ask a physicist

Scott DeWar · Aug 30, 2015

freyar said:
I believe I've found some news articles about this thing, and it just seems to be a fuel cell. You can kind of think of fuel cells as batteries that require, well, fuel. This is not really new technology, but I guess they claim to be able to make them smaller and cleaner. I can't say whether they have or not, of course --- part of how "clean" or "green" a fuel cell is depends on the fuel you give it --- but I don't think there's anything in principle that says they can't.

Umbran said:
Yes, it is plausible. It is a fuel cell, and the basic idea dates back to 1838 - https://en.wikipedia.org/wiki/Fuel_cell

Now, when they say early in that video that there's no burning - that's technically inaccurate. The fuel cell takes some specific fuel, and combines it with oxygen to get energy. What do you think "combine fuel and oxygen" is? Oxidation, aka "burning". A fuel cell does it without an open flame, mediated by a catalyst on a substrate(1) that takes a spare electron out of the process and feeds it into a wire.

Now, note something - you need fuel for this to work. In order to put one of those in your yard to power your house, you need it to be next to a fuel tank, or fed by a municipal fuel line, or something. So, yes, you can remove power lines and electrical distribution grids, but then you need to have fuel distribution instead. My house already gets natural gas, so I could install one of these in my basement. But for rural areas(2) that don't have gas service? Carting around big bottles of flammable liquids to remote areas is not itself cheap, energy-wise.

Also note that whether the energy is "clean" depends on the fuel used. If you use pure hydrogen, then you get water (3) and a bit of heat out - that's not bad, but pure hydrogen is actually pretty expensive. If you use a fossil hydrocarbon, you get CO2 out, which is not so good. If you use a biofuel (say, alcohol), then at least the carbon is coming from the biological carbon cycle - but typically you have to watch biofuels because their production processes are often not very clean.

first of all, the highlighted portions were the source of my concern. My question of how green / clean this is considering the use of hydrocarbon fuels. what is the exchange rate of say cu ft of natural gas to watt vs natural gas to watt of a nat gas generator?

second of all my points of interst as numbered

(1) I am guessing the two "inks" mentioned are the catalysts, the ceramic of silicone sand the substrate

(2)rural areas often are fueled by liquid petroleum gas: propane. They are usually in taks of various sizes sitting on the same side as where the kitche sits and brought into the house with copper tubing.

rural Missouri is about 99 percent run by this gas for heating and cooking.

another couple of hydrocarbon fuels you see is found on farms typically: agricultural diesel and ag gasoline.Chemically the same except for a green dye put in it. this fuels is not for road travel use as road taxes are not charged in this fuel and there are very heavy fines in finding the green die in your car's tank as a very convincing deturrant.

(3) water: I have seen a few water to fuel conversions out there, bu none are in the market yet. Where are they. Why are they not in use??

Umbran · Aug 31, 2015

Scott DeWar said:
what is the exchange rate of say cu ft of natural gas to watt vs natural gas to watt of a nat gas generator?

If you mean, what is the efficiency of the Bloom Box as compared to the traditional generator of the same type? I don't know. That's not a matter of theory, such that I can ballpark a guess. That's entirely dependent on the engineering details.

Something to note when considering the cost of a generation system - to honestly compare, you need to compare end-to-end. For traditional generation, you have losses from power transmission through wires to your home. However, there's a cost to distributing fuel in teh Bloom Box as well - say we are using those big propane tanks - you *drive a truck* to deliver it, so the gas used in that truck counts against the Bloom Box.

(3) water: I have seen a few water to fuel conversions out there, bu none are in the market yet. Where are they. Why are they not in use??

The only straight water -> fuel I know of is taking water and splitting it into hydrogen and oxygen. You can do this just by running a current through it - a normal high-school science experiment. The basic problem is that it takes a lot of electricity to do this, so it isn't terribly cost-effective - you don't get more Hydrogen energy out than you put electrical energy in. Another difficulty is that, when you take it to an industrial scale, impurities in the water matter. Since the water bubbles off as H2 and O2, any impurities are left behind, and they tend to gunk up production - getting on the electrodes, making them less efficient, and so on.

freyar · Aug 31, 2015

Yeah, Umbran said what I was going to say here. Water is the opposite of what you want from a fuel: its atoms are tightly bound, so you have to work pretty hard to get anything out of it.

