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ATLAS sees first direct evidence of light-by-light scattering at high energy

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Re: ATLAS sees first direct evidence of light-by-light scattering at high energy

#51

Post by mitchty »

soVeryTired wrote: post #31
ekimekim wrote: post #29

Earlier quoted context omitted.

Currently, very little. According to the article, you need crazy particle-accelerator-levels of energy to have them happen at all. However, by studying and understanding such events, we can test and improve our theories of physics, which may lead to breakthroughs in technology we can only dream of. For example, general relativity would have been considered completely useless knowledge for any practical purpose when i…

I've heard people make the argument about GR being essential to the GPS system before, but the skeptic in me has doubts. I don't deny that it's useful to have a correction derived from relativity, but is it really as essential as people make it out to be? If we hadn't known about GR when we built the GPS system, wouldn't we have realised there was an unexpected drift in the clocks and found a way to recalibrate them…

Yes its essential. Because the satellites are moving at orbital velocity, their frame of time is ever so slightly different than ours. To keep accuracy timewise without special and general relativity would be difficult if not impossible without understanding relativity generally. Could we have done GPS without relativity? Maybe, but we'd have to discover it with the satellites to be able to use it.

A quick note from: (i'm too lazy to do the calculations right now) http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps....

    If these effects were not properly taken into account, a navigational fix based on the GPS constellation would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometers each day! The whole system would be utterly worthless for navigation in a very short time.

Re: ATLAS sees first direct evidence of light-by-light scattering at high energy

#52

Post by anfractuosity »

raverbashing wrote: post #12

Just as a reminder, those photons are high energy ones (Gamma-ray range) so no light sabers, holograms or anything involving visible light I am afraid

Electrolasers kind of seem slightly light saber-esque. I'm assuming miniaturisation of them would be tricky though, as i'm assuming ionization with a laser requires a lot of power.

Re: ATLAS sees first direct evidence of light-by-light scattering at high energy

#54

Post by jerf »

Newtopian wrote: post #45
jerf wrote: post #36

Earlier quoted context omitted.

"Alternatively (and more likely) we would have decided the idea simply doesn't work." No, we would just keep beating on it until it works. It's a myth that science always precedes engineering. It's a very regular drift, and worst case scenario, if they just couldn't work it out, they'd build several ground stations in known locations, derive corrections as those drift, and upload corrections into the system periodica…

Given the choice between trial and error engineering and science backed engineering I much prefer the latter. That does not mean doing engineering without the science is impossible just so much harder. They both go hand in hand, nothing mythological here. Advancements in one leads to ideas in the other and vice-versa.

"Given the choice between trial and error engineering and science backed engineering I much prefer the latter."

That's not the question; the question is, are we forced to give up engineering if we don't have "the science" yet?

And the answer is an objective "no", from abundant past human history. There's this myth sold that science always precedes engineering that is very, very popular. I'm not even sure where it's coming from. Oral history in primary education, maybe. But it's a myth that engineers themselves can ill afford.

The vast majority of practical programming is programming running way, way ahead of the "science", which occasionally takes point samples of how 10 college sophomores behave under a certain limited experiment.

Re: ATLAS sees first direct evidence of light-by-light scattering at high energy

#55

Post by FiatLuxDave »

dspoka wrote: post #48
pif wrote: post #30

Earlier quoted context omitted.

High energies are not absolutely necessary. Couples of "virtual" particles can be created at any energy, it's just less likely for smaller energies. I think the real problem was the experimental set-up: particle accelerators use electric field to transfer energy to particles and magnetic field to control their trajectory and eventually to cause collisions, but all of this only works with charged particles. And photon…

what does virtual mean in this case? is this word mean something in quantum physics?

Virtual means that the particle does not obey the Einstein energy-momentum equation: E^2 = p^2c^2 + m^2c^4 . You might recognize the shorter form, where the particle has zero momentum, as E=mc^2.

More info here: https://en.wikipedia.org/wiki/On_shell_and_off_shell

Re: ATLAS sees first direct evidence of light-by-light scattering at high energy

#56

Post by jononor »

jerf wrote: post #54
Newtopian wrote: post #45

Earlier quoted context omitted.

