as one second is to a more than a week.

The detectors were within 15.3km, hardly a distance that can prove speed of light isn't involved
Any distance is great enough to prove the speed of the information travelling between the entangled particles. Even if the clocks were not precise enough, the difference between this "entanglement speed" and the speed of light is very large. As mentioned earlier, about 10,000 times greater.

Doesn't the delayed choice experiment show us that we can't know when to start the timer? All I want is for someone to hop on spaceship and fly out a couple light speed seconds to try the entangled speed experiment.

Nugatory
Mentor
Even if the clocks were not precise enough, the difference between this "entanglement speed" and the speed of light is very large. As mentioned earlier, about 10,000 times greater.
The way they reported their results is in fact a recognition of the precision of the clocks: "This is the smallest speed that could hide behind the accuracy of our clocks". Given a 50 microsecond light travel time, it seems that they had their clocks synchronized and accurate to about 5 nanoseconds, which is clearly feasible.
(Comeback City already knows this, of course. This comment is for OP and others following the thread).

Nugatory
Mentor
Doesn't the delayed choice experiment show us that we can't know when to start the timer?
I don't see how. Which delayed choice experiment are you thinking about, and how does it suggest that we don't know "when to start the timer"?

I'm not sure it may be possible to determine instantaneity, since that would require an agreed simultaneity.
HOWEVER
Experimentation can (and to my knowledge HAS) provided sufficient evidence that this occurs far greater than speed of light which is arguably enough to warrant that for general purpose may as well be considered instant.

DrChinese
Gold Member
All I want is for someone to hop on spaceship and fly out a couple light speed seconds to try the entangled speed experiment.
Go for it.

By the way: papers such as I cited are written by scientists who are quite familiar with the field they write about. If you read and understood their research, you would have the answers you seek.

There are plenty of other papers I could cite, but frankly, why bother? You only are interested in a very particular setup. A couple of light seconds is well past the moon, for example.

on spaceship and fly out a couple light speed seconds to try the entangled speed experiment.

https://www.technologyreview.com/s/428328/super-physics-smackdown-relativity-v-quantum-mechanicsin-space/

For example, physicists routinely measure the quantum phenomenon of entanglement by sending entangled pairs of photons from one location to another. In these experiments, the sender and receiver must both measure the polarisation of the photons, whether vertical or horizontal, for example. But that can only happen if both parties know which direction is up.

That’s easy to specify when they are close together. But it becomes much harder if they are separated by distances over which the curvature of spacetime comes into play. The problem here is that the answer is ambiguous and depends on the path that each photon takes through spacetime.

The experimenters can work this out by tracing the path of each photon back to their common source, if this is known. But then, how does each photon ‘know’ the path that the other has taken? Theorists can only guess.

Another problem arises when these kinds of experiments are done with the sender and receiver travelling at relativistic speeds. This introduces the well known problem of determining the order of events, which Einstein famously showed depends on the observers’ points of view.

That’s in stark contrast to the prediction of quantum mechanics. Here the measurement of one entangled photon instantly determines the result of a future measurement on the other, regardless of the distance between them.

If special relativity ensures that the order of events is ambiguous, what gives? Once again, theorists are at a loss.

Of course, the way to answer these questions is to test them and see.

Today, David Rideout at the University of California, San Diego and a few friends outline various ways to crack these nuts and they say these kinds of experiments ought to be possible in the near future.

That’s largely because the required experimental gear is standard in many optics laboratories, so qualifying it for use in space should be straightforward.

Two groups have already proposed to do these kinds of experiments in space. One group wants to put a package capable of producing entangled photons on the International Space Station, for beaming back to Earth. Another wants to keep the quantum equipment on the ground and bounce photons off a simple microsatellite in low Earth orbit, an option they say will be cheaper, easier and better.

Fundamental quantum optics experiments conceivable with satellites—reaching relativistic distances and velocities

http://iopscience.iop.org/article/1...8A9901A43D8B8271136.c2.iopscience.cld.iop.org

https://arxiv.org/pdf/1206.4949v2.pdf

Physical theories are developed to describe phenomena in particular regimes, and generally are valid only within a limited range of scales. For example, general relativity provides an effective description of the Universe at large length scales, and has been tested from the cosmic scale down to distances as small as 10 m (Dimopoulos 2007 Phys. Rev. Lett. 98 111102; 2008 Phys. Rev. D 78 042003). In contrast, quantum theory provides an effective description of physics at small length scales. Direct tests of quantum theory have been performed at the smallest probeable scales at the Large Hadron Collider, ~10−20 m, up to that of hundreds of kilometres (Ursin et al 2007 Nature Phys. 3 481–6). Yet, such tests fall short of the scales required to investigate potentially significant physics that arises at the intersection of quantum and relativistic regimes. We propose to push direct tests of quantum theory to larger and larger length scales, approaching that of the radius of curvature of spacetime, where we begin to probe the interaction between gravity and quantum phenomena. In particular, we review a wide variety of potential tests of fundamental physics that are conceivable with artificial satellites in Earth orbit and elsewhere in the solar system, and attempt to sketch the magnitudes of potentially observable effects. The tests have the potential to determine the applicability of quantum theory at larger length scales, eliminate various alternative physical theories, and place bounds on phenomenological models motivated by ideas about spacetime microstructure from quantum gravity. From a more pragmatic perspective, as quantum communication technologies such as quantum key distribution advance into space towards large distances, some of the fundamental physical effects discussed here may need to be taken into account to make such schemes viable.

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