We wrote about this in Orbital Index recently (https://orbitalindex.com) and so I did some background research: Loeb's paper [1], mentioned in the article, found that hypervelocity stars accelerated by massive black hole binaries could hypothetically reach as much as *half the speed of light*. This is not something you want to be anywhere near.
Relatedly, Chinese astronomers recently published research on 591 high-velocity stars in our galaxy [2, 3], and 43 of these were hypervelocity stars, moving at relativistic speeds and destined to eventually escape the Milky Way entirely. They used China's LAMOST optical telescope and ESA's Gaia and doubled the number of known high-velocity stars. Apparently this is useful for measuring the mass of the Milky Way.
"Chinese astronomers recently published research on 591 high-velocity stars in our galaxy [2, 3], and 43 of these were hypervelocity stars, moving at relativistic speeds and destined to eventually escape the Milky Way entirely."
All of those stars have velocities < 1000 km/s relative to the Milky Way, which is less than 1/3 of one percent of the speed of light. Calling that "relativistic" is just hyperbole.
What would life be like for an Earth-like planet that orbited a star like that? What about spaceflight (e.g. from a planet to a moon) within that solar system? And what would the rest of the universe look like to them?
Life, including spaceflight, would be pretty much the same as it is for us. Since the entire solar system would be moving, the relative velocities of the planets and moons would be the same as in any other solar system.
The rest of the universe would look quite different, though. Stars "ahead" of the solar system - in the path of its motion - would be noticeably blue-tinted and clumped together. Stars behind the system would be red-shifted. There are some great visualizations at [0].
There is a bad-but-good hard SF book about this general subject called Tau Zero. You’d definitely need to be going much faster than 0.5c to get anything close to the effects you’re describing, but that book is a fun read if you’re interested in the subject.
These numbers are wildly exaggerated, but isn't this also the wrong "direction" of relativistic time dilation?
In the frame of the planet orbiting the hypervelocity star, the rest of the universe is moving at 0.5c, so the planet would measure all clocks in the rest of the universe to be running slower by a factor of 1/sqrt(1-0.5^2) = 1.15. Astronomers on this planet would measure stellar lifecycles (and every other physical process) to proceed slower, not faster.
Not correct. "Relativity" is very literal. When you go fast, then relative to you, it's the entire universe that is moving fast.
So if you fly away on a spaceship at relativistic speed, then people observing you will see you moving slowly. However, if you observe them from your ship, you will think it is they who are moving slowly.
> These numbers are wildly exaggerated, but isn't this also the wrong "direction" of relativistic time dilation? In the frame of the planet orbiting the hypervelocity star, the rest of the universe is moving at 0.5c, so the planet would measure all clocks in the rest of the universe to be running slower by a factor of 1/sqrt(1-0.5^2) = 1.15. Astronomers on this planet would measure stellar lifecycles (and every other physical process) to proceed slower, not faster.
You have it backwards (easy to do with such a thought experiment!). The direction of time dialation originally stated was correct. Clocks on a body moving at ludicrous speeds will be slower (to the observers present, who are able to look at those clocks) than clocks on a body not moving at ludicrous speeds. Which is to say, if you did a loop around the galaxy at light speed (hypothetical, of course) and came back to Earth and compared your clock with a terrestrial clock, your clock would be waaaay behind, and you would be younger than the people you left behind. This effect is measurable and provable even in the orbit of our own planet (albeit, at a miniscule scale). And so while your time did in fact move more slowly, the overall effect is that you time travelled into the future because everyone else was moving much faster. This of course assumes that you are able to escape intact, which is most unlikely (again, this is just a thought experiment, for argument's sake).
In your defense, I know how easy it is to get turned around in such a thought experiment. Theoretically speaking from the perspective of the par-luminal traveller, it's easy to think "my clock moves slower, so am I slower?" In a way, yes, you are! Your reality moves slower relative to theirs, and for those not travelling at light speed time passes more quickly relative to your time. Objectively speaking, a human lifecycle on a very fast moving planet will equate to many, many lifecycles on a slow moving planet, if all other things are equal. And so while you are truly travelling much faster, your time moves slower, relatively speaking.
Remember as a kid being told about a hypothetical traveller that enters a black hole, and at one point they become a long spaghetti string and then simply freeze in time? That's because they are moving much faster than us, and from the outside looking in they appear to be frozen. The light emitted from their mass is but an echo. They have moved far beyond the light that we see. From the inside looking out, the universe would move much faster.
Again, this is an extreme example used for clarification, but it might not be as extreme as you think. Consider a solar system that gets caught within the grasp of a black hole. Initially the tidal forces tear it apart, but over eons that mass might begin acting like a normal solar system again, and harbor life once more.
Unfortunately, this is a very misleading explanation.
If you are travelling fast in a straight line, you will observe the clocks in the entire universe around you running slower. Every clock you watch through a telescope on your ship will be running slow. The entire universe around you will be moving in slow motion.
