Ask me any one question about space, please!

[Nibb31]

What are the cardinal directions in space?

prograde, retrograde, normal, antinormal, radial, antiradial?

The ISS uses forward, aft, port, starboard, zenith and nadir. It isn't always in the forward-prograde orientation. The ISS sometimes rotates on another axis to facilitate docking of supply ships.

Edited April 17, 2014 by Nibb31

[Sillychris]

I was at a kid's rocketry demonstration and one kid asked "What would happen if your satellite blew up?"

The guy said "Well, you'd have a blown up satellite"

What a crappy and unimaginative answer.

So yeah, what would happen if your satellite blew up? And it was made of mithril.

[Nibb31]

I was at a kid's rocketry demonstration and one kid asked "What would happen if your satellite blew up?"

The guy said "Well, you'd have a blown up satellite"

What a crappy and unimaginative answer.

I think it's pretty accurate really. What else would you expect with a question like that?

[Sillychris]

I think it's pretty accurate really. What else would you expect with a question like that?

Haha. I spoke up and told the kids that you'd get satellite debris orbiting the Earth potentially wrecking other satellites, plus the Chinese are assholes for doing that on purpose already.

You could also say "A piece might land in your bathtub" but that might scare the kids of exploding satellites and keep them up at night.

Comments about lousy satellite craftsmanship and no-return policy would also be acceptable.

[Seret]

prograde, retrograde, normal, antinormal, radial, antiradial?

The ISS uses forward, aft, port, starboard, zenith and nadir. It isn't always in the forward-prograde orientation. The ISS sometimes rotates on another axis to facilitate docking of supply ships.

By definition cardinal points can't be based on your own orbit, they should be the same for everybody regardless of how they're moving. I'm genuinely interested, since I know next to nothing about navigation in space. Is the concept of a cardinal point even relevant? Does it change when you move from one body to another, if so, is there an equivalent used for interplanetary navigation?

[Nibb31]

By definition, cardinal directions are really only relevant in a 2D environment, or by projection, a spherical surface. In a 3D environment, you need to add the Z-axis.

In space, your are always moving. You don't travel from a location (X,Y,Z) to another location (X',Y',Z'), but from one orbit to another. Therefore, an actual location in space, corresponds to a set of orbital orbital parameters (Eccentricity, semimajor axis, inclination, epoch...) rather than coordinates. So I would imagine that thinking terms of cardinal directions makes no sense in space.

[Seret]

Well exactly, hence my question. FWIW orbit is essentially a spherical space, with a vertical component which is why I asked if it was reckoned in relation to whatever body you were orbiting. I understand that in practical terms they use things like inertial updated by GPS or star shots, but I was wondering if there was an underlying conceptual framework or if it's simply redundant.

[victory143]

Closed universe or open universe?

[TarkinLarson]

Unfortunately I would have many questions...

If I could have two, that are closely linked I would choose the below in bold.

How fast does gravity travel? How do we know?

The idea of if a concentration of mass (like the Sun) disappeared it would take a few minutes for us to 'fall' out of orbit tickles me.

I always had a fantastical thought that gravity is so weak because it 'leaks' into other dimensions and so we only feel a portion of it's effects.

My brother had an interesting, but perhaps naïve view on gravity. He believed it was actually not mass attracting us together, but vacuum (or something in the apparent nothing-ness) pushing us. Matter was actually neutralising or blocking this force and was an area of 'null-gravity' so that is in fact why two massive objects appear to attract each other. Really they are being pushed, but the matter is blocking internal pushing. It also helped in justifying why items with short distances between had more 'attractive' force between them... the space between them was smaller than the space pushing them together from outside the system. It would also go to explain by the universe it expanding at an ever increasing rate (which if I remember correctly is the latest paradigm). Interesting thought exercise, but I'm pretty sure it's been proven wrong (How? I don't know).

Edited April 17, 2014 by TarkinLarson

[cantab]

So yeah, what would happen if your satellite blew up? And it was made of mithril.

