Faster than light...

[Infinit]

Hello I'm new and I’m not a scientists. I just wonder if there is no faster speed than light speed what then happens if we have two light beams pointed each against other and take 1 photon from 1 beam and 1 from other then if we measure speed of the photon from the one that is passing then what?
In common words if one photon is observer and "flying" against other then the speed have to be twice the speed of light! Is that right- from the relevant point of view?

Excuse me if there was such threat before!

[CraigD]

Originally Posted by Infinit

Hello I'm new and I’m not a scientists. I just wonder if there is no faster speed than light speed what then happens if we have two light beams pointed each against other and take 1 photon from 1 beam and 1 from other then if we measure speed of the photon from the one that is passing then what?

You’ve asked a variation of one of the fundamental question that the theory of Special Relativity addresses.

The question and its answer according to SR is easier to understand if avoid having any of our observers moving at exactly the speed of light (c) relative to one another. Consider instead the following example:
Alice is traveling toward Bob 0.75 c, as measured by Bob.
Carol is traveling toward Bob from the opposite direction, also at 0.75 c as measured by Bob.
What speed is Alice moving toward Carol, as measured by Carol?

Ignoring SR, the answer is 1.5 c.

However, using SR, the answer is .

The explanation of why this is so, though found in most introductory texts, is surprisingly complicated, so rather than reproduce it in this post, I refer the reader to this good explanation from the good old sci.physics usenet.

The addition of velocities formula can actually be used when the two velocities are 1 c, giving a result of , but I find this less easy to understand than an example with velocities less that 1 c.

[Infinit]

Wow thanks a lot. And sorry for my lack of knowledge.

Best

[watcher]

Originally Posted by CraigD

The addition of velocities formula can actually be used when the two velocities are 1 c, giving a result of , but I find this less easy to understand than an example with velocities less that 1 c.

it is quite simple though hard to accept.

photons do not respect special relativity.

[CraigD]

Originally Posted by watcher

photons do not respect special relativity.

I don’t know what you mean by this, watcher.

Special Relativity is, to a great extent, a theory dedicated to predicting the behavior of photons. It’s been extensively experimentally tested, without, to the best of my knowledge, any failures. If you are aware of an experiment showing photons “not respecting” SR, please post links and references, explaining and supporting your claim, as such information would be a very significant and interesting.

If you mean that photons lack the emotion of respect for or the intellectual capacity to understand SR, I agree – they’re fundamental particles, with very limited capabilities compared to even the simplest “thinking being”. The same can be said of most of nature, however. Nature appears to obey scientific laws, regardless of whether it understands or emotionally respects science.

[watcher]

i mean, photon had created so much counterintuitive /paradoxes for special relativity .

for example, why does matter(or Alice) don't decay(doesn't age) at the speed of light.?

[watcher]

Originally Posted by CraigD

Special Relativity is, to a great extent, a theory dedicated to predicting the behavior of photons. It’s been extensively experimentally tested, without, to the best of my knowledge, any failures. If you are aware of an experiment showing photons “not respecting” SR, please post links and references, explaining and supporting your claim, as such information would be a very significant and interesting.

the interesting part is... with two bodies at normal speeds, the measured speed difference (additive or subtractive) are very significant relative to the irrespective velocities of the two bodies.

as the two bodies nears c, the speed difference approaches zero.

at c, relativity becomes irrelevant

[Little Bang]

Infinit, colliding photon to photon should produce a photon with twice the energy but still be traveling at C. According to the latest Stanford linear accelerator experiment, if the energies are large enough you get an electron positron pair. I don't have a clue as to what their velocities were after the collision of the wave forms.

[CraigD]

Originally Posted by Little Bang

Infinit, colliding photon to photon should produce a photon with twice the energy but still be traveling at C. According to the latest Stanford linear accelerator experiment, if the energies are large enough you get an electron positron pair. I don't have a clue as to what their velocities were after the collision of the wave forms.

Little Bang, I think you’ve misunderstood some literature.

Photons can’t collide. They’re bosons, not fermions, which means in short that any number of them can occupy the same volume simultaneously. An intuitively useful example involves the behaviour of photons in a device such as a telescope or the human eye. In such a device, all of its gathered light is focused on a single focal point. If the photons interacted (eg: combining producing higher-frequency photons), the image would be a different color than when measured without any refractive or reflective optics. This is not the way things are.

Many experiments performed by the SLAC and other particle accelerator/colliers have produced both positrons (e+) and antiprotons (p-). However, they do so by colliding fermions, typically many in heavy atomic nuclei. The collisions can produce photons of very high energy, which in turn become particle/ antiparticle (fermion) pairs. If the antiparticles can be made to interact with other fermions, they can be detected, and even cooled and collected.

[Erasmus00]

Originally Posted by CraigD

Little Bang, I think you’ve misunderstood some literature.

Photons can’t collide.

Yes and no. In the standard model there is no direct photon/photon interaction. However, they can exchange a virtual fermion between them which allows them to pair produce. One such feynman diagram is in the image attached. In the diagram, time moves to the right. Two incoming photons exchange a virtual electron and produce a positron and an electron.

This type of scattering has been (to my understanding) observed at SLAC.
-Will