Also, imagine having the technology to send signals through the lens and get the attention of intelligent life on the other side.
At 10 billion light years away from the most distant lens it is 100% certain that they are no longer in a gravitational lensing configuration.
For a frame of reference, the Milky Way will be in the middle of its epic merger with Andromeda in about 5 billion years.
Conceivably, a civilization could predict in advance that two galaxies would form a lens configuration, and send a signal that arrived just as the lens formed, correct?
Might just not be us.
These distances and time periods are unfathomably long. I can see predicting the alignment of galaxies but predicting a civilization with an adequate evolution stage will exist at the right spot, at the right time is very different. Any civilization with this power of prediction probably has a level of advancement that makes the difference between humans and amoeba look positively non-existent, and probably wouldn't bother with broadcasting lowly radio waves into the universe.
I can't imagine the universe and evolution of life being so deterministic and predictable especially over this time scale, no matter what tech you have.
> probably wouldn't bother with broadcasting lowly radio waves into the universe.
I bet we would be very glad to receive such a transmission, even when knowing full well "replying" isn't a realistic option (both due to technology limitations and the RTT meaning that even if the reply receives were descendants, they'd be so far removed as to be entirely another ship-of-theseus civilisation)
A gift in a cosmic dying sigh could be motivation enough.
"Should anyone receive this, know that, as far as life forms go, you were not quite alone and life existed beyond yours. We're sending this knowing full well we'll be long gone, but during all of our civilisation history we could only hypothesise that we were not. We hoped but never knew, may this transmission relieve you of the doubts we had; you now unambiguously know."
2. Unless we find faster than light communication (which, with our current understanding of physics is about as likely as humans jumping to the moon) there is nothing we could use it for other than definite proof that other life has evolved in the universe. Interesting data, but they're most likely extinct for billions of years already and even if they're not, the compound gravity lens will have moved out of alignment by then so we have no means to send a message back.
Also, other stars can come to align in the future. Makes me wonder if we can antecipate other cases like this and create a future schedule of "To Observe" so future generations can look at them. Although, these generations might be so distant from ours that might not even be considered of the same species
However, beamed sources don't fall off that way.
A search for optical laser emission from Alpha Centauri AB - https://academic.oup.com/mnras/article/516/2/2938/6668809
> ... This search would have revealed optical laser light from the directions of Alpha Cen B if the laser had a power of at least 1.4–5.4 MW (depending on wavelength) and was positioned within the 1 arcsec field of view (projecting to 1.3 au), for a benchmark 10-m laser launcher
For comparison, with our measly human technology...
https://www.ukri.org/news/uk-science-facility-receives-85m-f...
> The Vulcan 20-20 laser is so named because it will generate a main laser beam with an energy output of 20 Petawatts (PW) alongside eight high energy beams with an output of up to 20 Kilojoules (KJ). This is a 20-fold increase in power which is expected to make it the most powerful laser in the world.
Or even five decades ago (TODAY!) ... https://en.wikipedia.org/wiki/Arecibo_message
> The entire message consisted of 1,679 binary digits, approximately 210 bytes, transmitted at a frequency of 2,380 MHz and modulated by shifting the frequency by 10 Hz, with a power of 450 kW.
https://www.seti.org/seti-institute/project/details/arecibo-...
> The broadcast was particularly powerful because it used Arecibo's megawatt transmitter attached to its 305 meter antenna. The latter concentrates the transmitter energy by beaming it into a very small patch of sky. The emission was equivalent to a 20 trillion watt omnidirectional broadcast, and would be detectable by a SETI experiment just about anywhere in the galaxy, assuming a receiving antenna similar in size to Arecibo's.
What you get from lasers is very high gain in the direction it is pointed in, but it's still subject to the inverse square law.
It's capable of being enough gain to be interesting, to be seen from a great distance.
If you engineer it so the gain is enough to outshine the rest of the parent galaxy in the direction it is pointed, then that's effectively good enough because the galaxy is also following inverse-square and you'll continue to outshine the parent galaxy even as you and it both get weaker, but it's still falling off inverse-square.