An administrative note for the thread: Fall term will be starting soon at my university, so that means teaching classes, attending committee meetings, and generally having more students around to ask questions, etc. That's all in all a fine thing, but it does mean I'll have less time for posting here. So, this is kind of a "last call" for new questions --- I'll try to answer everything that comes in by Weds or Thurs this week, but I can't promise to answer anything after that. When I close the thread, I'll try to leave it so people can still comment, assuming I can figure out the administrative features.

Janx · Sep 17, 2015

I just thought of a new question, based on seeing a science news article title about 2 black holes colliding with each other.

How "fast" is the speed of gravity?

This is a flexible question, I'm not talking about how fast an apple falls from a tree, that's a googleable formula.

In the case of those 2 black holes, they are far away. What astronomers are watching now, happened a long time ago, and thus possibly even their collision and resulting changes in gravity (due to the new combined mass) has also already happened.

How long does it take for a new gravity shift to to be measured/or have impact at a set distance away?

Consider Star Trek: Generations where the crazy guy blew up stars in order to affect gravity to shift the travel path of the Nexus. For movie purposes, that was pretty fast.

freyar · Sep 17, 2015

Janx said:
I just thought of a new question, based on seeing a science news article title about 2 black holes colliding with each other.

How "fast" is the speed of gravity?

This is a flexible question, I'm not talking about how fast an apple falls from a tree, that's a googleable formula.

In the case of those 2 black holes, they are far away. What astronomers are watching now, happened a long time ago, and thus possibly even their collision and resulting changes in gravity (due to the new combined mass) has also already happened.

How long does it take for a new gravity shift to to be measured/or have impact at a set distance away?

Consider Star Trek: Generations where the crazy guy blew up stars in order to affect gravity to shift the travel path of the Nexus. For movie purposes, that was pretty fast.

Since you're not talking about the acceleration of an object due to gravity (the apple falling), I gather you mean the speed of gravitational waves. In general relativity or any similar theory of gravity, gravity waves travel at the speed of light in vacuum. So when those black holes collide, the gravitational waves ("ringing of spacetime") travel outwards at the speed of light. Incidentally, the Advanced LIGO (that's the Laser Interferometer Gravitational-Wave Observatory) is just coming on line this month and has a very good chance of detecting gravitational waves from colliding black holes within the next few years. That would be the first direct measurement of gravitational waves. We have indirect evidence from watching two pulsars which are orbiting each other; their orbit is decaying precisely according to the prediction of energy loss to gravity waves (and the observations have won a Nobel prize).

So the timing in ST: Generations is a bit fast. It would really take a few minutes for light or gravitational waves to get from an exploding star to a planet at an earth-like distance. That's not the only issue, either. To create a gravitational wave, the explosion of the star couldn't be spherical but would have to be quite asymmetric. It's been a while since I've watched the movie, but I don't think that explosion was really odd-looking enough.

Janx · Sep 17, 2015

freyar said:
Since you're not talking about the acceleration of an object due to gravity (the apple falling), I gather you mean the speed of gravitational waves. In general relativity or any similar theory of gravity, gravity waves travel at the speed of light in vacuum. So when those black holes collide, the gravitational waves ("ringing of spacetime") travel outwards at the speed of light. Incidentally, the Advanced LIGO (that's the Laser Interferometer Gravitational-Wave Observatory) is just coming on line this month and has a very good chance of detecting gravitational waves from colliding black holes within the next few years. That would be the first direct measurement of gravitational waves. We have indirect evidence from watching two pulsars which are orbiting each other; their orbit is decaying precisely according to the prediction of energy loss to gravity waves (and the observations have won a Nobel prize).

