The further distant galaxies are to us, the faster they are moving away. So how far would they need to be to appear to move at light speed (even if we can't see them)?

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Re: extraterrestrial life, if we presume life could form anywhere, what are some of the material factors that would prevent technological (as we know it) development? E.g., a planet without atmosphere would maybe prohibit ignition, a planet with an extremely low temperature might affect electrical storage, etc. Or, what are some of the technologies that could conceivably be produced in very different environments, and are there technologies that come to mind that would be physically very improbable to be produced in non-Earth-like environments?

What's the best evidence against flat earth?

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We are living on the cool surface of a hot planet. Is there any practical reasons why geothermal isn't the answer to most of our energy needs? What I'm asking is there a reason that geothermal has gotten so little funding over things like fusion.

All the energy we could want is only 10-20km away.

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Given the water density and weight, what would happen if a person skydived into clouds?

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Cost.

Longer answer to an almost identical question a few weeks ago.

That depends on what you call "appearance".

If you take the distance today and consider how fast it increases then it is the speed of light divided by the Hubble constant, around 14 billion light years. We can see light these galaxies emitted in the past, it's only ~1/3 the distance to the edge of the observable universe, the current location of the matter that emitted the oldest light we see today.

We can see things where the distance between us and them always increased faster than the speed of light because of the expansion history of the universe: Initially the distance between us and the emitted light increased but as the universe got older and the expansion rate decreased the light started catching up.

Okay but that doesn't really answer my question. How far away would they have to be to be moving at, from our point of view, the speed of light?

I see your line about the Hubble constant etc but it seems to be just a commentary about that distance, not how far away a galaxy needs to be to be moving at SoL.

And I use the word appears, because it would appear to be moving at the speed of light from our point of view on earth.

How does a wireless charger work? And why don't we see it being used for other appliances?

The axial tilt of the earth is a significant contributor to the production of seasons. The tilt is decreasing, so we should be seeing less extreme seasonal changes. Due to climate change we are seeing more extreme seasonal changes. How do models that predict future impacts of climate change account for the changing axial tilt? It seems like we must have some indications of the severity of seasonal fluctuations from 40,000 years ago to show what would be happening absent our impact. Are those measurements precise enough to be used in models of a few decades or less?

Is a dying star the only known source for iron to be made?

How deep can you find usefull minerals?

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Saw an article a while back about Earth's core possibly reversing directions. What affect would that have on day to day lives of people?

Nothing will ever appear to be moving at the speed of light from our point of view, because if it's moving at C away from us information will never reach us.

The hubble horizon is the distance where something would be moving faster than C due to expansion right now. But that's 14+ billion lightyears away, so even if we could receive information from those galaxies to see them, it wouldn't arrive for many billions of years.

But it will never arrive if it was emitted after the object was already past the hubble horizon.

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So how can this Hubble Horizon exist in a distance smaller than the observable universe? Everything outside that Horizon should not be visible because it's moving faster than light right

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Firstly, don't do this. It's unsafe and illegal in most places to make a skydive when you don't have a clear view of the intended landing spot. It's also very easy to become disoriented! It's particularly dangerous to jump at the same time as other skydivers with limited visibility!

But if you were bailing out of an aircraft in immediate distress and this took you through some clouds... you'd get wet, and probably not enjoy it. You're hitting moisture droplets (and many times... FROZEN moisture droplets) at the 200 kph terminal velocity that you're falling. There will be some sting on any exposed skin.

And finally, remember that clouds aren't an isolated phenomenon... they're indicators of weather activity, and that activity is USUALLY an UPDRAFT... rising currents of air are taking warm moist air from near the ground and lifting it up into colder layers to form condensation. While that's not much of an issue for you during freefall... once you pull the ripcord and have a parachute hanging over your head... an updraft can be a significant issue for you in getting to the ground in the place and time that you'd prefer to, unless you have a lot of experience controlling your descent rate. Experienced skydivers can handle a variety of steady winds, but gusts and turbulence can pose a real danger. In the fore mentioned ejecting from an aircraft in distress, this is an even bigger issue as they may have reduced ability to control the parachute and can be held aloft long enough to cause significant exposure injuries!

If nebulae are composed (at least partially) of gas, wouldn't that gas just expand outwards in a vacuum and disperse?

