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So the track goes something like this...

...Just few hours ago, NASA observed unnatural disturbance on radar near Earth's atmosphere. However, satellite images and telescopic observations revealed nothing weird.

No one knew that thousands of aliens who do not come in visible range of Human eye have just crossed Earth's atmosphere. All their belongings including their spaceships surpasses visibility range of humans. Neither X-ray and/or night visioned cameras or satellites could trace them. They hardly made sound, so it is still unclear whether their sound/noise range too exceeds human's hearing range or not.

...Suddenly out of nowhere a laser like ray started raining from cloudless sky and everything that touched that laser began turning to ashes.

Questions

  1. How would hero come to know about such alien invasion? (Consider Hero working in NASA)

  2. Can human eye's visibility range be altered in any case?

  3. What can be the ways to see alien troops?

Edit:(From the comments below)

  1. All types of sensors in satellites could not capture these aliens, be it temperature or IR.
  2. How would hero come to know about such alien invasion?-> Here by this question I meant, *How would hero being in NASA would detect (any possible technique, scientific method) alien troops coming towards the Earth?*
  3. My third question is actually the primary question. What techniques are available for use to see invisible aliens?
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    $\begingroup$ Does this include being invisible in the IR or Gamma as well since we have satellites that can check for those too? $\endgroup$ – rangerike1363 Dec 16 '16 at 14:28
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    $\begingroup$ How we did find an unnatural distubance using radar (in the high Mhz to Ghz frequency range) but then can't find anything using our radio telescopes and satellites, many of which can detect signals in the same ranges? $\endgroup$ – kingledion Dec 16 '16 at 14:31
  • $\begingroup$ Visible by temperature cams? Also, the aliens from The Silence in Dr Who spoiler-alert are genetically engineered in such a way that anyone who looks at them forgets them the moment they look away again. They were there since the invention of the wheel. Earth belongs to them. There is no need for weapons for them. Just a thought ;) $\endgroup$ – Ayutac Dec 16 '16 at 16:17
  • $\begingroup$ Are they also completely transparent so that they don't appear as deep black shapes? $\endgroup$ – AlexP Dec 16 '16 at 16:29
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    $\begingroup$ I have edited the question to clarify the context @Zxyrra, Ayutac and AlexP $\endgroup$ – Karan Desai Dec 17 '16 at 13:03
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As far as we know in our universe there appear to be four forces that mediate interactions. Since you seem to want true invisability that immediatly restricts the type of matter possible.

The Electromagnetic force Mediated by Photons this force interacts with all charged particles The Strong force mediated by gluons interacts with "color charges"(found only in quarks/antiquarks) that always result in net color neutral particle.

The weak force-Mediated by heavy twins of the photon the W+/- and Z bosons(only force that can violate Parity and Charge parity Symmetry and allows quarks to change flavor/color -responsible for radioactive decay... kinda weird force (but very weak strength wise 2nd weakest)

Gravity-Infinite range interacts with all particles with nonzero mass. Weakest force strength wise.

The condition of true invisibility(no immission on any part of the spectrum or any interaction with photons what so ever) imposes that all particles involved in our hypothetical aliens can not possess any charges as if they had a charge no matter how miniscule they would not be impervious to electromagnetic reactions. This means all the particles involved in the aliens existence must be chargeless not just the net total (neutrons while net neutral actually are composed of smaller charged particles thus do react to photons)

These conditions immediately impact your aliens as all Bosons are composed of quarks which are not electrically neutral (all quarks have (-/+)2/3 or (+/-)1/3 charge) This means that your aliens are immediately restricted to non baryonic matter.

