Hacker News Reader

Comment by freeslave | Hacker News Reader

If the star is 120 million light-years away, does that mean this explosion actually happened 120 million years ago?

Always blows my mind that because light travels through space, you can literally see into the past. Imagine, way way down the line, eventually meeting a civilization from a far away planet, and they could potentially show you actual photographs of Pangaea.

How big would the telescope need to be to be able to produce high-res images of Earth from a distance of 200M+ light years away?

I realize that a civilization could be 50M light years away and witness Pangea, but the idea of a civilization 50M light years away observing Pangea and then persisting data for 150M years presents it’s own challenges.

Check out my reply to a sibling comment. TLDR: huge

More than Pangea… if they mastered the ability to resolve vast distances they could show us the herds of dinosaurs roaming the world, and then their total extinction, a Timelapse of the rise of human civilization, ancient Egypt and the Roman Empire, if only we could truly meet a civilization way down the line, or perhaps they would come to meet us, given their obsession.

It's cool to think that first contact between two space-faring civilizations could potentially involve a mutual exchange of detailed historical imagery that neither civilization could have captured on their own.

Indeed, though it’s probably likely in the history of the universe this has never happened, as what are the odds of two distant civilizations separated by many light years meeting and both offering historial imagery to each other? The chance to capture relevant images will have passed if they do not specifically plan for it.

Probably unlikely, yeah. It would have to be the result of several lucky and serendipitous events, I'm sure. The idea of seeing into the past seems so other-worldly though, it's hard not to dream :-)

Perhaps if there is a large reflector in space somewhere, that light could be passing back by us today! If we had a large enough collector, we could create an image.

In theory black holes could do that. Some photons will slingshot around it and go straight back.

We haven't seen a photon ring yet, and when we do, there won't be that much information in them. Perhaps after technology has advanced another 1000X...

Unfortunately, that is not possible. Earth isn't a star and not big enough to provide this much data even outside of our Solar System, let alone millions of light years away.

To add some back of the napkin calculations to this:

Angular resolution of the earth at 66 million light years away would be approximately 2e-17 radians. Using the Raleigh Criterion [0] for lens size for the visible light spectrum (700nm for the best case scenario), you would need a lens with a diameter of about 4e10 meters. That's about the radius of Mercury's orbit around the sun.

If you want to see dinosaurs, say 1m resolution, that's about 1.5e-24 radians at 66M light years, needing a lens of diameter 5E17. The entire solar system has a diameter of ~3e14. So even if your lens was the size of the solar system you'd still be off by a factor of 1000 trying to resolve the dinosaurs.

At these scales you start running into some pretty fundamental engineering and physics problems with building a telescope this big.

Warning: this math may not be totally right, I'm just procrastinating some PDE homework right now, but the scales should be roughly correct.

[0] https://en.wikipedia.org/wiki/Angular_resolution#The_Rayleig...

> you would need a lens with a diameter of about 4e10 meters.

Probably a mirror rather than a lens. And actually all you need is two mirrors separated by 4e10 meters, not a single mirror of that diameter:

https://en.wikipedia.org/wiki/Astronomical_interferometer

The problem would be the amount of signal you can collect. Might be interesting to do a calculation of the number of photons that would be detectable at that distance per unit solid angle to figure out roughly how big the mirrors would have to be to capture an image in a reasonable amount of time.

> So even if your lens was the size of the solar system you'd still be off by a factor of 1000 trying to resolve the dinosaurs.

What's the effective size of a gravity lens? You know, the kind where you park a satellite out at 550 AU and image whatever's directly on the opposite side of Sol (or an equivalent distance in proportion to some remote star's gravity).

The idea of using sun's gravitational lensing effect as a telescope has been proposed seriously (https://en.wikipedia.org/wiki/Solar_gravitational_lens). this has much smaller resolution then what you mention but perhaps can be used to witness other planets in ours and nearby galaxies in our cluster.

I’m imagining an advanced civilization with a telescope capable of doing observing dinosaurs, but only ones being thinks that it an interesting use of the telescope. So they work super hard and get exactly one Earth day’s worth of observing time to look at dinosaurs w/ no possible rescheduling. When the day finally comes… clouds.

This is admittedly a bit over my head, but does that mean it is technically possible that an image of earth could be captured from the distance needed to see Pangaea?

Thinking about this has sparked a bunch of questions I hadn't thought of before. For instance, does information encoded with light degrade over long distances in a vacuum? If not, it does seem like a mega-lens could potentially capture such an image right?

"For instance, does information encoded with light degrade over long distances in a vacuum?"

Since the vacuum is not totally empty, it does as far as I know.

