Deep Space and Interplanetary Exploration - Houston, we are go for launch!

[rebs]

Some people are enamored of their own criticism, me thinks.   ;)

[Natasha Kerensky]

Some may think it ignorant to want to actually press the envelope of Planetary exploration.  But I agree.  I would also wish to have seen more energy, drive, and financial support for actual efforts in colonization.  At least the drive and energy of the Apollo Program and it's results.  Imagine where we would be if that had continued.

Human space flight is not held back by lack of financial support.  NASA spends $10 billion annually on it and growing.  The problem is how very inefficiently the funds are spent.  That’s due to parochialism and deals with the devil that were made during the Apollo buildup and since.  But I shouldn’t get into specifics or risk a warning for delving into real-world politics.

Apollo was unsustainable financially and in terms of risk/flight safety.  We needed a better approach.  Unfortunately, Shuttle was not it.

[rebs]

It absolutely is underfunded compared to GDP growth during the same time.

[Natasha Kerensky]

It absolutely is underfunded compared to GDP growth during the same time.

Depends on the time period.  It grew explosively during Apollo, then came back down and leveled out.  There have been periods since when it grew faster or slower than inflation.  But that’s driven by the projects and work being done, not by inflation or set growth rates.  We don’t make decisions by saying spend X% on human space flight.  It’s not an entitlement program.  We make decisions by saying we want to do programs A, B, and C and then adding up their budget needs.

NASA’s budget is as much as the budgets of all the civil space agencies in the rest of the world combined.  NASA spends as much on human space flight alone as China spends on all its space programs — civil and military, unclassified and black, manned and unmanned — combined.  It’s not that we’re not spending enough.  It’s that we’re not spending it well.

[Daryk]

If you know how much China's black space budget is, I know some people who'd like to know too...

[Wrangler]

There must be considerable amount money dumped into it.  I'm curious if they will go through their endeavors.  There was report last year of their ambitions to build a mile long structure which has been reported from being massive space station to an actual interplanetary vehicle to be used to go to Mars.

[Maingunnery]

Those statements are woefully ignorant of the state of research in these topics and scientific process in general.  They’re also grossly unfair to the physicists and astrophysicists doing the work.

This is how science works.  You make observations.  Certain observations don’t match law or theory.  So you come up with a hypothesis that does explain the observations.  You figure out way(s) to test that hypothesis.  Sometimes those tests cost money.

Observations raising the issue of dark matter have been around since the 1930s.  In the early 1990s, I worked with a theorist on large-scale structure who tried to explain it without initial dark matter fluctuations.  He didn’t ask for grants.  He worked it out on chalkboards and laptops.  We’re finally going to put his and dozens and dozens of other dark matter theories to the test with JWST observations.  JWST is a very expensive telescope, but it or something very much like it is required to see the young universe early enough to rule-out or rule-in certain dark matter hypotheses.  No one is involved on JWST to test the impossible or for grant money.

I was at one of the two institutions whose extragalactic supernova observations in the 1990s first raised the issue of dark energy.  I had a small hand on a panel of astrophysicists who defined the follow-on to JWST, which, among other things, will measure thousands of extragalactic standard candles, reducing mathematical uncertainty about the accelerating expansion of the universe, which will rule-out or rule-in certain hypotheses about the dark energy driving it.  No one is involved in the Roman Space Telescope for the sake of “epicycles” or because only theoreticians can test dark energy.

The natures of dark matter and dark energy are hard to solve not because researchers enjoy writing grants (they absolutely do not) nor because our profession rewards going down known dead-ends (it absolutely does not — terrible way to get published, win awards, and advance up the ladder).  Dark matter and dark energy are hard to figure out because they’re not luminous sources of light like stars.  We can only observe their influence indirectly.  It’s like trying to understand something by looking at its shadow.  A circular shadow could be cast by a circular disk, a ball, the base of a column, the base of a cone, or any number of other objects.  Trying to test for disks, balls, columns, and cones without being able to see or touch those objects — only the shadow — is immensely difficult.  Dark matter and dark energy have the same problem, only magnified many-fold.

I think that Dark Matter and Dark Energy have a bad reputation because the means to properly test them were missing, so the layman would just hear of a lot of random theorizing without any definite results. So I am quite happy to hear that JWST will play an important role for testing them.

[Natasha Kerensky]

If you know how much China's black space budget is, I know some people who'd like to know too...

All sorts of non-government organizations watch this stuff.  Spacecraft have to go up so it’s not that hard to figure out by counting launches, tracking satellites, using other publicly available information, and doing comparisons to known programs with known costs.  The major cost centers cannot be hidden.  Are these kinds of estimates exact?  No.  Are they close?  Certainly.

