Questions about the Speed of Light

[Ankh-Morpork Guard]

...but that's still judging the entire universe soley on the way Earth works...

 

[Umbran]

...but that's still judging the entire universe soley on the way Earth works...

If you want us to list every single observational experiment that involved relativity, we'd be here a long, long time. Suffice to say, the entire visible universe does seem to strongly adhere to Einstein's predictions.

Lots of people are on the kick of having a problem with "judging the entire universe based on what happens on Earth", these days (EN World and elsewhere, ime). As if it were a bad thing. Never mind the fact that assuming such until proven otherwise works. Never mind that there's no particular reason to assume otherwise.

 

[WayneLigon]

...but I thought laws couldn't be violated

Its an interesting idea, really. We set so much into these laws and what we say IS right, yet we really don't have anything beyond theory when it comes to the universe as a whole. Kind of seems to me that claiming these are unbreakable laws of physics would be like having lived away from rivers and natural water sources one's entire life and claiming that, due to this, the entire world is covered in land.

Well, no good scientist ever calls an law or theory 'unbreakable'. Science by it's very nature never 'proves' anything. It describes, but is (should) always be aware that it might not be seeing the complete picture. It's just that by every experiment we've done, things like the speed of light barrier and the laws of thermodynamics do indeed seem unbreakable. Evidence has piled up to such a large degree that it's hard to think of a situation where all of the work on relativity would be rendered moot but it could happen tomorrow. Then we'd look at the results of all those past experiments and say 'Oh, we were looking at X but Y was what was really happening."

There was a nice anecdote I saw recently about Einstien (and hopefully he really said it). He remarked that this years test in his class would use the same questions as last year. "But Dr. Einstien," his assistant remarked. "These students have talked to those from the previous year, of course; won't they know many of the answers already?" "Ah," said Einstien. "The questions are all the same but this year the answers are all different."

 

[Ankh-Morpork Guard]

Lots of people are on the kick of having a problem with "judging the entire universe based on what happens on Earth", these days (EN World and elsewhere, ime). As if it were a bad thing. Never mind the fact that assuming such until proven otherwise works. Never mind that there's no particular reason to assume otherwise.

I don't have any problem with it at all. Just something to think about, really. Sure, we now have a bit more knowledge from other planets in our solar system, but that's still just our solar system, and the universe is a big place. Its fine to assume that these laws really are laws...but what if they aren't?

There's as much reason to assume they ARE correct(as they've been proven to be here on Earth and in our solar system) as there is that they AREN'T(i.e. the universe is a big place and has a habit of constantly surprising us).

I've got nothing against science, at all. Its just when people start saying things that aren't possible when we really have such a limited scope on what IS and ISN'T possible that gets me. Again, the universe is a big place.

here was a nice anecdote I saw recently about Einstien (and hopefully he really said it). He remarked that this years test in his class would use the same questions as last year. "But Dr. Einstien," his assistant remarked. "These students have talked to those from the previous year, of course; won't they know many of the answers already?" "Ah," said Einstien. "The questions are all the same but this year the answers are all different."

I like it.

 

[fuindordm]

I think Babong, PBartender and others have covered the ground here pretty well...

Regarding photons, your typical photon such as the Sun spits out is a wave packet. The particle/wave paradox really isn't that big a deal. Do a google image search on 'wave packet' and you'll get lots of pictures of them. Where the electromagnetic energy is concentrated, the fields look like a sine wave, and outside that region the amplitude of the wave decreases rapidly to zero.

When a wave packet (essentially a little ball of energy) interacts with something on scales much larger than the waves in the packet, it acts like a particle. When it interacts with something with sctructure much smaller than itself, it acts like a wave. When you're in the middle ground, you can get elements of both behavior.

But light is in all important essentials a wave, and its behavior can always be predicted correctly by treating it as such, even if in some situations it would be an unbearable pain in the butt to do so.

