Miscellaneous Science Questions

[thoughtfully]

Can I derail for a second to ask?: thoughtfully mentioned Feynman's QED - how accessible is the text/what level of knowledge does it assume, and how much ground does it cover? I have a very superficial grasp of QFT which I would like to expand on, is all.

It's intended for a general audience. There are (Feynman) diagrams, but not too many equations or heavy lifting. Might be a few small ones like E=hν.

The scope is the same as the title. Quantum Elecrodynamics is the quantum field theory of EM.

[Copper Bezel]

I'll definitely look into it when I get a chance. It really does sound like required reading and doesn't look to be hard to find.

[Xanthir]

How is the magnetic field not photons? What does photon mean to you? For me, a photon is a little shortcut for talking about the particle-ish behavior in QED, but not so important. Light and magnetism are both just shades of QED. If you want to split out the radiative term in a propogating field as the 1/r^2 bit (I think it was that term that gets that name) and give it a special "radiation" name, then you can do that, but all these things are photons.

The radiative term is exactly what I was talking about, and from Copper Bezel's post it sounds like exactly what she was talking about, given her post started "not in the form of radiation". If you want to call the EM field a photon field, that's fine, but saying it is "made of photons" doesn't make a whole lot of sense. It sounds to me equivalent to saying the Higgs field is "made of Higgs bosons" (though I guess with enough acceleration...).

Experiencing an EM field, tho, relies on exchanging photons. If you change/move an EM field, the effect propagates outward at the same speed that photons do; it won't happen "at c" if the photons are traveling slower than c.

[jewish_scientist]

Light is literally an EM wave.

I thought of EM waves as the sum of many photons, and that light refers to photons and/or EM waves.

Okay, what is the minimum number of photons that makes a wave?
The whole point of early QM and the double-slit experiments is that it's wave-like down to individuals.

Newtonian physics is almost always use and always taught before Einsteinian physics, even though Newtonian physics has been proven wrong. It is just the same with EM waves. The EM wave model is not correct, but is good enough most of the time. Imagine trying to teach a high school physics class on how moving magnets differ from stationary magnets using QED; it is just not practical even though it is correct.

[doogly]

Yeah but if you are just at the Maxwell level of E&M understanding, which is as you say a very nice level to be at (and I wouldn't say it's been proven "wrong," more "incomplete," perhaps), then Maxwell is not saying "An EM wave is the sum of many photons." If you are going to invoke photons, you should invoke them in a way that is meaningful.

[Xanthir]

Also the conversation topic is literally about quantum effects (the virtual-particle sea comes from quantum mechanics), so trying to use a pre-quantum understanding of EM to intuit behavior has a good chance of leading you wrong.

(Note: I don't actually understand EM at that level either, but I have a pretty good layman's understanding. It usually works out okay for me? Pay attention to doogly, tho; they actually have a degree in this stuff.)

[Frenetic Pony]

So I've become lost on something, or rather lost on an entire part of this subject due to one question.

How can one determine how much gravity say, a photon, generates? Its energy should be relative to your velocity compared to its velocity (reference frame) so how gravity does it then have?

[doogly]

It's velocity is c, regardless of your reference frame. This is the big deal.

[thoughtfully]

The energy of a photon can vary depending on the relative motion of the origin/destination (remember, no observations of light unless you are in its path or on the set of Starr Wars!). This is reflected in the wavelength/frequency, however.

[gmalivuk]

Like massive particles do, hotons have varying amounts of energy and momentum, which more or less what "generates gravity". It just doesn't depend on their velocity the way it does with massive particles.

[Carlington]

I would like to ask something related to your responses: does redshift doing its thing change the gravitational field we measure for a photon? E = hf but motion of the light source changes f and thus E...and thus g? (I feel like this should be correct, even just by analogy with how the mass we measure is frame-dependent, because mass-energy)



...fake-edit: I am too close to sleep to formulate this thought properly but it's momentum too, isn't it. Not just mass or energy but momentum.

