Speed of light

[kalexia]

If someone slows down when approaching the speed of light, viewed by a stationary observer, and stops when at the speed of light, then why do we see starlight etc? Why does the light not take an infinite time to reach us, since time dilation states that a spacecraft flying at c would?

Also, why does light travels at c no matter what speed you're going at?

[Janus]

If someone slows down when approaching the speed of light, viewed by a stationary observer, and stops when at the speed of light, then why do we see starlight etc? Why does the light not take an infinite time to reach us, since time dilation states that a spacecraft flying at c would?

While the stationary observer sees time slow for the other person, it does not effect the amount of time the observer measures for the object to travel any given distance. For example: At .866c, the observer will observe the traveler as aging At half his own rate, but will still measure that it takes the traveler 1.15 years to travel 1 lightyear. He will just see the traveler only age 7 months during that time. IOW, the rate at which the observer sees the traveler age at does not affect the relative velocity at which the observer measures the traveler as moving.

 

Also, why does light travels at c no matter what speed you're going at?

 

The speed of light is controlled by just two physical constants: Permittivity and Permeability. These constants deterime the strength of electric and magnetic fields. For example, the permeability of a substance determines how strongly two magnets separated by the substance will act on each other through the substance. Permittivity does the same for electric charges.

 

Vacuum has a set permittivity and permeability. Two magnets or two charges separated by a vacuum act on each other according to these constants. These constants are not dependant on motion. If I am sitting on a moving train and holding two magnets apart in a vaccuum, they will attract each other with the same force as if I did the exact same experiment standing on the ground.

 

The above is part of what is called the Principle of Relativity; physical laws do not change due to relative motion.

 

Now if the values of permittivity and permeability in a vacuum are set by physical law, and thus do not change with motion, then the speed of light in a vacuum© cannot change with motion either.

[CraigD]

If someone slows down when approaching the speed of light, viewed by a stationary observer, and stops when at the speed of light, then why do we see starlight etc?

One explanation of this is to note that, being comprised of fundamental particles (photons), light has not “moving parts” that an observer can “view slowing down”. In a sense, photons can be thought of as “frozen in time”.

Why does the light not take an infinite time to reach us, since time dilation states that a spacecraft flying at c would?

Special relativity, of which time dilation is a prediction, states no such thing.

 

It does state that the mass of a spacecraft with nonzero rest mass approaches infinity as its speed approaches c, and that therefore the energy required to accelerate it to a speed approaches infinity as that speed approaches c. Because the energy available to any location in space is finite, this implies that an object with nonzero rest mass can never be accelerated to c.

 

Photons have zero rest mass. Though somewhat mathematically vague, one can deduce from this that, at speed c, photons have a finite mass of [math]\frac{\hbar v}{c^2}[/math], where [math]v[/math] is the photon’s frequency.

So, for example, the apparent relativistic mass of a single photon of red (543 THz) light masses about [math]4 \times 10^{-26} \, \mbox{kg}[/math].

Also, why does light travels at c no matter what speed you're going at?

Though Janus states a well-accepted answer to this question, another answer is: “because it does.” That is, it is so because we always observe it to be so.

 

Though in a sense an unsatisfying a answer, one can reasonably consider the theory of special relativity to be an explanation of the observed fact that c is constant.

[Guest chendoh]

Photons and c are children of the universe....

They will act as they need to.

 

A photon asks; Would you like me to go faster?

And c replies....faster than what?

[sanctus]

 

It does state that the mass of a spacecraft with nonzero rest mass approaches infinity as its speed approaches the speed of light, and that therefore the energy required to accelerate it to a speed approaches infinity as that speed approaches infinity

 

I guess you wanted to say "as that speed approaches c", otherwise it would be logical even from classical mechanics