Tamago wrote:

One thing that really bugs me about the whole speed of light (c) issue is the theory that an object's mass increases as it approaches c, are they talking abut the size of the object or the number of particles increasing?

My understanding of relativistic physics is sketchy at best, but I'm pretty sure the answer is "neither."  It's better to think of the object's density as being the quantity that's increasing.  Got a cubic centimeter of aluminum that weighs 3 grams?  At 99% of c it's still a cubic centimeter of aluminum, and it still has the same number of atoms as before, but now it weighs several times as much.  It got denser, in a certain sense... or something like that.  (At least, relative to you as you watch it from Earth; if you were flying on the spaceship with it, your scale would tell you it still weighs 3 grams.  It's very confusing.)

Another thing that gets to me is how scientists from Einstein onwards seem to mistake the illusion of time slowing down with actual time slowing down for the object that travels near c.

That's because scientists from Einstein onwards don't really believe that there is such a thing as "actual time."  One of Einstein's great insights was that time is relativistic.  Time, that is, works the same way as mass from the previous example.  Put an astronaut wearing a wristwatch on a rocket at noon, then shoot it into the sun at 99% of c.  According to our clocks on Earth, the rocket arrives at 12:08 (because it takes about eight minutes to get from the Earth to the Sun at near light speed), but when we use Hubble to spy on the astronaut's watch, we see it says something more like 12:01.  If she now turns the rocket around and flies back at the same speed, her watch says 12:02, but our clocks say 12:16.  In a certain sense (a phrase I'm using a lot in this post), she has time-traveled fourteen minutes into the future -- our future.  But in a certain other sense, Earth time-traveled fourteen minutes into the past -- her past.

All this is pretty well-established theory, I think.  What I don't understand is the asymmetry.  So far I've been talking as though the Earth is stationary and the rocket is moving, but of course that's not the only perspective we could watch from.  What if we watch from the rocket -- construing the rocket as the stationary point?  From the astronaut's perspective, Earth is moving away from her at 99% of c.  But if Earth is moving at 99% of c relative to the rocket, then shouldn't time appear to travel more slowly on Earth than on the rocket?  At 12:01:00 rocket time, shouldn't an Earth clock appear to read 12:00:08 or so?  And when the astronaut returns at 12:02:00 rocket time, shouldn't the Earth clock read 12:00:16?  But that contradicts what I said in the previous paragraph.  Both these things can't be true, but then which gets to be the fixed point -- Earth or the rocket -- and why?

Last edited by satyreyes (08-09-2009 06:41:18 PM)

Satyr, I can help!

This same seeming paradox perplexed me for a while, but then I read the Elegant Universe. I didn't get a lot of it, but the resolution to your asymmetry issue is actually pretty simple. If a rocket is traveling away from the earth at .99c, then the rocket and the earth experience the same issues relative to each other. For the rocket, time goes slower, and for the earth... time also goes slower. Seems crazy, but it's because they're in different frames of reference. They can in fact both be slower than the other. And as long as the velocity between the two objects is constant, that is no acceleration is going on, that's how it is. It's acceleration that jumbles things. And that is actually the distinction between special and general relativity, special relativity describes a system where acceleration is zero. Sadly, general relativity is still beyond me, so I can't explain how a rocket that travels and then comes back gets there faster than it should, the so-called twin paradox. And just to make sure I'm clear on this, the time dilation produced is NOT an illusion. As satyr pointed out, there is no objective standard for the flow of time that you could claim was more valid than either measurement.

Regarding the "objects get bigger" issue, yes, they do get bigger. Not just in mass, but in displacement as well. In other words, an object passing by you at .99c is considerably longer than the same object sitting in your hand. This is not an illusion either, it really is longer. It's called a Lorenz distortion, named after the guy who developed the equations. Even wilder, the mass and displacement of an object both approach infinity as they approach the speed of light. Which suggests that if an object with mass reached the actual speed of light, it would occupy the entire universe.

Nanami's Rose Groom wrote:

Eh? So why are photons not as big as the universe?

Its unanswered questioned like this that future historians will wonder why scientists of our time couldn't answer without inventing an even more unlikely theory to attempt to answer it.

Tamago wrote:

Nanami's Rose Groom wrote:

Eh? So why are photons not as big as the universe?

Its unanswered questioned like this that future historians will wonder why scientists of our time couldn't answer without inventing an even more unlikely theory to attempt to answer it.

Because photons have no mass, as I mentioned in the first post of this thread   No mass means they can hit light speed without becoming infinitely massive.  (Oddly enough, a photon that is actually traveling at light speed does have "relativistic mass," because zero times infinity is not always zero, but it's not a very large mass.)
ETA: Beaten by Stormy as usual! 

