Standard of Evidence: What 'definitive proof' is required for the mainstream scientific community to accept the conclusion that 'UFOs are alien spacecraft'?

That's a great question, and it's indeed quite complex. I'll try my best to explain it in plain language.

Imagine this: if someone told you they discovered a brand new species of octopus at the bottom of the Pacific Ocean – one that glows, and even uses tools – what would you think? You'd probably say, "Really? Do you have photos? Did you catch one alive? Let scientists study it!"

See, your first reaction would be to demand "evidence," and evidence that can withstand scrutiny. Scientists facing the claim "UFOs are alien spacecraft" have a similar mindset to yours, except their standard for "evidence" is far, far higher. That's because if such a conclusion were proven, it would be a monumental event, completely overturning humanity's worldview.

We can categorize evidence into several levels, from "unreliable" to "undeniable":

Level 1: Legends and Rumors (The current level of most UFO evidence)

• What it is: This is most of what we see online today. For example, a blurry, shaky video of a light spot taken by someone; a pilot's "feeling" that they saw an aircraft defying the laws of physics; and various anecdotal accounts like "I was abducted by aliens."
• Why it's insufficient: Because these things are too easily "explained away" as something else. That light spot could be a drone, a balloon, or even a reflection from a plastic bag; a pilot's "feeling" can be influenced by high-altitude environments and psychological expectations; and anecdotal accounts are impossible to verify. In the eyes of science, these don't even come close to being "evidence"; they are merely "clues" or "hearsay."

Level 2: Reliable Observational Data (The threshold for scientific interest)

• What it is: This goes beyond a single person's account. For instance, multiple independent, different types of equipment, at the same time and same location, all capture the same inexplicable object.
  • For example: A fighter jet's optical camera captures it in the sky, while the jet's radar also locks onto it, ground-based military radar stations pick up its signal, and even a nearby research satellite records its infrared signature.
  • These data must also show that the object possesses "supernatural abilities," such as instantly accelerating to Mach 20 and then making a right-angle turn, or disappearing directly from the sky. These are feats that no known aircraft or natural phenomenon can achieve.
• Why it's still not "conclusive": While this would be incredibly astonishing, rigorous scientists would first suspect a collective equipment malfunction, or an extremely rare natural phenomenon we don't yet understand (like a peculiar plasma). They would exhaust every possible explanation using known science, and only when all known possibilities are ruled out would they seriously consider the option of an "unknown intelligent construct."

Level 3: The Real "Hard Evidence" – Tangible Objects for Analysis

This is the most crucial step, the leap from "conjecture" to "science."

• What it is:
  1. Craft debris: A piece of metal or composite material, unequivocally artificial and originating from beyond Earth. What makes it "unequivocal"? For example, its isotopic ratios are unlike any element found on Earth; its metallic crystalline structure cannot be replicated by any of our technologies; it contains new elements not found on our periodic table. This "piece of metal" itself would be a top-tier scientific paper.
  2. A complete craft (whether crashed or captured): This goes without saying; it would be a game-changer. Scientists would study it like a new continent, from every angle: materials science, physics, energy science, computer science, and so on. Its very existence would be irrefutable proof.
  3. Alien remains or a living specimen: If an alien organism could be obtained, even just a single cell, and its DNA (or other forms of genetic material) could be analyzed to prove its origin of life is completely different from Earth life, then everything would be settled.

Level 4: Repeatable and Publicly Verifiable (The ultimate standard of science)

This is also the most important point. Science thrives on "challenge and debate." The evidence you present must be verifiable by any competent laboratory worldwide, yielding the same conclusions.

• What it is: For example, if you obtain an alien alloy, you can't just hide it in Area 51 and study it yourself. You'd have to distribute samples to top laboratories in China, Russia, Europe, and Japan. Each would conduct independent research, and ultimately all would conclude: "This thing genuinely could not have been made on Earth." Only then would global scientific consensus be achieved. Any claim of "I have a secret, but I can't tell you" amounts to zero in science.

To summarize:

Therefore, for the mainstream scientific community to accept that "UFOs are alien spacecraft," what's needed isn't more blurry videos or eyewitness reports, but tangible objects that can be placed on a lab bench, subjected to repeated "torture" and analysis with various instruments.

This tangible object must clearly demonstrate its two main attributes: "non-terrestrial origin" and "intelligent construct," and the verification process must be open and repeatable.

Until that day arrives, scientists remain skeptical, not because they are rigid in their thinking, but precisely because the very nature of science demands such rigor. "Extraordinary claims require extraordinary evidence" – that is the scientists' motto.