What is the relationship between the physical and our senses? If something is physical, must it necessarily be the case that we should be able to perceive it, at least in principle, directly with our bodily senses, or indirectly by expanding our innate senses with technology (e.g., the neutrino detector)? Can something be physical even if there is no known or conceivable way of perceiving it, directly or indirectly, with our senses?
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1Can you give an example of something that you consider non-physical? – JimmyJames Mar 11 '24 at 15:36
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"no known or conceivable way" I feel like this would be a better question if this phrase was replaced with 'no possible way'. But I might be misinterpreting your question. Are you asking whether physicality depends on human perception? Kind of a 'if a tree falls in the woods...' sort of question? – JimmyJames Mar 11 '24 at 15:42
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@JimmyJames The question was prompted by the comment section discussion under this answer. As for examples of non-physical things, that's a point of contention. This answer offers some examples, such as consciousness (purportedly). – Mark Mar 11 '24 at 16:00
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Would the universe during the cosmic dark ages fit the definition of "no known or conceivable way of perceiving it, directly or indirectly"? – JimmyJames Mar 11 '24 at 16:21
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@JimmyJames Maybe, although I'm pretty sure someone could argue that we can indirectly observe or infer what happened then with our senses today. – Mark Mar 11 '24 at 16:24
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OK. What about things outside of our light cone? It's definitely impossible us to directly or indirectly perceive them. But in the context of the question, is it 'conceivable' that someone could? – JimmyJames Mar 11 '24 at 16:28
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@JimmyJames An even better potential counterexample would be parallel universes in a multiverse, unless someone comes up with conceivable ways of perceiving other universes (indirectly) with our senses. – Mark Mar 11 '24 at 16:30
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I'm not making an argument, I'm just trying to make sure I understand the question. I'm not sure if that's a 'yes' or a 'no'. – JimmyJames Mar 11 '24 at 16:31
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@JimmyJames I see. My answer would be "I don't know", since I don't know all the things that smart people can conceive of. For all I know, it is epistemically possible that someone could come up with a revised version of general relativity that might overcome the apparent limitations imposed by light cones. – Mark Mar 11 '24 at 16:36
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I meant more like I can conceive of being an alien living in a galaxy outside of earth's light cone. But I think I have an idea of what you are asking. – JimmyJames Mar 11 '24 at 16:50
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@Mark "Is something physical if and only if we can perceive it (directly or indirectly) with our bodily senses?" In a nutshell, yes. This all depends on various definitions, but this is the basic idea, yes. – Speakpigeon Mar 11 '24 at 17:23
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1"Physical" evolved by extension from "ponderable matter" of everyday objects, to micro-particles and then fields. Accessibility to senses (with repeatability and public agreement on the outcomes) was only one of the extension conditions, subject to consistent interaction laws was the other. As light cones and parallel worlds suggest, we are ready to give up the first. But ghosts, qualia and abstract objects (as interpreted by realists about them) suggest that we are not inclined to give up the second. Their behavior is "too far" from "ponderable matter" even if they are "sensible" somehow. – Conifold Mar 12 '24 at 00:09
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You may be able to surmise the existence of something that is imperceptible but then construct a mechanism/device that can render them perceptible and then decidedly physical: telescopes, microscopes, Petrie dishes and so on. Then there are illusions, mirages and such that are perceived as physical but with consideration determined not so. – civitas Mar 12 '24 at 01:06
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Referring to perception is an erroneous restriction. We can't perceive planets in far off galaxies, but that doesn't stop there being planets in far off galaxies or make them unphysical. Physical things interact with other physical things, but might do so very weakly or infrequently, so that any form of detection is a challenge.
Marco Ocram
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I agree, of course. But how would you define "physical" and delineate it from "unphysical"? – Jo Wehler Mar 11 '24 at 20:53
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@JoWehler Hi Jo, I hadn't thought about it, but will ponder and get back to you if anything springs to mind. I wonder if it is possible that the only definition is that physical things are things we consider physical. In other words you would delineate physical by naming all the categories of things considered physical. – Marco Ocram Mar 11 '24 at 21:22
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For example, we can mention matter, energy, fields, forces, spacetime etc and say they are physical, and everything else is unphysical. Might that work? – Marco Ocram Mar 11 '24 at 21:24
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@MarcoOcram I would think the delineation is that unphysical things cannot interact with each other. Though you could argue that my unphysical thoughts about your face might interact with your unphysical thoughts about my friendliness via my very physical fist. – DKNguyen Mar 11 '24 at 22:52
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You're ignoring the "expanding our innate senses with technology" part of the question. Telescopes that we use to see planets in far off galaxies count as such technology. – HolyBlackCat Mar 12 '24 at 07:53
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@HolyBlackCat I didn't ignore it. We don't have telescopes that can see all of the planets in the Universe, and we never will, according to current science, because parts of the Universe are receding from us so rapidly that light from there will never reach us. – Marco Ocram Mar 12 '24 at 08:39
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@HolyBlackCat There is an argument that something no longer exists if something literally has no potential to ever affect you again in any way (such as things that have receded beyond the even horizon). – DKNguyen Mar 12 '24 at 14:34
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Where exactly do you want to go with that question?