I still hold that it would be possible to send and detect signals set with intention with not too much more advanced technology than what we have.
That's what "density" means. (i.e. the amount of something per unit volume)
> noise level
A photon will travel thru space forever without losing energy, unless it hits something. What noise are you talking about?
I'm talking about the https://en.wikipedia.org/wiki/Noise_floor, in particular the unavoidable receiver noise caused by the cosmic background radiation.
A single photon is not a viable communication signal, certainly not at interstellar distances. In practice you need to send out some sort of modulated beam. Even very narrow beams have nonzero dispersion, so the further you get the lower the signal energy will be at an antenna of a given size. So to get more energy you'd need a bigger antenna, but that in turn means receiving more of the background noise as well. In practice there is a minimal signal strength level at which it is still practical to receive the signal.
Long story short: A photon will go on forever (unless it hits something), but a radio signal rapidly spreads out so much that no realistic receiver will be able to recover it from out of the cosmic background noise.
Interestingly, if you send out a single photon from a radio antenna not even the universe itself will have 'determined' which direction it even went until it DOES interact, because there would be a Quantum Mechanical superposition/indeterminacy similar to the famous slit-experiment, if you were dealing with one photon at a time.
So even the thought experiment itself is complex due to wave/particle duality.
Light beams (or similar sources of EM waves generated by individual electrons or nucleus) are made by photons. We can record individual photons.
Maybe, radio waves are made of photons, but nobody confirmed that yet, so I can safely say «no». If you can confirm that, Nobel prize is yours.
Are radio waves quantized? Of course, at Planck scale.
Is it possible to form a single 100kHz photon using a macro antenna? I hope for «yes», but I have no idea about «how».
Radio waves are photons; photons are quantum entities that have particle- and wavelike behavior simultaneously.
Please, say something useful.
And why do we ignore the most common eclipse, the 'terrestrial eclipse'? Happens literally all the time. Also called 'night'.
Another thought that occurred to me, we humans are short lived and trying to think about the length of time such a message would take to travel far exceeds out lifetime. Even the thought of humanity lasting that long is difficult. But imagine if there were intelligent life forms that lived a single life on galactic timescales. To them, this discussion of sending a message that reached someone wouldn't be so pessimistic.
The relationship is (must be) symmetrical. Were this not so, it would violate a principle called "Maxwell's Daemon" (https://en.wikipedia.org/wiki/Maxwell%27s_demon).
Lensing works in reverse except for time delays which make the idea much more complex. The object's past is projected to us now, but our past would be projected to somewhere that the far object no longer occupies. Double lensing makes this even less reversible.
When the light we are now seeing was emitted, the lensing wasn't in place. In fact, the galaxies doing the lensing hadn't even evolved to the state that we see them in.
So if we sent a response to what we see now, it wouldn't make it back to the lensed objects.
That's just for single lensing. Double lenses are a massive coincidence of events at 4 points in time and space (emission, first deflection, second deflection and observation). That means that light going the other way wouldn't have the two intermediate points in the right place at the right times so it all breaks down for us and the object we see. There are some points that would be double lensed in the reverse direction but the locations and times for the source and observer have only very vague correlation to our location and the location of the object we see.
So lifeforms on the other end of this cosmic "lens[es]" cannot use it to see us better, because in fact it makes us look further away from them than we are, from their perspective.
If I understand right, objects further than a redshift of z ~= 1.8 can't be reached by any signal we emit, and the second galaxy is at a redshift of z = 1.885. But I don't know how precisely (standard deviations rather than decimal places) the distance to the outbound cosmological horizon is being approximated, so it might be reachable by a signal sent by us:
https://upload.wikimedia.org/wikipedia/commons/8/88/Home_in_...
Not sure what the practical analogy would be. You can't use an exploding telescope?
The question of at what distance and relative velocity are the two locations so far apart that light can never make it from one to the other (due to expanding universe) is a completely separate issue.
Underlying paper: https://arxiv.org/abs/2411.04177
The sky is huge and we are moving, so surely some would happen in our lifetimes?