There must be considerable amount money dumped into it.  I'm curious if they will go through their endeavors.  There was report last year of their ambitions to build a mile long structure which has been reported from being massive space station to an actual interplanetary vehicle to be used to go to Mars.

You have to be really careful reading breathless reports in popular news outlets about eye-catching futuristic space activities going on in other countries.  If you read them critically, they're almost always about small-scale studies.  In this case, it’s a study analyzing the dynamics of really large space structures.  They’re not launching such a structure.  They’re not testing such a structure.  They’re not building such a structure.  They’re not designing such a structure.  There’s no government approval or wedge of funding for such a structure.  They don’t even have a purpose for it.  They’re just investigating some of the physics of such a structure.  It’s one of five such studies sharing about $2.3 million worth of awards.  So something on the order of only $460,000 is going to this one study.  A few engineers’ time ginning up a computer model.  That’s it.

https://interestingengineering.com/china-developing-23-million-megaship-miles-long

These kinds of articles generate clicks but there’s little substance to them.  The space sector it littered with studies and proposals that go nowhere.  The vast majority do not because there’s no customer willing to pay for the real thing, it’s uneconomic, it’s too expensive, it’s technically infeasible, etc.  Around 2000, I funded a NASA study at about 10x the cost of the Chinese study above to do the first serious re-look at solar power satellites since the 1970s.  Obviously, 20 years later, we’re not solving the climate crisis or supplying energy by beaming solar power from space.  Just because a couple engineers are looking at something doesn’t mean that something is on the path to becoming reality.

The boring backdrop to the article about a study of mile-long space structures is that the space station China is currently putting up is only about 1/5th the size of the International Space Station.  China’s space program is a catch-up program, it’s not a leap-ahead program, as much as we might want it to be.  That’s not to poo-poo past and future accomplishments.  But we have to separate reality from speculation.

[Daryk]

Going back to post #679, I finally had enough time to read the article.  That looks like a very reasonable explanation, and I hope the JWST can help prove it!  :thumbsup:

[Dynodragon]

Some really good posts over the last couple of pages so I wonder if I may be allowed to post this question here;

Large masses warp space-time and that shows up as gravity, in the absence of a graviton particle how do objects interact with the curved space-time? How do they know(wrong word I know) that space-time is curved?

Secondly, why are photons which have no mass affected by gravity? The equation for working out the force of gravity between 2 objects is G*M1*M2(over distance squared), shouldn't that equal 0. (I'm guessing I'm wrong that photons have mass=0)

Feel free to go PASS!

[kato]

I'm guessing I'm wrong that photons have mass=0

Photons have a "imaginary rest mass" on a scale of 10^-51 to 10^-47 gramm that has been proven experimentally. The exact value depends on the wavelength apparently.

See e.g. https://www.sciencedirect.com/science/article/pii/S2211379719330943

[Sabelkatten]

As I understand it photons don’t "interact" with gravity as such. Rather they simply follow the straightest line possible through the curved spacetime.

[kato]

Interaction would mean that the photons warp spacetime further through their own mass.

[worktroll]

Secondly, why are photons which have no mass affected by gravity? The equation for working out the force of gravity between 2 objects is G*M1*M2(over distance squared), shouldn't that equal 0. (I'm guessing I'm wrong that photons have mass=0)

Because of Uncle Albert - the infamous E=MC² means that there's a direct equivalence between energy and mass. Not only can you convert M into very large quantities of E (via fission, fusion, or antimatter), E exerts a gravitational force of its own - but it's very very  small.

There's a very classic example of this, the precession of perihelion of Mercury's orbit. Short form, the planet's orbit didn't quite obey Newtonian physics. They allowed for all the other planets' impact, and still not right. In the end it required Relativity theory to explain, in a vast simplification you had to allow for the energy the sun emitted which provided a small but critical effect. This does affect the other planets, but at too low a level to be easily observed; Mercury happens to be close enough for it to be a small factor.

Then there's the opposite - gravitic lensing.


Re your first question - I don't think you can use gravitons in the curved sheet model. Short form, it's a model, it doesn't attempt to be a theory. A bit like wave or particle models - both work for many things, but there is only Zool the wave-particle duality. The universe is not only weirder than we imagine, it's weirder than we can imagine.

[Daryk]

I'm happy to wait for the actual PhD to explain this one.

[Natasha Kerensky]

Large masses warp space-time and that shows up as gravity, in the absence of a graviton particle how do objects interact with the curved space-time? How do they know(wrong word I know) that space-time is curved?

In short, they don’t.  Spacetime curvature is a mathematical model for our puny primate brains.  It’s not what’s actually happening in nature.  In fact, spacetime curvature, which was introduced by Minkowski, was initially called “superfluous erudition” by Einstein, but he later agreed that the model jived with his equations.

