Science Questions

If you shone two lasers directly into each other, what would happen?

Sun, 13th Jun 2010

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DavidWhalley94 via Twitter asked:

If you shone two lasers directly into each other, what would happen?


We put this question to Dr Graeme Hirst, from the STFC's Central Laser Facility...

Graeme -   Well that’s a good question.  If you shone them directly into one another, then you're quite likely to break the lasers unfortunately. But if you imagine you're going to misalign them slightly so that the beams crossed, but they don't go down the throat of the opposing laser, then if you were to do that in a vacuum, if you just have one light beam crossing another then to all intents and purposes, nothing happens.  Once you get to extreme physics conditions where the laser intensity does become spectacular - and in some experiments we do, we’re getting there - you might imagine that you can perturb the vacuum.  You can change the vacuum with the intensity of light you need.  But mostly, this is interesting if you cross the two beams over in some kind of material, and if you do that, you can achieve all sorts of interesting effects in the material by combining two laser beams, through the properties of the material.


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Well, i do believe a percentage of that energy will manifest to actual matter given by the formula E=Mc^2. We have shot many beams of photons at each other, and this is an experimental fact. Many of the photons will not collide though, and just continue on, falling into the same state as the partner photons that surround them. Mr. Scientist, Tue, 1st Dec 2009

It depends on the degree of coherence between them if they are of the same single frequency they will form an interference pattern where they reinforce and cancel each other.
If they are from thermal sources with a wide spread of frequencies there will be little or no interaction.    syhprum, Tue, 1st Dec 2009

In the absence of any other material light beams do not interact in any other way than to produce localised interference based power density variations. Soul Surfer, Sun, 6th Dec 2009

Light doesn't seem to influence light electromagnetically, strange isn't it. One way to look at it is to say that Bosons " Bosons (such as photons) do not notice each other and can be at the same location at the same time without any problem. That's why different rays of light can cross each other without any problem and continue to go straight ahead as if nothing happened. Because bosons do not notice each other, it is very difficult to build a stable structure out of bosons."Here

But then we have Helium4 that also are supposed to be bosons but seems to take up 'place' inside SpaceTime. But as far as it comes to photons it seems to be true. They can be superimposed, all photons into 'one' sort of without problems.

This guy have an alternative suggestion to how light may 'propagate'. Does Light Exist Between Events? which seems interesting to me. Not that I say that it is the 'right one' but I'm starting to question the idea of 'propagation', and every little thing :)
yor_on, Mon, 28th Dec 2009

The experiment was done at the Stanford linear accelerator in 1997. The collision was between a laser beam of 6 X 10 ^14 Hz and a gamma ray of 10^25 Hz which produced an electron anti-electron pair. That would suggest the collision energy would have to at least equal, M = fh/C^2. Ron Hughes, Mon, 28th Dec 2009

But look here too.
yor_on, Mon, 28th Dec 2009

If photons do not interact then how do we explain the above experiment? It would appear that we do not understand the photon. One of the problems is how we have pictured the photon graphically. Since the late eighteen hundreds we always describe the photon with a mathematical representation which is the sine wave. When I visualize light going through a polarizing grating I see a sine wave entering the grating at one angle and coming out the other side shifted by ninety degrees. That is a picture of what happens but obviously does not truly represent the truth. Ron Hughes, Mon, 28th Dec 2009

A single photon is not described by a sine wave.  If you add up a bunch of photons in just the right way, you get what is called a coherent mode, which is what people generally mean when they talk about classical (i.e. non-quantum) light.  jpetruccelli, Mon, 28th Dec 2009

A single photon is not a wave? Please enlighten us and describe a single photon. Ron Hughes, Mon, 28th Dec 2009

I haven't come across a really concise non-technical description of a photon.  In essence, a photon is the smallest excitation of the free electromagnetic field.  You use them to describe the interaction between matter and the electromagnetic field.  Matter can interact with the electromagnetic field to either generate or absorb photons.  The classical sine-wave you would find coming from a laser, for example, is a particular way of "stacking" photons known as a coherent mode.  These coherent modes behave like the classical light described by Maxwell's equations.

The problem is that you can't think of photons as little particles zipping around space.  They're excitations of an underlying field that permeates all space and their emission and detection are modeled by probabilities of interactions of matter with the electromagnetic field. jpetruccelli, Mon, 28th Dec 2009

That's a really good description.

Is it correct to say that a photon is the smallest detectable excitation of the free magnetic field? Geezer, Mon, 28th Dec 2009

Geezer, you could say that, but I'd be a bit nervous about the description of a photon as being the smallest detectable excitation of an EM field simply because quantum mechanics is all about observations/detections.  If it's the smallest unit of the field we can observe, does it make any sense to talk about smaller units of the field? jpetruccelli, Mon, 28th Dec 2009

As always JP
It's a joy reading you.

They will have to do something about you
A Christmas star in your book or something _:) yor_on, Mon, 28th Dec 2009

JP - Beats me! In a practical sense I imagine it's highly unlikely we will ever observe smaller energies. I'm just not sure that means they could not exist.

Perhaps I'm concerned that we don't assume too much about the nature of space until we get a better appreciation of what it is. On the other hand, I could just be waffling! Geezer, Mon, 28th Dec 2009

This already happens inside a laser resonance cavity (= inside the laser).
The two beams (= the two electromagnetic coherent waves) interfere making a standing wave.

About photons interacting with themselves, this doesn't happen (excepting with an extremely low probability, at extremely high energies). lightarrow, Tue, 29th Dec 2009

Interesting topic but while discussing the interaction and noninteraction of photons, please don't forget the Hanbury, Brown, and Twist effect, or bunching of photons. sciconoclast, Thu, 17th Jun 2010

Well what about when 2 weak unseeable Laser lights interact will that area of Interaktion be Seen? Please explain for Kids? Markus Baumgartner, Fri, 28th Mar 2014

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