However, everyone else observing you speeding by in your ship will observe your clocks running slow. They will observe you moving in slow motion.
This is an apparent paradox - in fact, it is the quite famous twin paradox.
The resolution is that this only holds for non-accelerated motion. Flying in a loop around the galaxy requires not just high speed but also a constant acceleration, giving you the additional effects of general relativity, which complicates the situation by quite a lot, but resolves the paradox.
However, an inhabitant on a star moving at relativistic speed through the universe still falls under the original case - the inhabitants on the star will see the rest of the universe moving in slow motion, while the rest of the universe will see them also moving in slow motion.
> The resolution is that this only holds for non-accelerated motion. Flying in a loop around the galaxy requires not just high speed but also a constant acceleration, giving you the additional effects of general relativity, which complicates the situation by quite a lot, but resolves the paradox.
Very interesting response! Thank you for that. I do see why my comment was misleading now. If you don't mind, could you clarify a few things? I'm just really interested in the subject.
I understand your point about travelling in a straight line: If a traveller were to travel at the speed of light speed away from a theoretical stationary point then both the observers at the stationary point and the observer within the travelling vessel would appear to eachother as frozen (assuming the traveller had equipment to measure the place they left from), because light wouldn't be able to catch up in either direction. That makes sense. When I said "From the inside looking out, the universe would move much faster" I shouldn't have used the term "looking out", that was definitely misleading now that you've mentioned it.
> However, an inhabitant on a star moving at relativistic speed through the universe still falls under the original case - the inhabitants on the star will see the rest of the universe moving in slow motion, while the rest of the universe will see them also moving in slow motion.
Your last statement got me thinking... so let's say there are two equally sized vessels that are somehow able to communicate instantaneously (for arguments sake, let's say the travellers are immortal wizards who have a telepathic link).
Let's say that one of the vessels is caught in the orbit of a black hole and is travelling at nearly the speed of light, and is constantly accellerating as it gets closer to the singularity (ignoring that tidal forces would destroy the vessel, because of it's wizard shielding).
The other vessel is lacadaisically making it's way around a star the size of our own. And let's say that the star and the black hole are slowly drifting away from eachother.
My question is this: Will the wizard inside of the vessel travelling around the black hole age more slowly than the wizard travelling around the normal star? Keep in mind that they will never meet (i.e. never share the same world line), but they do still have a telepathic link and can compare their clocks.
The thing is, instantaneous communications do not and can not exist in our universe. In fact, there is no such thing as two things happening "at the same time" if they are in different locations. If you observe two things happening at the same time from where you are standing (say, you are see two stars exploding at the same time and you know they are both at the exact same distance from you, so you know light took the same amount of time to reach you), another person who was passing you at high speed at the same time will say that you are actually wrong, those two stars exploded at completely different times, after doing the exact same measurement as you.
We talk about the "speed of light" being a constant, but this is misleading. The "speed of light" is actually the "speed of causality". Breaking the speed of light, with some theoretical instant communication, would be breaking causality.
In the earlier example, if those two stars each had an emergency beacon that sent you an instant message just as they exploded, you would receive both simultaneously. But the person travelling past you would be very confused about how that could happen, since he clearly observed them exploding at different times.
> The thing is, instantaneous communications do not and can not exist in our universe. In fact, there is no such thing as two things happening "at the same time" if they are in different locations.
I do understand what you're saying, and while I agree that observations differ based on the location of the observers, there's one thing that still baffles me. Quantum entanglement.
Let's say that we replace the wizards in my example with a pair of entangled particles, and in fact, for this thought experiment let's pretend that humans and "observers" don't even exist anymore - there is noone left in the universe to observe what is happening anywhere.
Wouldn't our current understanding state that the wave function would be identical between the particles regardless of where those particles are located in spacetime? In effect, wouldn't the wavefunction be "happening at the same time"? Mind you, I'm not suggesting that the particles are communicating, simply that their waveforms are identical. It's really baffling to me that one particle could be in an area subjected to massive gravitational distortion while the other is not (a completely different world line), and yet they are always in sync. Thoughts?
It is a deeply weird thing and we do not have a theory yet to explain this satisfactorily, but the one thing we do know is that no information can actually be transmitted this way, so it does not break the speed of causality.
> It is a deeply weird thing and we do not have a theory yet to explain this satisfactorily, but the one thing we do know is that no information can actually be transmitted this way, so it does not break the speed of causality.
I've read that as well, and I concur that popular opinion states that quantumly linked particles do not appear to violate causality, because we don't believe they are actually communicating at all, let alone communicating faster than light.
