AMA about Physics

[Magmoo]

One the great gaps between the scientifically literate and those who are less so is the proper understanding and use of terms. The whole "theory" issue for example. Are there any fundamental terms and concepts that you experience people miss using that you would like to clear up?

[Burning]

Another thing I thought of (in terms of theories and skepticism etc) is that something can be far from "set in stone" (the way I would consider Newtonian physics or the Pythagorean theorem to be "set in stone") can still be experimentally valid and more importantly, practically useful. For example non-real numbers are pretty esoteric to most people and didn't even get accepted as worth consideration by academics until a few hundred years ago, but mechanical and civil engineers use them all the time to allow them to build physical things that we walk on/in.

It could follow that a theory/model that later proves to be off the mark, could in the present be somewhat useful for a number of practical applications, not to mention being a necessary stepping stone to a more accurate theory.

What you're describing is not uncommon in the history of physics. The big famous one is classical physics being supplanted by relativity and quantum mechanics. Without all the developments of classical physics, physicists would never have been able to find the places where our common sense notions start to break down. And of course, classical physics wasn't just good an useful before the advent of modern physics; it's still useful today for a great number of applications where quantum and relativistic effects are orders of magnitude to small to be noticed. Your electronic devices need quantum mechanics to be understood and your GPS relies on relativity. Other than that, your everyday life can be described perfectly well with classical physics (granted, in the 21st century your everyday life involves a lot of electronic devices.)

On a smaller scale, both electricity and thermal physics were first conceived using fluid models. We can see the remnants of this in modern language: electric currents and heat flow. The fluid models aren't really useful any more, but electromagnetic field theory and statistical mechanics needed the accomplishments of the scientists who worked in the fluid models to get off the ground.

[Burning]

One the great gaps between the scientifically literate and those who are less so is the proper understanding and use of terms. The whole "theory" issue for example. Are there any fundamental terms and concepts that you experience people miss using that you would like to clear up?

I'm going to broaden this a bit to misconceptions that bother me. There is a bit of an element of misuse or misunderstanding of terms to what I'm going to talk about, but I'm going to be going beyond just the meaning of words. Just wanted to be out in the open about that.

Anyway, there were two things that particularly sprung to mind with this question.

1. "Uncertainty" is not a bad thing

There seems to be an expectation among a lot of people that science will give absolute answers. That's not actually possible, even in principle.

There is no measurement technique that is perfect. Measuring the same thing twice can result in two different answers. The better the technique, the smaller the difference will be, but you can't eliminate the possibility of discrepancy completely. Furthermore, there can be outside influences that can introduce a variation. When we talk about repeating a measurement, we mean that we do our best to have everything the same the second time as it was the first. There's still going to be stuff beyond our control, however.

"Uncertainty" is our estimate of how large our assorted variations might be. No measurement is really complete without an uncertainty. The measurement result is the scientist's best estimate of the real value. The uncertainty gives the range of values that the scientist is reasonably confident contains the real value.

Of course everyone wants small uncertainties. But a large, realistic uncertainty communicates important information. A small but unrealistic uncertainty is worse than no uncertainty at all.


2. Science does not require "experiments," and experiments do not require "controls."

OK, this is really two misconceptions, but they are extremely similar. Both are based on a true principle, but have the problem that they're too specific.

I'm a bit sensitive to the first one, having a background in astrophysics. I have actually been told by people that astrophysics is not a science because you can't you can't perform experiments on stars, planets, etc.

To be clear, science requires objective observations. If you have those, you can evaluate the predictions of the scientific theories. Experiments are observations where you have some control over what happens, not just what you look at. That control makes it easier to get the data you need. I'm sure there are astrophysicists that would like to have a star-in-a-jar that they could fiddle with. But it's not necessary to have that control, and it doesn't make your observations more objective.

Similarly, in all observations whether they are part of experiments or not, you need to be able to isolate the factors you are interested in from all the others that might be present. The control subject or control group method that you learned about in grade-school science is one technique for this. It is excellent when you are trying to understand how person/animal/system responds to outside stimula or influences. It's not much use if you are trying to study the intrinsic properties of something. Furthermore, in experiments where a control is appropriate, it is not necessarily adequate.

Too many people have been taught the importance of a control without really understanding its role properly. So they judge results based on whether there is something that they recognize as a control, without considering whether or not there's something that doesn't really look like a control but that serves the same purpose.

[tombombodil]

One the great gaps between the scientifically literate and those who are less so is the proper understanding and use of terms. The whole "theory" issue for example. Are there any fundamental terms and concepts that you experience people miss using that you would like to clear up?