It would cost more than all the money in the Shire to replace.

More seriously, for me the interesting aspect of this question isn't the physical side (which I know) but the impact on infrastructure. If a communications satellite fails, does it cause an outage? If so, how is service restored and how quickly? If not, why not? Presumably it varies depending on use and orbit. And how, if at all, is an investigation done to figure out why it failed?

[Yoha]

How fast does gravity travel? How do we know?

Lightspeed; derived from theoretical models, themselves derived from various empirical observations [1].

My brother had an interesting, but perhaps naïve view on gravity. He believed it was actually not mass attracting us together, but vacuum (or something in the apparent nothing-ness) pushing us. Matter was actually neutralising or blocking this force and was an area of 'null-gravity' so that is in fact why two massive objects appear to attract each other. Really they are being pushed, but the matter is blocking internal pushing. It also helped in justifying why items with short distances between had more 'attractive' force between them... the space between them was smaller than the space pushing them together from outside the system. It would also go to explain by the universe it expanding at an ever increasing rate (which if I remember correctly is the latest paradigm). Interesting thought exercise, but I'm pretty sure it's been proven wrong (How? I don't know).

It is usually seen as particles (gravitons) going being emitted from one body in all directions and being caught by the other; when caught, they transmit a slight amount of momentum in the opposite direction of their own velocity (for some reason). This explain the m₁ × m₂ in the gravitation formula (the amount of graviton emitted is proportional to the quantity of mass, and the probability that an object catch gravitons is proportional to its mass).

The "÷ r²" comes from the fact that gravitons are emitted in all directions at the time; so, all gravitons emitted at time t form a sphere whose surface is proportional to r² where r is the distance traveled by the gravitation since t (the radius of the sphere). The "G" constant handles the 4À factor for the area of a sphere and unit conversions.

“naïve†→ are you French by any chance?

[1] https://en.wikipedia.org/wiki/Speed_of_gravity

[Yoha]

How big is space?

http://www.numberphile.com/videos/pi_universe.html

[TarkinLarson]

That's answered a few of my questions. I was suggesting that gravity was an unusually 'weak' force in the context of the 4 fundamental forces (weak nuclear, strong nuclear, EM and Gravity). You explanation is a little simpler and often the simple ones are the best.

I do enjoy that we (at least from your explanation) do not know why gravitons transmit momentum in the opposite direction. There's always something new to learn isn't there?

No, I am British. My spell checker must be French .

Thanks again.

Edited April 17, 2014 by TarkinLarson

[Yoha]

If you had a machine that could teleport you to a random place anywhere in the universe, what is the probability of you landing somewhere where you would:

a). die within a second?

. survive for longer than a day?

As far as we know, the only place where man can survive is on Earth, in the lower atmosphere, not too far from the ground, not too far from shores, not too far from vegetation. Let us be generous en say that we still have a quarted of the surface as viable. Assuming ten meters of height, the volume is:

Vok = 4/3 À ( (r+10m)³ - r³) = 5.10 ×10¹⁵ m³

Now, the universe is roughly a finite sphere of radius 46Gly [1] and assuming an homogeneous distribution, the probability is:

p = Vok / Vtot = 2 × 10^-65

Basically, it is more likely that you will turn out as a Kerbal tomorrow when waking up.

[1] https://answers.yahoo.com/question/index?qid=20110926045225AACoIsm

Edit: the previous probability is more focused on ; as to a), you can actually survive in space for as much as two minutes, so the probability is quite close to 1

Edited April 17, 2014 by Yoha

[Yoha]

I do enjoy that we (at least from your explanation) do not know why gravitons transmit momentum in the opposite direction. There's always something new to learn isn't there?

I do not know the actual theory about the graviton and it may happen that we have a theory of how the gravitons work. What we do not know however, is it gravitons (as a particle) exist altogether.

No, I am British. My spell checker must be French .

I have this problem, even though my whole system, Firefox install and profile and the website I visit are in English .

Thanks again.

Happy to help!