I mean if you take our current level of understanding and regress to a time with insufficient technology it's quite easy to imagine for example that radiation is a physical effect, that is impossible for us to perceive directly and quite hard even to detect indirectly as the effect is often quite small and easily overlooked. So it would be something physical that we couldn't directly or indirectly perceive and you could imagine that there are other things like it, that we have yet to discover.
On the other hand if we can't see them, can't hear them can't perceive them in any way shape or form and don't see their effect by any of those senses, how should we know that they are there in the first place? Like it's not the "being physical" part that is the problem but the "BEING" part in the first place. How would we even know that we don't perceive them if we don't perceive them?
Now the "obvious" counter might be the existence of particles which were predicted in theory before they were discovered in nature, for example because a calculation showed a violation of conservation of momentum and/or energy or such fundamental relations that we believe to be fixed and which we therefore assume to must hold even if our observations don't readily confirm that. And where we thus assumed that something is there even if we didn't see it's effect (yet).
But there are two problem here a) the (yet) part so does it count as not being physical or are we just not aware that it is and b) wouldn't we have already seen the effect of that thing with our senses indirectly? Is contradicting our logic already perception enough or does it need to be a primary sense? Like after all we do consider measurements done with a computer to be primary enough so not sure it's valid to discard that.
So yeah in order to categorize something as physical we need to be aware of it's existence in the first place and for that we need to see it's effect on us and/or our environment (by our senses). So yeah that is kinda a requirement. On the other hand it's perfectly possible for things to exist without our knowledge and it's kinda arrogant to make our perception a requirement for their existence.
So idk consider a mysterious field, modifying a mysterious property that has an effect that we can't see but that our alien neighbors could see. If we don't see it, then we don't think about it, it takes up no space in our environment and we don't consider it physical because we don't consider it at all. Once we meet those aliens and they tell us about it, we become aware of it's existence and it suddenly becomes physical (or always was) due to the fact that we can know indirectly perceive it (by communicating with the alien via our senses).
Though what if the alien is lying? Are we then perceiving something despite the fact that it doesn't exist? Do the concept of "lies" and in general "impressions" become physical because they can be perceived? The thing is our senses do often confuse the cause and effect of a an action. Like a sound or picture isn't scary, it's just the effect that the object has upon us, though the cause of that effect might very well be physical and measurable.
haxor789
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1"Once we meet those aliens and they tell us about it, we become aware of it's existence and it suddenly becomes physical" I would consider that a 'conceivable way of perceiving it'. – JimmyJames Mar 11 '24 at 15:39
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Two cents.
"Physical", in most cases, means "being a form of matter-energy, or a property of matter-energy (eg charge, or a conservation law)".
Matter-energy can be such that is detectable or sometimes undetectable (eg a form of "dark matter").
Even if undetectable though it can be inferred to exist due to other reasons (eg the discovery of a new planet due to anomalies in orbits of known planets and known laws of motion).
So the answer is no, something physical need not be directly or indirectly perceptible. It may not be perceived yet still be physical.
Nikos M.
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Isn't inference a form of indirect perception, though? Using a device to detect something rather than direct sensation is itself a form of inference, after all; we infer the presence or properties of something rather than being personally aware of it because of the response of the device. – Idran Mar 12 '24 at 14:16
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In what sense is the inference that sth like dark matter must exist (due to some theoretical reasons), a kind of perception of dark matter? – Nikos M. Mar 12 '24 at 14:22
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I was responding to "even if undetectable though it can be inferred to exist...". I'd consider the sort of inference you describe a form of indirect detection/perception; that is, I wouldn't consider the word "undetectable" accurate in that case. We describe exoplanets whose presence we only know of via gravitational effects on its star as "detected", after all. – Idran Mar 12 '24 at 14:25
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Ok what you say can be taken as an indirect form of perception, but I don't want to limit the inference only to these cases. Dark matter is inferred to exist purely on theoretical grounds. – Nikos M. Mar 12 '24 at 14:27
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Isn't it also inferred to exist via the required gravitational mass of the galaxy vs. the total amount of perceived visible matter? Dark matter is literally just "the matter we can't see that contributes to the gravitational mass", after all, whatever form that takes. – Idran Mar 12 '24 at 14:29
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True this is the theoretical reasons for dark matter, however I would not take the gravitational mass an indirect perception of dark matter since by definition is undetectable (as far as we assume at least). – Nikos M. Mar 12 '24 at 14:31
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So the inference of dark matter is only a theoretical possibility but that is enough to make the point that physical need not always be perceived. – Nikos M. Mar 12 '24 at 14:34
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Ultimately this comes down to the question of what 'physical' means. This is a not a simple question. As mentioned in Section 4.2 of this article on Physicalism:
Along with the concepts of space, time, causality, value, meaning, truth and existence, the concept of the physical is one of the central concepts of human thought. So it should not be surprising that any attempt to come to grips with what a physical property is will be controversial.