Ninjad: okay yeah I think y'all have me covered

[Flumble]

I'm reverse-time ninja'ing all of you.

The photon does change frequency when you start accelerating*, right? Thereby changing its energy and space-bending powers, too?

*except if you accelerate in a particular direction.

[gmalivuk]

Yes, the energy and momentum (and thus gravity-producing potential) of photons changes depending on your reference frame. But since time and space also change, the different apparent stress-energy you observe balances out the different times and distances you measure, so that you'll calculate the same* predictions for where things will go.

* Same in the sense that you'll agree on statements like, "The particle reaches observer A at the moment A passes B," not, "The particle reaches observer A at time t."

[Frenetic Pony]

Yes, the energy and momentum (and thus gravity-producing potential) of photons changes depending on your reference frame. But since time and space also change, the different apparent stress-energy you observe balances out the different times and distances you measure, so that you'll calculate the same* predictions for where things will go.

* Same in the sense that you'll agree on statements like, "The particle reaches observer A at the moment A passes B," not, "The particle reaches observer A at time t."

This is exactly what I'd thought, but it seemed too weird to take at face value Thanks!

[eSOANEM]

This is what it means when people say a law of physic is Lorentz covariant. They mean that all parts of the equations transform in the same way so that all observers agree on suitably formulated predictions (like those gmalivuk talks about). Basically, the easiest way to get this is to formulate everything in terms of tensors because then, provided you have a well-formed equation (well-formed in the sense that it makes sense in the sort of way that "x = y+" isn't) it will be Lorentz covariant.

[Ansorri]

If I had a stick that was long enough to reach Mars, and waved that stick quickly to Saturn, would the top of that stick travel faster than the speed of light?
Or, if the magically long stick is too absurd, if I had a laser that could go from Earth to Mars, and I wave that laser quickly over to Saturn, what happens to the light as it travels over to saturn? Does it break apart?

This question stems from this thought: If I am standing on Mars, and turn on a flashlight, it would take maybe 5 minutes (I'm guessing) for that light to hit Saturn. But from Earth, I can wave a stick, or laser pen from Mars to Saturn in a split second.

Am I making any sense with this question?

[gmalivuk]

Yes, the spot from a laser pointer can move faster than light, and need not break apart or anything along the way.

There's nothing relativity breaking about the photons you release now eventually ending up light years away from photons you released a second ago. Both destinations still have to wait for the light speed delay from you, and thus they can't transfer any information that way.

[Flumble]

Also note that you can "wave a laser pen" in the sense that you can quickly change the direction it's pointing in, but it will take minutes before the light stops reaching Mars and even more minutes before it starts reaching Saturn.

In the case of the stick: no, you can't wave the stick at more than the speed of light. The far end will become heavier and heavier (because of relativity) as you accelerate it.

[jewish_scientist]

Regarding the stick, it would bend so that the tip would reach its final resting place long after the base, thereby keeping its velocity below the speed of light. If the stick was perfectly rigid, and could not bend (a.k.a. Born rigid), then it could not rotate under any circumstances*. I would suggest looking into the Ehrenfest paradox for more information.

*Unrelated question: could a Born rigid body accelerate linearly, and would it be effected by gravity.

[Xanthir]

The important part of the stick "paradox" is that, in real life, solid objects are held together by electromagnetic force, which is modulated by photons, so the wiggle in the stick can't travel faster than the photons communicating/coordinating the force. Thus, the minimum transfer time is still lightspeed.

[eSOANEM]

↶
*Unrelated question: could a Born rigid body accelerate linearly, and would it be effected by gravity.

Born rigid bodies are just hella broken. You'll get paradoxes whenever different parts of it accelerate at different rates and therefore will also get them if ever there is some part of the body which is not free-falling/not accelerating uniformly in a uniform field (when you extend to the gravitational case).