It takes light, the fastest thing that can be, 100,000 years to cross the Milky Way.
The Sagittarius Dwarf Spheroidal Galaxy is currently in the process of being consumed by the Milky Way and is expected to pass through it within the next 100 million years.
So, unless you're even more optimistic about life extension technology than I am, not in our lifetimes, no.
Relative to us. For the light itself it takes no time.
https://www.universetoday.com/149214/if-we-used-the-sun-as-a...
Seriously, we could build that, it's at the limit of our tech but if it was either we walk on the moon again or build SGL, I'd pick SGL
walking on the moon was beyond our limits when it was announced
JWST was insanely hard and almost cancelled a few times, look at it now
The SGL would be much, much harder than the JWST would be, and the JWST was already stretching our capabilities.
The SGL needs to be 650AU away from us. Voyager 1 and 2 are currently 165AU and 120AU away.
The JWST is 0.01 AU from us.
And you can only look in one direction after the probe finally gets into position. Once you're 650AU away, it's not really feasible to move "sideways" far enough to look at something else.
In 1957 Sputnik 1 had an apogee of ~900km from the Earth.
By 1969 NASA was sending rockets ~385000km to the moon.
By 1979 Voyager 1 & 2 were reaching Jupiter ~5AU from Earth.
We went from 900km to 5AU in 22 years.
If SpaceX achieves their stated goals of lowering $/kg to orbit and rapid re-usability with Starship it will unlock things like asteroid/lunar mining and space based manufacturing which will allow the construction of the kind of infrastructure needed to make distances like 650AU achievable in reasonable time frames.
That is precisely why we must transition to space based resource extraction and manufacturing.
There are practically infinite resources at our fingertips on the moon, the asteroid belts and eventually the gas giants.
What we need to unlock this are the means to economically launch a minimum viable self replicating infrastructure into space to take advantage of this.
The feedback loops that will ensue should we succeed will allow us to save Earth ecology, radically transform the human condition, and unlock the ability to explore the universe in ways that we can only imagine.
Do you work in something related to Astro?
To see earth, the lensing would been to be focused on where Earth was 2x ago. Still possible in theory, and you might even argue just as likely as a fully reflecting curve. But you'd not call it "back towards us". It would need to be "curved to where earth was".
The idea being that a spacecraft traveling at 99% of light speed can't ordinarily catch up with light reflected by Earth. But if the light curves, and the spacecraft can travel directly towards where the light will end up (spacecraft traveling "as the crow flies"), it might be possible to catch up.
Same way I might be able to catch up with Usain Bolt at a track event if he's forced to run on the track, and I'm allowed to run across the turf in the middle.
From "Hear the sounds of Earth's magnetic field from 41,000 years ago" (2024) https://news.ycombinator.com/item?id=42010159 :
> [ Redshift, Doppler effect, ]
> to recall Earth's magnetic field from 41,000 years ago with such a method would presumably require a reflection (41,000/2 = 20,500) light years away
To see Earth in a reflection, though
Age of the Earth: https://en.wikipedia.org/wiki/Age_of_Earth :
> 4.54 × 10^9 years ± 1%
"J1721+8842: The first Einstein zig-zag lens" (2024) https://arxiv.org/abs/2411.04177v1
What is the distance to the centroid of the (possibly vortical ?) lens effect from Earth in light years?
/? J1721+8842 distance from Earth in light years
- https://www.iflscience.com/first-known-double-gravitational-... :
> The first lens is relatively close to the source, with a distance estimated at 10.2 billion light-years. What happens is that the quasar’s light is magnified and multiplied by this massive galaxy. Two of the images are deflected in the opposite direction as they reach the second lens, another massive galaxy. The path of the light is a zig-zag between the quasar, the first lens, and then the second one, which is just 2.3 billion light-years away
So, given a simplistic model with no relative motion between earth and the presumed constant location lens:
Earth formation: 4.54b years ago
2.3b * 2 = 4.6b years ago
10.2b * 2 = 20.4b years ago
"The helical model - our solar system is a vortex" https://youtube.com/watch?v=0jHsq36_NTU
(IANAastronomer, but I have opinions on any given topic...)
