Absolute invisibility in cosmos [closed]

Question

I am a mathematician and not a physicist, so I have a lack of understanding subjects of physics. I would like to ask which physical objects are absolutely invisible by us -I mean which objects are invisible by human eye or any human device that we have or we can create- so we can approach them only by logic and mathematics.

Answer 1

... which objects are invisible (to) any human device that we have or we can create ...

That's a pretty tall order, since we do not know what devices or detection methods may become available in the future. At present we have no way of directly detecting dark matter, and we can only infer its presence from its gravitational effects on galaxies and galaxy clusters - but we may find a way of directly detecting dark matter in the future.

If the laws of general relativity are correct then we can never have a way of observing events beyond the event horizon of a black hole (provided we remain outside of the event horizon ourselves). However, we know that general relativity and quantum mechanics are not consistent with one another, so it is not entirely inconceivable that some future theory of quantum gravity (to which general relativity is only a good approximation) may allow us access to events beyond the event horizon.

Answer 2

As gandalf61 alludes to, there are different methods of detecting different things in our universe, and as we learn more, we can invent new methods of "seeing" things we couldn't "see" before.

There was a time when all we could see was those wavelengths of electromagnetic radiation to which our eyes were sufficiently sensitive. Then we invent optical telescopes, which increase the sensitivity our our eyes. Now we can see things we couldn't before.

Then we invent photographic film, and stick that in our telescopes where our eye would have been, and develop the film. Then we invent solid-state detectors to replace photographic film, with the same results.

Then we invent rockets, and put our solid-state telescopes in orbit where there's no shimmering atmosphere in the way, and see a whole new universe of previously-invisible objects.

Then we repeat this whole process with detectors that are sensitive to wavelengths of light that our eyes do not respond to at all: gamma rays, x-rays, ultraviolet, infrared, microwave and radio waves. Same result.

In each of these examples, we are working with electromagnetic radiation that directly interacts in one way or another with matter. If we now wish to see more than that, we need detectors that respond to signals other than those which consist of electromagnetic radiation.

We have recently taken the first baby steps to make telescopes which are sensitive to gravitational waves and to neutrinos. These tasks are enormously more difficult than inventing photon-based detection schemes but based on the successes of those baby steps, the expectation is that we will eventually have telescopes which can render visible whole new worlds in the universe.

So, stay tuned!

Answer 3

If I may I will make a list, things that we can't do or that are problematic:

TIME

• We cannot observe anything in the future
• We can observe things happening in space in the past but we can't choose
• Its problematic to observe things that happen very slowly
• Its problematic to observe things that happen very very quickly

EM Waves (Photons)

• Anything happening in the Universe can be observed if it sources EM Waves (Photons) or somehow interacts with
• Most likely mankind takes EM Waves for granted and makes assumptions, there could be many many things, events, phenomena that do not source or interact with EM Waves (Photons).

Gravity Waves, Gravity Lensing, Gravity bending

• Gravity events can be observed now but effect of Gravity is inverse square of distance so we can observe massive events that are not too far away (eg collision of two black holes).

Dimension, Length, Scale

• There are limits to what we can observe when dimensions get extremely small (or extremely large of course). Again there is strong reliance on EM Waves (Photons). Quantum Physics can make things difficult too.

There are more. Its clear that throughout Physics tools like logic and mathematics have been fundamentally essential. The most difficult unknown, that even logic and mathematics has not yet helped us with, is how the Universe came to be. Big bang is unhelpful, it just begs the question, what was here before the Big bang? Where did it all come from. There is something fundamental missing from our understanding.

Answered in good faith.

-

Answer 4

We know of things that are invisible. We cannot see them because we do not receive light from them.

Light is an electromagnetic field. That means it is generated and reacts to objects with an electric charge. Light reflects off a mirror because electrons in the mirror can move. Light sets the electrons vibrating and is absorbed. The vibrating electrons emit new light, which is the reflection.

Windows have positive and negative charges in equal amounts which are tightly bound. They do not vibrate much in response to passing light. Little light is reflected or absorbed, so it mostly passes through. But glass isn't perfectly invisible.

Black paint absorbs light, so it is black. If it was perfectly black, you would see nothing when you looked at it. So you can tell it is there because otherwise you would see something. This applies to other objects.

Black holes have such strong gravity that nothing, not even light, can escape once it enters. So they are in that sense perfectly black. But we can see them in a few different ways.

• Strong gravity bends light away from a straight path. Stars and galaxies behind the black hole look displaced or distorted.
• Objects near a black hole are sucked in. On the way, they are violently compressed and ripped apart. They heat up to millions of degrees. We see bright light from them.
• Objects farther from a black hole orbit it. We can see them. From the orbital parameters, we can infer the mass of what they are orbiting. If the mass is large enough and the object gives off no light, we infer that it is an invisible black hole.
• In theory, black holes give off Hawking radiation. But for any reasonable sized black hole, it is far far too faint to detect.

