Q: Have aliens ever visited Earth?

Physicist: No.

Space is big.  The distances involved are ridiculous, the energies are ludicrous, the costs are somethingelseous.  The New Horizons probe (due to reach Pluto in 2015) is the fastest vehicle ever created.  At its top speed of 16.26 km/s (36,400 mph) New Horizons would take over 80,000 years to reach Alpha Centauri (the nearest star system).

(The following paragraph is wrong.  There’s a redaction here: My bad: Have aliens ever visited Earth?)

A quick calculation shows that the fastest that a ship can reasonably travel, using its own fuel, is about 11% of the speed of light (0.11C).  This calculation assumes that the ship is essentially all hydrogen tanks, that all of this hydrogen is fused into helium, and that all of the resulting energy is converted into thrust.  However, even at this speed, it would take about 40 years to get to Alpha Centauri.  Any realistic method used to get from one star system to another involves millennia of travel time.

(This last paragraph was wrong.)

Adding to that, beyond exploration, there aren’t any good reasons to leave your home solar system.  The resources you’re likely to find are about the same as the resources you’d find in your home system (nearby stars tend to form in regions with similar chemical make ups), while the costs of getting there are amazingly high.

It has been proposed that aliens may have a technology that would allow them to travel faster than light (FTL).  Sadly, there’s a lot wrong with that.  Unlike the sound barrier, which was an engineering problem, the light barrier is written into the universe.  In fact, based on everything we know about time and distance today (I’m talking relativity here), the question of FTL travel doesn’t even make logical sense.  However, if you’re interested in technology based on unknown (contradictory) science, then there are plenty of experts to talk to.  Scientists almost never tell you what you want to hear.


On a personal note, if I made the trip to another star system and found intelligent life, I would spend the rest of my life there telling everybody about it and bragging to the native population.  If aliens have been to Earth, they’ve been suspiciously cool about it.

Mathematician: I disagree with the physicist’s answer in one significant way. While it’s true that the distances in our universes are insanely large, and that the speed of travel is capped by the speed of light, the theory of relativity can actually step in to save the day (when it comes to aliens traveling enormous distances).

If aliens were able get up to speeds approaching that of light (using, say, a Bussard ramjet) then they would experience a twin paradox type situation (essentially, time would slow down on their ship compared to the passage of time on Earth). Hence, while a trip from Alpha Centauri to Earth (traveling at almost the speed of light) would take something like 4.5 years from the point of view of us earthlings, to the aliens such a journey could take very little time (thank you time dilation and length contraction). The closer they got to the speed of light, the less time would elapse (for them) on their journey (and the more energy such a trip would require, with an infinite amount of energy being necessary to travel exactly at light speed). If they ever did travel at exactly the speed of light (which is almost certainly impossible) the entire trip would take exactly 0 time from the viewpoint of the spaceship. Due to these relativistic effects, Aliens could theoretically travel enormous interstellar distances while experiencing very little aging. The members of their species on their home planet(s), however, would continue aging at the usual pace, so long journeys (of, say, thousands of light years, which is still not much compared to the hundred thousand light year diameter of the milky way galaxy) would presumably lead to the death of all of one’s planet bound acquaintances. Therefore, such a journey would only make sense for an explorer who had no intentions of ever returning home, or for large star ships that acted as their own colonies. Perhaps it’s also worth mentioning that when aliens actually arrived at Earth after such a trip, our civilization would have changed dramatically from the time they departed. See here for an article that discusses some of these questions in greater detail.

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34 Responses to Q: Have aliens ever visited Earth?

  1. Scott says:

    Why 11%? That’s pretty specific. Can’t you just make a bigger ship with more fuel tanks, or is this an optimization problem balancing mass & available resources with total energy available from the fusion reaction?

  2. The Physicist Physicist says:

    The calculation I did was: take the mass of four protons at rest, subtract the mass of one helium 4 nucleus, take the difference as the energy produced by hydrogen fusion, then balance the energy and momentum of the helium going in one direction and a single photon in the other. I figure that the maximum speed of a ship can’t exceed the maximum speed of it’s ejected material. I did this calculation a few years ago, but I think it’s right.

