> Since g forces are based on gravity

They're not, they follow f=mA.

A vessel with sufficient thrust could crush astronauts into a thin paste even if it's already in orbit.

To be fair, we don't fully understand how gravity functions at galactic scales

> they are microwave frequencies, right?

Yep - I'm confused about /u/bullett2434 saying microwave frequencies are higher than radio frequencies, when the dramatic majority of our radios are using microwaves these days…

> Except microwaves are way higher frequency than radio waves which we detect all the time.

Uhh microwave ovens, WiFi, and bluetooth all use the same radio band around 2.4GHz, and the definition of "microwave" is frequencies between 300MHz and 300GHz…

> what is the lowest frequency longest wavelength photon that is observable by state of the art equipment?

We're still receiving photons from the first moment that the universe was transparent, they're called the CMB

> Where do photons go if they've been emitted but are destined to never be absorbed

They just keep going forever.

We're still receiving photons from the first moment that the universe was transparent, they're called the CMB

> would these photons traveling ad infinitum define the edge of the universe

There's no edge.

> Can a Ion Thruster ever Match the Thrust of a Chemical Rocket?

In theory, sure, but in practice the VASIMR project is trying to approach that threshold and already has numerous drawbacks.

Haven't heard much out of them lately though, and the power requirements make it pretty impractical.

And that power requirement is always going to be an issue with electric thrusters: objects in LEO have about 32MJ of energy per kilogram vs an object sitting on the earth, and launch to LEO takes about 8 minutes, so you're looking at 67MW per ton (first approximation crude napkin math) with even a 100% efficient ion thruster just to match already existing rockets - and I don't think we have any source of electrical power that gets anywhere near that requirement.

As a cross-check, this post says Falcon 9 is pushing 26GW at launch, and with its ~545 metric ton launch mass, that's 47MW/ton so apparently my napkin is a bit off, but well within an order of magnitude.

Seems like we're gonna be sticking with chemical fuels for a while, and using ions for more sedate in-orbit maneuvers where a small thrust for a long time is just as useful as a lot of thrust for a short time.

This PBS SpaceTime video may interest you

Not enough air, and it's full of nitrogen.

LOX is 1141g/L at its condensation point (~54K) and I think rockets make it even colder to increase the density a bit higher, while atmospheric air is only ~1.3g/L.

Compressing it at ~1000:1 would take some pretty epic equipment, and then it'd be way too hot.

Even if you somehow manage to sort that out while still having a launch TWR > 1, air is still only ~19% oxygen or so, meaning the fuel wouldn't be able to burn effectively.

Furthermore, the atmosphere gets even thinner within a few dozen seconds of lift-off, so you'd still need to carry oxidizer anyway - and the little bit extra it takes to get up to that point is far lighter and simpler than having the rocket itself run on atmospheric air for half a minute.

It's much more sensible to process it as much as possible on the ground, and load LOX into the rocket.

Having said that, companies that strap their small rockets to aeroplanes are technically already using atmospheric air for that phase of the flight - but the type of engine required is radically different, and those rockets have to be relatively tiny because aeroplanes can only carry so much mass.

> I like that you are taking it realistic though.

This is a sub for hard science after all, there's others better suited for creative writing ;)

> what would you think it would sound like if we could hear everything without consequences and death and not universal atmosphere, just like... our ears would be able to hear loud noises just fine and no destruction from sound... just imaginative... just a what if.

Ever been near a rocky seashore on a windy night?

Like that but louder than a jet engine 24/7 (mostly from the sun) with occasional gut-thumping cracks thrown in (from large celestial events) and maybe a couple discernible tones from millisecond pulsars depending on how loud of a sound your medium could carry before hitting vacuum at one extreme.

It would be pretty unpleasant

Sound works by particles in close proximity repelling each other and thus passing pressure ripples along - so we either have sound everywhere and no chance of orbits or life, or vast swathes of emptiness with occasional islands where life could occur.

Fwiw it's thought that some of the variations in the CMB were from 'phonons' ie packets of sound energy in the plasma - see here

Also note that we have problems with the sound of rocket exhaust shredding concrete at launch pads (which is the main reason why we spray water under rockets when they lift off, it breaks up the shockwaves) - so astronomical events would trivially destroy everything over and over again if their sound could reach us.

The sun alone would hit something like 140dB from here, so we'd all be deaf - and that's assuming we don't get roasted and shredded simultaneously by Earth sweeping through this atmosphere of yours at 30km/s before losing enough speed to fall into the sun.

The universe isn't here for us, we're just a byproduct of it searching for better ways to increase entropy - which life is quite excellent at, since the whole concept of life is that it actively seeks out available energy gradients to ride, and cracks stored energy out of local minima.

That there's unimaginable mountains of available energy out there for us to do something interesting with is nice for us and nice for the universe - but it really doesn't care if we're the ones to go get it, or if it eventually burns some other way.

I want to have a nice time, so I definitely think we should go out there and grab it - but I also think we should do the most interesting things possible with it rather than squandering it, and if ever we find anyone else out there helping the universe with its goal of increasing entropy and being interesting, hopefully we can increase how complex society is in association with them rather than do something stupid and wasteful.

And fwiw, damaging our ability to increase entropy by riding the energy gradients on this planet too hard before we get sustainable space colonies going is definitely under the banner of 'stupid and wasteful' in my book - but at the same time we shouldn't be so careful that those space colonies and further leveraging of energy gradients available elsewhere never happen.