Gravity is a force field, just like a magnetic field.  A gravitational field acts on masses moving through it, just like a magnetic field acts on magnetized objects moving through it.  Maxwell developed the equations describing electromagnetic fields; Einstein developed the equations describing gravitational fields.

Fields mediate energy through waves.  Waves can be quantized as particles.  Electromagnetic fields mediate energy through light waves.  The quantized particle of a light wave is a photon.  Gravitational fields mediate energy through gravitational waves, which we have detected in recent years (look up LIGO).  In theory, the quantized particle of a gravitational wave is a graviton.  But unlike photons, we cannot detect individual gravitons and may never be able to.  We currently can only detect some of the largest gravitational waves in the universe, which would be compromised of ginormous numbers of gravitons.  We have no idea how to detect gravitational waves small enough to discern the existence of individual gravitons.  Therefore, although they must exist, gravitons remain undetected and theoretical.

Secondly, why are photons which have no mass affected by gravity?

Covered above, but to reiterate, photons do have mass.  For a practical example relevant to this thread, we have flown solar sails in space, which work by gajillions of photons imparting inertia to the sail when they collide with it (just as gajillions of air molecules impart inertia to a cloth sail when they collide).  If photons were massless, they could not impart inertia to the sail.

Hope this helps.

[Daryk]

I always understood photons to have momentum, not necessarily mass.

[Kit deSummersville]

I always understood photons to have momentum, not necessarily mass.

Well you need m and v to get p.

[Sabelkatten]

Actually not, mass is stlll zero. It’s more like energy times velocity. For most everyday purposes it can be treated as mass, thought.

[Natasha Kerensky]

Photons have no _rest mass_.  Rest mass is the mass of a particle as measured by an observer in the same frame of reference (moving in the same direction and with the same velocity) as the particle.  If I could move at the speed of light and weigh photons moving in my direction, the scale would read zero.

But because mass cannot achieve the speed of light, photons and mass can never exist is the same frame of reference.  When measured from the frame of reference of a mass, photons do have mass.  This is how photons transmit energy to masses, like solar sails.

The mass of a photon is:

m=hf/c^2

Where:

h=Planck’s constant
f=frequency of the photon
c=speed of light

Hope this helps.

[Daryk]

That's a new equation for me, but I haven't actively studied physics for around 28 years now...

[Natasha Kerensky]

That's a new equation for me, but I haven't actively studied physics for around 28 years now...

Here’s the derivation:

https://einstein.stanford.edu/content/relativity/q1647.html

(Only good thing we ever got from that mission...)

[Daryk]

That would seem to imply gravity lensing varies with the frequency of the photons being lensed.  Do we know?

[Natasha Kerensky]

That would seem to imply gravity lensing varies with the frequency of the photons being lensed.  Do we know?

No, a gravitational field is not a medium (air, water, ice, clear plastic, glass).  It cannot refract light.  In the vacuum of space, all photons travel through a gravitational field at the speed of light in a vacuum, regardless of their frequency/wavelength/mass. 

Depending on their wavelength, different photons travel at different speeds slower than the speed of light in a vacuum when traveling through a medium like air, water, ice, clear plastic, or glass.  This disperses white light into its constituent rainbow colors, as seen in a prism (as I’m sure you know).

Related to this is the phenomenon of gravitational redshifting.  As photons move away from a massive object, they lose energy.  But because photons cannot move slower than the speed of light, they lose energy by stretching out their wavelength.  Because photon wavelength is related to photon mass, photons lose mass as they move away from a massive object.

But to be clear, gravitational redshifting does not create a dispersive effect.  Because all photons must travel at the speed of light, all wavelengths are redshifted by the same amount when traveling away from the same massive object.

[Daryk]

Gravitational redshifting is new to me.  What's the magnitude of the effect?

[Sabelkatten]

Gravitational redshifting is new to me.  What's the magnitude of the effect?

Well... A photon inside the event horizon of a black hole would lose all energy and disappear if it was traveling straight out.

In more normal circumstances it's useful for measurements, but I can't think of any "ordinary" circumstances you'd really notice any physical effects.

[worktroll]

Gravitational lensing is a real observed effect. It's not photons trying to escape the event horizon; it's photons passing near any large mass getting deviated. The Sun produces detectable lensing, if you're observing photons that passed near it - the stars on the other side appear slightly out of position when nearly behind the Sun in line of sight, than when not.

[Sabelkatten]

Yes, lensing gives obvious visual effects. Gravitational redshift is rather more limited... It’s not like we live next to a neutron star!

[Daryk]

Order of magnitude?  ???

[Sabelkatten]

Order of magnitude?  ???

Redshift? Completely dependent on circumstances. I'd guess stars really close to the core BH of the Milky Way looks noticeable redder.