  3. some other physicist says:

    That’s not right. The ejected material will still have positive velocity relative to the ship’s inertial frame, and so will always accellerate the ship further. You could get arbitrarily close to the speed of light with ANY source of fuel – even mentos in coke bottles – given enough of it.

    It’s true that efficiency of conversion of mass to energy is a big factor in determining whether near-light-speeds can be reached in reasonable time – especially if you’re carrying your own fuel. But there are more efficient methods than hydrogen fusion. What’s wrong with matter-antimatter annihilation? From statistical arguments, any alien civilization that we overlap with in our cosmic time window will be far more advanced than us, and so we can think beyond our own current technologies.

    That said, I agree that it’s very unlikely that aliens have visited us in the past. The distances are crazy and there’s no evidence for it.

  4. Pingback: My bad: Have aliens ever visited Earth? « Ask a Mathematician / Ask a Physicist

  5. The Physicist Physicist says:

    I was so wrong, and you (some other physicist) are so right.
    I can’t believe I have to give up my “5 years right” chip.

  6. C student says:

    aren’t there multiple faster than light travel theories out there….? Aren’t we talking about civilizations that could be potentially millions of years beyond humans in science and technology? Sorry gentlemen, but we humans may not be the peak of evolution in the universe after all :(

  7. The Physicist Physicist says:

    I’d agree with that.

  8. Jim Ward says:

    You don’t want aliens, even friendly ones, visiting earth. If you compare it to the visit of the Europeans to North America, about 95% of the American Indians were wiped out by alien diseases. So it would be with us. Another argument against alien visitation is the Polynesian islanders before the Europeans. Garbage from European ships, nails, and planks would wash up. The islanders knew something was out there, but they didn’t know what it was. Keep your eyes peeled for alien trash.

  9. The Mathematician The Mathematician says:

    Diseases carried by aliens would be extremely unlikely to effect us. With many creatures on our own planet, we are biologically dissimilar enough that much of what infects them cannot effectively infect us. With aliens the biologically dissimilarity would be greater still.

    Trash from alien ships would similarly, be extremely unlikely to fall to earth, even if there were aliens in the vicinity. If the aliens are in orbit, the trash would similarly be in orbit, and if the aliens hurtle the trash at us, unless it is huge or remarkably flame retardant it would burn up while falling through our atmosphere.

  10. christopher says:

    isint it theoretically possible to bend to points of space together, allowing a ship that works on this principal to effectively cross an entire universe without moving more then a few hundred feet? i dont think any laws of physics are broken by this. but even so, that does not mean such a device could be built.

  11. The Physicist The Physicist says:

    You’re talking wormholes! Even if the construction of wormholes was theoretically possible (it doesn’t look good) you’d still need to establish the two ends. So, although you may be able to “fold space”, you still need to travel to the far side to set it up.

  12. Terry Hayward says:

    Given the possibility that some of the pieces of rock floating around in space came from a planetary impact, maybe it is possible that we have been visited by alien life in the form of microscopic life during meteor impacts.?

  13. The Physicist The Physicist says:

    You’re talking about “panspermia“, which is a definite possibility!
    At least in part because of how patient microbes can be.

  14. julz julia says:

    heheheheehhheheheheheheheheheh the aliens shd let us be!!!!!!!!!!!!!!!

  15. conner says:

    Although it may be possible that aliens have visited earth, doesnt mean its likely or even plausible. Technically theres a possibility that winnie the pooh is a real. Anyone who knows a thing or two about space would understand why its unlikely to believe that aliens visited earth. simple as that.