It would be wonderful if we can restore the environment here to a more pristine state for our species' psychological health, but the universe doesn't care about that - that's all on us.

Keep in mind that Mars and Venus would both be counted as "earth-like exoplanets" if we spotted them from similar distances.

We haven't seen any chemical signatures that are unique to life in any exoplanet atmospheres so far afaik - but that's not saying much since numerous biological processes mirror geological ones but just go faster, at least in terms of processes that produce enough gases to show up in atmospheric absorption spectra.

In short, we have no evidence that cellular life (let alone anything bigger) exists on other planets - but also no evidence that it doesn't, due to the limited abilities of our best instruments.

We could barely see evidence of our own existence with current tech from more than a few dozen light years away - and the most obvious evidence for sapient life here that's distantly visible is the radical change in atmospheric composition over the past hundred years or so, which is only visible (with our tech) from locations that lie exactly on our equatorial plane such that Earth passes between those locations and our sun every year.

In terms of radio transmissions, we're struggling to communicate with the Voyager craft and they've only just passed the heliopause…

We'd never detect alien tv/radio transmissions even from the nearest stars, and that's assuming that we're even listening at the right time to catch the narrow high-power transmission window - terrestrial communications have invested hard into more efficient methods as well as widespread encryption, meaning that 1) there's dramatically less RF blasted into space compared to 50 years ago, and 2) what is broadcast is essentially meaningless noise.

If you can't reach controls while you're strapped into a seat, you could only ever be a passenger - and that industry is still nascent

It is slowing down due to tidal forces, and transferring momentum to the Moon which is making the Moon's orbit larger…

> But matter can't be created nor destroyed right?

Sure it can, nuclear reactors turn mass into energy - or is it even matter if the released energy is actually the binding energy of a nucleus which appears as mass?

Energy can't be created or destroyed though (except by the big bang apparently), only transformed from one form to another.

> How did small atoms expand themselves to become size of galaxies.

They didn't start off as atoms, the Universe was far too hot for atoms to form for about 380,000 years - and the CMB is the remains of the light that was flying around at the moment when things were cool enough that atoms could form and the universe became transparent.

If you're wondering why it didn't all collapse into black holes at that density, we think that might be where the black holes at the center of galaxies came from…

Also, the expansion doesn't have a border or edge, it's more useful to imagine new empty space being injected everywhere all at once, like infinite raisin bread rising.

> I thought big bang was like a supernova where a massive amount of matter exploded

Nope, it's a time-like surface from which energy and spacetime poured forth, and could reasonably be described as a white hole - you can draw a ray in literally any direction you like, and it'll eventually intersect the big bang at the moment of our universe's creation.

> which external force was applied on universe to stretch it?

We have no compelling evidence for anything outside our 3+1 perceivable dimensions, and what we can see is same-ish in every direction for as far as we can see, so why not an internal force?

It's called dark energy fwiw, and it's an ongoing field of study since we know almost nothing about it

Space junk is hypersonic (LEO orbital velocity is ~7.5km/s ie ~mach 22) and thus burns up when it hits the atmosphere - and if it's large enough to not all burn up before it sheds most of its speed, it'll plummet to the ground rather than floating around in the air until taken down.

If you're thinking hydrogen fuel tanks, their walls are much thicker than weather balloons and would drop like a stone if one somehow survived reentry (eg protected by some larger structure that burned up).

> Now if an object is lets say 200 light years away, would it take 200 (Earth years) to travel to said object?

It would take 200 years for photons or other massless particles (eg gravity waves) to travel from there to here or vice versa - but it would take a manmade spaceship hundreds to thousands of millenia because we don't have yet any rocket engines efficient enough to get there faster.

Nope, we only do hard science here, not neospiritualist fantasy

The first two are in good spots to look at stuff already, and there's probably not enough political will to send another similar mission with more modern sensor tech - especially when they took 50 years to get where they are now, and currently available rocket engines aren't much better than what was around in the '70s in terms of ability to propel something beyond the heliosphere.

It might make more sense when we've got nuclear rockets happening - some of those have mind-boggling Isp compared to chemical rockets, at least on paper!

Or perhaps if there's an upcoming favourable alignment of the interplanetary transport network or something..

Steam engine boilers have been doing something similar for over a hundred years - boiling occurs when the partial pressure of a liquid exceeds ambient atmospheric pressure, so you can just jack up the pressure to force a hot liquid to not boil, and they kinda do this by themselves by simple thermal expansion in a filled closed volume.

From memory, steam engine boilers commonly operated at something like 300+°C to get sufficient working pressure for efficient operation, and there were numerous boiler explosion disasters before we worked out how to make them strong enough to withstand the insane pressure.

Conversely, if you reduce atmospheric pressure, water will boil at a lower temperature - and in fact there's heaps of videos on youtube where folk chuck a cup of water in a vacuum chamber and make it boil until it freezes from the evaporative temperature loss, and also explains why it's really difficult to make a decent cup of tea or coffee at high altitudes.

Because diffraction, and most exoplanets have been detected by star transits where even if the star only shines on a single pixel in the detector, we can still look for periodic brightness changes.

The total quantity of energy is fixed, space is expanding, and entropy always increases on average, so the heat death of the universe is inevitable according to all accepted theories of physics.

There may be some fun hypotheses around with new universes spawning, multiverses, and other oddities, but they're just that - hypotheses, not accepted theories.