Like Xanthir says, the resolution comes from seeing that the fact that rigidity must be transmitted by local forces inevitably rules out anything that behaves like a Newtonian rigid body at anything except the low-acceleration/weak-gravity limit.

[jewish_scientist]

Yes, the spot from a laser pointer can move faster than light, and need not break apart or anything along the way.

There's nothing relativity breaking about the photons you release now eventually ending up light years away from photons you released a second ago. Both destinations still have to wait for the light speed delay from you, and thus they can't transfer any information that way.

What do you mean by information?

[doogly]

Literally anything. But you can take ...---... as a basic sample notion of information.

[jewish_scientist]

Oh, so it is information like in Information Theory.

[morriswalters]

Yes, the spot from a laser pointer can move faster than light, and need not break apart or anything along the way.

The spot doesn't move. There is no spot. Neo.

Oh, so it is information like in Information Theory.

Turn a light off and on. You have now transmitted information. Anyone that can see that light, sees it go off and on. Fancy it up a bit and you get "...one if by land two if by sea!"

[Quizatzhaderac]

What do you mean by information?

An (in my opinion) easier way to understand this is what isn't information.

Imagine you have a long chain of dominoes stood up But rather than doing it he normal way, you have them all set up to fall over based on a timer.

The timers can be set up to mimic a natural progression (let's say .1 seconds between dominoes), or faster, or have the whole chain fall at once, or even fall in reverse order (ie, A falling, then B falling onto A, then C falling onto B).

A natural chain of dominoes falling can send information, but the rigged setups can't. The rigged setups can superficially look like an initial tip over event spread extra-quick, but a careful examination shows the dominoes fell at their times because of the timers.

The laser pointer dot (Or a shadow) is a similar thing).
If a dot i at s=0 at t=0 and s=2 at t=1, that doesn't mean information traveled at a speed of 2, because the dot isn't where it is at t=1 because of where it was a t=0, but because somewhere else a photon was already on it's way to s=2.

[phlip]

Say you are at point A, and you know someone at point B. Is there literally anything you can do, that will have effects that B can measure? Is there any measurement B can make, where the results will differ depending (directly or indirectly) on actions you take?

If so: there's a transmission of information from A to B (namely: the information about what actions you are taking). If not: there isn't.

It's a bit of an simplification and pretty handwavey, but it works well enough at the intuitive level.

So, consider the long-stick proposal: someone at point C is poking you at A with a long stick, and then spins the stick around to be pointing at B. Well, yes there are actions you could take that B could measure - you could attack something to the end of the stick, or paint it different colours, or whatnot. There's actual matter that will leave A and then arrive at B, which is, in itself, information, and can also carry additional information.

But consider the laser proposal: someone at point C is pointing a laser at A, then spins it to be pointing at B. Now, at A, there's nothing you can do to affect this. You know that once the laser stops pointing at you, sometime later the laser will be pointing at B... but the photons that will be arriving at B are currently somewhere en route from C to B and aren't anywhere near you. It doesn't matter what you do with the photons that are currently arriving at A, it won't affect what B sees. So you can't use the laser to send information from A to B.

[morriswalters]

The spot is an artifact, it doesn't exist.

Say you are at point A, and you know someone at point B. Is there literally anything you can do, that will have effects that B can measure?

Maybe. Assume mirror A at the receiving end and that A reflects towards a target at B. The path from C to A and then to B, is longer than the direct path from C to B. The source will always shine on the B first before the reflected light from A. The direct path is always shorter. Given the information from A though, B can measure the time it takes the laser to traverse the arc which is the information transmitted by the source. A can't measure that time, it doesn't have all the information. The time it takes B to get that information is the time it takes light to go from C to A and then B.

[phlip]

↶
Assume mirror A at the receiving end and that A reflects towards a target at B.

Sure, but someone at B won't get that reflected light from A until, by definition, lightspeed delay from A to B. A still can't do something to get information to B faster than light, is the point.

