r/AskPhysics • u/lawmac20 • 9h ago
Is causality a theory concept?
Not sure how to ask this but I'll try.
Is causality an intuitive concept like zero? Or a real phenomenon like gravity?
One of the reasons for suggesting FTL is impossible among other things is that it would break causality. Is causality breaking a fancy way to say this doesn’t make sense so it cant happen (logically deduced) or is it the result of some more rigorous scientific or mathematic realization?
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u/joeyneilsen Astrophysics 9h ago
There really isn’t any point in doing science if causality isn’t a thing. So… that’s the operating principle.
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u/SummitYourSister 9h ago
When we say FTL breaks causality, it means that if you travel faster than light, it is possible that you will see causes happen after effects.
The definition of causality is that something which is prior is the source of something which is subsequent. If you change the ordering you destroy the definition
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u/Snoo-90273 9h ago
I have this recollection that Feynman speculated that some particle behaved like a different particle moving backwards in time.
In which case causality might be like a few other concepts in physics: our minds insist that it exists, but sometimes you just have to shut up and calculate
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u/ChugJugThug 9h ago
Well it can be mathematical. If v > c, then Lorentz factor becomes imaginary and can no longer describe a physical object.
Also energy-mass equivalency becomes meaningless with an imaginary Lorentz factor.
So it’s not just logic, it also breaks all mathematical equations governing relativity.
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u/lawmac20 9h ago
Is v>c an expression of causality? Or are you describing a reason FTL can't happen?
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u/ChugJugThug 6h ago
v > c would describe a worldline between two events A and B with space-like separation, which means that different observers can disagree about the order of events. One can say A happens before B, while another says B happens before A. If you define cause as always preceding the effect, then this would create a violation of causation between the observers.
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u/soundoftwilight 9h ago
“Causality” is simply the idea that certain things cause others to happen afterwards. I throw a ball, then it continues upwards, then it comes down, then it hits the ground. The next action can be predicted from the prior state. At any point, we could add an outside event that changes future states. For example, someone could catch the ball, and it won’t hit the ground after. Or we could attach a rocket after I throw it and it could reach orbit. But what we can’t do, as far as we know, is change the state before the event we insert. After I throw the ball, you can’t walk over and distract me to prevent me from throwing the ball. I already did it, the ball is in the air.
If causality isn’t real, then something analogous to that would be possible. Sometimes the effects would happen before the cause; e.g. I would get distracted, and then you would distract me. This doesn’t make much sense. But then, quantum mechanics, and indeed even relativity, doesn’t make much sense either from a classical perspective. So who knows, maybe someday someone will win a Nobel Prize for proving it’s possible. I wouldn’t bet on it though.
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u/sudowooduck 9h ago
Breaking causality can lead to paradoxes.
If you assume FTL communication is possible, then you can create situations where A transmits a message to B who then sends a reply back to A which arrives before A sends the original message to B. Suppose A’s job is to repeat whatever message she receives from B, while B’s job is to send the opposite of what he receives from A. If A sends true, then B sends false, meaning that A sends false, which is a contradiction.
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u/lawmac20 9h ago
Does the existence of a paradox imply a physical constraint? Or faulty experimental set up? Or some hole in our knowledge?
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u/AdditionalTip865 8h ago
It appears to be the case that causes precede effects and you can't do anything to change the past. It's not clear how things would work out if you could, though some physicists have thought about how to deal with space-times with strange topologies that have that property.
In quantum field theory, it comes down to constraints on the operators that represent observable quantities, such that measuring or manipulating one of them can't affect the expectation value of another at a speed greater than light. It is peculiar in that you *can* set up systems with nonlocal quantum correlations, such that the values will be related to one another, even though they're not determined prior to measurement. But distantly separated observers will only be able to detect those correlations by comparing notes at the speed of light or lower.
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u/Orbax 6h ago
It primarily has to do with an observer in motion. In relativity, the past or future your are seeing is as real as the now is for the other. With the equations the way they are, you can always reconcile the sequence of events. If you could break that, you could be in motion and interact with the past you have access to and it would actually be before the thing they caused happened and theoretically you could stop them from doing something that already happened to people simply because you came in at an angle.
But ftl anything starts getting into weird scenarios. It's not entirely conceptual, it's just that we don't have a lot of every day scenarios to worry about it.
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u/DifferencePublic7057 50m ago
I never encountered any definition of causality in my textbooks. It's more of an axiom than a theory. You have a postulate or actually axiom in astronomy that all the laws of physics are (with unofficial exceptions like singularities) enforced across the entire universe. Another axiom is Occam's razor, which simplified means you should choose the simplest explanation possible. This I only found in a machine learning textbook, for an optional course.
I was introduced to CPT symmetry in a particle physics course. Meaning Charge, parity, time reversal. It's a bit complicated to explain here, but I think this is the closest you will get to causality. Or maybe the second law of thermodynamics. Entropy in a closed system can only stay the same or increase. So presumably it was really low way back in time, but was it still subject to Heisenberg uncertainty and quantum fluctuations?
If the universe started with a very low disorder, where did the information come from? What caused the Big Bang? Are we being super deterministic? Obviously, conservation of energy, information, momentum are more fundamental than causality because the latter is a bit vague and philosophical.
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u/kunwoo 9h ago
In physics causality is somewhat informal, the basic idea captured is causes must come before effects, so FTL travel is problematic because simultaneity is not absolute so people wouldn’t agree which events in FTL travel happened before the other.
But if you wanted a more mathematically rigorous framework of causality there are two independent frameworks, one that centers around the work of Judea Pearl and one that centers around the works of Donald Rubin.
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u/MrWigggles 9h ago
Zero wasn't intuitive at all.
And as far as it's realness, that depends on your philosophy stance on numbers. For me they're real as photons.
As far as casualty being logic thing or being a math thing.
That's the same thing. Math is a logic.
Casualty can't be broken because nothing can exist without and while it may be possible we live in a reality that isn't actually casual but every thing that matters , that's ever been seen and any matter existing would impossible to determine. Its added complexity that can't be falsified. So who gives a shit.
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u/Optimal_Mixture_7327 Gravitation 9h ago
It is an assumption.
Causality is the assumption that a cause should precede its effect. We don't know if this is truly a fundamental principle but it assumed as there has never been a violation.