Domenico,

There is no question about this. Human life is certainly possible in weightless conditions. Many hundreds of humans have lived and worked in these conditions over the last 50 years.

Gravity would still be an identifiable phenomenon - indeed, in a microgravity environment experiments such as the Cavendish experiment would be even easier to perform!

Thanks James, do you mean that life is possible for a full life long or just for periods of time?

Domenico,

We do not yet know if the adverse effects that *are* known to arise from microgravity are severe enough to prevent a full human lifespan from being lived in reasonable health.

I don't quite see how that affects your argument.

If life were to evolve in a microgravity environment, it could still know about gravity.

Thanks for your answer James.

Within the dominant paradigm of sciences, you are completely right, because the gravity is objective, and gravity would have been anyway discovered through the centuries and the emergence of sciences.

My question was not clear: I was thinking about the subjective experience of “gravity’, not the scientific concept, but, so to say, the feeling of being attracted, heaviness, weighing or the fact that what goes up will then go down. Is this subjective experience a prerequisite for a person to reach an understanding of the scientific concept of gravity? And is this subjective experience a prerequisite for science to “discover” this concept? In the case of microgravity, what would have been the subjective experience of heaviness (not a scientific concept)? In microgravity (or in macrogravity) would we have developed the same subjective experience of heaviness?

Domenico,

> . Is this subjective experience a prerequisite for a person to reach an >understanding of the scientific concept of gravity?

No.

I do not need to experience particulate radiation (alpha, beta rays) in order to understand it. Nor do I need to experience time dilation in Special Relativity to grasp its impact.

In a microgravity upbringing the notion of acceleration would still be true (things would still be able to be moved through the application of force). And an acceleration is indistinguishable from gravity.

The elder clouds (sentient molecular clouds) would simply describe gravity to their offspring as like 'a continual push from a magnetized plasma loop acting on another loop' - that being the closest that a sentient cloud might get to a bona fide rocket.

The general notion that we need real-world analogues to understand an idea is not tenable. We develop tools of the mind that allow us to transcend the limitations of our body and mentally explore many more regimes than we could survive in.

I suggest that you take a look at the following paper that summarizes the complex and pervasive interconnections between the outputs of the vestibular apparatus and many areas of the brain shown to be important for human cognition. Zero-G environments would tend to "open the loop" between physical interaction with our environment and learning/cognition.

Hitier, M., Besnard, S. & Smith, P.F. (2014). Vestibular pathways involved in cognition. Frontiers in Integrative Neuroscience, 8, 1-16.

Frank,

An interesting link.

From the abstract, " a new pathway is *hypothesized* via the basal ganglia, *potentially* involved in spatial learning and spatial memory. "

My emphasis.

Astronauts/cosmonauts/taikonauts have shown no obvious difficulties in learning new tasks - that I know of. If there are influences from microgravity on the developing mammalian brain, that is another question.

I think that Domenico's question tended to the more metaphysical - can one *know* a thing without experiencing it? I could be wrong.

I think James Garry's answer is reasonable but incomplete. We might consider David Hume's answer. He said that all ideas come from impressions. While we can accept the idea that there is gamma radiation, for example, without feeling it, by recognising that our experience of heat and colour connects to the e/m spectrum, we cannot imagine what gamma rays might feel like, for example. I had a student once who could not see the colour red. He wasn't colour blind, it was invisible to him and his mind painted over red spaces. He thought he could see through red objects. I asked if he could imagine what the colour might be, based on the colour wheel you get on a computer settings. He couldn't.

A question is, do any of us 'understand' gravity? We are all prisoners in Plato's cave at some level. Can one 'know' a thing even if it is experienced? Kant would say you can't, at least not based on experience. We see the world through the lens of the mind.

Only the endpoint rationalist might get at knowledge, but that would require one to work out how the world must be based on Descartes' internalism and foundationalism.