[Jasel]

I've got one that will blow your mind in your research. What is gravitational lensing and what can we do with it?

Enjoy and good luck.

[Yoha]

I was at a kid's rocketry demonstration and one kid asked "What would happen if your satellite blew up?"

The guy said "Well, you'd have a blown up satellite"

What a crappy and unimaginative answer.

So yeah, what would happen if your satellite blew up? And it was made of mithril.

Mithril or not does not change much. What is interesting is that when it explodes, several parts are ejected in different directions and you end up with several objects in different orbits.

First, if we ignore Earth/Kerbin (I like physics because we can pretend that things are negligible even though there are blatantly not so), we can clearly imagine the different parts to be to drift farther and farther away from each other. We should expect that same behavior to happen in orbit; but they won't drift in the same way.

As all KSP players know, giving an impulse in a direction will slightly change your orbit either by inclining it or making the orbital period longer or shorter. In the later cases, your pieces will indeed drift apart more and more, along the orbit. In the former case, they will just go in different directions (maybe more intuitive); but if the orbital period is unchanged, it would mean we should expect the parts to meet again (actually come very close) after each semi-orbit.

[Yoha]

I've got one that will blow your mind in your research. What is gravitational lensing and what can we do with it?

You all know that gravitation attract mass. However, in finer models it actually attract energy; because there is an overly big amount of energy in mass (E=mc²), we can usually just skip it to the usual handy formulas. However, it means that even massless particles are subdued to mass, including light (photons).

Now, a photon carries a lot less energy than a particle with mass so we need a very massive object to have visible effects. Take for example a galaxy, or even a cluster of galaxies; it will be the "lens". Consider an object behind it (for example another galaxy). The light from the object is emitted in all directions and deflected by the "lens". Looking at [1], you can see how this behaves like a lens [2].

This is exactly what you can observe at some spots in the sky [3]; it is no more than a (blue) galaxy behind a lens (red galaxy). With some luck, lens could *potentially* help us see far galaxies in more details.

[1] http://imagine.gsfc.nasa.gov/Images/news/lens_fig1.gif

[2] http://upload.wikimedia.org/wikipedia/commons/e/ef/Lens1.svg

[3] https://upload.wikimedia.org/wikipedia/commons/1/11/A_Horseshoe_Einstein_Ring_from_Hubble.JPG

[Yoha]

What are the cardinal directions in space?

Depends. If you are staying around Earth, use [1]; in the Solar System [2]. Outside that, we don't really need one at this point.

[1] https://en.wikipedia.org/wiki/Earth-centered_inertial

[2] http://en.wikipedia.org/wiki/International_Celestial_Reference_Frame

[K^2]

I do enjoy that we (at least from your explanation) do not know why gravitons transmit momentum in the opposite direction. There's always something new to learn isn't there?

We do know, actually. Albeit, graviton theory is incomplete, due to certain problems in quantizing gravity, we have a general theory for gauge bosons which covers this question. After all, it doesn't matter if it's supposed to be a graviton or a photon. Two opposite charges attract in a very similar way to two masses, and must also exchange photons that cary momentum the "wrong way".

And the solution is actually quite simple. A classical particle has momentum proportional to its velocity. Any on-the-shell particle, which can freely propagate through space, must also have that property. But a virtual particle, does not. All forces are mediated by virtual particle exchange, and all of these particles can propagate one way, and carry momentum for a different direction. At that scale, energy and momentum behave more like a charge. In fact, energy-stress tensor, which is generalization of these concepts to a general coordinate system, is the conserved charge of the General Relativity, just like electric charge is a conserved charge of Electrodynamics.

I could probably get into this a bit deeper, but understanding propagation of force carrying particles requires good understanding of Partial Differential Equations, Green's Functions, Fourier Transforms, and some theorems from Complex Analysis. The math on this is pretty hairy.

P.S. I really like that this is turning into a community Q&A thread.

