# A Why light beams attract or repel each other even when they don't have charge

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1. Nov 10, 2017

### SunRay-dvsh

Hi,

Can someone please explain as to why light beams attract or repel each other even when they don't have charge. Seems like it behaves like two current carrying parallel wires. There is very little material about this which goes completely above the head.

Thanks

2. Nov 10, 2017

### BvU

Is that good or bad ?
Can you indicate where you got this idea about light beams that attract or repel each other ?

3. Nov 10, 2017

### Demystifier

Perhaps he is talking about bunching and anti-bunching in quantum optics. If so, it is caused by specific entanglement between photons in a light beam.

4. Nov 10, 2017

### Mentz114

There is a gravitational effect see

5. Nov 10, 2017

### vanhees71

6. Nov 10, 2017

### pervect

Staff Emeritus
Parallel light beams going in the same direction don't attract each other gravitationally. Parallel light beams going in the opposite direction do attract gravitationally, about 4x as strong as one gets by a naive calculation.

7. Nov 11, 2017

Staff Emeritus
Before we determine why something is true, we need to determine if it is true.

8. Nov 14, 2017

### SunRay-dvsh

9. Nov 14, 2017

### SunRay-dvsh

Hello,
I couldn't find any reference to bunching or anti-bunching. I hope this article helps: http://discovermagazine.com/2010/jan-feb/083

Thank You

10. Nov 14, 2017

### vanhees71

11. Nov 14, 2017

### Demystifier

Now I get it. This is not a force between light beams as such, but a force between waveguides of the light beams. The light beams induce the dipole moments in the guides, and those dipole moments cause the force. The effect is very similar to the Casimir effect. For more details see http://lanl.arxiv.org/abs/physics/0509073

12. Nov 14, 2017

### SunRay-dvsh

THANK YOU VERY MUCH FOR THESE REFERENCES ! Its easier to understand, though i still don't get how the transverse force is measured.

13. Nov 14, 2017

### SunRay-dvsh

Thanks,
They have explained it very well, the original research paper in nature didn't really explain it nicely.

14. Nov 14, 2017

### SunRay-dvsh

Basically, if someone is shooting a Really powerful laser at you, you can use a tiny weak laser to deflect it ?!

15. Nov 14, 2017

### Demystifier

No.

16. Nov 14, 2017

### Demystifier

That's why you need to read theoretical papers, not experimental ones.

17. Nov 14, 2017

### SunRay-dvsh

18. Nov 16, 2017

### Zafa Pi

Expository articles about old experiments can be fun and informative. It's a shame modern experimentalists can't do the same with their own work.

19. Nov 16, 2017

### Demystifier

20. Nov 16, 2017

### vanhees71

Hm, the introductory textbooks for the experimental-physics course do not too bad a job. Of course, theoreticians, sometimes have problems to understand these books due to a lack of math in these books ;-)).