Is there some gravitational source retaining the gas?

This one might be a bit weird, but can a System with outet Wilds scale work? If so how? Also how can Luna moth live a week without eating?

I do not think this is quite right, though perhaps I misunderstood you. See this StackExchange discussion as well as the paper linked in one of the responses. From the abstract of that article:

> we can observe galaxies that have, and always have had, recession velocities greater than the speed of light.

Yeah, it's definitely not right in the details. Maybe I shouldn't have emphasized "ever" so much. But my goal was more to dispel much bigger misconceptions that went into the question rather than cover all the little details of how these horizons work.

I don't think there's really a good way to answer these complex questions that were formulated based on a lot of complete misunderstandings of what these words even mean. At least not while still sticking to just words. But saying "nothing will ever appear to be moving at C from our point of view" was probably a bad choice. But it would be true if the hubble sphere never changed in size.

The fact that we can see light from galaxies with recession velocities higher than C is one of those facts that is truly awesome to point out, but is pretty much guaranteed to just confuse someone who probably has only learned about cosmology through popular science.

On Titan, we observe rivers/ lakes of liquid methane. Are there any theories as to exactly how it interacts with the solid ice & what unique erosional features may form?

If have a weird idea locked in my head for a few days now and it's kinda speculative.

Let's assume its possible to set up an array of scalable mirrors or prisms relatively close to the sun. The goal is creating a focused beam of sunlight.

1. Which body in our solar system would be the best candidate to point the ray on if you want a long or constant target to aim this beam at?

2. How big would the mirror array / prisms have to be to realize it? (I'm aware it might depend on the distance towards the sun, but maybe theres a "sweet spot" when you have a designated target)

3. Would the beam be visible to the human eyes?

Maybe someone around here can help me clean my head up by answering this.

At what rate does iron sublimate?

As I understand it, the faster the molecules in a substance are moving the higher the temperature. If this is the case, how come wind makes things feel cooler? Wouldn't the molecules be hitting us at a higher rate, it seems that wind should make things feel warmer.

For a gas like air, temperature is related to the spread in molecular velocities, not the average molecular velocity.

Wind feels cool due to convective heat transfer. In still air, your skin loses heat to the air a millimeter away, but that air is now a bit hotter itself, and it can transfer heat back to you. In a breeze, the air next to your skin is constantly replaced with new air at the average temperature of your environment. This is also why clothing keeps you warm-- it creates a barrier between the air next to your body and the air in the environment so they don't mix.

Someone who believes in flat earth is denying dozens of obvious sources of proof. It's difficult to guess what is "best" for arguing with a person in that state of mind.

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Every single point of mass has a gravitational field, including tiny elemental particles such as protons or neutrons. The force of attraction between two masses is governed by Newton's law of gravitation, such that larger masses attract smaller ones and mass begins to gather in small pockets, "it's getting closer together" and then, eventually, "it's a star!"

TLDR: Yes, the gases themselves are gravitational sources.

Why isn't the amount of energy absorbed by a photon striking a target based on the distance traveled? Eg, if I have a 420nm wavelength with a target 420nm from the source. Polarization filters in place.
Why is the same amount of energy absorbed if it is 210nm from the target vs 105 or 215nm?

Many parts to this question:

Q. How do we know that the universe expansion is really the explaination for the observed redshift of light from stars.

1. As i understand it, space is a quantum soup of virtual particles. Shouldn't these virtual particles able to absorb light energy?
2. is it possible the magnetic and electric permeability of free space is changing over time? Wouldn't that change light waves in space?

Q. If one has crossed the event horizon of a black hole, is one traveling faster than light and thus going backward in time wrt the outside universe on the other side of the event horizon?

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Is Gravity just a by product of Electromagnetic Force and its relationship between objects, or is it probably its own thing...we know magnetically polarizing things can "beat*" Gravity. (Floating art using magnets for example.)

• I know it's not beating Gravity it is using force to resist effect of gravity, but to make objects float and not just fly off would require both forces to be comparable would it not?

Everything emits electromagnetic waves, everything is impacted by Gravity. Both Gravity and Electromagnetism decay at 1/R^2.

I do know that GEM (Gravitoelectromagnetics) is a fairly new (relatively speaking) concept of physics? (Maybe mid 1800s?).

