Has a gravitational force been observed between massless particles? by 12ShotsThenHome in askscience

[–]AsAChemicalEngineer 3 points4 points  (0 children)

To add to the answers here, he's the starting reference for those interested in the topic:

  • Tolman, Richard C., Paul Ehrenfest, and Boris Podolsky. "On the gravitational field produced by light." Physical Review 37.5 (1931): 602.

A creationist told me that science, under uniformitarianism, basically assumes that things have always occured as they do now. Is this true? If it's true isn't that a problem? by BookerDouglas in askscience

[–]AsAChemicalEngineer 65 points66 points  (0 children)

I have never even heard of uniformitarianism, but after a quickly skimming the Wikipedia article, I'll now pretend to be an expert on it. I won't comment on its relation to geology where the idea seems to have gotten more attention, but from a physics perspective I can say this:

Yeah, science isn't invincible. It cannot defend itself against Last Thursdayism and it cannot defend itself against perfect and completely deceptive coincidence which reminds me of a bit in the play "Rosencrantz and Guildenstern are Dead" where the two titular characters bet on coin tosses using a coin which comes up heads nearly every time. The universe R+G live in is utterly unfair because Hamlet was already written therefore R+G cannot survive their own play. In a similar fashion we have to assume that the universe is being honest and fair with us when it reveals behavior through experiment. It we cannot do this, we can't move forward and do things like build computers and radio telescopes.

If it's true isn't that a problem?

Not really. I'm emphatically uninterested in the myriad of 'gotcha' carefully constructed what-ifs which do nothing but make sure that the natural laws are somehow not what they appear to be to plain, but careful eyes. Nobody else has even come close to making a formal system of generating knowledge about the natural world that works as well as science.

Time is relative. I can understand this on a basic level, but how can time be faster in the past? by cinymin in askscience

[–]AsAChemicalEngineer 19 points20 points  (0 children)

That quote is fine until the "one physicist has suggested..." part which then becomes wild speculation which contradicts our current understanding. While trained physicists aren't immune to pulling ideas out of their rear, that article you link doesn't provide names or other resources... might just be made up even.

Anyway, the core idea here we're discussing is that different observers will disagree on how much time has passed between events. In plain and old special relativity, this disagreement arose because different observers were moving relative to one another. To preserve the speed of light as being the same for everyone, moving observers can no longer assume they'll measure the same distances and times as other observers.

General relativity makes things a bit more interesting, not only do observers moving relative to one other disagree on times and distances, but observers in different places now disagree as well. Curvature in spacetime (e.g what we call gravity) now becomes the cause for this. The classic example is the portion of time dilation we experience compared to global positioning satellites because we're deeper in Earth's gravitation field than the satellites are. We don't even have to leave our solar system to see this effect.

Now back to the galaxies accelerating away... Our universe is indeed expanding and that expansion is accelerating. This is pretty surprising considering matter is attractive! You would think that gravity's attraction means that the universe would either: slow in expansion until it "falls back" into itself like a big crunch (think of a baseball being thrown up and eventually falling back to Earth) or slow in expansion to some residual expansion rate and just coast (think of a baseball being thrown so hard it escapes the Earth).

You can check out the FAQ for more info, but in short our current model of the universe requires we fill all of space with a substance with negative pressure we call "dark energy" which is responsible for this accelerating expansion. We have no idea what it is or why it's there, but essentially it has a constant density which doesn't change even if you increase the amount of space. This is very different from say the behavior of a gas which decreases in density as you increase the volume. Like any form of energy, this "dark energy" curves spacetime. This means those far away accelerating galaxies seem to be working in slow motion; time seems slower over there. This isn't anything to freak out about too much though because to aliens in that galaxy it is our galaxy that seems to be operating in slow motion and time is normal for them.

Edit: That quote is actually not fine until the "one physicist" part. It's wrong much sooner. Even with a constant (not accelerating) expansion, it is still true that father galaxies are moving faster away than closer galaxies! Blow up a balloon halfway and then draw a grid of dots onto it equally spaced out. Now blow up the balloon some more. You'll see that even though the distance between every adjacent dot has increased the same, the n-th far away dot has moved n-times as far as the closest dot in the same amount of time!