The way I interpreted this claim is that those particals are more akin to a deterministic algorithm, which is to say, once you set it free it will always result in the same answer based on the seed. I'm still very confused though about how those particles could remain in sync in vastly different spacetimes. That to me, is a flaw in the conjecture, and one that cannot be tested. We've based our understanding of quantum entanglement only upon that which we can observe, and so we truly don't know if a quantumly entangled particle will be "behind" if it is in a denser gravitational field. It's unprovable, because even if we were to travel into that spacetime, we could not escape to share the results with other observers.
Thank you again for your input. Your argument forced me to mentally reconcile "observable reality" from "actual reality", and that was a fun and intriguing thought experiment, albiet probably pointless since none of it can be proven. Our observations do not define reality, they only measure it to the best of our ability. And most of the time when we measure reality, apparently we fuck it up (i.e. breaking quantum links, or just having incorrect assumptions).
To expound upon my point, consider that we currently believe that our own solar system is rotating around a supermassive black hole, and in fact all large galaxies are believed to contain supermassive black holes at their center. Our solar system is subject to gravitational time dilation from those black holes, and so while our world lines may never meet with a galaxy far away with its own supermassive black hole, objective time differences will absolutely be present.
It would not. The universe would actually age slower. However, it would also be much shorter along the axis of motion, and the stars in front of you would be much further along in their evolution than the star behind you. So you would perceive any given star to be evolving more slowly, but you would keep encountering older and older stars as you travel.
If a man goes on a 50 year voyage and returns, he will not have travelled in a straight line, like a planet zooming through space would. He would have to accelerate and change directions, which brings in the effects of general relativity which complicate things. So these two are not equivalent cases.
For a star that got ejected at near light speed from the galaxy, any orbiting bodies would probably be flung in other directions and no longer orbit the star.
Assuming the planet miraculously hung around for the ride, after the initial acceleration, it would be no different then what we experience here on earth.
Remember that everything is relative and perspective is key. A question about speed depends on who you’re asking.
To us, the other star and it’s planets are traveling at near light speeds away from us. To that stars system, it’s Earth that’s traveling at near light speed away from them.
Space flight would be the same and so would gravity and time. To them, nothing changes within their stars system because it’s all within the same reference frame.
I guess the night sky would be the main difference for people on that planet. They’d see the galaxy they left behind as red shifted and anything they’re speeding towards as blue shifted.
Since they’ve left the galaxy and was red shifted, they probably wouldn’t see as many stars with their naked eyes.
There is a story (I am certain it's apocryphal, but who knows?) of someone trying to get out of a ticket for running a red light by claiming that the light of the semaphore had been blueshifted because of their speed, so they had seen it as something similar to green. The story then goes that the judge decided to calculate how fast they would have been going for red light to appear greenish because of blue shift, and then issued them the mother of all speeding tickets for going some fraction of the speed of light in a 30mph area.
Edit: according to [0], you would have to be driving at 30855km/h (2237mph) to see a red semaphore as green, which comes up to ~3 millionths of c.
I can't imagine even an inner world surviving. If a star is ejected at 150,000 km/sec and Mercury got an acceleration one part in 7,500 more it would be ejected. Is the gravity field flinging the star really that flat over that much distance??
> Remember that everything is relative and perspective is key.
This has always bothered me a bit, everything being relative. You can validly say that we're the ones moving at near light speed and those stars we're talking about are actually the stationary objects; there's literally no way to tell the difference.
Couldn't you say the same thing about the earth and the sun? Why do we insist that we are moving around the sun, and not the other way around if everything is relative? Well I understand the reason actually, and it seems that there is good reason to believe one interpretation of relative motion is the correct one, conforming to physics.. and the other view of relative motion isn't actually what's happening in reality.
It actually annoys me that we still teach kids that the Earth revolves around the Sun and the ancients with their silly earth-centric system were wrong. Just the math is (kinda [1]) simpler, thats all.
First, its a lie. And second it breeds contempt for the ancients that said and discovered many great things. I dont mind telling them that heliocentism is a great (amazing) approximation. But I really dont appreciate how from K-University previous work that has been outdone is dismissed [2]
[1] the higher order terms are sill there in the heliocentric system, they're just smaller in magnitude.
[2] Another example is atomism and how the physicists who rejected it are mocked in undergrad classes. Those guys were bloody titans and their models and methods are the cornerstone of modern engineering (try designing a bridge w/out continuum mechanics but using MD)
>Why do we insist that we are moving around the sun, and not the other way around if everything is relative?
Try to express the trajectories relative to earth of other planets in our solar system and you will quickly understand why we use a heliocentric model.
> A planet orbiting the sun is accelerating toward the sun.
How do we know that the sun isn't orbiting the fixed earth, and accelerating toward the earth? Everything would look _exactly_ the same to everyone.
Yes, the math works out much easier and more neatly, so that is likely what is happening in reality. But from a purely relativistic framework.. you can consider any point in space the frame of reference, and all motion relative to that "fixed" point.