Googling just gives me an answer of "they aren't the same thing because we have no evidence they could be (is this true or just a lazy take because we just dont know?)

Curious me has to ask. If both forces protrude "forever" and become their strongest at a certain "break point" and both decay at the same 1/R^2 Rate, and both impact all objects are they effectively the same thing?

The magnetism (haha) between the two seems pretty dang strong.

Is it?

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In real world application Photons lose energy all the time colliding and scattering off objects.

If a Photon can travel a straight line in a vacuum it will travel that line forever without loss of Energy.

However in space for example there is just so much stuff out there (including other photons) to collide into that photons are constantly be redirected rerouted or absorbed by other particles and objects.

This is what leads to depletion of energy, adjustment of wavelengths that is call red shift.

Actually the tilt decreasing would increase the rate of extreme weather events. Every 26000 years or so seasons on the planet flip North to South. Or winter in the US would be in July, and Summer in Australia would be in July.

The most recent was about 18000 years ago now (that allowed people to traverse the Bering Strait and settle North and South America from Asia), or the last ice age we are currently coming out of. In another 8 to 10K years we will likely re-enter a minor ice age. This is due to the sheer volume of landmass in the North vs the South, and as such Snow sticks better...reflects more light back...so it stays colder, for longer, more snow, etc etc etc.

So as the tilt declines air and sea currents become more tropical (focused between the tropics and the equator) and as such more freakish weather can pop up almost anywhere at any time due to the increased amount of water entering the atmosphere due to the tropical heating effects on waters that didn't really experience it before.

As this procession continues weather patterns normalize as the system balances....and then like 13000 years later it starts to repeat as the function repeats itself in the inverse.

As for the measurements themselves based on tilt, they can be factored in, but it's largely irrelevant given the time scale of the procession 26000 years or so is a long ass time...and basically irrelevant for what happens in the next few hundred years.

For the most part you going to have same or similar weather right up until a couple centuries before it flips and starts going back the other way.

(If this makes sense. Explaining these types of procession can be tough at times.)

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This NASA page says:

> The greater Earth’s axial tilt angle, the more extreme our seasons are, as each hemisphere receives more solar radiation during its summer, when the hemisphere is tilted toward the Sun, and less during winter, when it is tilted away.

It has this to say about the 26k year cycle you're referring to:

> Axial precession makes seasonal contrasts more extreme in one hemisphere and less extreme in the other. Currently perihelion occurs during winter in the Northern Hemisphere and in summer in the Southern Hemisphere. This makes Southern Hemisphere summers hotter and moderates Northern Hemisphere seasonal variations

So I guess I still have the same question, but complicated by the fact that models would have to account for a different effect in different hemispheres.

Why does Ganymede have a global magnetic field, while so many other large, similarly massed moons don't?

Why is Titan the only moon with a substantial atmosphere?

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What's the relationship between radiation, convection and conduction? Are they all just electrons exchanging energy with photons, where the only difference is whether the photons go directly between the substances or whether the atmosphere is involved? Do they happen in equal amounts whenever something is heated? I understand each phenomenon individually, but not how they fit together. It's not like atoms decide whether to engage in a quantum process or a regular Atomic one, is it? Sorry if my question doesn't make sense.

Wireless charging is a type of inductive charging, which works via an application of Faraday's Law. Essentially, if a magnetic field changes inside of a wire loop, it will induce a current in that loop. So, wireless chargers producing a time varying magnetic field, and the phone has a wire loop, so a current is induced, which is used to charge the battery.

Why isn't is used much outside of small, personal electronics? 1.) it's not super efficient. Only the energy that goes into producing the magnetic field that goes through the loops actually goes into creating power, the rest is wasted. 2.) It produces quite a bit of heat for how much power it makes. Make enough power to power something big, and it's going to get real hot.

Magnetic fields in planets/moons is caused by having a rotating, magnetic core, which Ganymede does. The size does not matter.

The deepest mine in the world is AngloGold Ashanti's Mponeng gold mine, near Johannesburg in South Africa.

By 2012, the operating depth had already reached 3.9-km below the surface, and later expansions have resulted in digging below the 4-km mark.

The deepest man-made hole in the world is ~12 km called the Kola superdeep borehole in Russia.