The shock to physicists around 20 years ago is that the expansion rate itself is becoming larger than just the situation described above.

Could I have a bag of neutrons? And if so, what would it look like, would they be reactive? by rmanx90 in askscience

[–]AsAChemicalEngineer 1 point2 points  (0 children)

The residual strong force, aka nuclear force, has a significant drop off with distance. The strong force itself, also called the color force, which involves quarks and gluons does not drop off with distance.

This manifests in color confinement, or why we don't see free quarks. By the time you've spent the energy pulling two quarks apart an appreciable distance, the system spontaneously decays into more quarks-antiquark pairs and preventing the free quark from happening.

The reason the residual strong force drops off with distance so quickly is that the particles which mediates the interaction are massive mesons (quark-antiquark pairs). Unlike the photon which is massless and has essentially infinite reach, massive bosons can't propagate so readily. Mathematically it means the potential energy exponentially decreases with the exponent containing the mass,


Could I have a bag of neutrons? And if so, what would it look like, would they be reactive? by rmanx90 in askscience

[–]AsAChemicalEngineer 12 points13 points  (0 children)

Simply put, in this situation the gravitational force is just larger. The residual nuclear force is also repulsive at roughly <0.7 femtometers. This repulsion is what keeps the star from completely collapsing. This however only works up to a point because GR has "instabilities" which allow gravity to grow faster than this repulsion if you have too much mass. This then triggers the formation of a black hole.

Edit: I need to spent some time reading up on this, but there seems to be two competing explanations for why neutron stars can support their size without collapsing: (a) Neutron degeneracy pressure holds them up and (b) the residual strong force is repulsive at short distances, and that repulsion is due to vector mesons rather than the attractive scalar pions.

My impression is that both play the leading role, but at different depths inside the neutron star. This makes sense to me since neutron stars inner layers boast densities higher than atomic nuclei. I would appreciate if someone corrected me or provided clarification here.

Could I have a bag of neutrons? And if so, what would it look like, would they be reactive? by rmanx90 in askscience

[–]AsAChemicalEngineer 26 points27 points  (0 children)

No, the nucleon density in atoms is comparable to the nucleon density in neutron stars. This is why the idea of a neutron star being a giant nucleus has some traction. REC's point though is that because the dominant force in the two situations are completely different, the comparison is probably not that useful.

Bosons have integer spin (such as 1, 2 or 3) whereas fermions have half-integer spin (such as 1/2, -1/2, 3/2, 5/2). In what situation does a boson have for instance spin 3, and a fermion spin 3/2? by MarklarE in askscience

[–]AsAChemicalEngineer 5 points6 points  (0 children)

/u/JanEric1, here's a good write up on the topic:


There's also a really lovely argument that forbids massless spin-3 particles because to have such particles (combined with Lorentz invariance) requires an energy conservation rule which is nonphysical.

AskScience AMA Series: I am Jessica Pierce, a bioethicist who has recently focused my work on animals. AMA! by AskScienceModerator in askscience

[–]AsAChemicalEngineer 16 points17 points  (0 children)

Has in your opinion the benefits of animal testing in scientific and medical settings outweighed the harm that often comes to these animals? If animal testing is only sometimes justifiable, what criteria would you use to minimize needless harm and are the criteria restricting when animal testing can and cannot be performed sufficient today?

Stephen Hawking megathread by AskScienceModerator in askscience

[–]AsAChemicalEngineer 1 point2 points  (0 children)

If the gravity of a black hole is so strong as to prevent light (which is the heat we're discussing) from escaping, then how does something maintain rigidity? And if it cannot maintain rigidity, the the natural conclusion is that everything collapses to the center leaving only curved empty space.

You seem to be keeping separate the ideas of physical rigidity and whether light can escape. In reality, the two concepts are deeply linked and if not even light can escape, then rigidity cannot exist.