Given the choice between trial and error engineering and science backed engineering I much prefer the latter. That does not mean doing engineering without the science is impossible just so much harder. They both go hand in hand, nothing mythological here. Advancements in one leads to ideas in the other and vice-versa.

"Given the choice between trial and error engineering and science backed engineering I much prefer the latter." That's not the question; the question is, are we forced to give up engineering if we don't have "the science" yet? And the answer is an objective "no", from abundant past human history. There's this myth sold that science always precedes engineering that is very, very popular. I'm not even sure where it's c…

Isnt the more relevant question "does engineering benefit from (basic) science?"

Possibly amended to include "to a degree that justifies the spending"

Re: ATLAS sees first direct evidence of light-by-light scattering at high energy

#57

Post by soVeryTired »

mitchty wrote: post #51
soVeryTired wrote: post #31

Earlier quoted context omitted.

I've heard people make the argument about GR being essential to the GPS system before, but the skeptic in me has doubts. I don't deny that it's useful to have a correction derived from relativity, but is it really as essential as people make it out to be? If we hadn't known about GR when we built the GPS system, wouldn't we have realised there was an unexpected drift in the clocks and found a way to recalibrate them…

Yes its essential. Because the satellites are moving at orbital velocity, their frame of time is ever so slightly different than ours. To keep accuracy timewise without special and general relativity would be difficult if not impossible without understanding relativity generally. Could we have done GPS without relativity? Maybe, but we'd have to discover it with the satellites to be able to use it. A quick note from:…

Nice, thanks for the info! I'd never seen the inaccuracy quantified like that before.

Re: ATLAS sees first direct evidence of light-by-light scattering at high energy

#58

Post by dekhn »

soVeryTired wrote: post #31
ekimekim wrote: post #29

Earlier quoted context omitted.

Currently, very little. According to the article, you need crazy particle-accelerator-levels of energy to have them happen at all. However, by studying and understanding such events, we can test and improve our theories of physics, which may lead to breakthroughs in technology we can only dream of. For example, general relativity would have been considered completely useless knowledge for any practical purpose when i…

I've heard people make the argument about GR being essential to the GPS system before, but the skeptic in me has doubts. I don't deny that it's useful to have a correction derived from relativity, but is it really as essential as people make it out to be? If we hadn't known about GR when we built the GPS system, wouldn't we have realised there was an unexpected drift in the clocks and found a way to recalibrate them…

I think it's reasonable to believe that if we hadn't found GR, and implemented GPS, that observant scientists would have found GR.

The reason is that for econmics reasons, GPS developers work hard on noise reduction. Once you eliminate all the "technical" forms of noise, you're left with the "scientific noise".

Re: ATLAS sees first direct evidence of light-by-light scattering at high energy

#59

Post by jessriedel »

phkahler wrote: post #49
rrmm wrote: post #2

Neat-o. A little bit more info at https://en.wikipedia.org/wiki/Two-photon_physics . Basically two photons with enough energy to interact via virtual pairs created. It will be interesting to see the cross-section rates and maybe hopefully some new physics (as the wikipedia page weakly hints).

>> Basically two photons with enough energy to interact via virtual pairs created. Or could it be that a photon bends space-time as though it had mass, and they simply follow the local curvature and it looks like scattering? Just speculating, because for some purposes photons act like a solid particle with their energy equivalent mass traveling at speed c.

It is not gravitational. That effect would be far, far too weak.

Re: ATLAS sees first direct evidence of light-by-light scattering at high energy

#60

Post by ars »

baq wrote: post #38
pif wrote: post #30

Earlier quoted context omitted.

High energies are not absolutely necessary. Couples of "virtual" particles can be created at any energy, it's just less likely for smaller energies. I think the real problem was the experimental set-up: particle accelerators use electric field to transfer energy to particles and magnetic field to control their trajectory and eventually to cause collisions, but all of this only works with charged particles. And photon…

> Couples of "virtual" particles can be created at any energy doesn't the energy need to be at least the mass of the virtual particle?

No. For example the weak force is created by the very heavy W and Z bosons.

The uncertainty principle says you can "fudge" the energy if your time-scale is short enough, and this is what happens. A result of this is that the distance the weak force can operate over is limited, since the timescale has to be short, so the particles don't have the time to travel very far.

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