So the timing in ST: Generations is a bit fast. It would really take a few minutes for light or gravitational waves to get from an exploding star to a planet at an earth-like distance. That's not the only issue, either. To create a gravitational wave, the explosion of the star couldn't be spherical but would have to be quite asymmetric. It's been a while since I've watched the movie, but I don't think that explosion was really odd-looking enough.

gravity comes in waves?

I'm picturing the bowling ball on a mattress example to reflect how it impacts a marble rolled across the bed past the indentation made by the heavier ball. That's pretty static (no waves), which also doesn't likely trigger my question of speed.

I imagine, gravity as waves being applicable in my blowing up a star example. That is more akin to tossing a pebble into a pond and the ripple of waves traveling outward having a certain speed. This sudden change in the presumably static system takes time to propagate the effects.

Is this closer to what you meant by gravity waves or is there some kind of graviton thing like photons?

Umbran · Sep 18, 2015

Janx said:
gravity comes in waves?

We think so. *EVERYTHING* comes in waves. *YOU* come in waves. For you, the wavelength is very, very, very short, so you look like a solid object.

But, we can consider light to be particles (photons) or to be waves in an electromagnetic field. We can consider gravity to be particles (gravitons, not yet observed, and we don't have a good quantum theory of gravity to tell us how gravitons act), or as waves in a gravity field (also not yet observed). The gravity field determines the curvature of spacetime, so waves in the gravity field mean waves in the curvature of spacetime.

I'm picturing the bowling ball on a mattress example to reflect how it impacts a marble rolled across the bed past the indentation made by the heavier ball. That's pretty static (no waves), which also doesn't likely trigger my question of speed.

Imagine it is a water bed. Now imagine the bowling ball is a little lopsided, and spinning. It'll wobble as it spins, and that will produce ripples in the water bed.

freyar · Sep 18, 2015

Janx said:
gravity comes in waves?

I'm picturing the bowling ball on a mattress example to reflect how it impacts a marble rolled across the bed past the indentation made by the heavier ball. That's pretty static (no waves), which also doesn't likely trigger my question of speed.

I imagine, gravity as waves being applicable in my blowing up a star example. That is more akin to tossing a pebble into a pond and the ripple of waves traveling outward having a certain speed. This sudden change in the presumably static system takes time to propagate the effects.

Is this closer to what you meant by gravity waves or is there some kind of graviton thing like photons?

Umbran gave a nice answer already, but as I thought of some things I wanted to say before I saw it, I'll go ahead and add them.

There are really two ways to look at your question(s). We can think about things in terms of classical "old-fashioned" physics, as if we'd never heard of quantum physics. What is gravity like? A good analogy is electricity and magnetism. You can have static electric and magnetic fields. If you have ever sprinkled iron filings around a magnet and watched them line up with the field, that's an example of a static magnetic field. If you've ever seen someone hold on to a van de Graaf generator, their hair stands on end because of a static electric field. Those fields are like the basically static gravitational field that holds us to earth or hold the earth in orbit around the sun (or, in relativistic terms, the static but curved spacetime). On the other hand, electromagnetic waves are also familiar --- good classical examples include radio waves, which are created by moving charges. Similarly, moving masses create gravity waves, which, yes, mean ripples in spacetime.

As Umbran says, in quantum physics, everything is a wave and a particle, simultaneously. When we think of a photon as a "particle" of light, we really mean it is a very tiny wave that moves at the speed of light. Those radio waves I mentioned before are really conglomerations of lots and lots of individual photons. Similarly, a gravity wave would be a conglomeration of lots and lots of gravitons particle/waves (we do have enough understanding of quantum gravity to say that --- they are a common feature of all quantum gravity theories I know of). Those static fields, like the gravitational field of the earth or a star, are created by what we call "virtual particles." These are really more little waves, but they're waves that don't look like waves moving at light speed. They're more like localized ripples. So the gravity of the earth that we feel is due to lots and lots of little graviton ripples.