The idea that physical means something that can sensed directly or indirectly is a fairly common interpretation but not the only one.
I would also make a distinction between something being 'physical' and being 'real'. A planet is an inherently a physical thing. What about a hypothetical planet? Is that physical? If it existed, I think we would all agree that it was physical. Another way to look at your question is whether being real is a necessary condition in order to be physical. But that leads to another question of what it means to be real.
I don't believe anyone from any side of these debates has conclusively defeated the other arguments, but I lean toward the idea that 'physical' means something of or relating to things that are detectable by our senses. By that I mean, for example, that the lightbulb is an inherently physical concept. It's defined by its physical properties and even the idea of a lightbulb is physical. In other words, you cannot separate the idea of a lightbulb from its physical definition.
JimmyJames
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@civitas I would say that's a metaphor based on the concept of a literal lightbulb. – JimmyJames Mar 12 '24 at 15:01
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Can something be physical even if there is no known or conceivable way of perceiving it, directly or indirectly, with our senses?
There could be completely undetectable physical things - we just wouldn't know about them.
The closest thing to undetectable that we do know about is dark matter - it's only mostly undetectable, it has mass so it affects gravity.
There could be undetectable parts of physics that don't interact with the particles that make up our every day life and don't have mass, that would be maybe 100% undetectable. There might be analytical ways to infer their existence, but there's no guarantee.
TKoL
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I would argue that "dark matter" is undetectable as it is only a theoretical possibility (the other possibility being some equation is wrong, or sth else being the case). So it is only theoretically conjectured to exist – Nikos M. Mar 13 '24 at 12:12
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@NikosM. I suppose you could say that, but of course it's still worth pointing out that we think it exists because of things we have detected, which means that if it exists, it's detectable - but only just, and not very well. – TKoL Mar 13 '24 at 12:14
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I would not count the theoretical reason we think sth like dark matter can exist as a detection of it.. – Nikos M. Mar 13 '24 at 12:16
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@NikosM.: All proposed explanations of dark matter either interact with regular matter in some way, or stipulate that our understanding of physics is incorrect such that dark matter is not required to exist. There is no such thing as totally non-interacting dark matter - nobody has seriously proposed such a thing. – Kevin Mar 13 '24 at 16:57
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@Kevin I am not aware of dark matter interacting as usual matter. In any case even undetectable dark matter as a theoretical possibility makes my point. – Nikos M. Mar 13 '24 at 17:00
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@NikosM.: Undetectable dark matter is not a theoretical possibility, it's something you made up. – Kevin Mar 13 '24 at 17:01
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@Kevin dark matter is undetectable by definition, else it would be called usual matter. If some people propose theoretical ways to verify its existence is another matter. So you can try to address what I said with references. – Nikos M. Mar 13 '24 at 17:04
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@NikosM. when I google the definition, it says
(in some cosmological theories) non-luminous material that is postulated to exist in space and that could take any of several forms including weakly interacting particles ( cold dark matter ) or high-energy randomly moving particles created soon after the Big Bang ( hot dark matter ).Nothing in there about being inherently undetectable, only weakly interacting. – TKoL Mar 13 '24 at 17:06 -
@TKoL I agree completely with what you quoted. The only difference is that we seem to play with words. Currently there is no way to detect dark matter except due to theoretical reasons for its existence. Agree with that? – Nikos M. Mar 13 '24 at 17:08
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@NikosM. at the moment we can't directly detect dark matter. Sure. I feel like you're arguing with something I didn't say, instead of reading the words I did say. – TKoL Mar 13 '24 at 17:10
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@TKoL at the moment we can't detect it even indirectly unless one counts purely theoretical book-keeping reasons as detection. That is what I said. – Nikos M. Mar 13 '24 at 17:11
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@TKoL you are saying that is indirectly detectable, something I disagree with, else we agree. Its existence is only theoretically conjectured. – Nikos M. Mar 13 '24 at 17:14
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What is physical can mean different things when viewed from different perspectives. But I would try to give the highest level qualification.
Let's say we live in a computer simulator (the Matrix), a bunch of programmers and architects are actively maintaining this system.
Then, everything falling under the rules of this simulator would be - physical.
But most of the exceptions to the rules, a code that does not apply to the simulator's inhabitants, something reserved for God, like programmers entering the simulator - would be non-physical.
TheMatrix Equation-balance
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No. Qualia (for instance) is directly observable but not physical.
In general, for something to be physical it should be observable with an approach that satisfies the criteria of scientific method. This (among other things) requires observability by different observers who can communicate their results to each other.
So, something is not physical if:
- After observing it the observer inevitably dies before being able to communicate it (say,occurs beyond the event horizon of a black hole).
- After observing it, the observers cannot communicate to each other (for instance, get separated into parallel universes)
- After observing it the observer gets duplicated or split in such a way that each copy reports different observations
- The observer has no suitable language to communicate the observation (for instance, new smell sensation)
Anixx
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