FullOfStarships

FullOfStarships t1_jcoc9ps wrote

Reply to comment by Pegajace in Where do photons go if they've been emitted but are destined to never be absorbed, and would these photons traveling ad infinitum define the edge of the universe (even if space itself were still larger)? by mysteryofthefieryeye

This answer is simultaneously completely correct, and completely wrong.

TL;DR the question you are looking for is "edge of the Visible Universe", and the answer is "yes, that is the dictionary definition of Visible Universe".

The Cosmic Microwave Background was created 370,000 years after the big bang. The photons that reach us have travelled for the lifetime of the universe, minus 370,000 years. These have a redshift of 1,100.

The Cosmic Neutrino Background was created about 10 minutes after the big bang. The neutrinos that reach us have travelled for the lifetime of the universe, minus 10 minutes. These have a redshift of 10^10.

You can only go another ten minutes further back in distance / history. That's it. No more.

More to the point, if you could get back to the exact "zero" point, the radiation would be infinitely redshifted.

In fact, that point is recognised as an Event Horizon. Apparently it actually emits Unruh radiation. It perfectly describes the edge of the "Visible Universe". This is "our universe".

There are good theoretical reasons to believe that the big bang created space billions, trillions, quadrillions, etc... times bigger than the Visible Universe, which has the same physical laws as us. But, for all we know, the universe one micron "further away" than the event horizon could be dragons packed nose to tail. We would have no way to know.

Ironically, if there is an intelligence which exists 99% of the distance to the edge of our visible universe, they would see a sphere the same size as our visible universe, but centred on them.

The visible universe is centred on the observer. To an utterly irrelevant extent, people on the opposite sides of the Earth perceive slightly different edges to their visible universes.

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FullOfStarships t1_jco9rvz wrote

Reply to comment by OffusMax in Where do photons go if they've been emitted but are destined to never be absorbed, and would these photons traveling ad infinitum define the edge of the universe (even if space itself were still larger)? by mysteryofthefieryeye

This is wrong (except for any difference proper motion that exists at the time the photon is emitted).

Cosmic redshift is an expansion of space.

Your analogy requires that the train is stretched by the expansion.

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FullOfStarships t1_jco9jx0 wrote

Reply to comment by triffid_hunter in Where do photons go if they've been emitted but are destined to never be absorbed, and would these photons traveling ad infinitum define the edge of the universe (even if space itself were still larger)? by mysteryofthefieryeye

The big bang was what scientists call "very hot". So hot that the atoms were in a plasma (the "fourth state of matter") for the first 370,000 years. Towards the end of that time, the whole universe had cooled to the same temperature as the surface of the sun. That's your mental picture - surface of the sun but everywhere. White hot and glowy.

As it cooled a bit more the plasma "condensed" into gas as we're familiar with (the "third state of matter"). This is much like water vapour (third state) condensing to become water (second state).

Gas is transparent (you can see through the atmosphere) instead of glowy, so the photons that had been trapped (outrageous simplification) in the plasma were released. Fly, my pretties.

That's from about 370,000 years after the big bang. Expansion of the universe has redshifted (cooled) those photons by a factor of 1,100 - from ~5,000K (visible light) to 2.7K (microwaves).

But, frankly, that's peanuts.

Between the 2nd and 20th minutes of the universe, hydrogen was fused to helium. This phase of the evolution of the universe is under appreciated.

Start with ~10^80 protons.

Over a period of about 20 mins, ~10^79 helium atoms were formed by fusion. Strewth.

Don't forget that those fusions produce neutrinos, and they don't have a transparency problem. Can't cage those beasts.

The "Cosmic Neutrino Background" has been redshifted by 10,000,000,000 times since then.

Neutrinos have an absolutely tiny mass. So small that the neutrinos that came from SN1987A arrived at the same time as the photons after racing each other for 100,000 years.

The CNB may be the only neutrinos in the universe which have slowed down so much that they are no longer relativistic. There is no currently conceivable way to detect them, but we know that they are still there.

So, there's your answer - photons could be redshifted by 10^10 (ten million times more than the CMB) and they'd still "exist" as a moving probability wave. If the wave happens to interact with matter, then there will be a collapse of the waveform, and an incredibly low energy photon would be detected.

Much more boring answer...

Photons emitted near a Black Hole's event horizon are redshifted as they ascend. In theory, they could be redshifted by any amount, only depending on how close they were to the event horizon when they started out.

The issue is not whether they still exist, but whether there is any practical way to detect them.

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FullOfStarships t1_jcabdpw wrote

Reply to NASA wants new 'deorbit tug' to bring space station down in 2030 by DevilsRefugee

NASA needs to build on their own previous research on electromagnetic EDL - "Magnetoshell Aero Capture":

https://www.nasa.gov/directorates/spacetech/niac/2012_Phase_I_magnetoshell/

Click through to the PDF for detailed info on their modelling, EG aerocapture of a 60t spaceship at Mars replaced a 20t heatshield with a 1t magnetic system. Also 20km/s aerocapture at Neptune.

"This means that for any given breaking drag forces on the Magnetoshell will be three orders of magnitude larger than the aerodynamic forces on the spacecraft. With the ability to rapidly and precisely modify the drag in varying atmospheric conditions, much larger braking forces can now be contemplated at low risk, enabling very aggressive aerocapture maneuvers."

The thing that makes this suitable is that when you turn the system on it basically kicks in the atmospheric reentry much higher than normal. You then adjust the magnetic field strength to accurately target the deceleration that you have pre-planned.

Some concepts that I've seen literally put the system on a tether behind the spacecraft, and it acts like the parachute behind a drag racer or a plane landing on a short runway. Nice feature of this is that it's a dynamically stable system - it just brakes in a straight line.

Just what this needs.

I'm honestly disappointed that SpaceX aren't testing this. Not because it's their patriotic duty to do everything interesting in spaceflight, but because they're launching 100 F9US this year alone, and every Starlink mission should be testing this during U/S disposal post mission (after all payloads have deployed).

Exactly like they tested reentry of the first stage long after MECO / stage separation / payload in its way to orbit. Zero risk to mission success.

First target should be to slow down FH core stages so they can survive F9-style reentry and recovery.

Also, those simulations were based on "room temperature" copper coils to reduce the technical complexity of the system during testing phase.

There are now very low mass / high performance thin film superconducting tapes which would be perfect for this job, I think. There will be LOX residuals to provide the cryocooling.

I'd love to see a world where every Starship uses this to reduce the performance requirements on the heatshield. Starship docks behind ISS, switches on the MAC, targets the reentry, then un-docks, reboosts itself, and lands safely.

(Before you say "why Starship"? ISS is about 4x the dry mass of Starship. If MAC is in regular use in Starship EDL, then it will work on a combined Starship / ISS system by just running the MAC for 5x longer than normal.

If any other provider is going to be in a position where they have 100t+ spaceships in regular use, ground control teams with years of experience with ISS rendezvous, MAC built into the system [I can dream], etc, etc, then perfect. But I'm not seeing it.)

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FullOfStarships t1_jca45ks wrote

Reply to comment by Pharisaeus in NASA wants new 'deorbit tug' to bring space station down in 2030 by DevilsRefugee

Agreed.

You could build an ion engine to do this with a tiny amount of propellant, but that would take years which completely misses the point.

Sort of like taking foot off the gas and letting a car drift to a stop.

Instead, ISS needs something like brakes. Press the pedal and the whole thing slows down. Just like modulating the brake pedal so that you stop at the lights. In this case, you apply just the right amount of brakes at just the right time to splashdown in the Southern Pacific (Point Nemo, as it's known).

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