# Relativity velocity of photon

#### logearav

Joined Aug 19, 2011
243
Two photons are travelling, one in the east and another in the west. Then the relative velocity between two is
(a) 2c (b) c/2 (c) c^2 (d) c
I know the velocity of light is constant, i presume the answer is option (d). But the term relative velocity confuses me.
I look forward for revered members' help in this regard

#### Wendy

Joined Mar 24, 2008
22,206
Measure velocity depend on the observer. It is not linear or intuitive. The only constant is light is traveling the same speed no matter where you measure it, the rest is caused from that. All other speeds adjust themselves around that.

#### logearav

Joined Aug 19, 2011
243
I can't understand Bill

#### Wendy

Joined Mar 24, 2008
22,206
Neither do I entirely.

Theory of Relativity is based on one premise. The speed of light is a constant, always. So if something is shining a light towards you while traveling 0.9C away from you the light is still arriving at the same speed. What changes is red shift, the light drops in frequency due to the velocity difference.

Same thing if the object is traveling towards you, the light is blue shifted, the speed of the photon hitting you is the same.

A photon always hits you at the same speed, always.

It also has ramifications concerning time shift. The rate of time flow between the two objects is different.

This and more are covered under the Theory of Special Relativity, which has been confirmed experimentally many times over the last 90 years or so.

http://en.wikipedia.org/wiki/Special_relativity

#### polychromeuganda

Joined Dec 12, 2011
8
Old answer: (as given by the previous respondent) The speed of light is constant, and then given that something else is changing (frequency, time, mass, etc.) In the case of light beams they are traveling at the speed of light in all frames of reference, yours, mine, on a satellite in orbit, a spaceship travelling past at 90% of the speed of light etc. (A discussion of the doppler effect usually follows next...)

New Answer: Until last summer, the answer was as above. The FTL result from Europe was confirmed and experimentally repeated this fall. The implications of this unanticipated result are unclear. A constant speed of light as a universal speed limit has been experimentally confirmed in dozens of ways. The resulting predictions of special relativity have been experimatally confirmed to an extraordinary degree of accuracy (many significant digits). No postulate has been advanced to resolve this apparent paradox ... yet.

#### thatoneguy

Joined Feb 19, 2009
6,359
The FTL particle, Tachyon, was only travelling a few parts per million faster than c, not double, or even 1% faster, just barely faster. It was also a subatomic particle, and Einstein's relativity didn't cover quarks.

#### BillO

Joined Nov 24, 2008
990
As Bill says, the velocity is dependent on the observer, or frame of reference.

Case 1: If the observationsi made from the frame of refernce of one of the photons, then the velocity between them is measured as C.

Case 2: If the observation is made from a frames of reference at the center of motion, then the velocity between them is measured as 2C.

Other cases could involve all sorts of other measurements ranging between C and 2C. The special theory of relativity gives us the tools to reconcile the different observations such that, once properly applied in each case, the observers can agree that they are seeing the same thing.

#### Wendy

Joined Mar 24, 2008
22,206
I have a minor problem with the #2 statement. Each photon sees the other as traveling at 1C, you are mixing apples and oranges. To the observer in the middle both photons arrive at 1C traveling in two directions, it is his reference that matters in this case. Their relative velocity is not 2C, because it is the speed of the observer measures that matters. The moment you declare the speed relative to the photon the point of reference changes.

Like I said, very non intuitive.

#### BillO

Joined Nov 24, 2008
990
I have a minor problem with the #2 statement. Each photon sees the other as traveling at 1C, you are mixing apples and oranges. To the observer in the middle both photons arrive at 1C traveling in two directions, it is his reference that matters in this case. Their relative velocity is not 2C, because it is the speed of the observer measures that matters. The moment you declare the speed relative to the photon the point of reference changes.

Like I said, very non intuitive.
Bill, you shouldn't have a problem with this. There are infinite different frames of reference and all are valid. That's the whole idea of special relativity. To the observer in the middle, assuming approaching photons, the distance between them is closing at 2C. Ergo, to him their relative velocity is 2C. This is a totally valid observation, and is supported by relativity, not denied by it. Niether one of them is exceeding C, not even to this observer, so no tenet of SR is being violated.

#### ErnieM

Joined Apr 24, 2011
8,082
If you apply the Lorentz–Fitzgerald contraction to anything traveling at c then you get a contracted length of zero.

Therefor for any particle traveling at c then entire universe is infinitesimally thin, and that particle gets anywhere in zero time (relative to it's own frame).

#### Wendy

Joined Mar 24, 2008
22,206
Due to the mass increase you can never hit the speed of light, and it only gets harder as you get closer.

A photon is massless (though I have read contradicting information), it is also timeless. A photon only has now, no past or future.