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View Full Version : Light moves 300,000 kmps relative to what?



Halimahnosa
2011-Jan-25, 06:00 AM
I keep listening to astronomy cast and they're always talking about things going like a certain fraction of the speed of light and the different properties it would have if it were. I keep getting hung up on this concept when talking about things with mass going relativistic speeds. Let me explain. Right now relative to my computer I'm at a dead stop, but relative to the center of the milky way I'm going along at a pretty good clip. So lets say instead of a supermassive black hole you had a computer identical to my own at the center of the galaxy. Since things get more massive the higher your speed, does that mean my mass would be greater relative to the computer at the center of the galaxy than it is is relative to my computer

Strange
2011-Jan-25, 09:52 AM
The important thing is that it is all relative. You will see the apparent mass of someone moving relative to you increase. On the other hand, from their point of view, you are the one who is moving and so they will perceive your mass as increased.

Similarly, everyone measures c (the speed of light) as being the same.

Jeff Root
2011-Jan-25, 02:54 PM
What Strange said is exactly correct.

Although Einstein predicted an increase in relative mass, and that
prediction was accurate, it was found that the idea is both confusing
and unnecessary. Rather than increased relative mass, it is enough
to say that the kinetic energy increases. So instead of using the
formula for relativistic mass, use the formula for relativistic energy,
and you will be able to discuss the same physics without getting
into certain problems that the concept of relativistic mass has.

To answer the thread title question: Light moves at 300,000 km/s
relative to the observer. Nomatter who the observer is, where he,
she, or it is, and nomatter how he, she, or it is moving, as long as
the measurment is "local". Nonlocal measurements are generally
not useful. You have to really understand what you are doing to
get a meaningful nonlocal measurement, and it has limited value.

You and the center of the galaxy are so widely separated, in so
many ways, that you really are nonlocal to each other for the
purpose of measuring your relative kinetic energies, or relative
masses. I personally wouldn't attempt to calculate it.

But there are other things you could use as examples, such as
the kinetic energy (or relativistic mass) of an electron speeding
toward the screen of a CRT at a sizeable fraction of c compared
to that of an electron in the screen.

-- Jeff, in Minneapolis

Argos
2011-Jan-25, 04:35 PM
The speed of light is an invariant, and everybody will agree with that.

caveman1917
2011-Jan-27, 02:15 PM
Only for inertial observers.

ETA: unless one is taking the notion of locality to be at the limit where the size of your apparatus goes to zero, but then one is not talking about any real measurement anymore

swampyankee
2011-Jan-30, 01:50 PM
Relative to what? Whatever instrument you're using to measure the speed of light. If you're using two instruments, one of which is moving and the other of which isn't, both will give you the exact same value for the speed of light, relative to the instrument.

Strange
2011-Jan-30, 01:55 PM
Relative to what? Whatever instrument you're using to measure the speed of light. If you're using two instruments, one of which is moving and the other of which isn't, both will give you the exact same value for the speed of light, relative to the instrument.

Top answer

HenrikOlsen
2011-Feb-03, 03:12 PM
The strange thing about the theory(ries) of relativity is that they're based on the counterintuitive observation that the speed of light is constant relative to everyone and it has been shown to be so in every experiment done so far.
BTW, it's 299,792,458 m/s, not 300,000,000 m/s

swampyankee
2011-Feb-03, 11:36 PM
I think we should redefine the meter so that the speed of light is exactly 300,000,000 m/s. After all, the change would be only 0.07%, and who'd notice that? ;)

grapes
2011-Feb-04, 01:16 AM
I think we should redefine the meter so that the speed of light is exactly 300,000,000 m/s. After all, the change would be only 0.07%, and who'd notice that? ;)Even better yet, let's define a meter so that the speed of light is exactly 1,000,000,000 m/s. To make it metric, of course.

O wait, that'd be a foot...