Chapter 575 Is Electromagnetic Wave Light?

The speed of electromagnetic waves is similar to the speed of light.

As Wheat finished this sentence,

Faraday was taken aback, then suddenly slapped his forehead with a crisp "pop".

So that’s it...

No wonder this number had seemed familiar to him.

2.97969X10^8m/s—doesn’t that practically match the previously calculated speed of light?!

But...

Why is that?

You have to know:

In this era, the scientific community already has a fairly clear understanding of mechanical waves:

They are caused by the propagation of disturbances, resulting in the transfer of momentum and energy within matter.

At the same time, mechanical waves can be divided into two types: longitudinal and transverse.

For example, waves along a string and sound waves, and of course mixed waves as well.

Besides the differences in type, the speed at which waves propagate varies as well.

For instance, the speed of sound is 340 meters per second, measured by Dehan, a Brit.

He observed cannons by eye in 1708 and determined that, at 20°C, the speed of sound is about 343 meters per second.

As for the person who measured the speed of sound in water, it was Coladon.

He conducted a precise little experiment on Lake Geneva—the Geneva that’s a place—and calculated that the speed of sound in water is 1435 meters per second.

There are also string waves and even light waves; their values have all been measured by now.

From Faraday’s point of view,

Electromagnetic waves originate from electric and magnetic fields, and the oscillation frequency of the electric field is inherently high.

Compared to other phenomena, the speed of electromagnetic waves shouldn’t possibly be low.

But this ’unlikely to be low’ at best means a few tens of kilometers per second, just a bit faster than John Michell’s 1760 estimate of seismic wave speed.

Yet based on current measurements, the speed of electromagnetic waves is actually close to the speed of light?

From Faraday’s... or truthfully, from every distinguished person present’s perspective, it’s clear what this coincidence means.

In physics, such magnitude-level coincidences basically don’t exist; substances whose important values are this close at ultra-high scales must be related in some way.

Seeing Faraday deep in thought, Joule hesitated a moment, then asked:

"Luo Feng, could we have made a mistake during the measurement process?"

Xu Yun glanced at him.

As someone from the future, Xu Yun could quite understand Joule’s thinking.

Faced with phenomena that could shake one’s worldview, it’s perfectly normal to feel doubtful.

He saw Xu Yun gently shake his head and explain:

"Mr. Joule, as you saw during the test, we collected no fewer than fifty sets of pitch data."

"Although the calculated results may still have some deviation, such deviation would at most affect digits after the decimal point, and as far as ’magnitude’ goes, it’s extremely accurate."

"Furthermore..."

As Xu Yun spoke, he pulled out the earliest classical wave equation from the table, pointed at the equation, and continued:

"We can actually start with the wave equation and calculate the speed of electromagnetic waves purely mathematically."

When Faraday and the others heard this, their eyes immediately shifted to the equation.

A few seconds passed.

Newman, who’d had little to do so far, suddenly snapped his fingers and circled μ0ε0 with his pen:

"Right! We can reverse-engineer the wave speed from the equation—ah, we really should have thought of that sooner!"

As previously mentioned:

The wave equation for electric fields is ▽2B=μ0ε0（?2B/?t2）.

The wave equation for magnetic fields is ▽2E=μ0ε0（?2E/?t2）.

If you compare the wave equations for electric and magnetic fields, you’ll see they are identical—just swap E and B, that’s all.

This proves the waves for both exist at exactly the same speed; compare that to the speed term of the classical wave equation, and another insight emerges:

The speed of electromagnetic waves can be reverse-engineered from the wave equation for electromagnetic fields.

Which means.....

V=1/√μ0ε0.

Here, μ0 is the absolute dielectric constant, valued at 4π×10^-7m·kg/C2.

ε0 is the vacuum dielectric constant, valued at 8.854187818×10^-12C2s2/kg·m3.

The former’s unit can be written as N/A2, and the latter as F/m.

But according to normal history,

Whether Farah or Ampere,

These units wouldn’t be officially defined until the International Electrical Congress of 1881.

But just like curl as mentioned earlier,

By 1850 the scientific world already knew of the concept; it’s just that the notation was still C2s2/kg·m3 for now.

Like the unit of electrical charge, the coulomb, which was also formally defined at the 1881 Congress, yet had been widely used long before then.

The reason for holding that Congress in 1881 was mainly because America and Asia didn’t have comprehensive systems for these units, so the event formalized them through official definition.

Asia here mainly refers to Neon, and it’s connected to the Meiji Restoration; let’s not elaborate further.

As an aside,

Seven units were defined at that conference, namely:

Coulomb, ampere, volt, ohm, farah, henry, and siemens.

Of course,

Some readers may ask at this point:

With the technology level of 1850, how could they measure those values in a vacuum?

This relates to Xu Yun’s own experience writing novels—a reader once asked, ’Could 1850’s data really be so precise?’ and it’s a similar question.