# Why did Einstein use light to define simultaneity?

• quantumphilosopher
In summary: This summary is about the conversation between a philosopher and a physicist. The philosopher discusses how most things are measured by EM signals and how this includes the speed of light. The physicist explains how this is an important discovery and why it is important. The philosopher then discusses how time's rate might slow down for those moving at high speeds and how this related to distances contracting. The physicist then explains that this is why laws of physics still hold for those moving at high speeds, despite the fact that they are traveling at a lower speed than those stationary. Finally, the philosopher discusses how one scientist, Albert Einstein, won the race to figure out how relativity works.
quantumphilosopher
That is, why did he use signals whose speed is invariant?

Because 99.99999% of the world we deal with involves electromagnetic interaction.

Zz.

That is, why did he use signals whose speed is invariant?
What else would he use?

quantumphilosopher said:
That is, why did he use signals whose speed is invariant?
One advantage in using light signals to synchronize clocks, is that the procedure is easy to analyze from any frame. But you don't have to use light to synchronize clocks--you can use any kind of signal, even sound if you wanted (and could arrange it). But then the analysis from different frames is complicated by the fact that speed of the signal is frame--and direction--dependent (and must be computed using the relativistic addition of velocities formula).

quantumphilosopher said:
That is, why did he use signals whose speed is invariant?

There's two reasons.

1) it agrees with experiment

2) Making the speed of light isotropic also makes "physics" (i.e. momentum) isotropic. Thus synchronizing clocks with light-beams also makes the momentum, p, of an object moving left to right with a velocity v equal and opposite to the momentum, p, of an object moving right to left with a velocity v.

Synchronizing clocks in a non-standard manner destroys this important relationship.

quantumphilosopher said:
That is, why did he use signals whose speed is invariant?

quantumphilosopher,

We measure most everything by EM signals, visible light being just one small subset of the EM spectrum. And for astronomical distances or very fast moving bodies, what else could be used besides EM? Even when you use a ruler, your eyes make the judgement via receipt of the light signals.

Before Einstein's era, everyone assumed that light's speed would add & subtract like billiard balls. But by Einstein's era, theory and evidence both were making a convincing argument that light's speed was invariant, that it did not add & subtract like material bodies do. That even though you and I are traveling at half the speed of light wrt each other, that the sole beam of light passing us both by must be recorded at speed c per each of us.

That said, the entire point of it was to determine how you and I could make predict the other's observations, while at the same time we both record speed c for the sole beam of light. No one needed to build a model for light acting as billiard balls, since that model had long existed. The new buzz of the day was invariant light speed, and so the race was on the determine how a universe could exist under such a context, while preserving Newtonian mechanics for everyday experience known to be true and accurate.

Well, Albert won the race. His insight was the realization that time's rate must slow down for others of relative motion. The higher the speed, the more the time rate contraction. Since x/t=c=X/T, then so too must distances contract given t<>T. That bodies do not contract in and of themselves in some unwaivering aether (Lorentz's view), but rather because of the way in which we measure the very dimensions of space & time.

The electromagnetic szar of the day was Henrich Lorentz who published his theory first. However about 6 months later in 1905, a 3rd class swiss patent clerk nobody trumped him and the szar ditched his own theory in favor of Einstein's. This took some 4 years to happen since no one understood Einstein's theory. Max Planck read Einstein's 1905 paper, understood it, and made him famous. The irony is that this leading physicist of the day (Planck) would eventually have many debates with Einstein over his own specialty area of quantum mechanics, over the completeness of quantum mechanics and the validity of quantum uncertainty and locality. It seems that Einstein was eventually proven wrong on these matters years later.

Because light speed is invariant, it is a cosmic speed limit for all motion. The perspectives of two observers of relative motion are related via the common reference of invariant light speed.

pess

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## 1. Why did Einstein choose light as the defining factor for simultaneity?

Einstein chose light because it travels at a constant speed in a vacuum, regardless of the observer's frame of reference. This means that the speed of light is the same for all observers, making it a reliable tool for defining simultaneity.

## 2. How does using light to define simultaneity affect our understanding of time?

By using light as the defining factor for simultaneity, Einstein introduced the concept of time dilation, which states that time can appear to pass at different rates for different observers depending on their relative motion. This challenges the traditional idea of time as a constant, universal measurement.

## 3. Are there any limitations to using light to define simultaneity?

Yes, there are limitations to using light to define simultaneity. This concept only applies to events that occur in a vacuum and does not take into account the effects of gravity or other forces.

## 4. How did Einstein's use of light to define simultaneity contribute to the development of the theory of relativity?

Einstein's use of light as the defining factor for simultaneity was a crucial step in the development of the theory of relativity. It helped to shape the concept of space-time and the understanding that time and space are relative to the observer's frame of reference.

## 5. Has the use of light to define simultaneity been proven through experiments?

Yes, numerous experiments have been conducted to test the validity of using light to define simultaneity, and they have consistently shown that the speed of light is constant for all observers. This provides evidence for the accuracy of Einstein's theory of relativity.

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