  16. jordan says:

    i find these statments invalid as
    #1 your are statements are stated as if they are running off of are current technology and not intelligent beings who have more then probably in some circumstances (there being scientifically calculated atleast more then 50,000 intelligent species in our galaxy (the milky way) alone) lived a tad bit longer then 1000 years longer then we have making their technology and understanding of science sooooo much greater then ours that they would be able to analyze why you couldnt travel faster then the speed of light and create a constant exception (as we have been doing) simple example metal cant fly so we created the shape that bends these rules and applied the principles neccessary

    #2 that they could doop our whole civilization (as any intellligent being who does not want to interfere with key development of a species would do. Hmans are still learning key lessons through experience and disaster,lessons on discrimination propaganda and other lessons i would want somebody who i would give the power to kill me know

    and #3 fact about the advancement of technology is that it moves exponentially (doubling previous progress repeatedly so answer this. what is 1000 steps forward exponentially? that would be how many years ahead there technology would be ahead of ours

  17. Kyle says:

    Aliens have been to earth the evidence is ill refute able. They have came many times and as different species. Anyone who doubts the existence of aliens is as dumb and out of touch as the people who believed the world was flat and that evolution isnt real. There are infinate alien life forms.

  18. Imam says:

    This planet is 4.4 billion years old, in a galaxy containing 300-400 million stars like our sun, each star has at least 1 planet orbiting it, there are 100 billion galaxies in the observable universe and the universe is estimated to be 14 billion years old. So I think the probability of aliens visiting earth it’s in 4.4 billion year lifespan is very likely. The aliens might or might not be maybe 1000 years ahead of us. But what if they were more? Like 1,000,000 years ahead of us in technology?

  19. Patrick says:

    I don’t think aliens ever visited Earth or ever will assuming that they are the product of what we call a good old fashioned sexual intercourse.
    Isn’t brain tissue always supposed to be organic?
    I mean, if any alien race would decide to explore space, rather than pieces of metal they would like to go themselves, or not?
    So if they are organic, and in theory would be able to realize very high speeds, how would they avoid that their organic tissue drops out of their nose holes with the outreageous G-forces.
    A human body can sustain 16G for a couple of seconds, let the human body sustain 16G for a minute and he is dead.
    What acceleration would be necessary to get anywhere near a velocity which makes a bit quick travelling possible?
    I always think about this, it’s romantic to think they visited Earth or will, but unfortunately I don’t believe in it for this reason.
    All comments or ideas are welcome, it’s for sure a curious topic to talk about!

  20. skyler says:

    physicst you say you cannot travel parsecs of space i call bs michio kaku and hawkings BOTH believe that you can travel across the universe but you dont think so, so thats it? what about the fact that hawkings proved that if you travel near the speed of light 7 days in space is equivilant to 100 years on earth you sir are ASSUMING that the civilization that is capable of manipulating spacetime ONE doesnt exist or TWO is not interested in our planet, so what do you say about the abudence of pre BC texts that clearly speak about creatures visting earth in ezikel 1:3 it says from heaven came a BURNING SHEILD and from that shield came 3 CREATURES not gods not devils not angels CREATURES what about the book of enoch which clearly states that an angel took him to heaven and showed him the ROUND of the earth and the history of the earth you cannot for sure say that aliens have never visted us due to the fact that you were not there and there is starting to become more evidence to prove we have been visited before the nephilim are a good example of “angels” mating with humans says it in the book of enoch

  21. skyler says:

    the book of enoch the mahabaratta and also the sumerian bible all have the same story of 25 beings descending to earth and knocking humans up creating giants there was a race of giant red headed asians that died out a few thousand years ago average skeleton found was 7 feet tall the tallest was 9 feet goliath himself claimed to be born from a nephilim, chances are you follow some bs creationist view that has no explination how humans became the way they were same thing with evolutionists they have no explination and evolution as we are force fed was proven wrong when 15k pygmy neanderthals were found in mt java but they were from over 100k years ago and they found a descendant from one of the neanderthals alive he was very small man with a 20% smaller skull just like the javan neanderthals get off it its obvious we have been visited i havent even talked about hybrasil or the 17k year old city in japan underwater with the largest pyramid in the world even larger than the great pyramid in egypt on also the sphynx was c-14 dated to 7 or 9k years bc and also the face was not human we have no idea what it was honestly a physicst is a horrible person to ask about aliens wheres a fuckn historian you are one close minded physicts which is sad because science is about the exploration of the unknown and its close minded people like you who are the reasons why we still follow the belife we orbit the sun and not the galactic center im curious are you a string theorist or a particle physicst and sorry for any gramar errors i could care less about eloquence on a blog i just feel that someone who isnt into crazy crack pot theories needed to comment i base all my theories off of historical texts and eye witness accounts like nasa banning the 80 hours of footage where there is clearly a jupiter sized ufo in fixed orbit around the sun actually siphoning radiation from the sun and you can also clearly see this object has a magnetic force field because its in the corona of the sun and causing solar flares and either the solar flares are siphoned or deflected but im not a physicist