Last edited by satyreyes (08-10-2009 09:57:51 AM)

A white tabletop contains no pigment; as a result, it is colorless under full-spectrum light.  But if you turn off the full-spectrum light and shine a red flashlight on the table, you observe that the table is now red.  Normally, for something to have a color it has to have pigment; but under abnormal conditions (say, under red light) you can make something without pigment have color.  If you hadn't seen the table under the white light, you might conclude it had red pigment.  But you do know better, so instead you might call this illusory pigment the table's "apparent pigment."

In the same way, a photon has no mass; as a result, it has no momentum when it is at rest.  But if you make it go light speed, you observe that the photon now has momentum.  Normally, for something to have momentum it has to have mass; but under abnormal conditions (say, at c) you can make something without mass have momentum.  If you didn't know from prior observation that photons are massless, you might conclude they have mass.  But you do know better, so you call this illusory mass the photon's "relativistic mass."

Does that make any sense?

Stormcrow wrote:

Put another way, when we say something appears one way and is another, we suggest that there is some deeper basic level of reality about the way it is, but not the way it appears. But when I talk about time seeming slower for something moving away from me very very quickly, there is no deeper level of reality to compare it to. Relative time is as good as it gets. Same with momentum (and by implication, mass) and length.

But isn't there at least one quantity -- the speed of a photon -- that is the same no matter what frame of reference you use for your measurement?  Not for a minute saying I understand it, but I'm pretty sure physicists believe that light in a vacuum travels 186,000 miles per second no matter what crazy things you might be doing.  That is, if you were going .99c with respect to Earth in the same direction as a nearby photon, you would still clock that photon as traveling at c -- not .01c -- even though south of the relativistic horizon things look like they're moving slower when we're moving almost as fast in the same direction.  You can't measure a photon's mass at rest because you can never get a photon to be at rest with respect to any frame of reference.  So isn't a photon's "relativistic mass" just about as un-relative a quantity as could possibly exist?

I wonder if light and gravity are two sides of the same coin, as gravity in itself doesn't seem to have mass or can be properly represented by particles.

I hope you are catching my drift.

Tamago, you may be surprised to learn that gravity has a carrier particle, as do all the fundamental forces (nuclear weak, nuclear strong, electromagnetic). Gravity's carrier particle is, quite simply enough, the graviton (pronounced "grav-uh-tawn"). When somebody speaks of "gravitonic" technology, they're talking about devices and weapons that make use of the force of gravity through its carrier particle to achieve the intended effect of the device in question.

Actually, the graviton is hypothetical, and may well not exist. If it does exist, it's unlikely it has locality, so even thinking of it as a particle may be incorrect. For this reason, the gravitational force may be fundamentally different from EM, strong and weak forces, all of which have mediating particles whose existence is confirmed. Oh, by EM force, I mean light, sort of. Light is EM radiation, so any force exerted by it is EM force.

As for black holes... it's not mass that defines them so much as density. In other words, you could make a black hole out of a baseball if you could crush it down small enough. Small enough in that case would be on the order of an angstrom I'd guess, which is a little ludicrous. You'd kind of need another black hole to generate that kind of pressure. Anyway, it's typically stars that collapse into black holes as I understand it, so you have a lot of mass, and when the fusion process powering the star stops, or rather decreases below a certain threshold, the star might collapse inward. The density of the sun isn't all that much, even though it's mass is gigantic, because the fusion keeps the mass blown out pretty far. I don't think the sun is a black hole candidate in any case, not the right kind of star. Anyway, the mass that is ADDED to a black hole after it becomes a black hole would be pretty trivial. Things in the universe mostly move AWAY from each other, and even a black hole wouldn't change that. Gravity peters off pretty quickly when you move farther away.

The Hawking radiation thing is really weird. Seems to have something to do with temperature and entropy... I may do some more research later, but no time right now. Oh, and the whole "black holes are portals to somewhere" thing is purely speculative. There is no evidence that other universes even exist, though there's no reason they can't either. It's kind of like believing in god. No reason god can't be real, but scientifically speaking, there's just no evidence.

Nanami's Rose Groom wrote:

Actually mathematically it is a concept that makes sense.

The spirit of Ambrose Bierce who resides in my body would like to point out that this is probably the best argument against the existence of gravitons.

But seriously, I know nothing about how gravitons are supposed to work.  I like the mattress metaphor you run across a lot in popular science books, where massive objects cause an indentation in space that everything else nearby tends to fall into, or else circle around in orbit.  This metaphor elegantly explains why photons are displaced by massive objects even though they themselves are massless.  Are gravitons supposed to somehow model the same idea only with particles -- which challenges my imagination, but then, what doesn't? -- or are they part of a different gravitational "metaphor" entirely?