I'm going to broaden this a bit to misconceptions that bother me. There is a bit of an element of misuse or misunderstanding of terms to what I'm going to talk about, but I'm going to be going beyond just the meaning of words. Just wanted to be out in the open about that.

Anyway, there were two things that particularly sprung to mind with this question.

1. "Uncertainty" is not a bad thing

There seems to be an expectation among a lot of people that science will give absolute answers. That's not actually possible, even in principle.

There is no measurement technique that is perfect. Measuring the same thing twice can result in two different answers. The better the technique, the smaller the difference will be, but you can't eliminate the possibility of discrepancy completely. Furthermore, there can be outside influences that can introduce a variation. When we talk about repeating a measurement, we mean that we do our best to have everything the same the second time as it was the first. There's still going to be stuff beyond our control, however.

"Uncertainty" is our estimate of how large our assorted variations might be. No measurement is really complete without an uncertainty. The measurement result is the scientist's best estimate of the real value. The uncertainty gives the range of values that the scientist is reasonably confident contains the real value.

Of course everyone wants small uncertainties. But a large, realistic uncertainty communicates important information. A small but unrealistic uncertainty is worse than no uncertainty at all.


2. Science does not require "experiments," and experiments do not require "controls."

OK, this is really two misconceptions, but they are extremely similar. Both are based on a true principle, but have the problem that they're too specific.

I'm a bit sensitive to the first one, having a background in astrophysics. I have actually been told by people that astrophysics is not a science because you can't you can't perform experiments on stars, planets, etc.

To be clear, science requires objective observations. If you have those, you can evaluate the predictions of the scientific theories. Experiments are observations where you have some control over what happens, not just what you look at. That control makes it easier to get the data you need. I'm sure there are astrophysicists that would like to have a star-in-a-jar that they could fiddle with. But it's not necessary to have that control, and it doesn't make your observations more objective.

Similarly, in all observations whether they are part of experiments or not, you need to be able to isolate the factors you are interested in from all the others that might be present. The control subject or control group method that you learned about in grade-school science is one technique for this. It is excellent when you are trying to understand how person/animal/system responds to outside stimula or influences. It's not much use if you are trying to study the intrinsic properties of something. Furthermore, in experiments where a control is appropriate, it is not necessarily adequate.

Too many people have been taught the importance of a control without really understanding its role properly. So they judge results based on whether there is something that they recognize as a control, without considering whether or not there's something that doesn't really look like a control but that serves the same purpose.

You should do a series of "Pretty big things you wish people would comprehend"

[Lord Foul]

What I don't get about gravity, and weight, is how far does it reach? I mean, everything that has mass has gravity, and the greater the mass the greater the gravity. And gravity acts upon stuff with mass to give it weight, presumably after you've subtracted the gravity effect of the mass itself.

So... if the total mass of stuff in the universe is BRIAN BLESSED (a very large space number tending to infinity), why isn't the weight of everything absolutely ENORMOUS?

Or, put another way, how far does gravity reach?

You are on the edge of deep waters. The short version is:

(a) Gravity reaches forever, but gets very weak as you get far away.
(b) However, the main reason that the force of gravity on you from a huge universe is so small is that you're surrounded by the stuff so the pull from the stuff on one side gets cancelled by the pull of the stuff on the other.
(c) "Weight" doesn't actually have a good scientific definition for complicated reasons. However, our every day experience of weight is the force of gravity from Earth alone.

EDIT: If you're interested I can give a longer answer in my Physics AMA thread.

Thanks for the explanation.

It does kind of make sense to me now, with the cancelling out effect. It's pretty obvious really, when you think about it. The one bit I'm still curious about is your line:

(a) Gravity reaches forever, but gets very weak as you get far away.

So, is gravity a function of both mass and distance, then? At what point do you stop falling towards Earth and start falling towards Mars, relative to the distance between them? (and assuming for convenience that all other space matter happens to be in perfect balance at that moment)

Also, does a fat person fall faster than a thin person because their mass is higher and therefore their gravitational pull on the planet is slightly greater?

[Gyro LC]

Also, does a fat person fall faster than a thin person because their mass is higher and therefore their gravitational pull on the planet is slightly greater?

Galileo had a classic experiment about that.

[Lord Foul]

The final part, about Einstein's theory, is interesting.

Well, the whole thing is fascinating and downright amazing, but you know what I mean...

If he's right and they are not moving, then that must mean the world is moving up at them, which is all very well except that the same thing happens wherever you are around the globe, and it can't be moving in every direction at once.

[BadMrMojo]

What I also love about this train of thought is that it goes back to the very first question in the thread, from almost 3 years ago.

