Flat earthers are a totally different thing. The shape of the Earth is a fairly simple physical fact about the universe. To deny it is to essentially deny even the most basic human ability to gather facts.
Perpetual motion and FTL travel are about physical laws, which are not known in full. We've got some really good theories with a lot of data to back them up, but the facts lie in the data, not the theories. The theories are subject to refinement and replacement as better data is obtained.
It is not known that perpetual motion or FTL are impossible. It is believed that they are impossible, because the best theories to explain current data say they are impossible. But there is room for new data and new theories.
It was not all that long ago that it was "scientific fact" that atoms were indivisible (that's what the word means, even), that velocities add linearly, that light propagates through a fixed medium, that matter is conserved, and that gravitational force is proportional to mass and inversely proportional to the square of distance.
I also don't see much "acceptance" of FTL as physical fact. I see it accepted a lot as a story device, but that's just because it's handy for making entertaining stories, not because there's any basis in physical fact. There's some "acceptance" of FTL as physical fact in that people don't completely rule it out, but that's entirely justified.
c is a bit more established than "scientific fact" like the behavior of atoms were once thought to be, it is very mathematical, as is the curvature of the earth being > 0. In principle new data could come in showing these things to be false, in the same sort of way twice 2 may not be 4.
We've only been doing this science thing in a serious organized way for between 100 and 200 years depending on how you count it. It's only since WWII that we've done it in a really big heavily funded way (cold war and all that). You're probably right, but I can't bring myself to be quite as dogmatic about it.
I do very much doubt FTL, not only for the physics reasons but for other circumstantial ones. If FTL is possible then the Fermi paradox goes from being an intellectual curiosity to a near metaphysical emergency. If there is FTL then either we are absolutely alone or there are aliens everywhere and they are hiding from us or for some reason we can't see them. That's because the first expansionary intelligence to discover FTL will fill at least the galaxy if not the entire universe in a geological eye-blink. Since ubiquitous aliens hiding seems less likely, if a pathway to FTL were discovered I would immediately conclude that we are alone in the galaxy at least and possibly beyond.
Our galaxy is estimated to be at most around 180,000 light years in diameter.
Even at a fraction of the speed of light, the amount of time to expand and fill the galaxy is still an eye-blink in geologic time. Even in evolutionary timescales it's still an eye-blink -- the Cretaceous era ended about 66 million years ago.
I think the depressing truth is that we are alone in this galaxy. See [1] for Michael H. Hart's extremely compelling discussion of this idea.
>If FTL is possible then the Fermi paradox goes from being an intellectual curiosity to a near metaphysical emergency.
Maybe. Maybe not. This is how UFO conspiracy theorists reason - "Here are some phenomena I don't understand, I can't think of any plausible explanation for them, therefore, must be aliens".
There may be some hidden reasons why we can't detect FTL civilizations.
Way off in tangent land now but: while much of "ufology" is indeed silly, "the aliens are here but they are not communicating" is a valid hypothesis for the Fermi paradox (with or without FTL).
There are numerous rational reasons that an intelligence exploring the universe might choose to be very very careful about initiating communication to the point of taking steps to avoid even accidentally revealing itself. Some of these are altruistic (e.g. prime directive and contamination concerns) and some are self-interested (see game theory, and consider the apocalyptic destructiveness of conflict between really advanced intelligences).
It seems implausible that all of the aliens would agree to ignore us, even less so for them to consistently arrive at that conclusion in an independent manner. Then again, Earth might be exceptionally boring and consistently uninteresting to spacefaring aliens. Who knows? Maybe life is really rare.
This is exactly right given our current understanding of the universe. FTL travel violates causality, which is a fundamental principle of the universe.
I wouldn't go quite so far as to say it's totally impossible though. There is a whole lot about the universe we don't understand and I can't rule out the possibility that there are entire branches of mathematics and physics that we don't know about.
It's one of a bunch of mathematical conclusions of Special Relativity (the most famous one being that objects at rest have a "kinetic" energy of mc^2). I'll try to explain it simply, without the math:
Imagine a friend and you, moving very fast with respect to each other. You measure your friend's clock as advancing slowly (due to time dilation), on account of the fact that he's moving with respect to you. But your friend measures _your_ clock as advancing more slowly than _his_, because motion is relative.
