# The Naked Scientists Forum

### Author Topic: How long would it take two photons, from Earth and the Sun, to collide?  (Read 2325 times)

#### Nivekeryas

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• Posts: 1
##### How long would it take two photons, from Earth and the Sun, to collide?
« on: 04/11/2011 00:36:35 »
Essentially, if you shot one photon of light towards the Sun, and simultaneously a photon was shot from the Sun towards Earth how long would they take to collide, considering that they're travelling towards eachother?

From my understanding of relativity, it would take 8 minutes 19 seconds regardless of the fact that they're travelling towards eachother, not 4 minutes 9.5 seconds as you would think intuitively.

Anyway. Help/explanations appreciated.

#### Soul Surfer

• Neilep Level Member
• Posts: 3345
• keep banging the rocks together
##### How long would it take two photons, from Earth and the Sun, to collide?
« Reply #1 on: 04/11/2011 09:09:24 »
Firstly photons do not collide they only pass through each other because photons do not interact with each other and secondly because they are both travelling at the speed of light towards a point half way between the earth and the sun the both take half the time i.e. around four minutes before they pass each other. when viewed from the earth and the sun.  If they both struck an object and lit it up it would of course be another four minutes before we "saw" the flash!

However from the point of view of the photons time does not exist it only "knows" it has been created and then destroyed even this has taken the whole age of the universe or a fraction of a picosecond.

#### abacus9900

• Jr. Member
• Posts: 39
##### How long would it take two photons, from Earth and the Sun, to collide?
« Reply #2 on: 05/11/2011 11:47:17 »
Essentially, if you shot one photon of light towards the Sun, and simultaneously a photon was shot from the Sun towards Earth how long would they take to collide, considering that they're travelling towards eachother?

From my understanding of relativity, it would take 8 minutes 19 seconds regardless of the fact that they're travelling towards eachother, not 4 minutes 9.5 seconds as you would think intuitively.

Anyway. Help/explanations appreciated.

We have to make a distinction between the speed of objects (like photons) and the *relative* speed of objects. For example, galaxies cannot move within spacetime faster than the speed of light (despite space expanding greater than the speed of light) however if you have two galaxies hurtling towards one another and each one is travelling, let's say, at 3/4 of the speed of light then their *relative* speed is more than the speed of light. This does not violate Einstein because neither galaxy is actually travelling more than the speed of light. So two photons travelling towards one another would take half the time to reach each other than would be the case if one of the photons was at rest.

#### imatfaal

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• rouge moderator
##### How long would it take two photons, from Earth and the Sun, to collide?
« Reply #3 on: 07/11/2011 10:13:43 »
Actually, you need to be quite careful - there is no absolute space (and thus all calculations must make sense whatever frame you chose), so you need to take care and define frames and use the relativistic velocity addition formulations.  Galilean addition of velocities only works when u.v over c^2 is very close to zero

Abacus you seem to be conflating the idea that galaxies can be moving apart from each other at seemingly higher than speed of light because of the expansion of the universe (which is correct) with a concept that there is an absolute space which you cannot move faster than light (which is incorrect).  if we ignore expansion - then whatever frame you choose to measure from, nothing will be moving faster than light

We had a recent discussion and I will dig out the link rather than rehearse the argument again

#### The Naked Scientists Forum

##### How long would it take two photons, from Earth and the Sun, to collide?
« Reply #3 on: 07/11/2011 10:13:43 »