  22. skyler says:

    also to the person who said our planet is 4.4 billion years old screw that the galaxy is 14.7 billion years old but lets go even further into the age i think the universe is far older than 14.7 because the expansion of light is how we guestimate if im incorrect please correct me but what if the first star didnt exist untill 4 billion years after the universe was created that would mean that the light could never catch up to a universe that is always expanding like ours aliens exist its the job of the media and idc what anyone says the illuminati runs everything they dont want us to know the truth that we arnt alone that god didnt create us in his image the annu did that were all just $ signs hence why you have a drivers lisence NO human needs a license only a corporation does and when were born we are given 2 documents that make us one our birth certificate and our soc number from the research ive done our government has comm 12 and other divisions that directly corospond and work with aliens like the peladiens(prolly butchered that lol) but believe mr scientist who probably isnt one to begin with i can be like dr phil and say ima doctor of psychology but that doesnt mean i have my phd ;)

  23. Paul MaDellan says:

    Most of the comments here, from well educated and qualified people, simply set out to try and disprove a theory rather than attempt to quantify a belief. Rather like someone throwing down a challenge to prove God doesn’t exist when in simplicity it’s impossible to prove something that does not exist. From an astronaut’s view there are many bibical stories that make sense. In the book of Genesis for example, ‘The sons of God came down’ (Aliens descending from a mother ship orbiting the earth) Apparently they mate with the maidens of eatrth and produce a hybrid race of giants. There is the accurate description of a helicopter, Radiation waves that turned Lot’s wife to a pillar of salt following an atomic destruction of the twin cities of Sodam and Gomorro. The description of an angel fits perfectly with a being in a small one manned aircraft, the outline of a cockpit defines a halo. The deliberate cause of a flood designed to kill off all descendants of the so called sons of God who decided to stay for good. Sure, all you brainheads can sit with your equations and scale rules and deliver your verdict but you are looking from the wrong angle. The aliens who visited this planet thousands of years ago were thousands of years ahead of us in technology.

  24. thatone12yearold says:

    me and my friends have talked about how it could be possible that aliens have come to earth and are the ones who helped the human brain advance quicker. that aliens are controlling us. although i think it is unlikely, i find it an interesting thought. some “proof” of that could be in how the Egyptians built their pyramids. i’m sure the wheel wasn’t invented yet, right? perhaps i have the wrong information…

  25. Stuart Mathieson says:

    Such is the infatuation with the idea ( UFO is assumed to mean flying saucer ) that the parameters of possibility (let alone probability) are simply ignored. If you belief in the biblical account literally you’ll believe anything. If you believe in metaphorical interpretations of it you can also believe anything.

  26. Stuart Mathieson says:

    We seem to be overlooking the time and fuel required to accelerate to an optimal velocity. Then there is the deacceleration time. I’m afraid infatuation exceeds everything including logical possibility.

  27. Deelynn Olson says:

    why couldn’t electromagnetic force be used ? no friction in a vacum right?

  28. Niko D says:

    I’m more curious to know, if aliens had sent a probe to observe us from a distance, would we even be aware of it? You’d imagine for practical reasons (as opposed to stealth), it would be pretty compact and sensors would absorb the electromagnetic spectrum, rather that reflect. How would be even know it was there?

    What I’m really curious about, is could a really compact ballistic (i.e. fuel-less) probe be sent, using an accelerator in space? How about a swarm of probes? Either on a one-way trip, or to swing around our sun and back home again. Is there a minimum size sensor could reach (for an electromagnetic scan of planets on the way), that would prevent that kind of tech from ever being made?