Turanil's comment on time not passing at all in the photon's frame of reference is right on. If you really want your mind blown, consider this:

Imagine you're a massless mind riding a photon. From your perspective, the ratio of time passing in the outside universe with respect to your own personal sense of time is infinity to one. You can think of that as existing at every point along the photon's trajectory simultaneously if you want.

But... the photon was created somewhere, and will be absorbed somewhere. So it only exists for a finite time in the universe. So 'as' you travel with the photon, you should see an infinite amount of time pass according to the rules of special relativity, and yet the photon doesn't exist for an infinite amount of time.

This is the kind of thing that used to keep me up nights when I was a young, naive physics undergrad.

If we're hoping for the future of space travel, keep in mind that special relativity is a *local* limit. The difference between special and general relativity is that the latter regognizes special relativity as the rules applying to flat space-time, and that all space-time is locally flat (in the same way that we can think of the surface of the earth as locally flat, at least on an everyday human scale). General relativity, however, allows for all sorts of strange configurations of space-time.

It turns out that the 'warp fields' of Star Trek are one plausible way of breaking the light barrier. The equations of GR allow a configuration of space-time that is flat in the middle (around a ship), contracting space in front of the ship and expanding space behind it; such a field could move faster than light globally while preserving the rules of special relativity locally. Cool, huh? All we need to make it work is large concentrations of exotic matter/energy with a negative energy density.

As it turns out, 70% of the energy content of the universe has a negative energy density--the famous cosmological constant that is making the universe expand every more rapidly. Negative energy densities are also observed in quantum mechanical experiments.

Research progresses on this front. We don't know yet how to harness this, or even if it can be harnessed (at quantum scales, it doesn't seem possible to create a negative energy density pulst that isn't right next to a larger concentration of positive energy). But the SF geeks among us have reason to be cautiously hopeful.

Ben

 

[JamesDJarvis]

as much reason to assume they ARE correct(as they've been proven to be here on Earth and in our solar system) as there is that they AREN'T(i.e. the universe is a big place and has a habit of constantly surprising us).

I've got nothing against science, at all. Its just when people start saying things that aren't possible when we really have such a limited scope on what IS and ISN'T possible that gets me. Again, the universe is a big place.

which is why we have the term "observable universe". Scientfic laws are not unmutable and really only apply to that observable universe we currentyl dwell in. If gravity (for example) didn't work like we think it does throughout the observable universe well there would be no observable universe.

 

[Thunderfoot]

Part of the problem(s) which have to be overcome if one is to actually arrive at a conclusion is
a) Is the theory of relativity closer to a law or is it mutable
b) In the approaching zero equation are we talking about zero or absloutely zero
c) If the theory of relativity does not apply, what is the formulae by which we equate faster than light travel.
d) One could argue that increase of speed does not increase mass, an SR-71 flying at Mach 4 actually weighs less than one flying at Mach 2 - is that due to the breaking of the gravity barrier or to some unknown law of physics and motion?
e) If we could travel at that speed, name three practicle applications beyond sending probes to other galaxies, solar systems.

As for the if a man walks from the front to the back of an object moving at the speed of light and the aspects of aging - I wouldn't hold my breath. Many scientists said that machine guns would never work on jet powered planes because the bullets move slower than the plane. However, an as then unknown law of motion proved that theory wrong.
Objects moving inside an object moving at speed X may increase and decrease speed by Y without affecting principal laws of motion. (ie the bullets were moving at the speed of the plane PLUS their base movement when fired from the aircraft) So the theory of if you can't age if moving at light speed is true, moving within that speed should hold bearing on aging because your base speed would still be the speed of light + the rate of movement. Direction has no bearing on the Theory of Relativity-
E=Mc2 Energy equals matter times the speed of light squared - Direction is not part of that equation.
But then - what do I know?

 

[tarchon]

Many scientists said that machine guns would never work on jet powered planes because the bullets move slower than the plane. However, an as then unknown law of motion proved that theory wrong.