In a similar way, we infer that dark matter exists. We look at a galaxy and add up the mass of all things we can see or infer from light. We look at how fast stars in the galaxy are orbiting. The matter we see or infer is only $20$% of the mass needed to explain the orbits. We infer that the missing 80% is something that does not interact with light. Other experiments eliminate everything made out of ordinary matter. We don't know what dark matter is, but we know it is there. Or we know that our theories of gravity are wrong.

In a similar way, we know that an even more mysterious thing called dark energy must exist to account for the expansion of the universe. There is far more of it than matter and dark matter combined.

Neutrinos are a particle with very peculiar properties. It does not interact with light. It very nearly does not interact with matter. It treats the Earth like light treats a window. But very occasionally one does interact with matter. $5$ light years of lead has a $50$% chance of stopping one. They are produced in huge quantities in the core of the Sun. They can be produced in particle accelerators. Huge detectors have been or are being built to look for the occasional interaction. They were discovered not by seeing them. But by seeing that conservation laws were apparently being violated in nuclear reactions, and inferring that the missing spin had to go somewhere.

There are other things we suspect do or might exist that we cannot detect. In some cases, we cannot detect them yet. In others, detection is beyond anything we could hope to build, but in principle possible. There are things that we cannot figure out way that we might go about determining if they exist or not. And there are things we do not expect it is possible to determine if they exist or not.

Magnetic monopoles are an example. They are a straightforward extrapolation of electromagnetism and quantum mechanics. There have been searches. But none have been found.

Proposed universes beyond the event horizon of a black hole are another. The math supports the idea that they could exist. But you would have to travel out of a black hole to get from one to here. There is no way, even in theory to do that. (Being emitted as Hawking radiation doesn't count as getting you. What got here wouldn't be you.)

Alternate universes from the Many Worlds interpretation are also a straightforward result from quantum mechanics. But again, there is no way to travel between them. We have no way of knowing if they exist or if they are a mathematical artifact.

Answer 5

I mean which objects are invisible by human eye or any human device that we have or we can create- so we can approach them only by logic and mathematics.

let's broaden this discussion to "entities" rather than just objects.

1. The vast majority of the total matter of the universe is considered to be dark matter. This is matter that is inferred to be there by various observations of the Cosmos, but after more than fifty years of searching and experiments, no one has been able to directly detect it or determine what it is made of. Some physicists suggest dark matter does not exist and that the laws of gravity are not exactly what we think they are over vast distances.

2. The vast majority of the total energy of the universe is considered to be dark energy. This energy accounts for the apparent accelerating expansion of the universe. No one knows what the source of this energy is. It is not possible to calculate how much dark energy the universe should have from first principles. We can only infer its existence from the apparent distances of distant objects.

3. Space is considered to be expanding. It is possible for distant galaxies to have recession velocities that are greater than the speed of light. This does not violate special relativity, because they are considered to be comoving with the local Hubble flow of space itself. No one knows what is actually expanding. Space does not appear to be a substance we can detect. We can only infer it is something that physically expands, although it is essentially made of nothing. Objects that have peculiar motion relative to the local Hubble flow time dilate by the factor we would expect if the local space is considered a rest frame.

4. Most people do not think that the Lorentz Ether exists. If it does exist it has the properties that any particle that moves relative to it length contracts and time dilates. Its properties mean we cannot detects its rest frame by observing the motion of particles relative to it. When you study the Lorentz Ether, it appears to be logically and mathematically there, but it is not possible to detect it.

5. Everything we think we know about the interior of black holes can only be inferred, because it is not possible to observe what is going on inside the event horizon from outside. When I drop an object into a black hole I can never observe it crossing the event horizon, but calculations that consider the proper time of the object predict that the object crosses the event horizon in finite proper time. If I drop a probe programmed to transmit a signal back to me at the exact time it arrives at the event horizon, I will never see that signal, so it can only be inferred it actually crossed the event horizon.

6. When a quantum particle is emitted its location is described by a wave function. The wave function is a description of the particle as a field of probabilities. This does not mean that the particle has a location, but we are simply ignorant of the location of the particle. It is worse than that. The popular interpretation is the particle does not actually have a physical location at any given time, but is in a superposition of locations.

7. The 'many worlds' of the many worlds interpretation of quantum mechanics. This interpretation of quantum mechanics is apparently the favourite interpretation of most most physicists. It does not appear to bother them that none of these parallel worlds (except for the one we appear to find ourselves in) are directly detectable and can only be inferred.

8. As user417360 points out in his answer, TIME itself is also one of these elusive entities.

Most of modern physics appears to be about the inferred properties of entities that cannot be directly detected and no clear description of what they are made of can be given. Most of the universe is governed by quantum mechanics. We can give a very accurate description of how quantum mechanics works in terms of mathematical equations, but we cannot come up with a single physical interpretation that everyone can agree on, so increasingly the universe appears to consist of inferred mathematical entities.