
#1
Apr1311, 12:47 PM

P: 5

When I think of the speed of light, I imagine something moving really fast.
Everyone knows that something moving fast hurts more than something moving slow. So why is something said to have "infinite" mass when traveling at the speed of light if the speed of light is a finite "measurable" speed? Why is this even considered "impossible"? If E = mc(squared) then doesn't it simply make energy proportianal to mass "since the speed of light doesn't change for the equation". Or does the E stand for "potential energy". Does anyone else relate to how I feel about this? Can't you just as justifiably take the speed of a train, write it down, and use that as an absolute? "which would produce a representation of energy that is only practical for use in a relitive sense, and cannot be applied as a number on it's own" 



#2
Apr1311, 01:38 PM

P: 1,262

[tex]E = mc^2[/tex] only applies to the 'rest mass' energy of an object. The total energy is [tex]E = \gamma mc^2[/tex] where [tex]\gamma[/tex] is a function of velocity, and becomes infinite when the velocity approaches the speed of light. 



#3
Apr1411, 07:33 AM

P: 5

Ahh, thank you very much.
I love wikipedia, but anyone I talk to will redicule me if I use it as my source, do you have any favorite books to recommend? 



#4
Apr1411, 08:01 AM

Mentor
P: 16,473

speed of light not a legitamate factor in relation to mass 



#6
Apr1411, 06:58 PM

PF Gold
P: 11,016

The view of mass increasing as velocity does is, from my recent knowledge, incorrect. The mass of an object is always the same. The momentum or kinetic energy of an object WILL increase, but not the actual mass of an object.




#7
Apr1411, 09:11 PM

P: 9





#8
Apr1411, 10:41 PM

PF Gold
P: 11,016





#9
Apr1411, 11:11 PM

P: 9

hmmm that is a good point. Does a particle traveling at a high velocity not contribute to the EM/SR Tensor? Energy definitely does, otherwise we wouldn't have gravitational waves, correct? So would a particle traveling at a high velocity not contribute more to the EM Tensor than the same particle at rest? When a particle has an increase in energy we see a corresponding increase in momentum, but not a corresponding increase in speed ( because as energy increases, velocity can only approach c asymptotically) If that energy isn't accounted for as an increase in kinetic energy, there where else can we account for that missing energy?




#10
Apr1511, 12:40 AM

P: 2,456

But, it will contribute to a DIFFERENT components of the tensor, than a particle at rest.
In fact, 2 test bodies moving at high speed flying parralel to each other attract slower (in a frame where we see them moving) than when they are at rest. The explanation is simple: if it takes, say, 1s for them to collide in their rest frame, then because time is dilated, it would take LONGER for them to collide if they are moving. There is a special case however, when there are MANY particles or bodies in a bound system with a total momentum of 0. In such case, the movement of particles create pressure, which creates 'more gravity' for an observer who is unaware of the internal structure of a system. Examples: 1. When you heat a body, its gravity slightly increases. 2. Proton. Most of the mass comes from the quarks, moving at relativistic speeds back and forth. 



#11
Apr1511, 08:54 AM

P: 9

I guess i stand corrected then. Thanks Dmitry67.




#12
Apr1511, 05:07 PM

PF Gold
P: 11,016

Dmitrey, I've heard of that before as well. However, are we sure that the gravity of a system will increase if you add energy, like in the form of heat to it? And if so, how does that relate to two moving objects moving together slower at relativistic speeds? Would adding energy not cause an increase in the speed of the movement of the particles, which would cause further time dilation?




#13
Apr1611, 12:08 AM

P: 2,456

Because when objects moving in the same direction, there is a rest frame where they are at rest. So you can calculate everything in that rest frame and then just calculate the dilation for the other rest frames. When bodies are moving in DIFFERENT directions, there is no such frame!
Compare: 1. Photon gas (photons moving in random directions) selfattract (and attract other bodies) 2. two collinear light beams, going in the same direction, dont gravitate to each other 3. two collinear light beams, going in the opposite directions, attract to each other 



#14
Apr1611, 12:11 AM

PF Gold
P: 11,016

As there is no rest frame for a photon, how can you relate to them using the same thing for matter?
Also, if I pick a frame of one of the particles, then everything else is moving in relation to it. Wouldn't that mean that everything else is still time dilated in relation to that particle? 



#15
Apr1611, 08:59 AM

Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 38,879





#16
Apr1611, 09:09 AM

Mentor
P: 16,473





#17
Apr1911, 12:58 PM

P: 47

you dnt wanna confuse relative mass and rest mass. 


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