[Yoha]

We do know, actually. Albeit, graviton theory is incomplete, due to certain problems in quantizing gravity, we have a general theory for gauge bosons which covers this question. After all, it doesn't matter if it's supposed to be a graviton or a photon. Two opposite charges attract in a very similar way to two masses, and must also exchange photons that cary momentum the "wrong way".

And the solution is actually quite simple. A classical particle has momentum proportional to its velocity. Any on-the-shell particle, which can freely propagate through space, must also have that property. But a virtual particle, does not. All forces are mediated by virtual particle exchange, and all of these particles can propagate one way, and carry momentum for a different direction. At that scale, energy and momentum behave more like a charge. In fact, energy-stress tensor, which is generalization of these concepts to a general coordinate system, is the conserved charge of the General Relativity, just like electric charge is a conserved charge of Electrodynamics.

Thanks for the correction. My intuitive understanding is that the graviton is basically a particle with negative energy, making the reaction of the receiver quite intuitive. Is it far from the actual theory?

[K^2]

Thanks for the correction. My intuitive understanding is that the graviton is basically a particle with negative energy, making the reaction of the receiver quite intuitive. Is it far from the actual theory?

No, not really. Exchange is symmetrical. If particle A emitted a graviton with positive energy that was absorbed by particle B, it's the same thing as particle B emitting a graviton with negative energy that is absorbed by particle A. Which happened first, absorption or emission, doesn't matter either.

Lets work out an example. Suppose, you have two particles, both of mass m, that were traveling with momenta p and q, have exchanged a graviton with momentum k, and are now traveling with momenta p' and q'. All of these are 4-momenta, so they contain information about both energy and momentum.

From conservation of 4-momentum:

p' - p = k

q' - q = -k

And we know that before and after, the two massive particles were on the shell.

p² = p'² = m²

q² = q'² = m²

I'm using natural units, where c = ħ = 1. Lets look at the mass of the exchanged particle.

k² = (p'-p)² = p'² + p² - 2p'·p = 2(m² - p'·p)

Lets look at that p'·p term. I can decompose it as p'₀p₀ - p'·p. Here, p₀ is particle's energy and p is ordinary 3-momentum. For sake of example, lets suppose that the particle was at rest to begin with. (In fact, we can always choose a coordinate system where it was so. So this assumption can be made without loss of generality.) In other words, p₀ = m, and p = 0. In that case, p'·p = p'₀p₀ - 0 = p'₀m. This is interesting. Clearly, if particle is no longer at rest, its energy, p'₀, has increased. In other words, p'·p > m². And that means, k² < 0.

So a virtual exchange particle can have negative mass. And, in fact, this is normal for virtual particles, regardless of whether they carry an attractive or repulsive force. The corresponding energy, however, can be positive or negative, depending on whether particle's energy increased or decreased in interaction, and on which direction in time you consider the particle to be propagating. This will be frame-dependent, by the way. Energy always is.

This should also give you some insight into why quantum gravity is so complicated. The fact that virtual photon in electrodynamic interaction has negative mass, or any mass at all, isn't a problem. Its always electrically neutral, so if you only consider electromagnetic interactions, photon will only couple to charged particles, making it pretty easy to account for all interactions. As a result, QED is pretty simple, as far as RQFT go. Then you can look at strong interactions. Well, gluons are much similar, but they carry color charge. So gluons can couple to each other. But there are only 8 gluon types, each carrying its own, constant charge, and so you can still account for most things. QCD is far more complex than QED, but manageable in a lot of ways. And then you look at gravity, and virtual graviton will have mass and energy, so it will be able to interact with other gravitons, and so gravitational interaction alters gravitational interaction, and the whole thing gets really bad really fast.

In practice, what you actually end up with is a non-renormalizable field theory, which sort of means that everything is infinite, and there is no way to deal with it. There have been some great results from effective field theory approach lately, so we can build a very good approximation for quantum gravity, but it has limitations. Specifically, it doesn't tell us anything about physics at plank scale, which is really the biggest puzzle people hoped QG would resolve.

[Jasel]

Total thread jacking of epic proportions. People have attention issues...