> It's Mega-Jansky... Why didn't they use Milli-Jansky
"All prefix names are printed in lowercase letters, except at the beginning of a sentence.
Unit names are normally printed in upright type and they are treated like ordinary nouns. In English, the names of units start with a lower-case letter (even when the symbol for the unit begins with a capital letter), except at the beginning of a sentence or in capitalized material such as a title...
When the name of a unit is combined with the name of a multiple or sub-multiple prefix, no space or hyphen is used between the prefix name and the unit name...
Examples: pm (picometre), mmol (millimole), GΩ (gigaohm), THz (terahertz)"
Source: https://www.bipm.org/en/publications/si-brochure
However yes, you raise a good question. I was surprised to learn that astronomy and astrophysics prefer CGS, so by convention the diameter of the Sun is given in centimeters, and the mass of the Sun in grams! For janskys there's no excuse though.
Can we then find more lensing with even more compounding on purpose instead of accidentally if we sift existing data for such dupes?
... so that ...
> ... "acts as a compound lens" ...
Not really -- not the sort of lens we're familiar with, one that concentrates light at a single focus. Technical methods can exploit these chance alignments to detect objects otherwise inaccessible, but not as coherent images.
I often see remarks like this one -- "Acts as a compound lens!" -- but that's not correct. It's more like this: https://arachnoid.com/relativity/graphics/curvature_diagram....
Such alignments are more likely to produce what's called an "Einstein ring" (https://en.wikipedia.org/wiki/Einstein_ring). Very useful, but not remotely a "compound lens".
See Figure 7 in (https://arachnoid.com/relativity/index.html#General_Relativi...) for an interactive gravitational lens simulator.
Q: We seem to want to use our sun for GL experiments. And that requires sending something out 500 AU out for focal length purposes. My question is somewhat theoretical so assume there are no alignment issues for the following thought. Are we not already at the "focal point/line" of some other 'entities' (sun/heavy body/galaxy/etc) gravitational lens?
Hmm... is that what this paper is talking about - the fact that the GL observation was made from Earth on some infinite focal length. Listening to other YT videos on the topic seemed to imply the resolution would be much higher. Or is the resolution location/technology related?
How would a compound lens lead to a better estimate of the expansion rate of the universe?
Our most powerful telescopes can see "back in time", by looking at stuff far enough away that it took nearly the entire age of the universe for the light to reach us.
I would guess that we can use this natural compound lens to "see farther" with our current telescopes than we might otherwise be able to see.
Our current best telescope, the JWST, can almost see to the very beginning of when it was possible to see, somewhere between 300k and 200M years after the big bang [0].
Somewhere in this time period, the universe cooled enough for normal matter to form.
The JWST still cannot see the actual 'edge' of when this occurred.
Maybe with this natural compound lens, we can see all the way to the edge.
And if we could see where the edge actually is, then maybe we can refine the estimate to a tighter range than [300k,200M], which would give us a better estimate of the expansion rate of the earlier universe.
[0] https://www.universetoday.com/168872/webb-observations-shed-...
> This unique configuration offers the opportunity to combine two major lensing cosmological probes: time-delay cosmography and dual source-plane lensing since J1721+8842 features multiple lensed sources forming two distinct Einstein radii of different sizes, one of which being a variable quasar. We expect tight constraints on the Hubble constant and the equation of state of dark energy by combining these two probes on the same system. The z2=1.885 deflector, a quiescent galaxy, is also the highest-redshift strong galaxy-scale lens with a spectroscopic redshift measurement.
With time-delay cosmography[1] one exploits that unless the source is perfectly in the center of the line of sight, then the photons that make up one lensed copy have traveled a different distance from the source than photons that make up a different lensed copy. This effect can be used to measure absolute distance and give an accurate measure of the Hubble constant.
With dual source-plane lensing[2] one exploits that if two different sources lensed by the same lens, one can take the ratio of the measurements between the two sources and get results that are significantly less affected by the lens itself and is completely independent of the Hubble constant.