[morriswalters]

I agree. I just repeated what you said and added something about the path to show why. It's high school physics, which is my limit. The only reason I got in was because the original point was foolish. There is no spot. And if there is no spot it can't move faster than light. Your cat might know that if you let him chase the reflection of your laser pointer.

[Xanthir]

Yeah, but trying to argue about the physical reality of some things versus other things gets philosophical, while the "can you actually affect anything (faster than light)?" argument is concrete and difficult to argue with.

[gmalivuk]

I feel like it's simpler to say dots and shadows aren't the sorts of things that carry information with them, than to say that there's no such thing as a laser dot or a shadow.

[Xanthir]

See Holes: Do They Exist?

[morriswalters]

Or, if the magically long stick is too absurd, if I had a laser that could go from Earth to Mars, and I wave that laser quickly over to Saturn, what happens to the light as it travels over to saturn? Does it break apart?

Yeah, but trying to argue about the physical reality of some things versus other things gets philosophical, while the "can you actually affect anything (faster than light)?" argument is concrete and difficult to argue with.

He's making a conceptual error. You correct it by telling him so. He would be better off thinking of laser light as trillions of little glowing nerf balls flying at 3.00×10⁸ m/s towards Mars.

I feel like it's simpler to say dots and shadows aren't the sorts of things that carry information with them, than to say that there's no such thing as a laser dot or a shadow.

In normal times I might agree. But in today's environment I'm no longer sanguine about it. But I suspect Ansorri would immediately recognize that there is no person looking back from his/her mirror. And the concept of information is more complex than the idea of reflection. However I have my own faulty concepts as I have demonstrated more than once. But anyway I have said what I wanted to say.

Just for laughs and giggles.

Spoiler:

You could point him towards this video of anti aircraft fire over Baghdad at nite during the gulf wars. The tracer fire demonstrates what happens when the laser moves, conceptually. Each tracer bullet flies straight, the curve in the video is an artifact of timing and the way your eye works.

[Quizatzhaderac]

The spot is an artifact

Just being pedantic, but you really should say an artifact of what. The cases where you leave the what implied are an artifact of something external to the experiment (which it is not, the dot was the subject of Ansorri's experiment), or a physical man made object (which is the exact opposite of your point).

Each tracer bullet flies straight, the curve in the video..

I'm guessing you mean the wave curve as the illusory motion and the parabolic/ballistic curve as "Straight"?

[morriswalters]

The spot is an artifact

Just being pedantic, but you really should say an artifact of what. The cases where you leave the what implied are an artifact of something external to the experiment (which it is not, the dot was the subject of Ansorri's experiment), or a physical man made object (which is the exact opposite of your point).

Each tracer bullet flies straight, the curve in the video..

I'm guessing you mean the wave curve as the illusory motion and the parabolic/ballistic curve as "Straight"?

Okay. And. Yes.

I did point out reflection twice, but I was more interested in being obscure. But the first was cursory. The second time I used a hammer. The spoilered bits were for fun. The tracers indicate what happens when the light beam moves. When I saw it during the Gulf War on TV I had to think about why it looked that way. The curve represents differential time in flight between each tracer as measured from the source. The perspective is skewed. It's the type of thing original questioner would see from an overhead perspective if he could see individual protons separated by some equal period of time. A curve descending towards the source, the farther from the source the longer the flight. And besides tracers are cool.

[jewish_scientist]

Here is a nice video where a teacher actually makes a rubber sheet to demonstrate GR on. I think my favorite part is where he actually gets the Earth-Moon system going for a little while.

[Sizik]

It's a little ironic that the rubber sheet model of gravity relies on gravity to work.

[elliptic]

You're not the first person to spot that: https://xkcd.com/895/

[Quizatzhaderac]

Do objects without an event horizon emit hawking radiation?

Looking at the math on Wikipedia: it appears that all space with changing surface gravity should emit it, but that objects much larger than their schwarzchild radius should emit much less radiation than the black hole they could make.