Hawking's result is a loophole that allows black holes to emit light, but the trick only works if black holes act like described earlier in our conversation.

Edit: I'm not arguing at new physics couldn't remove the singularity, and replace it with something of finite size, butour modern understanding doesn't allow this. Personally I think black holes act like GR predicts except very close to the singularity, then new physics takes over, but I don't know what that physics is.

Stephen Hawking megathread by AskScienceModerator in askscience

[–]AsAChemicalEngineer 2 points3 points  (0 children)

  1. It's just changing time 't' to 'it' where 'i' is the square root of negative one. The normal imaginary number rules apply, e.g i2=-1 and so on. The fancy word for this is "analytic continuation," which basically exploits that a lot of physics equations aren't sensitive to imaginary variables, by switching to them a lot of physics problems become easier to solve. I wouldn't take it too literally, it's a useful math tool.

  2. Cause black holes are almost all super cold. We haven't actually measured Hawking radiation because all the known black holes are too massive and thus too cold. We can see black holes via two measurements (a) their gravity, like if they are in a binary orbit with a star we can see, which looks like a star is orbiting around something invisible. In the case of supermassive black holes, the stars orbit it like the planets orbit the Sun. (b) Low density, colder gas around the black hole emits radio waves and high density gas, hotter gas actively falling in makes jets that are bright in X-rays (well, bright in everything really).

  3. "i think it’s much more likely a black hole is just a quark star or preon star." Perhaps, but you need new physics to replace general relativity to do this, the gravitational interaction becomes unbounded in strength inside the event horizon and unless new physics intervenes, none of the other forces can withstand it and prevent collapse into a point. Also you need to explain why black holes are so cold, while a neutron star (what happens to a star that dies, but is not quite heavy enough to make a black hole) are super hot. Quark stars are much much more likely to be very hot and mistaken for neutron stars. "a tiny black hole and a supermassive black hole would carry the same size and mass" This isn't how singularities in general relativity work. A black hole's event horizon radius is proportional to its mass.

Mass is a funny thing in relativity. For example, while photons are massless, if you have two photons traveling away from each other, the system of two photons has a mass. Another example is the mass of the protons and neutrons which are much much more massive than the quarks inside them. It is a combination of quark momentum and gluon field density which generates the mass of the protons and neutrons. Black holes just join this freaky parade of concepts of mass we humans are ill-equipped to understand intuitively, a black hole is empty vacuum everywhere, but still has a mass which is locked in the gravitational field.

Stephen Hawking megathread by AskScienceModerator in askscience

[–]AsAChemicalEngineer 16 points17 points  (0 children)

It's not obvious that if black holes emit particles, that the spectrum (e.g how many photons of X energy versus Y energy) would be a thermal distribution we normally associate with objects with a temperature. Thermal distributions come about from quantum systems that have many degrees of freedom and can emit energy in many ways, but because the emission is quantized aka photons, the statistics of what emissions are most likely is restricted.

This is wholly unlike a black hole which is... well... a vacuum that happens to have a funky geometry. A black hole isn't like a bunch of iron atoms being heated on your electric stove, because a black hole doesn't have many microscopic quantum parts... or at least we can't describe black holes as having many microscopic quantum parts in general relativity.

My view is that this is because of some unclarified relationship between geometry, and thermodynamics we've just uncovered a small part of. That you can "derive" the Hawking temperature entirely by geometrical arguments ignoring what Hawking initially did, but later realized, is a red flag for me. The highlights of that connection is this,

  • quantum field theories at finite temperature have this funky property of being "periodic in imaginary time." This happens because the time evolution in quantum mechanics (how stuff changes in time, duh!) and the partition function Z (how likely is your system to be in a certain state/configuration at some temperature) are basically the same math.

So here's the game--find the periodicity, you get the temperature.

  • If I look at a black hole, but consider imaginary time, you immediately in like 2 lines of algebra get a periodicity related to the mass of the black hole. And bingo. We found the periodicity of a black hole, and therefore we know its temperature.

