Quantum astronomy could create telescopes hundreds of kilometers wide | Hacker News Reader

Quantum astronomy could create telescopes hundreds of kilometers wide

scientificamerican.com

145 pointspseudolus35 comments

I remember how, as a physics student, I was pretty amazed to find that a CD I had lying around created inference rings from the sun shining on it and that I could quite accurately calculate the spacing of the track on the disk. I considered the sun to be a thermal light source - something that should not result in any interference. My conclusion at the time was, that the I was looking at single-photon interference and that the 'lateral coherence length' of the photons coming from the sun must be large. I didn't follow up on this, I'm ashamed to say. But it must me the prerequisite for the attempt to increase the effective aperture of the telescopes.

Another thought that came to me - we already know of a technique to store phase information from coherent sources for practically indefinitely, which is holography. Can anyone tell whether that would be an option for the use case?

edit: I should have spent a little more thought on this comment, but I mostly wanted to get it out of my head and maybe have others pick up on it. The issue with holography is, that the image is created by interference of a reference beam and the reflections from an object. Here, we don't want to image an object, but the light source itself. Maybe we can learn something about the light source when we have multiple holograms created with reference objects...

IIRC, thermal light sources have a coherence length of about 10µm. Otherwise reflection holograms wouldn't work when viewed in a thermal light source.

Also: https://en.wikipedia.org/wiki/Thin-film_interference

I shudder to think of the difficulty in maintaining phase coherence at optical wavelengths for the baselines they mention in this article.

They want to store the state of received photons on „quantum harddrives“, transport them to the same place, read the states out and let them interfere locally.

This is the mentioned paper from december 2020 about hour-long optical storage (by Chaun-Feng Li et al.):

https://arxiv.org/abs/2012.14605

(Earlier article from 2015 about this storage technique: https://newatlas.com/quantum-encryption-storage-drive/35553/ )

The state (phase) of the received photon will depend on the position of the receiver. Even if you have a perfect quantum hard drive, you still need to maintain the position of your receivers to within a wavelength. That will be hard to do if they are separated by a long distance.

There are already some crazy light sources with line width of less than a single Hz (not a typo) https://www.news.ucsb.edu/2019/019332/quiet-light

Still other work to be done, and worrying about what happens in the fibre optics, but it is becoming possible, especially if the coherence does not need to be long lived.

The perseverance of scientific obsession must not be underestimated.

It is also amazing how the right amount of curiosity, ingenuity and spirit can create opportunity for insight.

The optimism of scientific publications likewise

I sincerely hope that thanks to SpaceX the cost of launching telescopes to space will become small enough to allow multiple space-based telescopes (and radio telescopes). We are obviously some ways from that, but not that far away that we cannot start planning for when it happens.

Launch costs are only a fraction of the total cost of orbital telescopes, so don't get your hopes up.

For reference, the WFIRST telescope came with satellite bus and 2.4m mirror for free yet already has a 3.2 billion dollar price tag attached to it. Even the most expensive launch provider would be less than 10% of that cost.

It only weighs about 4 metric tons by the way, so there's that.

Just a quick reality check on the unrealistic assumptions some people have.

I wonder, if you know you were launching one every six months for the next 20 years, what would the unit cost be?

You might be able to use a commercial satellite bus for example. It would be fine if some would experience failures, as there would be the next one already lined up.

Just put different instrument(s) at the end of the optical path in each. As example, Hubble has five instruments. https://hubblesite.org/mission-and-telescope/instruments

One could start a design from something like CHEOPS with a 30 cm mirror. https://sci.esa.int/web/cheops/-/54032-spacecraft

Do we need that many telescopes though? It seems like one only needs many 5 at any given time.

There are currently 35 space-based telescopes in active service with 6 planned to launch in the next 12 months.

[0] https://en.wikipedia.org/wiki/List_of_space_telescopes

One can already see that the existing terrestrial and space telescopes have long queues for observing time.

Maybe more experimental instruments would be nice as well.

But how much of that telescope cost is because of high launch costs. Ie using the lightest materials, overbuilding because maintenance is impossible, etc.

Hard to tell in detail, but not as much.

The primary cost factors are instrument development, ground segments, staff (over years, e.g. 5-10 years development plus ~5 years primary mission plus extensions), testing, and building (no economy of scale here).

The most counter-intuitive observations is that the cheaper your launch-costs, the higher the portion they represent w.r.t. total mission cost. A "cheap" (e.g. sub-50 million dollar) telescope for example would have launch costs come in at up to 50% of the total cost when launched on a medium-sized launcher or with a second payload.

It's a multivariable optimisation problem and the best solution depends on the variables that you want to optimise for.

Yeah that would be cool, the Hubble telescope has been massively successful but I would have thought there would be much more projects like that. The James Webb one will be next, but I feel like that one's really risky; they took years to develop and build it, and it's going out to a point of space where it's infeasible to send someone up there to fix it if there is a problem.

