RELATIVITY QUESTIONS! (and other common queries)

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby thoughtfully » Wed Nov 30, 2016 12:51 pm UTC

Carlington wrote: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.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Copper Bezel » Wed Nov 30, 2016 6:45 pm UTC

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.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Xanthir » Wed Nov 30, 2016 10:34 pm UTC

Eebster the Great wrote:
doogly wrote: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.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby jewish_scientist » Thu Dec 01, 2016 6:43 pm UTC

thoughtfully wrote:
jewish_scientist wrote:
Xanthir wrote: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.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby doogly » Thu Dec 01, 2016 9:45 pm UTC

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.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Xanthir » Fri Dec 02, 2016 9:12 pm UTC

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.)
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Frenetic Pony » Mon Dec 05, 2016 3:06 am UTC

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?

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby doogly » Mon Dec 05, 2016 1:19 pm UTC

It's velocity is c, regardless of your reference frame. This is the big deal.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby thoughtfully » Mon Dec 05, 2016 1:57 pm UTC

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.
Last edited by thoughtfully on Mon Dec 05, 2016 2:01 pm UTC, edited 1 time in total.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby gmalivuk » Mon Dec 05, 2016 2:01 pm UTC

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.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Carlington » Mon Dec 05, 2016 2:21 pm UTC

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.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Flumble » Mon Dec 05, 2016 2:55 pm UTC

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.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby gmalivuk » Mon Dec 05, 2016 4:06 pm UTC

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."
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Frenetic Pony » Mon Dec 05, 2016 11:19 pm UTC

gmalivuk wrote: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 :shock: Thanks!

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby eSOANEM » Tue Dec 06, 2016 4:10 pm UTC

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.
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Magic stick

Postby Ansorri » Wed Jan 04, 2017 10:43 pm UTC

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?

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby gmalivuk » Thu Jan 05, 2017 5:35 am UTC

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.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Flumble » Thu Jan 05, 2017 9:25 am UTC

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.

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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby jewish_scientist » Thu Jan 05, 2017 5:01 pm UTC

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.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby Xanthir » Thu Jan 05, 2017 10:15 pm UTC

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.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby eSOANEM » Fri Jan 06, 2017 5:37 am UTC

jewish_scientist wrote:
*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.
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby jewish_scientist » Tue Jan 17, 2017 4:01 pm UTC

gmalivuk wrote: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?
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Re: RELATIVITY QUESTIONS! (and other common queries)

Postby doogly » Tue Jan 17, 2017 4:16 pm UTC

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