  29. Nick Zetton says:

    Two generations ago, it took your grandparents or great grandparents a week to travel from Europe to Ellis Island. Today such a trip is made in 8 hours, about 5% of the time, using technology we completely take for granted but that they couldn’t have possibly imagined. Sorry, but the predictions of early 20th-century physicists – given their limited present ability to explain, predict, and control even the limited physical forces they’ve been able to discover in the past couple hundred years, as it pertains to such matters, is to be entirely discounted. I say this with all due respect.

    In the first place, any civilization with such technological capabilities would likely be a post-singularity one. While I don’t prescribe to Ray Kurzeil’s aggressive timeline for achieving “the singularity”, I do believe we’re on the cusp of it, and it is an inevitability, arguably the next phase in our evolutionary development. And by “cusp” I mean 100 to 300 years – a relative blink of an eye in historical terms. The singularity in this context is an event horizon state wherein we will have developed machines capable of independent problem solving and creative capabilities. However, these true “thinking devices” will think creatively and independently with the mind-boggling efficiency that likely will far exceed the “dumb” computational capabilities of computers — and with perfect recall. We will develop the “Krell”, if you will. Of this I am certain. In essence, we’ll reverse engineer and digitize the human mind. So these future computers (for lack of a better term) will be capable of true creative problem solving but with precision and speed that is unimaginable. We will not be programming, per se, such computers in the future. We will ask them to solve problems: cure cancer, develop a “unified theory of everything”, develop a means of intergalactic travel – name it. We may even fuse with such technology in a biological sense. The impact on our civilization, once these machines are developed, and they will be developed, is entirely unpredictable. The world and our way of living will be inconceivable – literally. Our capabilities and what is possible and impossible will be clearly defined, not gray as most of it is today. We will eventually be able to completely predict, explain, and control all physical forces. Our technological capabilities and “power” would take a quantum leap in a very short time frame. I’d argue we’d be closer, technologically, to denizens of civilizations during the Bronze Age than any post-singularity civilization. One of the almost certain outcomes once this transpires is an enormous leap in human lifespan — potentially to one that is virtually unlimited. Life, and the perception of time itself, would be radically different from our own. We will know if intergalactic travel is possible definitively. And if so, how to accomplish it.

    So why would such a post-singularity civilization visit earth? It would not be to procure any natural resources we posses on this planet. We have nothing unique here that is not in abundance elsewhere amongst the galaxies or that such a civilization would be able to synthesize more efficiently.

    There would only be three reasons: 1. Pursuit of knowledge of other life forms and civilizations. Pure science. Curiosity. We’re already doing this, aren’t we? Many of the credible sightings might be unmanned probes, collecting data on life harboring planets on a mission lasting thousands of years. An example of this might be the JA Flight 1628 incident back in the mid-1980’s. But the next two reasons are more ominous. 2. Surveillance/Protection. We have nuclear capabilities that can already destroy planets. Such lifeforms may know that we will soon have the technology to take our war-faring ways into space. Let’s hope they’re not planning a preemptive strike. We wouldn’t stand a chance. Cases like the Edwards Air force base flap and Rendelsham incident could be evidence of this.

    But the third scenario might be the most ominous. Perhaps when asked, “What is the meaning of life?” a post-singularity “digital mind” might not quip that the answer is “42”. Rather, it might argue that the question is irrelevant. That life has no “meaning” but rather a purpose. And that purpose is “the survival of species” – at all costs. This is something we’ve already discovered in the pre-singularity era on earth. The lengths biological life forms will go to to insure “species survival” is as remarkable as it is well-documented by biologists who understandably marvel nature’s resourcefulness. And one way to insure the survival of species — against, say, a catastrophic event like your home planet colliding with a massive asteroid or the inevitable supernova of your home star — indeed “in general”, would be to intersperse your genes among suitable/compatible other species that exist on other planets once your species has figured out how to get there.

    I would not be so quick to discount the anecdotal accounts of Betty and Barney Hill or those gentlemen who went fishing one fine afternoon in Pascagoula, Mississippi back in 1973.

