- #1
antariksh
- 1
- 0
Just a thought, according to Einsteins relativity mass changes with speed and tends to become infnite as it approaches 'c'. Since photons too have mass, why doesn't their mass become infinite since they travel at c?
The momentum is related to wavelength:Gyroscope said:How is it possible for photons to have momentum if they are massless?
mathman said:Photons have zero "rest mass", so that the Lorentz transformation can't be used - ie. m= (0/0)*c2.
mathman said:Photons have zero "rest mass", so that the Lorentz transformation can't be used - ie. m= (0/0)*c2.
antariksh said:Just a thought, according to Einsteins relativity mass changes with speed and tends to become infnite as it approaches 'c'. Since photons too have mass, why doesn't their mass become infinite since they travel at c?
bernhard.rothenstein said:photon momentum and energy is a frequent topic on the forum. consider a tardyon (u<c) the momentum of which transforms as
p=gp'(1+V/u') (1)
E=gE'(1+Vu'/c^2) (2)
duordi said:We are not sure that light has zero rest mass or that it travels 100% of the speed of light.
But we do know it is very close to it. Make that very very close.
mathman said:Photons have zero "rest mass", so that the Lorentz transformation can't be used - ie. m= (0/0)*c2.
duordi said:No it isn't, but can you tell? That is the point.
If a person is accelerated to 99% of the speed of light and that person measures the speed of the photon passing him he will determine the photon is traveling past him at the speed of light.
So if two photons are traveling in parallel paths what speed do they measure of each with respect to the other?
duordi said:No it isn't, but can you tell? That is the point.
If a person is accelerated to 99% of the speed of light and that person measures the speed of the photon passing him he will determine the photon is traveling past him at the
of light.
So if two photons are traveling in parallel paths what speed do they measure of each with respect to the other?
duordi said:We are not sure that light has zero rest mass or that it travels 100% of the speed of light.
But we do know it is very close to it. Make that very very close.
yenchin said:Light does not travel at the speed of light? Then why do you call it "the speed of light"?
duordi said:Interestingly C was defined by definition in 1983.
Permittivity (E)of free space is defined by C and the Maxwell equations.
Permeability (u) is measured.
Which means C can not be determine by ( or is not determined ?) by the Maxwell equations.
Unless (E) can be measured.
http://en.wikipedia.org/wiki/Speed_of_light
Actually the reason c is a constant has to do with how the meter is defined.nakurusil said:You may wonder why is c defined and not measured to be 299,792,458 metres per second .
The reason is that any physical measurement has certain error attached to it, so that a number had to be chosen. To recap:
1. c is chosen
2. [tex]\epsilon_0[/tex] is defined based on c
3.[tex]\mu_0[/tex] is derived based on the values chosen at 1 and 2
So, all of the above guarantees that c is exactly 299,792,458
jtbell said:If at some point the speed of light is shown not to be constant, then the definition of the meter will surely be changed to reflect this.
c is constant by definition in relativity. In addition, there is ample experimental confirmation, so I don't think that c being a constant has much if anything to do with the meter is defined.MeJennifer said:Actually the reason c is a constant has to do with how the meter is defined.
That is fine, and you seem to be hard to convince , so I won't bother.nakurusil said:I do not think so.
MeJennifer said:That is fine, and you seem to be hard to convince , so I won't bother.
For others, the meter is defined in terms of the speed of light. One meter is the distance traveled by light in a vacuum in 1/299,792,458 of a second. So obviously c must be 299,792,458.
MeJennifer said:Actually the reason c is a constant has to do with how the meter is defined.
nakurusil said:I don't think so.
MeJennifer said:That is fine, and you seem to be hard to convince , so I won't bother.
For others, the meter is defined in terms of the speed of light. One meter is the distance traveled by light in a vacuum in 1/299,792,458 of a second. So obviously c must be 299,792,458.
rbj said:I don't think so.
he must have deleted that post, because i was about to tell him to check it out at the NIST site if he doesn't believe it.
some of the other stuff he said (like [itex]\mu_0[/itex] being derived) is not correct either.
nakurusil said:Check my post.
rbj said:what's there to check? you said two things that are demonstratively incorrect. check the NIST site to see how these get defined so that c = 299792458 m/s. and how [itex]\mu_0[/itex] is defined, not derived.
rbj said:what's there to check? you said two things that are demonstratively incorrect. check the NIST site to see how these get defined so that c = 299792458 m/s. and how [itex]\mu_0[/itex] is defined, not derived.
nakurusil said:c is constant by definition in relativity. In addition, there is ample experimental confirmation, so I don't think that c being a constant has much if anything to do with the meter is defined.
Actually, it is exactly the other way around, the definition of the meter is dependent on c:
http://en.wikipedia.org/wiki/Meter
and on the definition of the second.
nakurusil said:Look at "Overview" here:
http://en.wikipedia.org/wiki/Speed_of_light
[tex]\mu_0[/tex] value is chosen such that [tex]1/\sqrt(\epsilon_0 \mu_0)=299,...[/tex]
rbj said:baloney. you need to read up on some of this.
[itex]\mu_0 = 4 \pi \cdot 10^{-7} [/itex] in whatever units because of how they defined the Ampere. check it out yourself.
rbj said:a postulate is not a definition. a definition is something that is not a "truth" that is proposed. a postulate (such as the invariancy of c in SR) is something akin to a physical law.
we could define a meter to be the distance that sound in air at STP travels in 1/331.5 second and that would fix the speed of sound in air at STP to be 331.5 m/s. does that mean that the speed of sound is defined to be constant in the same way that c is believed to be constant?
c = 299792458 m/s because the meter was defined to make it so. now, if these VSL guys are correct (i don't think they are) then the distance between the two little scratch marks on the prototype meter will have changed, in terms of the present definition, if the speed of light actually did change sufficiently.
but i agree, that to speak of a changing speed of light, especially when the base units are defined as they are, is meaningless. the only numbers about the that we measure are ultimately dimensionless numbers. if [itex]\alpha[/itex] changes, that is meaningful, but there is no meaning in saying that [itex]c, G, \hbar [/itex] changes in and of themselves. if the number of Planck lengths in the Bohr radius changes, that is meaningful and, assuming the old prototype meter stick is a "good" meter stick (and it doesn't lose or gain atoms), then the number of Planck lengths between those little scratch marks will have changed.
but the fact that c=299792458 is a matter of how they defined the meter and SR could not have predicted or determined that.