It's basically black magic.

Stephen Hawking megathread by AskScienceModerator in askscience

[–]AsAChemicalEngineer 0 points1 point  (0 children)

His most important is of course the radiation result itself which I would say spawned an entire sub-field in physics, solving the problem of black holes and thermodynamics with Bekenstein and others, clarifying properties of the singularity and in my view how spacetime works (though that was more Penrose's jam). I would also cite his work on early universe cosmology and quantum gravity with Hartle and Gibbons.

What do you know of the work he actually did?

I already regret trying to rank physicists at all in only that it has led you to being rude. :/ Have a nice evening.

Stephen Hawking megathread by AskScienceModerator in askscience

[–]AsAChemicalEngineer -2 points-1 points  (0 children)

You are drastically, DRASTICALLY, underestimating Oppenheimer's significance with respect to the success of the development of the atomic bomb.

I half-consciously excluded his work in the development in the atomic bomb in my assessment. I won't disagree that his role was of great importance.

Even Hans Bethe, who has a Nobel Prize himself and was 2nd in command I believe, referred to RJO as their intellectual superior.

I wasn't able to find a quote to that effect, but I did find one from a letter to his mother, "I am about the leading theoretician in America. That does not mean the best. Wigner is certainly better and Oppenheimer and Teller probably just as good. But I do more and talk more and that counts too."

My impression is that we're evaluating scientific importance with different rubrics, my personal bias is towards ideas and contributions that change how we view the natural world. To that end, I see Hawking's work as having much longer legs if true.

Stephen Hawking megathread by AskScienceModerator in askscience

[–]AsAChemicalEngineer 18 points19 points  (0 children)

Yup. As wonderful as his research was, essentially none of it can yet be experimentally tested.

Stephen Hawking megathread by AskScienceModerator in askscience

[–]AsAChemicalEngineer 13 points14 points  (0 children)

He's a caliber of thinker that only shows up several times a century, certainly no lightweight! Someone like Einstein in comparison only once every couple hundred years.

Edit: I would add "several times a century" in a major field of study, not science as a whole.

Stephen Hawking megathread by AskScienceModerator in askscience

[–]AsAChemicalEngineer 3 points4 points  (0 children)

or even Oppenheimer

I would place Hawking above Oppenheimer. Funnily enough probably Oppenheimer's most influential research was in astrophysics including a paper on gravitational collapse that comes to mind.

The other two you mention are certainly one-class above Hawking, but from my lowly perch all of them look like giants!

Stephen Hawking megathread by AskScienceModerator in askscience

[–]AsAChemicalEngineer 51 points52 points  (0 children)

The particle tunneling picture is in Hawking's own words "heuristic only and should not be taken too literally." It gives you a useful mental image, but it's not something you need in order to make the arguments for radiation that he made. His insight was basically that in quantum field theory, the flux across a surface in vacuum depends on the space-time curvature. He showed that the taken-for-granted result that an empty vacuum stays that way doesn't always hold. The distribution of produced particles or radiation is thermal, which is a minor miracle.

What sort of particles is it?

All of them. While the details change, Hawking's argument doesn't care what kind of particle we're talking about, if it obeys QFT, it will be emitted. However the caveat is that if the mass M > kT, (Boltzmann's constant times temperature) then those particles don't participate much in emission. Once the black hole gets small enough and therefore hot enough, you can expect it to emit massive particles like electrons and positrons too at large rates too.

How should society determine what weapons are appropriate for private ownership? by BuckleUpItsThe in PoliticalDiscussion

[–]AsAChemicalEngineer 1 point2 points  (0 children)

Well I mean, neither has the US stockpile of nuclear weapons; and in any case you wouldn't buy a gun that shoots low velocity marshmallows for self-defense. Guns are undeniably designed to be accurate and lethal.

The question that whether the utility of a gun, e.g a lethal tool used to protect yourself and others, has on a societal level actually led to more harm than good is worth exploring.

My impression is that we agree that the answer is no, but I wouldn't call the above poster's argument moronic.