  30. Roy says:

    Most scientific discussion on this topic ignores the acceleration of technological development. Before discussing whether or not we have been visited by extraterrestrial civilizations, we should consider where human technology is headed. Long before we are in a position to build the first star ships (assuming that it does become technologically possible) humans will have achieved biological immortality, or evolved into machine-based non-biological beings. So, our time perspective will be become radically longer than what we currently experience. This is often referred to as the “Singularity”. It should be safe to assume that any other technological race achieving the technology to attempt star travel would have also transcended similarly prior to attempting interstellar travel.

    Such beings, if they visited Earth, would not have any interest in opening a dialogue with pre-transcendent humans any more than we would attempt to communicate with fruit flies. At best, they might leave semi-sentient monitors to report on human progress toward what they would consider true sentience.

  31. Matt says:

    My question to all the venerable mathematicians and physicists:

    How do you know aliens have Not reached our planet? Do you have any proof? None whatsoever. Isn’t it possible that these “aliens” are so advanced technologically that they have “ways” to travel effortlessly through the galaxy?
    What You are talking is about Your knowledge of physics and mathematics, and you know we are just starting to unravel the mysteries of “things” around us. But to proclaim so arrogantly that there is NO WAY for unknown/known to us “aliens” travelling inside any galaxy effortlessly is simply dumb and stupid.
    I once asked a friend who works at JPL if they “really” have seen/photographed any structures on the far side of the moon. My friend looked straight into my eyes and with a somewhat stern look said:”I simply cannot talk about that subject, I don’t want to lose my job.” No comment here….
    My point is this, arrogance of some of US scientists (especially the ones who work for the US government) is understandable, they have to mislead the sleeping masses…however, the American people are Not as dumb as the government wants us to be.
    You can do all your calculations and you will find out that it simply is impossible to travel long intergalactic distances in a short time….and you know what? You are correct, because of our still very young but growing science. Remember, it was Only 100 years or so ago that the car was build, the first plane was flown, the first toilet was invented etc etc, and we are sitting here, and blindly saying Aye Aye to these physicists and mathematicians whose (extremely limited) scientific knowledge is ONLY 100+ years old young….yet they arrogantly force others to believe that it is impossible to reach other planets in the same galaxy.
    Oh and by the way, I am a physicist from the University of Haifa, Israel.
    PS: Apologize for my grammatically wrong English

  32. justjo says:

    Question: What happens to an object traveling at (insert a number here) % the speed of light when it runs into a micrometeoroid?

    Answer: It goes boom!

  33. russell fink says:

    There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy

  34. Internecivus says:

    Isn’t it reasonable to assume that we are a species with ALMOST the engineering knowledge capable of building and harnessing the vast amounts of energy that of the mass of our entire planet is also able to build certain types of ships that use exotic forms of matter as an energy source to be able to conquer the vast distances in interstellar travel, or at the very least let us explore the Solar System in a way more timely fashion?

    Why do we need to go to Proxima Centari when there’s still plenty of mysteries to solve and things to explore here in our own backyard? If I was Elon Musk, Bill Gates, and supra-national group of space agencies, billionaire’s, and fortune 500 corporations, I’d be much more inclined to fund a project to build a self sustaining ship capable of undertaking a long-term mission to explore and analyze moons and asteroids for the purpose of mining operations and space tourism for the pure purpose of fund raising for more missions than I would, say, a one way mission to another star.

    In my head, I’m thinking something along the lines of a few very powerful magnets, some fission reactors, a quantum supercomputer or two and a few particle accelerators on an object half the size of Manhattan.

    As far as humanity is concerned, even if we can’t immediately get from say, Earth to the edge of the solar system in few hundred years, with even slightly more advanced propulsion and a real planetary wide effort to advance light based computing and communications, robotics, and space travel we can colonize the solar system, mine moons, asteroids and the asteroid belt, build space stations and jump start a space based economy.

    Sure it would take a hundred years and a tens of trillions of dollars, but with every company on Earth pitching in, every wealthy family and corporation, every nation state, every engineer, astronomer, physicist, computer scientist on Earth or at least, a good majority pitching in during their spare time….I mean it wont be Mass Effect but at least we could get to Cowboy Bebop, no?

    I need to get out more.

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