This matter does exist but only in the form of neutrinos and dark matter. The first of these particle types are often called ghost particles as they are only able to interact by the weakforce and gravitational force(due to turning out to carry a very small nonzero mass) factoring the weak strength of these forces and the sheer quantity of these particles passing through us at any given moment and having no reaction outside of rare direct collison with a subatomic particle in your body On to the mysterious "Dark Matter" standing in for matter we have no idea what it is. While dark matter is poorly understood we know it is everywhere thanks to Observations of star velocities not matching what would be expected if byronic matter were the only matter present. This has been further backed up by the wonderful result from general relativity known a gravitational lensing. This has provided overwhelmingly good evidence that something is out there and it makes up the vast majority of all matter in the universe(or that everything we know about gravity is wrong and just so coincidentally works for everything else in the universe). So we have something exceedingly predominate and has not been detected on Earth. This gives an inkling that Dark matter is likely far less reactive than Neutrinos whatever it is.

Now there could be other particles that have some magical properties we don't know however unless they interact with a force they are exceedingly unlikely to ever interact with us at all.

The limited range of the strong and weak forces makes them unlikely candidates for aliens as they aren't applicable outside the range of a nucleus/atom while the gravitational force is so weak it would be negligible unless your aliens were extremely massive and dense.

None of these options really would fit your goal story wise so they can not just be present as is.

Now a possible loophole would be optical limits imposed by focal length and size however they would still produce light only be on the scale of microns or nanometers, In which case your invaders would be more likely to be a disease or some kinda nanomachine. However a sentient disease might be pretty imposing so it could work for you maybe. Can something that small exhibit intelligence?

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  • $\begingroup$ If they don't interact with the electro-magnetic force at all, they couldn't even land on the planet or walk on the planet and NASA wouldn't have observed a disturbance in the atmosphere as they wouldn't just be transparent to photons, they'd be nearly intangible to all baryonic matter. $\endgroup$ – Shufflepants Jan 2 '18 at 21:42
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They're not truly invisible

Fundamentally, if something cannot be detected by any form of electromagnetic radiation (aka Light), then it cannot interact with matter. Physical contact, chemical reactions, lasers, all these things are based around the same fundamental force, electromagnetism. If your aliens can't be detected with electromagnetic radiation, they can't interact with the Earth at all; if they can interact, they can be detected.

How are they reducing their signature?

So our invaders are classically invisible: They can't be detected with visible light. Per the question, they also can't be detected in the Xray or IR bands. There's only a few ways they could do this.

Metamaterials

These are already being developed today around the world. Essentially metamaterials allow lightwaves to break around them, then reconstitute on the far side; by the time they reach a detector, there's little sign of the change.

These materials, however, are very limited in the wavelengths they can disrupt, so the aliens would be visible to detectors working in the radio, microwave, UV, or gamma wavelengths. Fortunately, our radar works at just those wavelengths (the question mentioned that these ships were detected on radar). The fix is easy, then - use radar. Nearly all weapons are already radar-controlled, so if the baddies are expecting us not to shoot at them just because we can't see them with our squishy organic eyes they're out of luck.

Active camouflage

Essentially, rather than trying to bend light around themselves, they're generating appropriate images and projecting them onto the outside of their hull. That is, if you look up at their ship, you're actually looking at a picture of the sky and clouds that they're projecting onto the bottom of the ship.

This works reasonably well - from one angle. If you and your friend both observe the ship from a mile or so apart, one or the other is going to see a discontinuity, and detect the ship. Also, this doesn't work against active detectors (like radar).

They're folding space

This is one of the weird ones. Rather than being classically invisible, these aliens are folding space around their ships so that they're always inside a pocket of spacetime that doesn't interact with the rest of the universe. They get themselves into position, then open a narrow window to shoot through.

This is true invisibility. Unfortunately, it doesn't fit with radar being able to pick them up. It also prevents them from seeing anything that's happening outside before they open their gun port, so if you can predict what they're going to do, you can have an entire army waiting outside when they open up (or, y'know...a nuke).

Handwavium

Dunno how they do it. It's not important. They're invisible, okay?

Okay. Not much we can say about that.

Okay, so how do we detect these guys?

Your question already gave the answer - radar. Radar is a very common tool; almost every part of the developed world is covered by radar, and most militaries have some form of mobile radar systems available, either ground based, ship based, or airborne. Basic portable radar systems could be easily adapted from police speed detectors for infantry use.