It makes me think that relativity is actually describing a more subjective experience, rather than the objective reality where we "know" the earth orbits around the sun.
> How do we know that the sun isn't orbiting the fixed earth, and accelerating toward the earth? Everything would look _exactly_ the same to everyone.
That wouldn't work out. You can take the earth as a fixed point and describe the Sun's motion relative to it, and it would be perfectly valid, but it wouldn't look as the Sun orbiting the Earth in any kind of almost constant speed elliptical orbit, it would look like a very different kind of motion.
Well, maybe, but I think you're mixing the everyday definition of "relativity" (i.e., things are relative) with what physicists mean by "relativity" (i.e., the Theory of Special or General Relativity, both theorized by Einstein).
> but I think you're mixing the everyday definition of "relativity"
Well, yes.. because Einstein's theory beautifully makes all the math work out, explaining one interpretation of motion as the real one. But what it did at the same time was show that there is no "fixed grid" of space independent of the objects themselves. Which is what leads to my uneasy feeling of how we ever can say one interpretation is more real than another -- i admit it may just be a nonsensical perspective.
As for what all physicists mean by relativity in general... I can highly recommend this old series from the National Science Foundation:
I was confused about this too, until a physicist friend pointed out that you can always tell which of the bodies actually did the acceleration to get up near light speed.
> Why do we insist that we are moving around the sun, and not the other way around if everything is relative?
Because it makes the math easier - that's all. If you fix your frame of reference to the Sun, the orbits of all other bodies become almost perfectly elliptical. If you try to pin the reference frame to any other body, you end up with complicated curves. But they model the same thing[0]. So the whole thing about "Earth orbits the Sun" is that it gives same results, but is much easier to work with.
Technically, for the easy math, the point you're after is the barycenter[1] of the Solar System - the center of mass, which, per Newton's First Law, can be used to center the reference frame, because it's not accelerating[3]. As it turns out, the barycenter of the Solar System spends most of the time within the volume of the Sun[2], and otherwise is very close to it. So for most calculations, you may just as well pin the reference frame to the Sun.
And then, when you fix your sights at the barycenter, you'll notice the movement of celestial bodies fall out pretty much straight from joining Newton's Second Law with the Law of Universal Gravitation - m₁a₁ = Gm₁m₂/r². Your model simplifies - you now realize the movement of celestial bodies is governed by the same laws movement on Earth is, and all the complexity of geocentric model was caused by needless coordinate transformation, due to a bad choice of the reference frame.
Also worth noting that historically, humans have developed the geocentric model to a very impressive level of precision - to the point that the "upstart" heliocentric model initially was worse at predicting movement of planets. It took some extra insights for the heliocentric model to beat the old ones[5] - and only then Newton came along, and people connected the effect with the cause.
--
[0] - If I recall correctly, if you were to take the path of a planet in a geocentric model and do a Fourier transform on it - that mathematical operation which represents a function of time as a possibly infinite sum of sine waves - you'd notice that the path of your planet is essentially a sum of two periodic functions. One would correspond to the movement of Earth around the Sun, the other to the movement of the planet around the Sun. This would give you a strong hint that your model is needlessly complicated, and can be recreated using much simpler curves.
[3] - Only forces from outside the considered system could cause it to move, which we're by definition not considering when talking about our system in isolation. And besides, they add up to negligible amounts anyway. Nice thing about forces in our reality scaling like 1/r^2 or worse[4] is that they very quickly add up to nothing with distance, which makes it easy for us to treat systems as isolated in calculations, and have the results match up to reality with extreme accuracy.
[5] - Like using ellipses instead of circles as the fundamental curve, because your competition that used circles moving on circles could just keep adding circles - they were doing a Fourier transform without knowing it, and each circle added a frequency component, increasing the accuracy of approximating the actual ellipse.
No, it absolutely isn't valid to say the earth is stationary while the sun rotates around it, you end up with crazy nonsense.
Technically, the earth doesn't rotate around the sun either -- they both rotate around the center of mass of the solar system, which happens to be very close the the center of the sun, so it looks like everything rotates around the sun.
Thanks. Still unsettling that while both perspectives are equally valid, Newtonian physics is so much more convincing than what came before (clockwork/earth at the center of the universe).
So while both relative views are equally true, the laws of physics (or at least mathematics) don't seem to hold both those views with equal esteem.
In Newtonian physics the sun is the preferred frame of reference.
However with the introduction of the mathematics of general relativity by einstein in 1915; we can prefer whatever reference frame we like and we get the same correct predictions. Predictions that are more accurate than Newton's.
At half the speed of light, I'd guess that the solar system would have its heliosphere stripped by interstellar or even intergalactic medium. The heliosphere is the region around a star where the pressure of the star's solar wind is great enough to push back against the ambient gas floating around in the galaxy. For some scale, we think Voyager 1 may soon exit the heliosphere of our solar system -- a first for man made objects.