Galilean relativity was pretty well known by the mid-20th century.

 

[Gidien]

Part of the problem(s) which have to be overcome if one is to actually arrive at a conclusion is
a) Is the theory of relativity closer to a law or is it mutable
b) In the approaching zero equation are we talking about zero or absloutely zero
c) If the theory of relativity does not apply, what is the formulae by which we equate faster than light travel.
d) One could argue that increase of speed does not increase mass, an SR-71 flying at Mach 4 actually weighs less than one flying at Mach 2 - is that due to the breaking of the gravity barrier or to some unknown law of physics and motion?
e) If we could travel at that speed, name three practicle applications beyond sending probes to other galaxies, solar systems.

E=Mc2 Energy equals matter times the speed of light squared - Direction is not part of that equation.
But then - what do I know?

Just to clear up some confusion... the Theory of Relativity works in all situations, if it is indeed true (which we have become pretty sure of due to many experiments). As you approach the speed of light parts of the equations of relativity approach zero, and if other numbers are being divided by those zeros they approach infinity. So, you can't really get to the speed of light using legal math, you can only take the limit as you approach it.

Something flying at mach 4 is not lighter than something flying at mach 2... in fact, it has slightly (very, very slightly) more mass and more weight. Now, if said jet has used up a lot of fuel to get to mach 4 obviously it will have less mass, and if it is higher up gravity will have a smaller effect, though that will not affect the mass of the jet.

And direction very much does matter in relativity, as anyone struggling with vector addition on a relativity test knows. If your spaceship is very close to the speed of light relative to an observer (having a stationary observer is the key to all of these questions, btw, so let's not leave it out), and you are walking around inside, you will walk very, very slowly, slow enough not to pass the speed of light. If your spaceship is at the speed of light (impossible) then you would not move at all. On the other hand, for you inside the spaceship, you can move freely, the spaceship is stationary, and everything else is moving very fast while slowing down in time. E=mc^2 is a cool equation that can be found by working the equations of relativity, but it is not the key equation in the theory.

 

[Umbran]

I Its fine to assume that these laws really are laws...but what if they aren't?

If they aren't, then we have no indication as yet what they are, and talking about them is more a matter of science fiction than science fact. Not that science fiction is at all bad, but it is important to be clear that you're engaging in speculation.

The big thing is understanding that when you ask a question, "What if X was true instead?" you can only analyze that assuming that everything else we know is still true. In some cases, that means that X being true is nonsensical. Our universe isn't built of discrete things that are highly mutable. It is made of a whole bunch of interconnected things, sometimes connected so strongly that you cannot change one of them alone and still make sense.

There's as much reason to assume they ARE correct(as they've been proven to be here on Earth and in our solar system) as there is that they AREN'T(i.e. the universe is a big place and has a habit of constantly surprising us).

With the exception that for the majority of physics, they've been shown to be be entirely consistent well outside our solar system. Thus my reference to the visible universe - we can see the effects of QM and Relativity out there, too.

I've got nothing against science, at all. Its just when people start saying things that aren't possible when we really have such a limited scope on what IS and ISN'T possible that gets me. Again, the universe is a big place.

The universe is large, yes. But it is also observably uniform is structure and laws. If we did have only proof of physics for Hoboken, NJ, then I'd support you. But when our laws really do seem to hold through a sphere several billions of light years across, you have good reason to start generalizing, don't you? While yes, it may be possible that the laws are different elsewhere, is that really the way to bet?

As it turns out, 70% of the energy content of the universe has a negative energy density--the famous cosmological constant that is making the universe expand every more rapidly.

If we're going to be picky - that's the popular hypothesis, but it isn't the only one. It isn't as if our universe isn't already full of constants for which we don't have an origin. Maybe the constant "just is". Einstein first proposed the constant as "it just is", without knowing where it might come from, like the charge on an electron.