Just like humid air fogs the picture.

Good point, I was being idealistic and imagining space as a perfect vacuum.

Under this perfect assumption you end up having a problem with the red shift caused by the expansion of the universe (which I didn't account for above) that lengthens the wavelength of the light you want to see, requiring an even larger lens.

On the lens side of things, as far as I'm aware (not a physicist, math PhD student who is just generally into this sort of thing) there isn't anything fundamental preventing you from collecting this light and building the lens, but from our current understanding of materials science I'm fairly confident it's currently impossible to construct a structure that will stay together that large.

That being said there may be ways around this problem, like I said not a physicist or engineer, but you are right that from an information theoretical perspective if you ignore dust and other things in the way then yes all the information is still there.

> Under this perfect assumption you end up having a problem with the red shift caused by the expansion of the universe (which I didn't account for above) that lengthens the wavelength of the light you want to see, requiring an even larger lens.

Redshift does not come into play at the 'small' scale of our galaxy since the Milky Way is a gravitationally bound system that does not itself expand with the Universe. Even within our local group (eg from Andromeda) it is not an issue since the local group is also gravitationally bound. Redshift only becomes an issue at much larger scales, if you are in another galaxy outside our local group.

Yup good point. Thanks for the clarification

Why not? If it can be seen from just a few miles outside the atmosphere, and the light continues to travel without obstruction through the vacuum of space, couldn't it be seen from millions of light years away as well?

Ask yourself: Are WE able to see the continental structure of an exoplanet millions of light years away with our current telescopes/imaging software?

Sorry, I should have clarified, that’s not at all what I meant. I’m thinking about if it’s bound by current theory, not current tech. Obviously we can’t do this today, or probably in my lifetime.

It depends on the observers reference frame.

https://en.m.wikipedia.org/wiki/Relativity_of_simultaneity

Unless otherwise stated the reference frame is generally taken to be the one where the observer is stationary

From a philosophical point of view, an event happens at the point when observed by an observer. For us, Humans on earth, this supernova happens now and not 120 million years ago.

I don’t think that’s right.

If a tree falls in a forest and we stumbled upon it today, I don’t think philosophy says that the tree fell today. I think it says we found a fallen tree today.

The supernova didn’t happen today, we found evidence of it today.

> I don’t think philosophy says

I'm a professional philosopher, and I and 100% of my colleagues agree with you.

Minkowski spacetime confuses matters, though. From the point of view of the photon we observe, no time elapses between its emission at the location being observed and its arrival at the telescope image sensor. It's as if the photon were born in exactly the right place and time to be observed by us, at that very instant.

So what we see is arguably happening in real time, regardless of distance.

Ehh. The measurement is happening in real time but we have well-defined ways to say when the event happened, and that's 120MY ago (given the reference frame where Earth is motionless).

Depends on what you mean by "real time". If by "real time" you mean "the spacetime interval between the event 'supernova explodes' and 'explosion observed on earth' is lightlike (or null)", then we witnessed it in real time. I can't think of any other definition that is observer independent.

I also argue that it is not well-defined, as the time span depends on the observer, as you say. Did it happen 120MY ago or 1MY ago? Both can be true for different observers, and none is privileged over the other.

Did it happen 120MY ago or 1MY ago? Both can be true for different observers, and none is privileged over the other.

Great way to put it.

I think even philosophy can withstand knowing the speed of light and incorporating it into the framework of 'when things happened' so that it agrees with our understanding of the universe instead of not.

Say you were immortal and witnessed a supernova from a million light-years away. Eventually, after another million years have passed, you meet another immortal who happened to be right next to the supernova when it happened. When talking to this other immortal, would you refer to the event as happening two million years ago (when you witnessed it), or three million years ago (when the other immortal witnessed it)?

There are many philosophical points if view thought.

Indeed, and some of them are inertial reference frames. ;-)

Yes

, assuming that the speed of light is the same in both directions, which we can’t prove.

https://en.wikipedia.org/wiki/One-way_speed_of_light

https://youtu.be/pTn6Ewhb27k

Would the fact that space between here and there is stretching during that time impact that?

The light has traveled for 120 million years, but the galaxy today is further away because the universe has been expanding the entire time. So the distance between the position of the object 120 million years ago, and Earth today, is 120 million light years. I think :)

It's not just objects getting more and more distant with time, it's spacetime itself stretching at every point.

So, during it's 120M years journey, the photon had to travel the initial distance + new distance added by expansion rate.

So when the photons we are receiving now left that star, that star was closer than 120M light years away. And that star is now further than 120M light years.

Reply on news.ycombinator.com