The way to reconcile this mathematically is to give up on absolute simultaneity of distant events. In other words, when two things happen in different places, at the same time according to your clock, they happened at different times according to your friend's clock. This is analogous to how you measure your friend to be in different places in different times (because he's moving with respect to you), but he measures himself to be stationary, always in the same place. To recap: If comparing things at different moments, "being in the same position or not" is relative to the observer. If comparing things at different places, "happening at the same time or not" is relative to the observer.
That's how causality comes in. If your friend tosses a very fast ball at you, and the moment you receive the ball you start towards him faster-than-c, you'll arrive where he is before he tossed it (if you went fast enough). That's what the equations say. Now the breaking of absolute simultaneity allows naturally for disagreements on what of two things happened first, assuming they happened far enough from each other that light couldn't cover the distance in time. But by putting faster-than-c velocities in the equations, we have your friend and you disagreeing on what happened first (him tossing the ball, or you arriving where he is?) in what (to him) is the same place. And it's not just a confusion of the mind: all your measuring and recording artifacts will agree with you, and all of his will agree with him. The mathematical conclusion is that if you can send information faster-than-c, you can send it back in time and break causality.
I want to make sure I understand "time dilation," as you describe it. I have no formal background in science or math. So, please bear with me. For the sake of making the math easy, I'm going to talk about traveling faster than the speed of light, but only for the sake of argument. You'll see why.
I understand it this way. Assume I were traveling away from my friend at twice the speed of light. My time of departure is 12:00:00. After 1 second (my clock) I have traveled 2 light-seconds. Light reflected off the face of my clock now travels towards my friend, reading 12:00:01. We'll call this view of my clock a "snapshot."
From my friend's point of view, that snapshot reaches him at 12:00:03. Yes?
Now, we allow for 1 additional second, my time. I have traveled in total 4 light-seconds. My 12:00:02 snapshot now travels towards my friend. It reaches him at 12:00:06, his time. Yes?
So, from my friend's point of view, my time has seemed to slow down. Is that what you mean by time dilation?
Try not to think of time-dilation in relation to "light travelling". Time dilation just means that time passes more slowly for you due to your speed relative to an observer. So imagine you have an identical twin, and you leave on a Journey to a nearby solar system at say 0.9x the speed of light. Your twin stays on earth and for arguments sake you can say he is "stationary" in comparison to you.
When you come back, he'll be much more "aged" than you. You had a clock on-board your craft, and it measured your trip as say "2 years". Likewise, your twin also had a clock and it measured your trip as having lasted "20 years". The clocks are not wrong. And trust me, this isn't some science-fiction type thing we're discussing. GPS satellites orbiting the Earth carry very accurate atomic-clocks, and they need to take time-dilation into account in order to compensate for the drift in values, simply due to the accuracy necessary for GPS. Just had a look on the wiki page, and the drift is approximately 38 microseconds per day.
When I first encountered this concept in high-school randomly, I was quite shocked at how "non-complicated" the equation for it is.
In that first link, where it says (referring to the diagram at the bottom right), "Observer moving parallel relative to setup [...]," I don't understand. Parallel to what? Parallel to the mirrors? Parallel to the beam of light? I don't find it to be clear. Can you tell me which it is? Thanks.
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P.S. Never mind. I follow the diagram now. I'm trying to follow the rest of it.
Ah, good observation! But what you're describing isn't relativistic time dilation. It is called "the (non-Relativistic) Doppler effect", and can be commonly experienced when a race car, an ambulance, or a train passes you: When it's approaching, the noise will have a higher pitch than when it's passed. This is because the frequency of things that move towards us appear higher than they are, and that of things that move away from us appear lower. It's also why stars that move away from us appear more red than they are (they are said to be "red-shifted").
Imagine the same scenario you described (there's actually no need to use faster-than-c speeds for it), but your friend has planted clocks all along the path that you're traveling. Once he's back at his starting position, he synchronizes them all (there are techniques to achieve this - Einstein famously proposed one). Now when you pass him and keep moving away from him, every second you take a picture of your own clock next to the clock planted on the path. In a non-relativistic world, the pictures will show that your clock is always synchronized with all of your friend's clocks. Making the observation this way rids us of the Doppler effect.
