Causality in current physics

In summary: I also heard time travel enabled such things as indefinite energy magnification, hypercomputation and stuff like that. These are hard to call physical because they're basically just undefined things that happen.These are all undefined things that happen, but they are still physical. These are all undefined things that happen, but they are still physical.
  • #1
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What is currently the common opinion on Chronology Protection manifested by Hawking almost 30 years ago?

There does not seem to be any fully accepted no-go result for Thornian time machines. Energy conditions can be violated in QFT, semi-classical results suffer from counter-examples, Novikov's and multi-world statements sound too artificial and only try to come around the real issue.

I also didn't see any discussion on the 2nd law of thermodynamics in the works by Visser etc. Does it somehow relate to the problem of causality though?
 
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  • #3
Demystifier said:
It also explains why the Novikov's principle is not artificial at all.

How exactly? You also say that the thermodynamic arrow of time prevents paradoxes on the macroscopic level. So, do you actually allow time machines, but forbid them to generate paradoxes a-la Novikov?
 
  • #4
ErikZorkin said:
So, do you actually allow time machines, but forbid them to generate paradoxes a-la Novikov?
Yes.
 
  • #5
Demystifier said:
Yes.

...a-and how is this not "artificial"?
 
  • #6
ErikZorkin said:
...a-and how is this not "artificial"?
The Novikov's principle says that multivalued solutions should be discarded. One may think that it is artificial, but it is not. The multivalued solution is in fact the same as an inconsistent solution, which is best viewed as something that is not a solution at all. If you have a differential equation for a function ##f(t)##, then a map that gives two different values of ##f## for the same ##t## is, by definition, not a mathematical function. So by requiring that the solution of the differential equation must be a function, one automatically discards those multivalued "solutions".

The above was a mathematical argument, but physicists sometimes object that it constrains the initial conditions, which they find physically unacceptable. But why do they find it unacceptable? Because that contradicts the "fact" that humans have "free will" - the ability to choose any initial condition they like. But if one accepts that "free will" is just an illusion (which many physicists do accept), then the constraint on initial conditions is not longer a problem. Nature will choose only consistent solutions (because inconsistent ones are not solutions at all), while a human trapped in such a solution may have an illusion that the corresponding initial condition was "her own free choice".
 
  • #7
Demystifier said:
The above was a mathematical argument, but physicists sometimes object that it constrains the initial conditions, which they find physically unacceptable.
I also heard time travel enabled such things as indefinite energy magnification, hypercomputation and stuff like that. These are hard to call physical
 

1. What is causality in physics?

Causality, also known as cause and effect, is the principle that every event has a cause that precedes it. In physics, this principle is used to explain the relationships between different physical phenomena and to predict future outcomes.

2. How does causality apply in current physics theories?

Causality is a fundamental concept in all branches of physics, from classical mechanics to quantum mechanics. It is used to explain the observed behavior of particles and systems, and to make predictions about their future behavior. In current physics theories, causality is a crucial element in our understanding of the universe.

3. Can causality be violated in physics?

According to the laws of physics, causality cannot be violated. This means that an effect cannot occur before its cause. However, there are some theories, such as certain interpretations of quantum mechanics, that suggest the possibility of retrocausality, where the effect can influence the cause. However, this is still a highly debated topic in the scientific community.

4. How does causality relate to the arrow of time?

Causality is closely tied to the arrow of time, which is the concept that time only moves in one direction – from the past to the future. This is because causality dictates that an effect cannot occur before its cause, therefore time can only move forward. The arrow of time is a fundamental principle in physics and is used to explain the observed behavior of particles and systems.

5. What role does causality play in determining the laws of physics?

Causality is a crucial factor in determining the laws of physics. The principles of causality are used to construct mathematical models and theories that accurately describe the behavior of physical systems. These laws are then used to make predictions and further our understanding of the universe.

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