Hot answers tagged causality
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Entanglement is being presented as an "active link" only because most people - including authors of popular (and sometimes even unpopular, using the very words of Sidney Coleman) books and articles - don't understand quantum mechanics. And they don't understand quantum mechanics because they don't want to believe that it is fundamentally correct: they always ...
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I wish to complete @Luboš Motl's answer, to which I agree. My point is on why people continue to make this mistake of an active link. This mistake is connected with one of the most interesting properties of quantum mechanics, Bell's theorem. One can argue that any physical theory is an hidden variable theory, the hidden variable being the description of the ...
19
Since general relativity is a local theory just like any good classical field theory, the Earth will respond to the local curvature which can change only once the information about the disappearance of the Sun has been communicated to the Earth's position (through the propagation of gravitational waves).
So yes, the Earth would continue to orbit what ...
15
It's not possible to communicate faster than light using entangled states. All you get out of entanglement is a correlation between the values of two measurements.; the entanglement doesn't allow you to influence the value measured at another location in a non-causal way. In other words, the correlation only becomes evident after combining the results from ...
15
Nobody has explained to me how Shor's quantum factorization algorithm works under the transactional interpretation, and I expect this is because the transactional interpretation cannot actually explain this algorithm. If it can't, then chances are the transactional interpretation doesn't actually work. (I have looked at some of the papers that purport to ...
13
Gravitational influences do propagate at the speed of light, not instantaneously.
The question of what would happen if the Sun instantly disappeared is actually a funny one in general relativity. The equations of general relativity imply as a mathematical consequence that energy must be locally conserved. Therefore, there is no valid solution to the ...
12
Shine a flashlight on a wall. Rotate the flashlight so the illuminated spot moves.
Q: How fast does the spot move?
A: It depends how far away the wall is.
Q: How fast can the spot possibly move?
A: There is no limit. Put the wall far enough away, and the spot can move with any speed.
Q: What is moving across the wall?
A: Nothing. The light that makes up ...
10
All observations are consistent with standard GR so far, but I don't think the speed of gravity, in particular, has ever been measured.
Experimental measurements of the speed of gravity was quite a controversy a few years ago when a paper came out claiming that the speed of gravity was very close to $c$ as measured by the Shapiro delay. To see papers on the ...
10
Just a nice analogue Prof. Jürgen Audretsch told me once:
Imagine at home you put one glove in your coat without looking (and noticing it's only one of the two). After exiting the train you notice it's cold and you pull out that single glove. At this very instant you know it's either the left or the right glove, and you therefore know which one is left ...
9
Suppose you and I have a conversation from a long distance away. We're at rest with respect to each other and communicate much faster than light. I say "How are you", and you wait a short time and say, "I'm fine thanks."
From our point of view, you were responding to my question. However, from a reference frame moving from me to you at relativistic ...
8
It's a combination of all these things and more. Most importantly, the TIQM interpretation is nonsense and all the positive words you hear about it are just unjustifiable hype promoted purely by John Cramer himself.
Ontology - or "realism", as it is technically called in quantum mechanics - has been falsified in physics in the mid 1920s and it can never be ...
8
A very simple example is the following:
Two observers A and B are spatially separated (one in London, the other in Cambridge). They are static with respect to each other and can therefore measure the same time. So, we can fix a particular time T.
The two events $(T,\mathbf{x_{London}})$ and $(T,\mathbf{x_{Cambridge}})$ are spatially separated - one cannot ...
8
Well, the problem in that paradox is that yes, one of the parties will measure the entangled particle to get the wave function collapsed and yes it will collapse for the other party. However, the other party will still have to measure the thing to learn what it is or has to wait for the initial party to send them a message telling what the wave function has ...
6
The formal statement would be that there is no one notion of time, and that one persons definition of time may be intermixed with another's definition of space. What is not correct is that time and space are wholly interchangeable, as Moshe says--there is a distinction between events separated in time and events separated in space. Causality theory is ...
6
Let me make an analogy which may be helpful.
The next time you're at the beach, watch the waves coming in. You will notice that the point where the wave breaks will often move faster than the waves do. This happens when the waves come in close to perpendicular to the beach.
The effect is that if you know the wave is breaking at point A, you can predict ...
6
Excellent question. You are correct about wavepacket spreading, and in fact you do get superluminal propagation in non-relativistic QM - which is rubbish. You need a relativistic theory.
You should read the first part of Sidney Coleman's lecture notes on quantum field theory where he discusses this exact problem: http://arxiv.org/abs/1110.5013
The short ...
6
In reality there's no such thing as a perfect rigid body. There will always be a delay in the motion propagating along the body.
Under "normal" conditions you don't notice this delay as it's infinitesimal when compared to the size of everyday objects you interact with.
However if you had a rod several light years long (assuming that's at all possible) the ...
5
I have come across a very simple question or critique of the Transactional Interpretation in an article "Nine formulations of Quantum Mechanics". The multiple authors of this 2002 AMJ paper are QM physics teachers.
They ask how "two particle" transactional handshakes work: are there "two handshakes across spacetime" or "one handshake across Configuration ...
5
From a physicist's point of view the quote is nonsense: gravity and electrodynamics are how matter moves matter on the macroscopic scale and we have perfectly good theories for both of them.
If the writer means "we don't actually know how it really works when you get right down to it; I mean not really know." then he's speaking pure, unadulterated ...
5
There can be at least two different flavors of paradoxes. In one, a result such as 2+2=5 is proved, and the problem must be either incorrect reasoning or a set of assumptions that was invalid. In the other type, exemplified by the EPR paradox, the correct result of an argument is so surprising that it seems like it must be a mistake.
Based on the ...
4
Causality becomes much more subtle whenever theories are statistical or probabilistic. When we see a correlation, it may be that one event caused the other, but it may be that there is a common cause or that there is just a chance correlation that would disappear if we do more of the same data gathering. For the notion of "common cause", the standard ...
4
Dear Nigel, causality is not an observable (quantity) with a value and a unit; so it is not identified with any operators.
Causality is a principle. In a broader scientific and colloquial context, causality is any property of the relationship between the cause and its effect.
However, in physics, we mean something more particular by causality. In classical ...
4
Recipe to generate events with space like separation (and be able to prove it):
Set up a radio transmitter at point $A$.
At point $B$ and $C$ which are arrange so that A lies at the center point of the segment between them we put radio receivers that will perform some action on receiving a radio pulse from A. We make the distance between them greater than ...
4
Let's be more rigorous. No-signalling has been proven safely and shouldn't be worried about. Nevertheless, you'd notice that the point of EPR paper was to show that if quantum mechanics is considered to be a description of "reality", then it is "incomplete". There is an approach, such as in operationalism, to say quantum mechanics isn't meant to be a ...
4
The answer is "no" if you take precise definitions of "continuity" and "causality". Then these are different concepts.
Indeed. The "background" for general relativity is a manifold which necessarily have some topological structure -- it locally "looks like" an Euclidean space and you just take standard Euclidean topology to your manifold. You can ...
4
It seems the FLP impossibility result may not hold for quantum systems (see this paper by Helm and section 5 of this paper for a criticism of the first), in which case you don't need any exotic causal structure for quantum mechanics to avoid it.
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It turns out that Asher Peres' original theoretical paper on delayed choice entanglement swapping is short and quite readable. It sets out the idea behind the experimental setup without getting distracted by practicalities.
Basically the idea is that, as the Ars Technica post says, Alice and Bob each make their choice of measurement on one of the two ...
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