Universal Speed Limit

[darkmatter42]

Can somebody explain to me, in layman's terms, how Einstein figure out the speed of light? And why did he declare it to be the "universal speed limit"? Why can't things go faster than light? 

[HallsofIvy]

First, Einstein did not have to "figure out the speed of light"!   Ole Roemer did that way back in 1676.  He did it by calculating the changes in times of eclipses of Jupiter's moon, Io.  A variety of people have since improved the accuracy.

[darkmatter42]

17 hours ago, HallsofIvy said:

First, Einstein did not have to "figure out the speed of light"!   Ole Roemer did that way back in 1676.  He did it by calculating the changes in times of eclipses of Jupiter's moon, Io.  A variety of people have since improved the accuracy.

I did not know that. That further proves that I should not believe everything I read in books

[bangstrom]

3 hours ago, darkmatter42 said:

I did not know that. That further proves that I should not believe everything I read in books

I have never read that Einstein was the first to “figure out the speed of light” so this is not a common impression. Roemer was the first to arrive at a reliable measurement and it was verified numerous times by various methods before Einstein. However, Einstein was the first to equate the speed of light with Maxwell’s constant c which was not considered to be the speed of light but a dimensional constant resulting from the combined effects of the dielectric permittivity and the magnetic permeability of a vacuum. These were also previously measured values. This means that a vacuum is not pure nothingness but it has measurable properties that give “empty” space properties similar to what we think of as density and elasticity.

Olaus Roemer discovered that the timing of the motions of Jupiter’s moon Io came early by one second every time Jupiter came 300,000 km closer to the Earth and they came late by one second every time Jupiter was 300,000 km farther from the Earth so he discovered that there was a constant relation between measures of observational distance and observational time in a constant ratio of one second for every 300,000 km of distance. He could have interpreted this as a conversion factor for converting between units of distance and units of time but he was looking for the speed of light so he called it a speed.

As described in Special Relativity, time slows and distances appear shorter when an object moves at relativistic speeds. If you could travel with a beam of light, your clock would stop and you could travel to any point in space in no time at all by your "frozen" perception of time. So, for light, there is neither time nor distance and light can go anywhere in an instant by its local "proper" time and nothing can travel faster than instant.

The very distant galaxies are a possible exception to the rule that nothing can travel faster than light. The galaxies appear to be moving away from us and, in theory, the most distant galaxies should be receding faster than c because the expansion of space acts as a tailwind blowing them away. Relativity always works both ways so an observer on one of those most distant galaxies would say they are standing still while our galaxy is receding faster than c.

If there is an error in the text books it is in calling c a speed rather than a dimensional constant. This would be an error started by Roemer and continued by Einstein.

C works as a universal conversion factor for converting between units of distance and time and it is nothing like a speed except that they are both values given in units of distance divided by time. C is the same for all observers independent of their individual velocities so this should be our first clue that c is a dimensional constant rather than a speed.

[bangstrom]

19 hours ago, JeffreysTubes8 said:

As an object gets closer to the speed of light it acquires more mass. 

When an object is accelerated to relativistic speeds, the rest mass remains unchanged but the relativistic mass is said to increase. The rest mass is its local mass but relativistic mass is how it appears to a remote observer. Accelerating an object to relativistic speeds requires more energy than predicted by Newtonian physics, and if the speeding object collides with another, more energy is released than predicted by Newtonian physics. To a remote observer, it would appear as if the object had gained in mass as it was accelerated but the gain in mass is only apparent relative to a stationary observer. The accelerating object does not actually gain in mass so locally its rest mass remains unchanged and it will not turn into a black hole..

[HallsofIvy]

On 8/25/2021 at 10:11 AM, darkmatter42 said:

I did not know that. That further proves that I should not believe everything I read in books

Please tell us what book you read that in!

It seems a lot more likely that  you simply misunderstood whatever you read.