As sibling comments describe, any planetary orbits that happen to survive would continue operating as usual, but if they were outside the heliosphere they would be subjected to interstellar gas traveling at half light speed in the other direction.
Also, each proton in the interstellar medium would become a cosmic ray with energy ~120 MeV. At 1 proton/cm^3, the intensity of this radiation on the planet would be 270 kW/m^2 (more than 100x the intensity of sunlight). If this can penetrate the magnetosphere (not clear to me it could, although I bet auroras would be extreme) it would erode the atmosphere and create large amounts of radioactivity.
Interstellar medium has a huge range of densities from approximately 10^6 per cubic cm down to 10^-4. However, the intergalactic medium is between 10^-5 and 10^-6 in high density filaments and gets much less dense in the voids.
As such in a cosmic void planets around such stars would be fine. Though it seems extremely unlike for such planets to exist.
Even at a comparatively low density surely it's many orders of magnitude higher than how often we encounter such cosmic rays on earth. I mean if we scale down the energy output by 10^-6, 270 mW/m^2 would still be way more than any known biological system could sustain, right?
> 270 mW/m^2 would still be way more than any known biological system could sustain, right?
It's negligible compared to our sun. All this from the top of my head (so double check if you really care ;) but our sun in winter at surface level is roughly about 100W/m^2 and in summer about 1kW/m^2.
Yes, but the point is that these are wildly different types of energy delivery. The sun delivers energy by gamma radiation ("plain old light") but here we're talking about alpha particles ("relativistic protons") which has a very different effect on biological systems. The fact that we are able to compare these two things by their total energy content at all is shocking to me.
To make this comparison more concrete, maybe we could consider how much alpha particle energy is present inside of a nuclear reactor running at peak energy output. I'd be surprised if it was as much as 1 W/m^2.
Protons at 120 MeV will deposit a significant amount of energy via nuclear collisions. The binding energy of a nucleon in a nucleus is about 8 MeV, so plenty of free neutrons will be produced, which will then create 14C by the (n,p) reaction on 14N. Back of the envelope gives this creating about 10 tonnes per second of 14C in the planet's atmosphere (assuming the proton density of 1/cm^3, and assuming the planet is Earth-like and all the protons struck the atmosphere). This is about 11 orders of magnitude higher than the production on Earth, and would result in such a high equilibrium concentration of 14C that the biosphere would likely be sterilized, even ignoring thermal effects.
We are talking a proton density of ~10^-8/cm^3 or lower in cosmic voids. At 1/cm heat alone is going to cook everything long before radiation has time to do anything.
Next solar wind actually becomes significant protection at those densities. It varies significantly and simulating what happens gets complicated, but it shouldn’t be ignored at those densities.
Anyway, the outer fringes of earths atmosphere for example is almost exclusively Hydrogen and Helium as it gets sorted by atomic weight. Free neutrons decay in a matter of minutes and therefore would almost entirely end up as more hydrogen. What’s a much larger risk is stripping the atmosphere off of any planet.
These planets would be accelerated at SMBH mergers, which would occur in galaxies. Yes, once the planet gets to a void it's hitting less gas, but it has to get out of a galaxy and galaxy cluster to get there, and that will take many thousands, perhaps millions, of years.
The outer fringes of the Earth's atmosphere are hydrogen, but the density there is so low that these cosmic ray particles would be unlikely to interact there. Also, the neutrons here are FAST neutrons (the (n,p) reaction on 14N is a fast neutron reaction) so they are traveling at very high, if not relativistic, speed, and would not have time to decay before they react (or are thermalized and become irrelevant to 14C generation from 14N).
Planets could be accelerated by SMBH mergers, but they wouldn’t stay orbiting a star through the process. It’s slightly more plausible for a one to form afterwards.
As to the density of earths hydrogen, that’s a function of earths atmosphere. Under sustained bombardment, assuming the planet kept an atmosphere, it’s going to have significantly less nitrogen in the upper atmosphere as that’s destroyed by collisions with relativistic hydrogen/helium.
Doesn't matter if it can penetrate the magnetosphere, if it's stopped that energy has to go somewhere and some of it is going to hit the planet. Even if it doesn't get blasted it's still going to get too warm.
If the motion is generated by passing close to a binary black hole pair and being sling shot away, if the planets were still in orbit around their stars, they would likely be in very perturbed orbits. Also, the nearby passage of a black hole is probably not something that life would survive due to radiation.
Sounds dramatic, but on my scale of existential dread, nowhere near up there with simply being trapped in a human body where there's no offsite backups and you (or anyone else close to you) can just get an aneurysm and die at any moment.
I don't think death of the individual is something to be dreaded. I suggest meditating on your own death until the thought no longer causes anxiety.
Death of the individual is a completely natural part of the cycle of life and should not be dreaded beyond the fact that it hurts like hell when it happens to someone you love.