Right now, using classical physics, you can calculate your gravitational effect upon the moon, for example. We can easily observe that the moon's gravitational pull has an impact here on earth - tides being the most obvious example - but in exactly the same fashion each and every single thing with mass has a corresponding pull upon the moon (insert "your momma's so fat..." joke). The number you come up with is infinitesimally small - completely beyond our capacity to measure - but mathematically demonstrable. The reason it's so small is because the attraction due to gravity is an inverse square of the distance - so twice as far away means four times less force. 4 times as far away is 16 times less force, etc. That adds up really quickly. If you think of a logarithmic chart that starts slowly and then suddenly spikes upward really sharply, it's the exact opposite of that shape. "Relatively strong, almost unchanged, getting weaker, plummeting quickly, and then almost infinitely small."

Taking that a step further (and slipping from physics into philosophy for a moment), any force also has a direction and whether you choose to stay at home or go out, for example, has a nearly infinitely small - vastly too small to measure but technically calculable - impact on the position of the moon. Not only does your decision make an immeasurably small impact on the moon, but also quite literally upon everything else in the entire universe. And then each and every one of those things has a corresponding effect on everything else. It's just so insanely, immeasurably, inconceivably, incalculably small that you'll break your calculator before figuring it out. Similarly, absolutely everything in the universe is exerting some influence upon absolutely everything else in the universe at every single moment and we have no capacity to ever take all of that into account. That makes the universe an infinitely complex system and every single decision you make - for good or ill - has an impact. Similarly, every single decision - while a deterministic result of a dizzying array of influences - can only be the result of a combination of factors so infinitely complex that it is no longer recognizable as causal. I find all that oddly comforting and applicable to a lot more than just gravity (which just happens to be my favorite example).


Meanwhile, back in the realm of physics, the theoretical quantization of time - suggesting that time consists of discrete, indivisible units - challenges that whole infinite gravitational range thing. Force is measured in Newtons, one of which is enough force to accelerate 1 kg by 1m per second^2. So one Newton of pull applied continuously to a 1kg weight will get it moving at 1m/s after 1 second, 2m/s after 2 seconds, etc...

If those seconds in that equation eventually become small enough to lose any meaning - a span shorter than the theorized smallest possible unit of time - then you start to introduce zero into the equation. If that's the case, then you can get to a distance at which an object's gravitational attraction is no longer just immeasurably small, but it actually becomes zero because it involves units and scales of time which are no longer valid. It's like a bank that can't calculate values lower than a penny. If you earn 0.4% interest on a dollar you've earned less than one penny - the smallest quantized unit of currency in this example - and therefore it has exactly zero impact on your balance.

Slipping back into the philosophy part, that means that everything is not inherently connected and the workings of the world are the deterministic results of nearby influences in finite complexity (incalculable, sure, but ultimately finite). You eventually reach a point where there's nothing to gain by looking closer. It essentially dismantles everything that I happen to hold rather dear.


tldr: the quantization of time terrifies me because it breaks my entire outlook on life.

[Burning]

Sorry for the delay in answering. I write slowly, and then have to rewrite because what I come up with is overlong and confusing. (If it still strikes you this way, then all I can say is "Just imagine..." ) I've tried to incorporate the various remarks made in the meantime, but I'm almost certainly missing something. Anywho...

So, is gravity a function of both mass and distance, then?

That’s right. If we’ve got two objects, 1 and 2, and they’ve got masses m1 and m2 and they are a distance r apart, the force of gravity between them is proportional to

m1*m2/(r*r)

(Pardon my crude notation. The forums don't handle formula well.)

Also, does a fat person fall faster than a thin person because their mass is higher and therefore their gravitational pull on the planet is slightly greater?

So you’ve seen the video that shows that the heavy thing doesn’t fall faster if there are no complications like air resistance. But the gravitational pull on the heavier thing is stronger, so how does that work?

It’s because of how force influences motion. If an object with a mass m is subjected to a force F, then it experiences an acceleration a = F/m. So for most forces, the mass of the thing the force acts on makes a big difference, but for gravity, the mass of the thing being accelerated cancels out. The gravitational acceleration of object 1 is equal to F/m1, but the force is proportional to m1*m2/(r*r), which leaves the acceleration proportional to m2/(r*r).

I’m actually putting the cart before the horse here in terms of discover. Galileo is the one who established that gravitational acceleration is the same for everything, but he didn’t have anything like the modern concept of force. Newton had something close to the modern concept of force and knew that the acceleration caused by the force had to be independent of the mass, so that guided him to the correct dependence of the gravitational force on mass. But people knew gravity was weird before Newton came up with a formula.