A ring of radio telescopes in high orbit would be neat.

I like the ideas of "solar gravitational lensing telescopes" and "lunar crater radio telescopes". The former would place a telescope extremely far from the sun and use its gravitational lensing effect, the latter would turn a crater on the moon into a giant radio dish. Here is a link for the latter.

https://www.nasa.gov/directorates/spacetech/niac/2020_Phase_...

The Moon could be good place for optical interference - on Earth you need to use fiber (or QHD's) to carry photons from one place to the other, but on the Moon you have a reasonable vacuum so you may be able to get a large base with only mirrors or line-of-sight communications.

Indeed, lower satellite costs open up lots of possibilities. Another project is LISA [0], a gravitational wave observatory made of 3 heliocentric satellites. Pretty wild idea.

[0]: https://en.m.wikipedia.org/wiki/Laser_Interferometer_Space_A...

Yeah would be nice after they had ruined ground based telescopes for the whole planet.

>I sincerely hope that thanks to SpaceX the cost of launching telescopes to space will become small enough to allow multiple space-based telescopes (and radio telescopes).

I think people have unreasonable expectations as to what SpaceX can do. I'm sure there are efficiencies to squeeze out of the business, but it's largely from the ops, and infrastructure around the rocket, not the rocket design itself ... design that has not changed significantly in 50 years.

Rockets themselves are as simple as you can get. It's fuel and exhaust and cargo . That's it. Reusability is fraught with issues and no guarantees it's cost effective (maintenance is expensive, reliability suffers, and decreased cargo/revenue due to needing to carry extra fuel for return). But even if reusability pans out, you're not going to get one (or more) orders of magnitude reduction in launch costs from where we are today. You'll get a few percentage points, maybe a few tens of percentage points and that's it.

Oh, we are to get an order of magnitude of cost reduction per kg on an orbit. There are attempts to measure it: https://everydayastronaut.com/raptor-engine/

They need to prove it. Talk is cheap and corporate PR is not very reliable source of information.

SpaceX has been known to talk bullshit ... or more charitably, they have been known to mix aspirational language into their projections. Case in point, I saw a TED talk with the SpaceX CEO claiming that rocket-based passenger travel is just around the corner and will be faster and more efficient than airplane travel... a claim that on face value is just stupid, bordering on insane.

Investing $100M of your own money into a rocket company and then 18 years later taking NASA astronauts to the ISS is equally crazy.

Most people will remember the achievements and not the bold claims.

And that is why spacex is doing so well. Europe, Russia and old US space contractors said same thing about falcon - and here we are

Coming "Soon"

Using the same rocket ten times instead of once is an order of magnitude improvement. Why shouldn't it be possible to do even better than that?

>Using the same rocket ten times instead of once is an order of magnitude improvement.

Reusability is not a panacea. Space Shuttle was reusable but it was not cost-effective and there were no cost savings. Also, reliability suffered and maintenance became prohibitive. In fact, I think the consensus is that the entire program was a mistake.

>Why shouldn't it be possible to do even better than that?

Because reusability has to contend with factors like:

1) Turn around time. How quickly can you refurbish a rocket.

2) Maintenance costs. How expensive is it to recover and refurbish the rocket.

3) How many times can you reuse a rocket.

4) Failure rates. Does reusing a rocket compromise quality.

5) Decreased revenue per launch. Reusable rockets need to carry spare fuel meaning, that your cargo space is cut (and cargo is revenue), and your costs are higher, because you have to still 'pay' to ship that spare fuel along with cargo.

Factors like those may make it that even if there are cost-savings, that those cost savings are not orders of magnitude lower, but maybe something on the order of 10%-50% - which is nice, and you can disrupt the market and make a good business out of it, but it's not anywhere close to the claims that are made. And again, SpaceX has not shown in practice that they can get ANY savings from reusability.

I wonder what would it take to send a pair of optical telescopes on lunar landers (unmanned?) and attempt to do it using vacuum instead of optical fiber as a medium to join them.

A lot of the things that make it complicated on Earth just aren't there on the Moon. Plus, we have a great excuse to keep a couple astronomers and engineers around to service the telescopes.

A telescope kilometers wide could image extrasolar planets.

Wouldn’t it be fun if our discovery of ETI were an image of its city lights.

Exoplanet imaging?

I love being surprised by new uses of exotic materials. Never thought I would read about quantum hard drives outside of science fiction.

What a great day is the day I received my first science book, learning about the great scientist named: Nikola Tesla, was very important to me, not just because he's very great in his field, but after this day I win a hero in my life. Some people like Super-man but my hero have electricity powers to show for everybody around the world, without the works of Nikola Tesla, maybe we don't have this great opportunity in Digital age to help each other in a remote way.

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