Secondly, almost any form of invisibility - except perhaps the pocket-universe one - is going to have one great big problem: heat.

Your alien ships are going to have to vent waste heat or they'll cook themselves That heat bloom is going to be detectable by IR sensors, even if nothing else is; follow it to the hottest point and stick a couple of nukes up their back end; aliens go bye-bye.

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  • $\begingroup$ 'if they can't be detected with electromagnetic radiation they can't interact with the earth at all': Messrs Strong and Weak N. Force would like a word. Shortly followed by Gravity. $\endgroup$ – Joe Bloggs Dec 16 '16 at 16:13
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    $\begingroup$ Interesting techniques. Never heard about Meta materials. Also Active Camouflage and folding space are really creative. +1 $\endgroup$ – Karan Desai Dec 17 '16 at 13:14
  • $\begingroup$ And regarding Radar, @kingledion has pointed out a logical loophole in the comment. So I will have to rethink about this. I am also considering option of radar being ultrasonic. Let me wait for few more answers if any. $\endgroup$ – Karan Desai Dec 17 '16 at 13:18
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  1. The internet seems like a good way to learn. Strangely enough, people tend to post videos of lasers raining from the sky and evaporating our houses.
  2. The spectrum which excites the cells on our retina is pretty fixed. However, some surgeries can remove some of the UV protection on the eye, permitting UV to be seen as a rather painful purple. Also, many people can see a little way into the IR spectrum, though not very far.
  3. The best way to see alien troops is to take a step back and answer a different question: "how can one sense the aliens?" You've told us they are invisible in X-ray, visible, and IR spectra. What are they visible in? Perhaps they can be heard? A classic mistake we often make when making "the ultimate bad guy" is that we focus so hard on why they are invincible that we forget to add explanations for why they aren't invincible. You, as the world builder, decide what can be done to see the alien troops. You may want to focus on why radar was able to spot them. Radar is another form of electromagnetic radiation, alongside X-rays, visible light, and IR light.

One great example of stealth being defeated by low tech is the F-117 in the rain. When the F-117 is in the rain, it is completely visible to radar because it creates a huge shadow in the radar image of the rain. Of course, the F-117 spends most of its time above the clouds, so it's not that big of a deal, but it is kinda funny.

Maybe throw flour at them? The only limit you have is your imagination -- its your world. Have fun with it!

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I'm not sure about the others, but location is simple once you know they exist. Step one: Saturate the area with...something. It can be lasers, it can be radio waves, it can be whatever you want. Step two: Look for a place that remains un-saturated, and see if it looks suspiciously like an alien ship. It's the same principle as throwing flour and looking for the flour-shaped outline. If they have mass and can interact with the world, then they have to take up space, and the space they take up can be seen and interacted with.

That said, if they're using phlebotinium to basically interact only through tiny windows and existing only in "the universe next door," then there's no way to detect them, as strictly speaking they don't exist in our universe - only the windows do, and by the time they open it's too late. So if that's the case then I don't know.

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This may not answer your question exactly, but there was a 2011 movie which dealt with "invisible" aliens invading Earth; I think the aliens were made solely of energy or electricity. I'll leave opinions out of it, but for the concept alone the movie might be worth checking out.

Here's a link to the full synopsis/plot on Wikipedia: https://worldbuilding.stackexchange.com/

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    $\begingroup$ That is clearly not the link that you are describing. $\endgroup$ – Brythan Dec 18 '16 at 3:27
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  1. If your radar is ultrasonic, we can detect them only once they enter the atmosphere, but before that, we'd be completely blind and only laser fire would tip us off to their existence and location.

  2. Likely not as we can pretty much only convert other forms of light into other things we understand (IR sensors make hot things red and cold things black)

  3. Echolocation type devices coupled with your type of radar are going to let you know exactly where they are once they get here and are in our atmosphere, however it does have a severe distance limitation as the farther they are away, the less likely you are to get a good signal back or even in a reasonable amount of time due to the limitations of the speed of sound.

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