I meant meditating in the "thinking about it" sense.
I generally consider not being anxious about future events you cannot avoid a good thing. The lack of anxiety does not mean that death would feel like a good thing, nor does lack of anxiety invoke irrational behavior.
"Natural" was maybe a too loaded word. Maybe "unavoidable" would be better.
This seems to be just another form of denial of existential dread to me. Draped in the ever fashionable "I can reshape anything by wishing upon it" but to be honest, i rather be around someone who suffers through and faces his mortal fears, then somebody thinking around them with some barely disguised religious rituals.
The later is much less likely to lash out, when his "protective" bubble against existentialist fear is penetrated. Its also indicative of a curious mindset, if you stare into your own emotional blindspot, to find out what you are made of.
Of course, it can become a self-destructive behaviour, destroying ones ability to function, which suppressing the fear in the end is.
I meant meditating not in the "sitting on the floor, breathing" sense but in the sense of ruminating and thinking.
I'm not an expert on death anxiety. There is the "normal" kind, and then there are conditions that probably require a therapist.
I am not suggesting life should not be cherished, nor that one should not consider each death a loss.
Lots of people have written about getting rid of it. You just probably need to skim through self-helpy books until you find some rationale to appease you.
Three points that helped me: It is a huge privilege to be born in the first place. The number of living people is infinitesimally smaller than the number of people who will ever exist (I'm referring to the fact that there is about 70B ways fathers and mothers genome can combine and you are only one of those options - as is any human of their parents genomes who has lived).
Being dead is just like being before you were born. Renaissance doesn't cause anxiety in me so I don't know why a future centuries from now on without me would frighten me.
There is the "imagine you are dead" visualization. Imagine you are dead, buried to the ground and worms are eating your corpse. Really vividly, focus for a few minutes. Then stop and continue your day. Notice the thought of being dead does not seem so frightening now you've imagined it.
Some mythologies purport an afterlife, if you prefer in believing the continuity of consciousness go ahead and do that - I have nothing against it.
I think everybody agrees patterns that are related to of us will remain in the world like ripples in the pond after we are gone. But beyond that I have no reason to believe in any sort of continuity.
Stoicism. Other than old works from Marcus Aurelius, Seneca, Epictetus, you can also find modern stoic practitioners, Massimo Pigliucci and Donald Robertson (there are others, but these are the best at teaching IMO). Robertsons book has additional chapter you can find somewhere online on exercises for meditation on death. But it's better to read it after https://www.amazon.co.uk/Stoicism-Art-Happiness-Teach-Yourse...
That's generally true, but it's not meaningful in this case. If I say "One in 10,000 stars in the Milky way is a hypervelocity star moving at relativistic speeds", there is no context to provide a threshold for the magnitude of relativistic corrections that are important.
(And if it's just about escape velocity, they should just say that. The Milky Way's escape velocity is 550 km/s = 0.002 c, for which relativistic corrections are extremely tiny: \gamma ~= 1.0000017.)
Yes, absolutely-- the general claim lacks enough specificity to fully evaluate it. Should there be a need for extremely high precision, even somewhat "slow" speeds would be considered relativistic in terms of the need to take such corrections into account.
Even better, half the universe would be in fast forward, everything behind you in slow motion! Watching a galaxy collission in accelerated time and a supernova in slowmo would be a unique experience.
Time dilation isn’t dramatic as you might think. For example, at 66% the speed of light, you only get a 1.3 (or 30%) time difference.
You have to get into the 90% range for something meaningful. Even at 96% the speed of light, it’s a 4x speed difference.
99.99% is 80x time difference.
99.999% is 240x time diff.
To have time speed up to the point of civilizations rising and falling in a matter of days, you’d need to be traveling at 99% (with a few 9s in the decimal places). Though at relative speeds that high, it would be difficult to actually see what’s happening.
Some events would appear faster because you're moving toward them, but once you account for that, all other clocks in the universe would tick slower from your perspective.
Yes, I think you are right to question whether they would observe the rest of the universe running in fast forward.
If a hypervelocity star passed by us, we would see them in slow motion because they are moving at high speed relative to us. At the same time, they would see us in slow motion because we would be moving at high speed relative to them. The effect of “the other clock is slower” is reciprocal.
The weirder and non-reciprocal effects would occur when the hypervelocity star is first slingshotted out to high speed. At that point they would be close to a black hole (deep in a gravitational well) and accelerating (non-inertial frame of reference). Under those conditions, their clock would run slower than our clock for observers on both sides — i.e., we would see them in slow motion and they would see us in fast forward.
You'd have no hope of ever exploring that universe though. Decelerating from 0.5c isn't really practical with any technology that we're pretty sure actually works. Sure, you'd see the rest of the universe on fast-forward, but you'd never be able to leave your own solar system.