At what point do you stop falling towards Earth and start falling towards Mars, relative to the distance between them?

This is a more complicated question than it seems at first, because Earth and Mars are both in motion. It’s fairly straightforward to figure out the location between them where the force of gravity of Earth is equal in magnitude to the force of gravity from Mars. But because they are in motion, that point is constantly changing. And the way you are personally moving when you are near that point is almost certainly under more influence from Sun than the either two.

To start with, let’s simplify to a two body problem for the moment, for example a spacecraft in the vicinity of Earth. The difference between the spacecraft going into an orbit and crashing is not simply a matter of how strong the force of gravity is. It’s a matter of how the spacecraft is moving. If it’s moving past Earth fast enough, gravity makes it change direction, but can never pull it in to Earth’s surface. If it’s moving really fast, then it has what’s called escape velocity, and the force of Earth’s gravity will never be enough to make it come back.

Now we move to the three body problem of Sun, Earth, and the spacecraft. Earth is orbiting the sun at a fairly good clip, so there’s no real possibility of anything moving slowly compared to both Sun and Earth. We could have the spacecraft moving slowly enough compared to Sun to fall into it, in which case if it gets close enough to Earth to feel it’s gravity, the two will whip past each other. It could be moving slow enough compared to Earth to crash into Earth, in which case it’s pretty much already moving at a speed where it’s orbiting the sun. (Pardon the ample amounts of hand-wavium here. It can get difficult to picture these things in your head, and difficult to express the ideas purely verbally.)

I thought the modern consensus was that gravity isn't actually a force that acts on anything. It's just a distortion of space-time caused by massive bodies [yo mama joke placeholder] that simulates a force. Things in orbit around other things are travelling in a straight line as far as they're concerned, but since space is curved around massive bodies [another yo mama joke placeholder] they appear to be travelling in an oval of some kind.

If he's right and they are not moving, then that must mean the world is moving up at them, which is all very well except that the same thing happens wherever you are around the globe, and it can't be moving in every direction at once.

Hoo boy. I knew we were going to get into this. Fair warning up front. All of my study of General Relativity (aka Einstein’s theory of gravity) has been self-directed and hasn’t progressed very far. But I’ll do my best with this.

I know Brian Cox understands this stuff far better than I do, but “standing still” is really a poor choice of words. The bowling ball and the feathers collide with the ground. Something has to be moving. Lord Foul’s observation that things fall all around the globe also points to what’s wrong with that articulation.

tombombodil’s description is closer to the one physicists actually use. However, rather than saying things travel on a straight line, it’s generally better to say that they take the most economic path available through space-time. They are the same thing, but you really have to be inhumanly comfortable with non-Euclidian geometry to not have the “straight line” description lead you astray intuitively. Probably the best job at making such a strange concept make some sort of intuitive sense is The Parable of the Apple from the text book Gravitation by Misner, Thorne, and Wheeler. I certainly can’t improve on it.

Now does this really mean that gravity isn’t really a force? Well, maybe. Certainly, general relativity is an amazingly successful description of gravity. In the areas where it makes different predictions than Newton’s theory, g.r. always wins. In the areas where it fails, no one else has come up with anything else that works either. And the view under g.r. that objects are really just moving freely, not under the influence of a force does neatly explain why the mass of the object makes no difference for its movement.

Still, it’s worth noting that actual working physicists still refer to it as one of the four fundamental forces. The goal of grand unified theories is generally to fit gravity in a unified framework with the other forces. Even Einstein, who came up with the not-really-a-force concept, worked on this problem. And we probably shouldn’t be too ready to disregard the fact that from our blinkered Euclidean perception, gravity behaves remarkably like a real force.

I'm going to close it there. It's long and rambly enough as it is. Let me know if I've raised more questions, missed existing ones, or have just given you a headache.

[keithcurtis]

This has been a fascinating thread so far. I read your answer concerning the value of math and science education aloud to my wife, who is a math teacher.

Also, I really loved your explanation regarding the... unsettled nature of science. I just wrote a short diatribe on social media regarding my dislike of the oft-heard statement: "The science is not yet in on _________". Half the point of science is that it is never "in", it just approaches "in".

My question is related to your statement that if the universe is only "close to flat", then we "just got lucky" and that scientists hate that. Is that not just an example of the anthropic principle? I.e. If the universe were not very near to flat, we couldn't wonder about it, so the very fact that we are here to wonder about it just means that the universe just happens to be close to flat? How much does this sort of conundrum affect the thinking of the scientific community?