The ubiquity of cameras in phones and the ease of uploading to one’s choice of social media sadly has not had a positive effect on credible videos/photos of Xenomorphs, Yetis, lake monsters or scout ships from Vulcan.
But all it will take is that first 100% proof.
@superwombat: Like this?
OR–
Maybe one such civilization has visited us, seen the cranks, drunks, and crazies, and put us on the Cosmic Quarantine list. Damn smart those aliens.
Interesting OP. At first thought, I guessed that it should be possible to see the gravitational effects of something beyond our horizon; that seeing planets at the edge of the observable universe being pulled in a direction does not entail that the “pulling” entity is within our horizon.
But, on thinking about it, I can’t think how that could be.
Gravitational waves travel in the same way as light waves, they are even stretched by Hubble expansion. So it doesn’t seem possible for an “effector” to be outside of our horizon, but the “affectee” within.
Of course it’s possible for the effector to be too dim for us to see, but that’s irrelevant to the OP.
Well that sounds like some variation on the Fermi paradox.
It doesn’t follow as a logical postulate, more of a probability thing.
Even without FTL, there has been time for a civilization to have visited every star in our galaxy and/or built structures that we would be able to see. But FTL makes the Fermi paradox much worse, as it makes all this require proportionately less time.
But it doesn’t logically entail that we would see ETIs.
I just want to say that it is a joy to read your posts, and that we are fortunate to have some scientific minds on this board. Astronomy and associated disciplines have been something that I always wished my brain could wrap around, but it turns out that the physics of it all was just not something I could grasp (and still can’t). Thank you for participating here.
Some good observations in that post, and I basically agree with the quoted paragraph.
However, I think it’s worth pointing out that when we have the kind of fundamental understanding that we do about the speed of light, it’s not really arrogant to at least make some extrapolations based on those fundamentals, which have nothing to do with technology. We can make those extrapolations based on the knowledge that the speed of light has amazing and totally counterintuitive properties, and is thus not like any other speed in nature. So it’s not at all like your analogies that relate to purely technological challenges, or, in the realm of speed, say, claiming that faster-than-sound flight is impossible, due to the nature of fluid dynamics or some other imagined physical obstacle.
First we have the underlying fundamental that the speed of light in a vacuum is a universal constant that is independent of any frame of reference. This not true for anything else.
A whole bunch of stuff then emerges from that mind-boggling fact. One is that space and time are inextricably interwoven, and there is no such thing as simultaneity at points separated by space. Another is that matter and energy are actually the same thing, and the translation between the two forms is a function of this same amazing universal constant, c.
This seems to give us good reason to believe, at least in terms of overwhelming probability, that not only is FTL almost certainly impossible, but so is any shortcut like “folding” space or transiting a wormhole, because the principle of non-simultaneity across space would seem to cause such an FTL transit to violate causality. At the very least we can say that if it turns out that FTL in some form is possible, then a great deal of foundational modern physics regarding the nature of spacetime is flat-out wrong, and if FTL is possible, very likely time travel would be possible, too.
Except that we know by experimental evidence that the assumed “non-simultaneity across space” of General Relativity is not really true, e.g. the phenomenon of quantum entanglement that causes entangled particles to correspond with each other regardless of the interval of separation in spacetime. Now, whether that is a phenomenon that can be exploited to transmit information or matter faster than the geodesic path is certainly questionable, or whether it gives rise to an underlying principle that would allow for superluminal transit is unknown, but we can say with confidence that Einstein gravity, while an incredibly good theory at explaining and predicting large scale gravitational behavior, is not a truly fundamental theory and is a “mean field approximation” of some more fundamental mechanism tying together all physical interactions (forces/fields/quantum behavior) that we just don’t understand yet.
So, while just using momentum transfer to accelerate an object with rest mass to a velocity of c is impossible to an almost absolutely degree of certainty, I don’t think we can actually say anything about what goes on below the veil of GR and QM and how it could potentially be exploited for superluminal transit by some future technological society with a better grasp on fundamental physics and access to energy sources that we can only speculate about. Which, again, is not to suggest that humanity will live in a future world as imagined by popular science fiction, running around in onsies shooting each other with energy beams that ‘stun’ or ‘vaporize’ depending on whether you remembered to reset the output level. (As an aside, I think that Star Trek’s phasers and matter transporters are even more fanciful than the warp drive, as the latter can at least be modeled in the mathematics of General Relativity, while the former would entail clear violations of basic thermodynamics.)