One other neat thing about gravity waves: like I said before, we're hoping to observe them from the collision and merger of black holes within the next few years. The fun thing about these gravity waves is that their frequency is expected to be in the same range humans can hear. So rather than having to plot things about the waves, you can just run the signal from the detector to a speaker and listen to the gravity waves (well, you'd want to take the experimental noise out first). Black hole mergers should sound like a "chirp" that starts out a low pitch and goes to a high pitch over the course of a few seconds or less. I've heard output from a number of simulations. This site has some audio for you. The LIGO experiment site also has some, but I'm having trouble getting that to play.

tomBitonti · Sep 20, 2015

There is a, I would imagine, very usual demonstration of relativity which puts a flash at a center point and sets it off with one observer moving past while another is at rest relative to the flash. The invariance of the speed of light is then used to determine the equations for transforming between the frames of the two observers. Along the way the notion of simultenaity is shown to be limited, and other results.

The issue, and my question, has to do with assumptions which are made in regards to each of the frames, in that these are assumed to be flat and that measurements can be reliably made, leading to the calculations and so forth.

My problem is that the calculations show that measurements are, generally, not quite as easy to make, as it would seem, and, because of this, the validity of those seems to depend on some additional assumptions (I'm guessing some statement about asymptotic flatness or thereabouts) but this part of the reasoning doesn't ever seem to be addressed. Can any insight in this regards be provided?

Thx!
TomB

Umbran · Sep 20, 2015

tomBitonti said:
There is a, I would imagine, very usual demonstration of relativity which puts a flash at a center point and sets it off with one observer moving past while another is at rest relative to the flash. The invariance of the speed of light is then used to determine the equations for transforming between the frames of the two observers. Along the way the notion of simultenaity is shown to be limited, and other results.

That example is frequently used to describe some of how relativity impacts the idea of simultaneity, yes. But nobody actually physically performs that demonstration. It is done in the imagination, a "gedankenexperiment" (thought experiment - a term first coined by Einstein, precisely because he couldn't actually do physical experiments at the appropriate speeds)

The issue, and my question, has to do with assumptions which are made in regards to each of the frames, in that these are assumed to be flat and that measurements can be reliably made, leading to the calculations and so forth.

Overall, the universe *is* extremely flat. Yes, there's some minor curvature around massive bodies, but we can correct for that.

My problem is that the calculations show that measurements are, generally, not quite as easy to make, as it would seem, and, because of this, the validity of those seems to depend on some additional assumptions (I'm guessing some statement about asymptotic flatness or thereabouts) but this part of the reasoning doesn't ever seem to be addressed. Can any insight in this regards be provided?

We don't usually go deeply into the assumptions in discussions with laymen, as they tend to clutter up the conversation. Let us remember that Einstein developed his theories in the first part of the 20th century (the first publication of Special Relativity was in 1905). Actually doing the experiment described above was out of the question.

But, the assumptions of special relativity are quite simple. And they have their own wikipedia article!

https://en.wikipedia.org/wiki/Postulates_of_special_relativity

Einstein assumed very few things:

1) The speed of light in vacuum is a constant in all inertial frames of reference.

2) The laws of physics are the same in all inertial frames of reference

Special Relativity then goes on to consider motion in those inertial frames of reference. An "inertial frame" is one in which space is described homogeneously (it has the same properties at every point), isotropically (it is the same in every direction we look), and as time-independent. It turns out that all inertial reference frames are in uniform linear motion with respect to each other.

You can hack out some interesting things from special relativity when considering uniformly accelerating frames, which are not inertial, but the base assumptions are frames that are moving in straight lines at constant velocities with respect to one another: So, for example, I am sitting on the ground, and a train goes by on a straight track at a constant speed.

Which all boils down to: Einstein assumed the outright simplest case he could for special relativity.

For general relativity, we consider spaces that are not necessarily outright inertial frames, but in which the curvature of spacetime isn't too big, so that we can think of it as being made of an infinite number of frames in which space is locally flat*. This is like finding the area under a curve by cutting it into an infinite number of rectangles and adding together their areas. In other words, special relativity is the arithmetic of spacetime, while general relativity is the calculus.

*This is why black hoes are such a big pain in the relativistic neck, because they are the cases where, at the singularity, the curvature of spacetime finally gets too big, and GR falls apart.