You guys/gals have gotten stuck so deep in time and space, I think it’s time to add mass!
Wasn’t there a notion that, at relativistic speeds, the mass of any object increases as speed increases? So that, for instance, no possible gun could ever get that theoretical bullet to move forward?
Yes, the faster a massed object goes the more energy it takes to provide further acceleration and that is equivalent to saying that its mass has increased. The mass increase can be measured in particle accelerators. (It’s also true that the faster a massed object goes, the slower time appears inside its reference frame.)
The need for ever increasing energy also implies that no massed object can go the speed of light. If the faster it goes, the more energy it takes to go even faster, then the math shows it would require an infinite amount of energy to reach light speed. Therefore only massless particles can go the speed of light.
Lots of other implications to this, such as the unfortunate fact that getting a spaceship to move fast enough to reach relativistic speeds so it could reach the stars in a reasonable amount of time would require more energy than any technology we can foresee.
But there have been dozens of threads on this. You should search through GQ for hours of fascinating reading posted by true experts.
There was, and it’s a flawed notion. Or at least more modern descriptions of relativity avoid it.
It originates from the fact that in SR, momentum is given by P = mv/sqrt(1-(v/c)^2), where m is the invariant mass. This has the property that P increases without bound as v approaches c.
It was noted that if you defined the “relativistic mass” by M = m/sqrt(1-(v/c)^2), you could write the momentum as P = Mv, and still maintain a sort of Newtonian expression, at the expense of having the mass be this weird quantity that varies with velocity.
So you can think of it as “mass is constant and how momentum works is weird”, or “momentum works as one would expect in a Newtonian world, but the mass is weird”. For various reasons, modern descriptions prefer the former to the latter.
But anyway, that doesn’t really matter to the “gun on a spaceship” issue because the bullet is stationary with respect to the gun before it’s fired. The “stationary” observer (i.e. the one which measures the spaceship’s speed as 0.95c) would perceive the bullet as having an abnormally large “relativistic mass”, but the gun that was doing the work of firing would not.
Right, it’s just another example of the over-dependence on mobil phones lately.
There’s a three-part answer: Ratings, ratings, ratings!!!
To get serious now (about time I did, right?) I’d like to get back to the OP, since the subject has come up in my local Bertrand Russell Society chapter.
Scientists don’t reject “time” as being not objective-- not exactly.
But what we Homo Sapiens do is divide time into past and future with an instant we call “present” sandwiched in between. At least, if we are “sophisticated” enough to realize that considering the “present” to not be a day or even a minute or split-second.
But even that is not quite “sophisticated” enough. Because to science there is no mathematics to distinguish any present moment. There is only the “arrow of time” (That is, there is increasing entropy in the future direction, which would make running a filmed event backward look bizarre in terms of causation). One way of putting it is that “past-ward” and “future-ward” are valid, but labelling segments of a world-line as past and future is not.
While we are in the habit of viewing events as “developing” there are only world-lines existing through space-time. Of course, 3000 A.D., January 1st, Midnight EST hasn’t “happened yet” but that is because we are “embedded” in time. A physics experiment would presumably work the same at that time, so it may as well be happening today or in 1000 B.C.
In addition, as indicated upthread, time is inseparable from space, so even in our “subjective” experience of “the present moment” we have a past “light-cone” and a future one with “elsewhere” for every other space-time point. But this has been extensively commented upon, in SDMB and um, elsewhere. (Unintentional pun.) My point is about the philosophical-scientific present-moment irrelevance, even for any individual world-line.
- Jack
Though no one can surpass Mr. Light’s speed, Mr. Light can be slowed.
And it doesn’t require a galaxy of ominous intergalactic black holes to do it. Rather a home basement laboratory using a Bose–Einstein condensate can do the trick.
You’re mixing two concepts, although that’s a very common mistake.
I’ve always advocated that we need to distinguish between C and c. C would stand for Einstein’s Constant, the theoretical speed of electromagnetic radiation in a perfect vacuum. (Yes, I’m aware that a more technical distinction would actually be necessary.) That’s the C we’ve been talking about.
Lower case c stands for the local speed of light in a real world setting. All the light we see by is slowed slightly because it passes through the atmosphere. Light is slowed even more by water, which is why pathways appear to be bent. Diamond slows light even more. And scientists can slow light to essentially zero speed. All of these are examples of c, not C.
Nothing we nor theoretically anyone else can do will affect C. Lower case c is as malleable as the speed of sound. Interesting, but not the stuff theory is made of.
Or if not bizarre, at least funny as hell.
300,000 km/s. Not just a good idea, it’s the law. 
Thanks for the clarification. I guess we should just say then that c <= C where c = C is the case of a perfect vacuum. My little assertion about Bose-Einstein condensation was contemplating the theoretical effects of Absolute Zero temperature that was explored in a NOVA documentary on the nobel winners of the race to near-Absolute Zero. A perfect vacuum would assume an absence of all particles and a temperature of Zero Kelvin. In a perfect vacuum, then, light is unimpeded by any form of matter and therefore chugs along at its theoretical speed rating.
But theoretical constants are valid so long as the theory holds and who knows… one day Einstein Jr. may come up with another theory and constant. Anyone gunning for the 2020 Nobel in Physics?
Einstein’s General Theory of Relativity is flawed somehow, we think, because it doesn’t mesh with Quantum Theory. But, so far, no one has come up with a new General Theory of Relativity that doesn’t have problems of its own.
The Special Theory of Relativity, on the other hand, has no known problems. No one is going to come up with a replacement unless and until we find something wrong with it.
I think that just creates more confusion, lower case ‘c’ in this context would be by the far the most likely to be understood as the speed of light in a vacuum.
When writing the speed of propagation of light through some medium, I think it is best either avoid using a lower case ‘c’ completely or make sure it is always qualified with a subscript (e.g. c[sub]air[/sub]). It’s extremely rare thesedays that you would ever see (an unqualified) lower case ‘c’ used to reference the speed of light through any other medium than the vacuum.