Just a recap - Maximus97 wanted questions pertaining to space so he could do the research and answer them via PSA youtube video. Just throwing it out there...

[How2FoldSoup]

Total thread jacking of epic proportions. People have attention issues...

Just a recap - Maximus97 wanted questions pertaining to space so he could do the research and answer them via PSA youtube video. Just throwing it out there...

Either way K^2's response is fascinating and (no offense to Maximus97) I trust his explanations a bit more. At any rate, Maximus can still use this questions and half of the research is already done for him and he still learns. Everybody wins.

Having said that, K^2, could it be said that we are effectively dealing with particles that have negative energy but have no choice but to do so? What I mean by this is when gravity is acting on you and said object. The pair would be exchanging gravitons one with positive energy and one with negative energy?

For anyone: How does the EM force propagate through the fourth dimension? Visualizing this has been one of the most difficult things I've ever tried to do and I still haven't succeeded. I know the flat land analogy but its so difficult for me to expand this to the 3rd dimension because of my inability to visualize the 4th.

I also just realized; K^2, wouldn't particles with 4 momenta break Heisenbergs uncertainty principle? Or am I understanding this wrong.

Edit: For Maximus, in your video you can tell people about voyager 1, how fast it's going, and what fraction of c it's moving at to show how far we really have to go to reach c. Then Math how long it would take to get to Alpha Centauri at Voyager 1 speeds. You should also explain the alcubierre drive to explain why we can theoretically circumvent relativity and travel FTL. They're good questions to answer and explain because it shows how mind boggling large the cosmos is.

Edited April 17, 2014 by How2FoldSoup

[K^2]

Having said that, K^2, could it be said that we are effectively dealing with particles that have negative energy but have no choice but to do so? What I mean by this is when gravity is acting on you and said object. The pair would be exchanging gravitons one with positive energy and one with negative energy?

I also just realized; K^2, wouldn't particles with 4 momenta break Heisenbergs uncertainty principle? Or am I understanding this wrong.

These kind of go together. What I described is a 1-particle exchange. Two particles pass each other, one of them emits a photon/gluon/graviton, the other absorbs it, and they part ways. Again, which does emitting, and which does absorbing doesn't matter. What matters is that one particle got exchanged. But that's not what really happens. Or more specifically, that's not all that happens. There can be exchanges of 2, 3, 4, and any number of force-carrying particles. And they need not be absorbed in the same order that they are emitted. Worse yet, there can be particle-antiparticle pairs that form from vacuum, absorb one of these force-carrying particles, and then emit another one. There is an infinite number of categories of infinite number of possibilities to how two particles can exchange energy and momentum. And they all take place.

Lets forget about interaction for a moment. Picture just one particle propagating through space. Since this is a quantum particle, it will have uncertainty in momentum and position. And so you can describe propagation of the particle as probability amplitude that changes in time. But alternatively, you can describe it as particle with precise location and precise momentum following one of precise trajectories. What you are uncertain of is which trajectory it follows. So instead of looking at it as particle being in many places at once, you look at it as particle following many paths at once. This is the Path Integral Formulation of quantum mechanics. While it tends to be fairly impractical in classical QM, it turns out to be indispensable in RQFT. Part of the reason is that it lets you describe interactions in terms of Lagrangian directly, and that looks after all the symmetries out of the box. Since all interactions are understood to be consequences of local symmetries, this is clearly useful.

Same idea applies to many-particle systems. Not only can particles take all possible paths, but the interaction can follow all possible combinations of particles being emitted and absorbed. The main requirement is that all conserved quantities are conserved. Momentum is one of these. So it's often useful to go from coordinate representation of the dynamics to a momentum representation. Mathematically, what that means is that we take Fourier Transform of the Green's Functions, so that they become functions of momentum instead of functions of coordinate. As a result, each particle path we consider has a very specific momentum associated with it. And for every possible trajectory of all the particles, the momentum is conserved. So I can still carry out all of the operations in the previous post as if we are dealing with classical particles that have definite momentum.