But a non-relativistic world isn't the real world! In reality, if you go fast enough (close enough to c), the pictures will show e.g. your clock marking 12:00:01 when the clock in the path marks 12:00:02. And your clock marking 12:00:02 when you encountered the next clock in the path marking 12:00:04. In other words, your friend (and everything that's stationary with respect to him) will measure your time passing more slowly. Using these example numbers, in what for them is the pass of 2 seconds, you (and everything moving at your same speed - like your clock) have only aged 1 second. This is relativistic time dilation. It's shocking, but it's a fact that's been established for 110 years (and its mathematical consequences help explain even the microelectronics you and me are using). Now this happens no matter what speed you move with respect to your friend. But for everyday speeds it's so small as to be imperceptible - it only becomes noticeable at speeds like that of GPS satellites.
An immediate question is:
> Wait, if I move away from my friend, from my point of view my friend is moving away from me. So I should measure his clocks advancing more slowly than mine.
And that is correct, and is exactly what would happen!
> But the pictures I took show the opposite?
No! They show your clock compared to different clocks. And these clocks on the path, that your friend synchronized, by your measurements they all started at different times. I.e. for you, none of these clocks are synchronized. For your friend, they all marked 12:00:00 simultaneously. For you, they all marked 12:00:00 at different moments. In non-relativistic physics, simultaneity was an absolute concept: If two things happened at the same time, they happened at the same time for everybody. In reality, if I'm moving with respect to you, what happens at the same time in different locations for me, doesn't happen at the same time for you.
My understanding of it is that it all goes to crap when outside observers manage to see the outcome of your event before they see the event started. Therefore, to be safe, keep your faster-than-light travel a secret.
Effectively, the speed of light is the rate at which Cause and Effect propagate through the universe.
The speed of light isn't an intrinsic property of light, it just so happens that light travels that fast and it was the first thing we were able to measure that was traveling at the limit.
If you were traveling at FTL velocities, you would effectively get there before you left.
> If you were traveling at FTL velocities, you would effectively get there before you left.
To be more exact, aren't you saying that you would seem to get there before you left, from the point of view of someone not traveling with you who is observing you from some particular vantage point?
You're still getting there at a point in time earlier than the universe permits. This is essentially the same as traveling backwards in time, and violating causality by allowing effect to precede cause.
To travel faster than light, you'd have to skip places in space instead of moving through them continuously. So you'd be constantly appearing out of nowhere.
You are way too sure about this. Everything we know tells us that this is a fundamental limit, but we don't know everything. For example, wormholes are still within theoretical possibility. Maybe there is some exotic aspect of reality that could imply FTL travel, causality be damned.
Everything we know tells us that energy being conserved is a fundamental limit, but we don't know everything. For example, exotic physics that allows us to pull energy from the cosmological constant itself is still a theoretical possibility. Maybe there is some exotic aspect of reality that could violate energy conservation.
My point being that, yes, these two are comparable, which is what the claim you are replying to said.
In fact, belief in FTL is sillier that violation of conservation of energy. We have no evidence for FTL of any kind. We do have some reason to believe that conservation of energy is in fact violated at cosmological scales by the increasing rate of expansion of the universe. (Perhaps future understandings will resolve this apparent violation with a more complete picture of "energy", but at the moment, what we know of as energy is not being conserved at that scale.)
I don't get the thought process that locks this possibility out. Imagine you are a simulated entity inside simulated universe - v100000 of Eve Online.
There are rules that you can see and access, and those you cannot. Both are in some sense irrelevant to your own perception of truth because they are determined outside the bounds of your empirical experience.
What good evidence do you have to refute such an explanation [surely its likelihood increases in line with our own ability to create the 'next turtle down'].
And in the early 1000s humans believed that we would never fly a few people strapped to a giant rocket to the moon. They believed that because the technology to do it hadn't been invented yet and we had no idea it was possible.
We have to learn how to do it and then we will do it.
Flying didn't violate the basis premises of physics. Also it exacerbates the issue wherein space doesn't appear to be full of aliens. Even a few races in the galaxy expanding with slower than light travel could fill the galaxy in tens of thousands of years. Now imagine how fast the universe fills up with ftl travel
You have atoms and quants and whatnot. Now you add one and one another. You keep adding and soon or matter there is a slip. Basically you get the universe to a buffer overflow. All still following the laws of physics. Like you get a cpu limited of his numbers you get the universe to a buffer overflow.
Now one could say, but the universe is endless. Which means you would need endless energy for that buffer overflow.
But who says you need to create that buffer overflow in the whole universe and not in a cube? And who says the universe is endless?
You can't do it, there are no hacks to do it. It's a law of the universe. No matter how many SciFi movies tell you other wise.
I just don't get the acceptance of this anti-scientific concept.