Physics question; re; Speed of Light conundrum

[SirFshAlot]

perhaps it's not so disconcerting when you consider that probably 99.9+% of the universe will always remain slow since there is only a limited amount of energy avialable to propel stuff.


[whine]
but I don't like my "knowledge" to be so conditional.
[/whine]

 

[Sohcan]

*cracks knuckles*

It's been a year since my last course of special relativity, but I'll try to answer it as best as I can.

The velocity of light (c = 3 * 10^8 m/sec) is constant in all reference frames, no matter their velocity. It extends from Maxwell's equations (lots of differential wave equations that I don't care to prove right now ), but basically the propogation of light depends only on the constants involved in electricity and magnetism, and not the velocity of the light's source or observer.

With the senario you're describing (light moves towards earth at velocity c, being moves away from sun at velocity .5c), after six months, from the perspective of a stationary observer in the rest frame, the being and the light are .75 light years away from each other like you described...simple enough.

The hard part is describing the distant as observed by the being moving at .5c. The being observes the light moving away from him at c, not c + .5c = 1.5c. Space gets contracted and time gets dialated by a factor gamma = 1/(1 - v^2/c^2)^.5, so at v = .5c, gamma = 1.15.

So after six months (from the perspective of the being), the being will think the photon is half a light-year away, not 3/4 of a light year. But six months from the perspective of the being is nearly 7 months from the perspective of the stationary observer. So after six months of "lab-frame time" from the perspective of the stationary observer, 6/1.15 = 5.2 months have passed as observed by the being, so the being thinks the photon is .43 light years away instead of .75 light years away. This is explained by the contraction of space; the being and the stationary observer observe the photon to be at the same location relative to the earth and the sun, but the being thinks that the distance between the earth and the sun has contracted.

I hope this explains it.




<< That's pretty disconcerting, though, to think that all the work done in physics is only useful to the point that objects, other than light, stay in a relatively slow state of motion. >>

That couldn't be more wrong...Special relativity explains mechanics at near c beautifully (at least on a macroscopic scale), and is very useful for energy and momentum in particle physics...it's actually quite easy to understand with algebra...they teach this stuff in the first or second year of undergraduate physics.

 

[ST4RCUTTER]

That's pretty disconcerting, though, to think that all the work done in physics is only useful to the point that objects, other than light, stay in a relatively slow state of motion.

As disconcerting as that might be, it's the situation. You have to go by a different set of rules when discussing matter at high velocities.

What really interest me is gravity. Gravity is a force created by the curvature of space due to matter or accelerating matter is it not? So gravity is a force created by the interaction of matter and space-time. What more do we know about this relationship? Are there any ways in which we can use this interaction for purposes of travel?

 

[SirFshAlot]

This is explained by the contraction of space

sure


I like the explanation, but it's above me, still.

Space gets contracted and time gets dialated by a factor gamma = 1/(1 - v^2/c^2)^.5, so at v = .5c, gamma = 1.15.

how firm is this?

 

[crzyc]

i have a headache from reading this thread

 

[Sohcan]

Space contraction and time dialation by the factor of gamma is actually one of the most empirically-supported parts of Einstein's special relativity...it's been proven to work with many different experiments. The best example is with the decay of muons...when cosmic rays hit the upper atmosphere, they produce muons, whose decay rate is well known. Given their extremely short lifetime and that they are moving at near the speed of light, they shouldn't reach anywhere near the surface of the earth...they should decay long before then. But since the muons are moving so fast, from their perspective the distance from the upper atmosphere to the surface is contracted, and they are able to reach the surface. So from the perspective of the muon, the don't have as far to travel, but from the perspective of a stationary observer, their lifetime gets extended. In both cases, they reach the surface.

 

[Urinal Mint]

Man, I love physics... being an engineer and knowing how to do this sh1t is fun fun fun

 

[SirFshAlot]

&quot;If you find this property of light hard to swallow, you are not alone. At the turn of the century physicists went to great lenghts to refute it. They couldn't. Einstein, to the contrary, embraced the constancy of the speed of light, for here was the answer to the paradox that had troubled him since he was a teenager: No matter how hard you chase after a light beam, it still retreats from you at light speed. You can't make the apparent speed with which light departs one iota less than 670 million miles per hour, let alone slow it down to the point of appearing stationary. Case closed. But this triumph over conflict was no small victory. Einstein realized that the constancy of light's speed spelled the downfall of Newtonian physics.&quot;

Brian Greene-The Elegant Universe

 

[SirFshAlot]

they produce muons, whose decay rate is well known

are we sure????

they should decay long before then

instead of assuming that muons are proof of a theory, couldn't we also wonder if we really have their properties understood?

