Momentum of photons

  • #1
Ebi
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Summary:
Can momentum of photons create impact forces?
We know that photons (light) are massless but they have momentum. Now suppose I am in the space far away from planets/stars that there is no external force exerts on me, if:
1- I turn on a flashlight (torch), would I be pushed in the opposite direction which the flashlight is facing (Newton's third law)?
2- If a star is shedding light on me (its photons hit my body), would I start moving due the impact of photos?
 

Answers and Replies

  • #2
Bandersnatch
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Yes. That's the principle behind solar sails. Also what was deemed to be the cause of the Pioneer anomaly.
 
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  • #3
PeroK
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Summary:: Can momentum of photons create impact forces?

We know that photons (light) are massless but they have momentum. Now suppose I am in the space far away from planets/stars that there is no external force exerts on me, if:
1- I turn on a flashlight (torch), would I be pushed in the opposite direction which the flashlight is facing (Newton's third law)?
2- If a star is shedding light on me (its photons hit my body), would I start moving due the impact of photos?
Yes, momentum is momentum and it's conserved!
 
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  • #4
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Yes, the momentum of a photon is given by h/lambda. And when the photon hits you and reflects back, the change in the momentum of photon is given by 2h/lambda (assuming normal incidence), you would start moving forward with that momentum.
The momentum of photons (from sunlight) have been used in the past to propel spacecrafts and are called as solar sails. There are efforts currently by breakthrough initiatives, to use ground based laser source to propel light sails to the exoplanet Alpha Centauri. Follow this link to learn more about that.
 

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