Stephen,

Quite. We *all* live in a moist utterly pitch black cave (the skull) and have no direct knowledge of the world - except by the faint electrical signals arriving through nerve fibres.

Our knowledge of 'red' is no more real than our knowledge of gamma radiation, or of gravity.

All of these, eventually are constructed models in mind (whatever that is).

So if one is willing to accept that 'red' can be 'known' then gravity (in a weightless environment, deduced and observed but not sensed directly) is equally well 'known'.

Sorry for my bad english.

Phylogenesis: if man appeared on earth more than 200,000 years ago, the concept of gravity can be considered as relatively recent. During the long period leading up to the discovery of gravity, man has experienced heaviness. Can we say that, phylogenetically speaking, the experience of heaviness preceded the discovery of the concept of gravity?

Ontogenisis: Jean Piaget’s researches (and those of his followers) show that the genesis of scientific concepts such weight and volume (and their differentiation) are acquired at 9-10 years old (weight) and 11-12 years old (volume). I don't know about gravity. But younger kids, we might expect, have developed experience of heaviness. Can we say that experience of heaviness precedes the understanding of weight as a concept and certainly also the understanding of the concept of gravity.

Today adults: does everybody have developed some experience of heaviness? Does everybody know about gravity?

These are facts. Question: is the experience of heaviness necessary to understand the concept of gravity?

As far as I see it, gravity is an abstract concept designed to explain, among other things, weight. However, some explanations in which gravity is invoked as an explanadum do not necessarily refer to weight, such as the motion of the planets. The motion of the planets is sufficiently explained by gravity and the planets' mass (but please correct me if I'm wrong). Hence, weight as a concept is not necessary to come up with gravity as a scientific concept. (In other words, gravity is necessary to explain the motion of the planets, but the motion of the planets can be explained without invoking weight.)

Of course, weight may still have been necessary for humans to have (culturally) come up with the concept, seeing how mass is experientially more abstract than weight. Hence, in a cultural history of ideas, the idea (/concept) of weight certainly precedes that of gravity.

However, given that as a scientific concept, weight is not necessary in order to come up with gravity, we can envision future scientists who live in a zero-G environment, were never familiarized with the concept of weight, never experienced the phenomenon of weight, but can still perfectly understand the concept of gravity (since they can work with the concept in their development and application of scientific theories). They may also then be able to infer weight as a derivative scientific concept, without ever experiencing it. It then depends on whether it is scientifically useful or even necessary for them to come up with this derivative concept... but that is where I will cease speculation for now.

Thanks Joachin,

Thanks for your answer.

If I understand you correctly, we don’t feel gravity, but we feel its consequence: weight.

To go further, we need to differentiate and articulate epistemology of sciences and epistemology of common sense (or common meaning).

Common sense feels heaviness, not weight (scientific concept).

Another example: felt temperature (common sense) versus the measured temperature. I assume that before the conception of thermometers (or any other tool to measure temperature), people could deal only with felt temperature. Related to this example, the questioning regards the relationship between knowledge of the common sense and scientific knowledge.

From the ontogenetic point of view (following Piaget's studies), the origin of knowledge (common sense knowledge as well as the acqusition of scientific knowledge) is to be found in the actions of the newborn child.

Thank you for your reply and for an inspiring conversation, Domenico!

I mostly agree - except that I think weight is also already an abstraction from experience. To adapt your mention of common sense, I believe that the concepts of weight or heaviness are "common sense abstractions", whereas gravity is a scientific abstraction.

My reason for this is as follows: In order to 'have' a concept such as weight or heaviness, one needs to know that phenomena of a certain kind (or type) fall under it. No such thing, however, is required in order to be able to feel the phenomenon itself. For instance, I think a stranded whale will probably feel the heaviness of its own weight, but it will not know what types of things fall under the concept 'heaviness' or 'weight'. The same may be true of prelinguistic children. They abstract from certain experiential phenomena in order to later arrive at the concept 'weight', and then, if they become educated in science, they learn to abstract further in order to arrive at 'gravity'.