 

[CQuinn]

No, the logical inference is correct.
If an independent observer was to map the point the photon reached at the half year
mark, to the point the traveler reached at that same time, then the distance between
the two points would be, as you said 4,401,900,000,000 miles.

From the travelers point of view the photon would also be at the right distance,
but the rest of the universe might appear askew subject to relatavistic effects
on time and distance as seen from the ship.

If I am reading that correctly, what Brian Greene is saying is because the photon
can be applied as part of the universal constant, it would not be subject to
relatavistic effects seen by the traveler or an outside observer.

EDIT: My explanation appears to be slightly off too, as Sohcan explanation
is much better.

 

[Sohcan]

<< they produce muons, whose decay rate is well known
are we sure???? >>

Yep, the decay rate of subatomic particles at many different velocities fits perfectly as described by relativity...there's many other observations that have fit special relativity.



<< instead of assuming that muons are proof of a theory, couldn't we also wonder if we really have their properties understood? >>

We've had no reason so far to question our understanding of these properties...over a half-century of experiments with particle accelerators have supported it.

Relativity is not the epitome of our understanding of physics...the equations for special relativity (dealing with motion at high velocities) fails at really small scales (sub-sub-atomic, at the structure of space-time), and general relativity (dealing with gravity and space-time) fails at the subatomic level. Eventually another theory will surpass it...but that doesn't mean we can't rely on relativity. For most purposes, it works beautifully. Special relativity replaced Newtonian mechanics, but that doesn't mean Newtonian mechanics is useless. Special relativity may be more accurate, but there's no sense using it to describe the motion of a baseball thrown by a pitcher. Each theory, and those not yet discovered, all have their place in describing physics.

 

[SirFshAlot]

over a half-century of experiments with particle accelerators have supported it.

hehehehe
sorry to be so skeptical, but;

how certain are we that particle accelerators are actually valid?

I've often wondered this before about accelerators. There has to be a slight possibility that something else is happening when these particles collide that is beyond our understanding.

 

[Sohcan]

<< how certain are we that particle accelerators are actually valid? >>

Well, if you question the very function of particle acclerators, you'll get nowhere.



<< There has to be a slight possibility that something else is happening when these particles collide that is beyond our understanding. >>

Oh, there definitely is a lot beyond our understanding in particle physics...the Standard Model used for particle physics is only about 20 years old:Q. But the theory is like a jigsaw puzzle...it works for a lot of stuff, but there's still a lot of gaps. Particle physics is a lot more complex than the typical electron/proton/neutron stuff taught in high school...literally hundreds of particles have been discovered so far, and we're just starting to get into the really high-energy realm of Higgs particles and quark matter. I have no doubt that our understanding of particle physics will change a lot in the next few decades.

You can't really outright question the very function of a theory and expect to get anywhere...you have to start with some problem that the theory doesn't explain correctly, and attempt to develop a better model. Einstein didn't just wake up one morning at say &quot;Hmm, I think I'll disprove Newtonian mechanics.&quot; He started with problems that physicists had trouble explaining and developed relativity from there.

 

[SirFshAlot]

Particle physics is a lot more complex than the typical electron/proton/neutron stuff taught in high school

yeah,
nuons, gluons, quarks, bosons, neutrinos, strings et. al. just came to my attention in the last year or so.

I found that if you don't go searching out this info as a layman, you are very much in the dark about the recent (relatively speaking) advances in science.


You can't really outright question the very function of a theory and expect to get anywhere

I know I can answer a lot of my own questions by further reading and study, but that could take more time than I have to really devote to an amateur interest.
At first glance to many theories, or assumptions, I am likely to jump on things that I consider to be assumptions based on assumptions.
I like to see science at the nitty gritty level of only one variable. That is when I am more likely to be open and accepting of the results.