If a star is moving at nearly the speed of light relative to us, doesn't that also mean that we're moving at nearly the speed of light relative to them?
If that's so, wouldn't that mean that the Earth is a "hypervelocity planet" in the article's terminology.. at least relative to some other objects in the universe?
Well, every cosmic ray makes you hypervelocity too I guess.
But to be meaningful you want to talk about speed relative to your local galactic frame or some such. Most of the mass around us is moving relatively slowly to one another. That makes these fast stars an anomoly. And it means their (relative) effect is going to be interesting.
These sorts of things are generally measured with respect to the galactic rest frame or the rest frame of the CMB. Since the vast majority of stars are moving much slower, hypervelocity stars are not on the same footing.
yes, but it means that everything around us - the sun, the solar system, the Galaxy are also "hypervelocity" relative to any such object, thus making our own "hypervelocity" label not that special.
When you accelerate away from me in an elevator, only one of us experiences additional forces during that acceleration. Only your body feels heavier as the elevator speeds up.
Here’s a minute physics video that shows how acceleration explains the twins paradox (the “common” explanation) https://youtu.be/0iJZ_QGMLD0
After you watch minute physics video, you might feel like you understand. Nope, can’t have any of that. Now, you must watch this Fermi lab video that says the common explanation is “not fundamentally correct” https://youtu.be/noaGNuQCW8A
Fermilab video is hard to grasp intuitively to me, but one important thing it did was it cleared that confusing misconception of acceleration, which was induced by [too many] other videos.
After watching these incorrect videos it was still unclear why acceleration even matters, because with 1g accel in space you can reach subluminal speed in a matter of days, and then, at the distant location, turn back with just 2x long 1g again, and then “brake” with 1g again to land on earth. While the twin on earth experienced that 1g all the time. One may even pick a distance and a (probably hyperbolic-y) route with rotations so that all of the journey would consist of a constant 1g for a flying twin, exactly as a sitting one experiences. Put them both into opaque boxes, knock senseless for a couple of hours at launch, and neither of them would even tell who is where, until opened.
I think the most important point if you want an intuitive explanation is that the observer who is in the same place at the beginning as at the end has aged more.
If you were to ask which of the twins is older at the time the second twin reaches the far away star, the answer is that the question doesn't make sense. They are in different places, so there is no way to compute a fixed time.
Imagine each twin is broadcasting a video signal of their face for the entire duration of the trip. The twin on earth puts a big red dot on the screen the moment that the traveling twin had reached the distant star in their frame of reference; while the twin on the ship would start putting a big red dot on their video the moment they reach the star in their own. Because the speed of light is limited, over the whole duration of the departure trip, for both twins, the latest image they see of their sibling will be younger than themselves.
When the twin on the ship reaches the star and starts emitting the red dot, they will have aged L/gamma * v years. By the time the twin on Earth sees this image, they will have aged L/v + L/c years = L(c+v)/c*v years, so they are [L(c+v)/c*v] / (L/gamma * v) = gamma(c+v)/c years older.
The twin on Earth will start emitting the red dot after L/v earth years, or L/gamma*v ship years. This signal will meet the ship after L/(gamma*(c - v)) ship years from the moment it was sent - so in total, L/gamma*v + L/gamma*(c-v) = L*c/gamma*v*(c-v) ship years. So, they are [L*c/gamma*v*(c-v)] / (L/gamma*v) = c/(c-v) ship years older than their sibling, which is gamma*c/(c-v) earth years.
While both see that they are older than their sibling at the time they first see their sibling's red dot, there is an asymmetry coming from the fact that one of them is at the same position as they were initially, while the other one is at a different position, relative to the star. Equivalently, there is an asymmetry that is caused by their different definitions of "the moment the ship reaches the star".
Acceleration isn't relative, velocity is. An object undergoes acceleration when a force is applied to it. Objects "know" when they are being accelerated, just like how you know you're being pulled down by gravity. The same can't be safe for velocity.
It's like wrong way driving: If everyone else is moving in the same direction / the same speed, only you are different, then maybe you're driving the wrong way / fast.
Where they become important depends on your precision of measurenment. GPS satellites don't move at 2% the speed of light, but relativistic effects do play a role for the system to work since they need to be very precise in measuring and broadcasting time.
It's odd that he didn't mention it in the writeup because his paper predicted that massive black hole binaries could accelerate stars to half the speed of light.
Makes me wonder, if a relativistic star passes through a decently thick dust cloud could its bowshock achieve fusion and glow brighter than the star? That would be quite the impressive lightshow
I did some napkin math and a star the size of our sun passing through intergalactic space (not a dust cloud obv) at the speed of light would only encounter about 1kg of matter per second on average.
(pi1.3e9m^2 cross sectional area 3e9 m/s velocity) / (1e-27 kg/m^3 density)
Your result is only about twice my own napkin math, so it checks out. That formula as written has a typo: to get mass per unit time, multiply (not divide) cross-section-speed by mass density.