Joachim,

The weight requires a measure, it is not the case for the heaviness. When it is very hot and wet outside, and if furthermore I am tired, I can then feel very heavy. In very great shape and in meteorological conditions more pleasant, I can feel light as a gazelle. But if I rise on the balance, I shall have the same weight, no matter the temperature. So I experience the real weight through an objective measure of it.

But in the everyday language, it is possible to confuse both (heaviness and weight) and in maybe people speaks about weight more than about heaviness when there own "weight" is concerned.

The weight of an object is defined as the force of gravity on the object and may be calculated as the mass times the acceleration of gravity, w = mg. That's the scientific definition, not the common sense understading of it. I don't think that Piaget was asking to children the scientific definition of weight. He was looking for the origin (genesis) of scientific thinking in the children active experience of the world. In that sense, and at that level, you are right to say that weight is an abstraction of common sense (through social exchanges).

Domenico,

I fully agree with you. I fondly remember a chapter from Popper's "Logik der Forschung" (The Logic of Scientific Discovery) in which he discusses the logical origin of the notion of measurement, and I think we can draw some cultural or even ontogenetic conclusions from these considerations. The basic question is: How do we get from purely subjective experiences to an objective measure? All we ever do, it seems, is collect *more* subjective experiences. But all the subjective experiences in the world cannot add up to objectivity... but those are big philosophical questions, and perhaps only tangentially related to the issue at hand.

What I think is that we have some capacity to abstract from experience by judging a given experience to fall under a certain type. That type being referred to by a concept such as 'heaviness' or 'weight' or even 'gravity'. But we certainly need social exchange in order to arrive at some of these abstractions.

Levitation is real. There are different kinds of levitation, permitted by physics. Some are pretty interesting.

Magnetic levitation is the most commonly seen and used form of levitation. Diamagnetic materials are commonly used for demonstration purposes. In this case the returning force appears from the interaction with the screening currents. For example, a superconducting sample, which can be considered either as a perfect diamagnet or an ideally hard superconductor, easily levitates in an ambient external magnetic field. The superconductor is first heated strongly, then cooled with liquid nitrogen to levitate on top of a diamagnet. In a very strong magnetic field by means of diamagnetic levitation, even small live animals have been levitated. Electrostatic levitation is the process of using an electric field to levitatea charged object and counteract the effects of gravity. Acoustic levitation (also: Acoustophoresis) is a method for suspending matter in a medium by using acoustic radiation pressure from intense sound waves in the medium. Sometimes ultrasonic frequencies can be used to levitate objects, thus creating no sound heard by the human ear, such as was demonstrated at Otsuka Lab, while others use audible frequencies. There are various ways of emitting the sound wave, from creating a wave underneath the object and reflecting it back to its source, to using a (transparent) tank to create a large acoustic field. Casimir force: Scientists have discovered a way of levitating ultra small objects by manipulating the so-called Casimir force, which normally causes objects to stick together due to forces predicted by quantum field theory. This is, however, only possible for micro-objects.

I've shamelessly copied the above text, as is, from the wikipedia article: Levitation.

None of the scientific studies so far have proven that any one of these effects are caused through meditation or other yogic practices. It could be entirely possible that the yogi used one of these levitation techniques (maglev, for example) to create an illusion of actually levitating from the ground.

Meditation is a good practice and helps to lower stress, and mental fatigue. Of course, there are other ways to achieve the same effect -- like  a long drive, or a hearty laugh with the family -- so there is no need to overestimate the importance of meditation. Any argument which claims that meditation, or some other brain activity, is able to do some incredible things like telekinesis, telepathy, levitation then at least be skeptical about it, if not outright reject it. One doesn't have to be a scientist to argue that things don't move/lift/vanish on their own. Apply something called as common sense.