But when I see people pronounce that faster-than-light travel is “impossible” because Einstein says so, I feel compelled to point that that General Relativity indicates no such thing and in fact forms of FTL, time travel, and other high weirdness are entirely consistent with particular solutions of the theory. Whether those can actually be physically realized or just represent a ‘physics’ that is no more applicable to reality than the arbitrary (but entirely simulable) mechanics of Warner Bros. cartoons is another question entirely, and one only answerable by digging into the more fundamental nature of reality.
Stranger
IANAP, but I’m pretty sure matter and energy are not “the same thing.” Matter is simply a form of energy. There are other forms of energy that are not “matter.”
Ah, yes. All credit to Mr. Munroe.
Been reading XKCD for years, it’s interesting how so much of it gets subconsciously absorbed 
But are they, in fact, “corresponding”? Granted, quantum entanglement is quite the mystery, the mind-boggling fact being that in the putative absence of hidden variables imposing a deterministic state, the state of one particle is instantiated only at the time of measurement. The inability to explain how this seems to set the state of the other one at any distance is certainly indicative of a major gap in our knowledge, but I don’t think it’s ever been suggested that some sort of superluminal communication is taking place, or that the transmission of information by this means could ever be possible.
At some level this is just semantics. But the physical reality is that energy has mass (or more accurately, momentum) and mass can be expressed as energy, making them equivalent.
For example, when the Higgs boson was discovered, it was found to have a mass of about 126 GeV (billion electron volts, a unit of energy). MeV and GeV are common units used to measure the mass of subatomic particles in particle accelerators. Likewise, radiation pressure – the energy that drives solar sails – is exerted by photons, even though photons are said to be massless (have zero theoretical rest mass). Photons exert radiation pressure directly proportional to their frequency (which determines their energy) because their kinetic energy imparts a transfer of momentum to whatever object they hit.
They say reading great literature changes a man. Munroe is great literature. We are all changed by xkcd, whether we consciously remember any particular episode or not. Cheers!
In fact, yes, a correspondence of the quantum states has been demonstrated across ‘distant’ (i.e. non-causal) intervals to a very high degree of confidence. This doesn’t mean that this is a “loophole” exploitable for communication or transportation (although there are people trying to work out a way to realize this) but interpretations of quantum mechanics that rely upon causal predeterminism (e.g. local hidden variables) have been invalidated by Bell’s Inequality and other interpretations that avoid the perceived perils of non-locality such as superdeterminism, Conscious Observer, or the so-called Many Worlds interpretation require such ridiculous gyrations or a complete abandonment of realism as we would understand it that make non-locality a minor issue by comparison.
Even without this, we know that General Relativity is not a complete (or perhaps even correct) theory because it breaks down at vary high gravitational gradients where the influence of gravity becomes comparable to that of the other physical forces, e.g. around black holes. And of course, we have the mysterious of dark matter and dark energy, which while hypothetically attributed to weakly interacting (‘invisible’) matter and some residual counter-gravitational field respectively, can also potentially be explained by some more expansive theory of which Einstein gravity is just a subset.
Fundamental and composite particles are described in terms of their energies because this is how they are measured by instruments in particle colliders, but there is a distinction between “mass” that has a specific set of properties, and more generally “energy”, which is really a description of field interactions. Noting that photons are “massless”, i.e. have a zero rest mass, is kind of a meaningless statement because photons by their nature are never ‘at rest’ in any inertial reference frame as they are always moving through spacetime at c. Photons do have variable energies with respect to the reference frame as determined by their wavelength, and a change to the wavelength in that frame corresponds to a change in local momentum, either by transfer between the photons and other particles with respect to that frame, or a change in the momentum state of the reference frame.
Now, it is true that most ‘mass’ is actually binding energy within collections of hadrons, i.e. atoms, and the Higgs boson, but it is not correct to say that mass and energy are strictly interchangeable qualities even just within the Standard Model of particle physics. In fact, mass, as we understanding it from a Standard Model perspective, is an emergent property of field interactions, while mass in General Relativity is some kind of fundamental property of energy in spacetime and, in theory, should be invariant on ‘local’ scales. The disagreement between these two ‘definitions’ of what mass is and where it comes from is more than just semantics; it is fundamental to understanding the basic nature of reality (or at least, what passes for it in our experience) and why gravity cannot be quantized while all other field interactions can only be adequately be described by quantum fields. There are physicists who believe that the Standard Model and gravitation are two totally different phenomena that are not fundamentally interrelated but that is far from the mainstream view, and in general it is well considered that the rift between quantum field theory and Einstein gravity represents some vast gulf of understanding of how physics actually works.