Space based solar power.

Ideally, you would point it at Earth and sell solar energy. Or point it at a solar panel to convert it to microwave frequency as beam that at a receiver on Earth to sell electricity.

Probably not visible to people. Depends how you build it, but for maximum efficiency you will be using a very tightly focussed beam.

Just like your laser pointer is making light using a LED, you can also have a visible light laser. You can only see the laser pointer side on if it passes through some particular like smoke, that bounce the light towards your eyes.

Home experiment.

Hold you hand in front of your face. Open your mouth as wide as possible and breathe on your fingers. Does it feel hot, cold or neutral?

Repeat, but close your mouth as narrow as possible so breathing is a tightly focused flow. Does it feel hot, cold or netural?

Other important considerations. Your internal body temperature is ~37°C, but your outer skin temperature is closer to 20°C.

When standing still you have an insulating layer of air around you. Your body is wanting to push out excess heat to prevent cooking itself. It loses that heat by radiating it, or by convection where some carrier rubs over your skin and carries away "heat". If the air is not moving, you have transferred as much heat away as possible and the air in immediate contact with you is saturated with heat. Example: hiding under the bed covers or wearing clothes.

Wind moving over your skin is transferring heat by convection. It is picking up heat from your body and carrying it away, which makes your outer skin feel cooler.

Practically, no, not even close.

Phytoremediation is the science word.

Lots of plants do pull "stuff" from the soil and water. They accumulate it in the plant tissue, then you can chop that tree or grass down, burn and collect the ashes to dispose of the hazardous material. It is usually targeted at removing heavy metals from soil or water.

Unfortunately, the Ohio train spill was not heavy metals. Phytoremediation won't work here.

Another reason it won't is the chemical spilled was burned. It resulted in a cloud of ash particles and some hydrochloric acid rain. The acid will have immediately reacted with any soil or substrate to form fairly ordinary salts, such as table salt. It is an acute problem, not a persistant problem.

Because of the way the expansion rate changed over time, as I discussed in my initial comment. The Hubble rate ("how much the universe expands in percent per year") decreased over time. A good analogy is the ant on a rubber rope problem where the rubber rope expands much faster than the speed of the ant, too - but the ant still makes progress on the rope and over time is less affected by its expansion. For the universe, this would be a perfect match in a scenario of constant expansion. That's not what we have, but it's reasonably close for the last 10 billion years.

Did the recent discovery of ancient galaxies mean that the Big Bang theory is now disproven?

Radiation is all about bodies emitting photons, where the amount of energy depends upon how hot the body is. That's why fires feel warm, infrared heaters feel warm, and the sun feels warm.

Conduction is about direct heat transfer. Heat is just thermal movement of the atoms in a body, so put that in contact with a colder body and the hot atoms run into the colder atoms and make them move faster, transferring heat.

Convection is the same as conduction, except that the transfer is done through air.

> The tilt is decreasing, so we should be seeing less extreme seasonal changes.

From 23.4 degrees to 22.5 degrees in 10000 years. Here is a plot. It's a very small effect. Around 0.01 degrees (or 1/2000 of its value) per century.

You can also produce it with accelerators on Earth (far too expensive to make it interesting commercially) and traces will be produced in natural high energy collisions (but some iron nuclei will be destroyed by these, too).

It's not reversing directions. There is some discussion if it is rotating slightly slower or faster than the rest of Earth, and if that changed recently. "Slightly" here means something like 0.001%.

Bullshit.

Redshift comes from the expansion of space, not from collisions.

If it came from collisions then we couldn't make out any individual redshifted objects because all the light would be scattered into random directions.

Where would the difference in energy come from?

> Q. How do we know that the universe expansion is really the explaination for the observed redshift of light from stars.

There is no other mechanism that can lead to the observed redshift/distance relation.

> As i understand it, space is a quantum soup of virtual particles.

It is not, this is just a myth in popular science descriptions.

> is it possible the magnetic and electric permeability of free space is changing over time?

If the chance has an effect on the universe then it also changes the ratio of wavelengths from different spectral lines, something we would observe.

> Q. If one has crossed the event horizon of a black hole, is one traveling faster than light and thus going backward in time wrt the outside universe on the other side of the event horizon?