Could we use stars like this as intergalactic vehicles!?
If (BIG if) we some how figure out how to launch a mass the size of a star or planet into an orbit around two galaxies. You could then launch your starship into its orbit around the apogee, ride the mass to the next galaxy, and de-orbit burn around perigee! In this way the orbit would practically be a store of energy as well.
The Delta V required to catch up with these things would also be an appreciable percentage of the speed of light. The star’s gravity would help, but it’s only going to take the edge off what will be an immense acceleration budget. Given that we basically haven’t hit even 0.01C, getting a planetary mass up to this speed seems impossible.
Plus, once you get wherever you’re going, you’d have to pay back whatever boost the star’s gravity gave you as you both escape its gravity well and spend down the velocity that the star itself was going.
You just need to let the star crash into your vehicle to get it up to speed, and then crash into another star to slow down. Building a vehicle (and passengers) to survive these events is left as an exercise for the reader.
Resources maybe, since you’d get sunlight from the star plus whatever material is caught in its orbit.
Fuel, no. You’d spend the exact same amount of fuel catching up to this star and eventually leaving its orbit as you’d spend just travelling across the universe alone at the same speed. You’d only save fuel if you used the star for a slight shot maneuver, where you steal a tiny amount of its energy to accelerate you, but this would preclude you spending the voyage in the orbit of this rogue star.
It could be fuel, if you consider fuel to be one of the resources a star can provide you.
If your fuel is something that is inside the star (or star's orbit), and you have a means of extracting it, then you can use the star as a refueling point.
You're right that using the star wouldn't require less fuel overall, but it could provide you access to extra fuel along the way.
could a black star swing around and accelerate wandering asteroids and comets; that would be a more frequent event, wouldn't it? what would be the likelyhood of running into an object like that in our solar system? wouldn't the friction with interstellar medium brake an object like that before it gets out of a galaxy?
> searches for extraterrestrial intelligence should check for radio signals coming from riders of hypervelocity stars
howso? wouldn't the ejection/acceleration process disrupt the planets around a star?
Still amazing how much action is happening in astrophysics these days.
I hate how the article plays loose with the wording: “nearly” the speed of light is not half the speed of light as comments have suggested about hypervelocity stars.
So a star moving that fast would last far longer from our frame of reference than a star in "normal" frames of reference? So it might be a fast-forward to the end of the universe if we speed up to catch it and hang out there for a billion years.
Would they qualify as stars? Would they be able to maintain integrity with the relativistic change of mass (across its volume) while still running the fusion process?
Indeed, in their local coordinate system, their mass hasn't changed, while from a still observer's point of view, such a star gains that extra relativistic mass and should in theory collapse once it reaches a certain speed. Of course, it would be strange if the same star would be collapsed or normal in different corrdinate systems. I guess the solution is that the extra relativistic mass isn't real, but a curious artifact that manifests as the higher than normal kinetic energy.
Black hole's aren't really defined by mass though - they're defined by spacetime curvature. Specifically, inside the event horizon of a blackhole, every possible vector winds up at the singularity.
A very speedy object however doesn't look like this: objects it passes have vectors which take them away from it. So while it can warp spacetime with that mass, it can't form an event horizon - there are vectors which lead things away from it and thus can escape.
[EDIT: I had to go do some reading to check up on this and it actually has an analogue to electromagnetism: moving objects develop gravitational effects perpendicular to their direction of motion, essentially this is one of the origins of gravitational waves - but it's also important to the overall point, which is that even if you had infinite gravitational acceleration perpendicular to your direction of travel, you've got numerous other degrees of freedom where this doesn't happen and thus escape vectors - so no event horizon though you might suffer a lot of damage and be spagettified anyway]
Black holes only work when you have a point force exerting uniform acceleration in all directions towards itself - beyond a key point, no attainable velocity ever results in an object not moving towards you.
Whereas a speedy object has an obvious one: just jump off the back of it (i.e. accelerate slightly against it's vector) and your trajectory now points you away from it.
From the perspective of the star, what impact does relativistic speed have on the rest of the universe? Does it also gain mass? Does that mean that an object accelerating to relativistic speed increases the mass of the universe from both frames of reference?
Relatedly, Chinese astronomers recently published research on 591 high-velocity stars in our galaxy [2, 3], and 43 of these were hypervelocity stars, moving at relativistic speeds and destined to eventually escape the Milky Way entirely. They used China's LAMOST optical telescope and ESA's Gaia and doubled the number of known high-velocity stars. Apparently this is useful for measuring the mass of the Milky Way.
[1] https://arxiv.org/abs/1411.5030
[2] http://english.cas.cn/newsroom/research_news/phys/202012/t20...
[3] https://iopscience.iop.org/article/10.3847/1538-4365/abc16e
P.S. HN needs a better way to make links.