As for superluminal transit (or direct fine manipulation of gravitational fields, et cetera), while it is rational to argue that this isn’t something we’re going to work out by an expansion of existing technology by making a “transtator” or whatever, I’ll point out that as of almost exactly two centuries ago, all of fundamental physics consistent of Newtonian gravity and the wavelike-description of light, plus some very crude understanding of thermodynamics. Steam engines were giant machines only suited for very low efficiency piston mechanisms such as water pumps, and “electricity and magnetism” were regarded as two separate types of parlor trick magic with no practical application. In the “Regency Era” it would have been impossible to imagine a world in which “machines” used these principles to do calculations at billions the times the rate of a human being, or to illuminate moving images upon a screen, or to communicate around the globe almost instantaneously as anything but mystical sorcery, but today these capabilities are so prosaic that most people don’t even question how they work (and they remain essentially magical to the public at large, but a type of magic grounded in our understanding of the physical world rather than fey or divine in nature). I certainly don’t think we’ll ever see an “Age of Sail” in interstellar space like that portrayed in Star Trek, or space wizards battling it out with laser swords and a command of an all-permeating mystical “Force” like Star Wars, but again I would not rule out a revolution in physics that allows access to mechanisms and energies that we cannot even dream about.
That is, if the human race manages to survive the existential threats of its own making and those it refuses to consider in blithe ignorance.
Stranger
Thank you for the thorough commentary. I will just note for clarity (and in my defense) that this is why I said that photons have a “theoretical” zero rest mass.
The significant thing about photons imparting momentum that I was trying to elucidate – perhaps not very competently – is that if they had any mass at all, at the speed of c that mass would be infinite. The idea that I was trying to get across is that what we have instead is photons imparting a small amount of momentum (to a solar sail, say) because 100% of their momentum is due to their energy and none due to any rest mass, in contrast to ordinary mass moving at low speeds where momentum is given as p = mv. Instead we have no m factor at all, and momentum is simply due to pure energy which is inversely proportional to the radiation wavelength:
𝑝 = \frac ℎ{𝜆}
There is a ‘loophole’ (so to speak) that can remove the possibility of causality violations from a transit through a wormhole. So long as neither mouth of the wormhole lies within the future time-cone of the other mouth, it would not be possible to create a closed timelike curve, so you could not affect your own absolute past. This does limit the utility of a wormhole transit system somewhat, but it means you could potentially get to a nearby star in less time than a light beam would cover the same distance.
Note that this effect is only possible because such a wormhole would be restricted to a particular, closely defined region of space-time. As there is no indication or proof that such a wormhole would be possible in real-life, this does not mean that this sort of travel could exist either; but we can’t use non-simultaneity to rule out FLT completely.
Here’s a link to a more detailed discussion of this possibility.
https://www.orionsarm.com/page/322
This seems to be exactly backwards. We can’t actually see the Big Bang, because it occurs behind the Cosmic Microwave Background, which is opaque. But the actual surface of the visible Cosmic Microwave Background is continually moving further away from us because of expansion and light-travel-time, and also getting older. So we are continually seeing parts of the universe which are further away from us, and which we have never seen before.
It appears that these locations are filled with an incandescent, largely featureless plasma, which kind of complicates things.
I think what Max is arguing is that, as things move away from us, we’ll see less stuff. If that stuff is moving away faster than the speed of light, then that would overcome the effect you mention. If not, then the effect you mention would grow faster. Right?
I can see Max’s point - the galaxies that emerged from the matter which we can now see as part of the CMB surface are now 46 billion light years away, and we will never see them in their current state, even if we wait 46 billion years for the light from their current position to reach us. Indeed, we can wait forever and that light will never get here - and neither will the effects of their gravity. But that is a concern for the far, far future.
The light from the CMB which is arriving now was emitted 13.7 billion years ago. Even though the CMB surface is receding from us at a speed faster than light, this ancient light can still reach us because it has traversed a wide region of space which is expanding but has finally reached our local area, where the expansion rate is subluminal.
Although the CMB will eventually get so far away in both space and time that no light from that location could ever reach us, we are still able to see the CMB and measure its characteristics. Billions of years from now the light from the CMB will fade from view, but that hasn’t happened yet.
This effector couldn’t be outside our horizon, because the horizon is the Big Bang. Nothing existed before the Big Bang, which occurred 380,000 years before the universe became transparent. Maybe there are weird things buried inside the hot, dense state that existed between the Inflationary Era and the surface of last scattering, but we will only need to wait 380 thousand years to see them.
Nothing older than that exists in our current universe, unless current models are seriously wrong.
I thought it would just stretch to longer and longer wavelengths but never actually disappear.
That is effectively the same thing.
In due course there will come a time when the light and gravity emitted by the CMB no longer reaches us, but we will still be receiving redshifted light and gravity from before that time.