How you describe the motion depends on your (largely arbitrary) choice of coordinates for the inside, but it's not time travel in any of them.

> Is Gravity just a by product of Electromagnetic Force

No, they are completely different things.

If you make something float then you exert an electromagnetic force upwards that's as strong as the gravitational force downwards, leading to a net force of zero. The same applies for e.g. things resting on a table, it's just easier to get it balanced there.

> Everything emits electromagnetic waves

Uncharged particles do not, and very cold objects have negligible emissions.

> everything is impacted by Gravity

... even uncharged particles, and it's essentially independent of temperature.

> If both forces protrude "forever" and become their strongest at a certain "break point"

There is no "break point".

An 1/R^2 law is the natural relation in 3 dimensions as the surface area of a sphere scales with R^(2). Deviations from that relation are special (they need massive exchange particles, for the weak interaction, or strong self-interaction, for gluons).

No...

https://www.reddit.com/r/cosmology/comments/xr0wlq/i_keep_hearing_all_this_stuff_about_the_james_web/iqcjc2o/

I am really asking about the trough of the wave. Is this a case where the answer is "it's also a particle Ramen"?

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What if pi was exactly 3?

If time travel were possible, would you end up in the same place on Earth, or would you end up in the same place in the universe, and the earth no longer be there because of it's orbit and the fact that the universe is always expanding?

Okay thank you we discussed this in physics class, but the teacher couldn't really answer the question :))

Very cool! Thank you!

We're perfectly evolved for life on earth. Could other life be perfectly suited to live within suns, on planets as far out as Pluto, or in black holes? Why do we say that life "needs water" - isn't it more the case that life on earth needs water, but elsewhere life could evolve to make do with other elements or compounds instead?

That was a great explanation thanks!

If you want a deep dive on the gross misunderstanding around the paper that sparked this perception, you could check out this thread.

What is dark energy??

Do you think we might be in a self perpetuated slow time envelope?

Right, I understand each individual phenomenon, but, for example, how does an entire atom moving faster result in the electron(s) of that atom emitting photons? And, conversely, how does a photon being absorbed by an electron become the entire atom vibrating with a little more energy? Shouldn't the electron receiving the photon just jump up a quantum level, then back down again when it in turn emits a photon, rather than any of that activity somehow making the entire atom vibrate more? This is what I'm missing...

Very complicatedly, I would say. Given the temperature and atmospheric pressure on Titan, both methane and ethane are at the triple point, and so goes between solid, liquid, and gas easily. This leads to a pretty complex chemical system and a cycle analogous to the hydrological cycle ("methane-ologic cycle"), the only other place in the Solar System. One of the famous features are that are seen to pop in and out of existence in the seas are the "Magic Islands", of which waves or bubbles might explain the transient nature.

Are photons affected by gravity like objects with mass? Or is it 'simulating' gravity and moving along the curved spacetime.

In Einstein's general relativity, he said that there is no difference between an accelerating frame and a gravity well. A photon will appear to curve down in both cases.

How does this suggest curved spacetime and not gravity's attractive effect on the photon?

How does E=mc^2 not translate to photons have mass? Is mass in 'energy' form not affected by gravity?

As a blackhole approaches a singularity, shouldn't time slow down so much so that it may never actually reach a singularity? i.e.it will forever be approaching a singularity.

One thing I'm looking for with this question is the time dilation in black holes. Thanks

E=mc² is the formula for the specific case of a stationary, massive object. That clearly does not apply to a photon. The full equation is E²=m²c⁴+p²c², which simplifies to E=pc for a massless particle like the photon.

"Being affected by gravity" and "moving along curved spacetime" are the same thing. That's the whole point of general relativity: gravity can be described as objects following geodesics in curved spacetime.

Wow that was fascinating, and exactly the type of phenomenon I would like to learn more about, thank you!

This might help:

https://physics.stackexchange.com/questions/72174/why-do-moving-particles-emit-thermal-radiation

How come there is a large area of Mesozoic terrain that stretches across the Atlantic Rift on top of ground younger than it?

You're welcome, glad I could help!

Is it too late to ask a question?

Are there places in the solar system where there are "pleasant" temperatures? Or are all places either a bazillion in the sun and minus a bajillion in the shade?