# The Naked Scientists Forum

### Author Topic: does light travel slower in a gravitational field?  (Read 3170 times)

#### thebrain13

• Sr. Member
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##### does light travel slower in a gravitational field?
« on: 29/12/2006 03:52:50 »
the title is self expanatory.

#### lightarrow

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##### Re: does light travel slower in a gravitational field?
« Reply #1 on: 29/12/2006 20:47:18 »
As far as I know, no, at least locally; however it's not a simple question as it could seem, even because the very notion of velocity it's not well defined in a curved space-time.

#### Heliotrope

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##### Re: does light travel slower in a gravitational field?
« Reply #2 on: 30/12/2006 01:10:44 »
Good point.
If the photon is moving along a curve in spacetime then it's continuously being acted upon by a force and therefore has a speed but not a velocity.
Would a gravitational field constitute a medium for a photon to travel in ?
If so, it's speed should be less than a photon in the vacuum.
Hmmmmm. Interesting.

#### ukmicky

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##### Re: does light travel slower in a gravitational field?
« Reply #3 on: 30/12/2006 01:41:37 »
Take two static points a & b and introduce a star to give you some gravity and the observer will see the pathway the photon travels as curved but its our view of space that is curved not the photons. The photon doesn't view curved space as curved, as far as the photon is concerned it is traveling in a completely straight line and its travel between the two points will always be reached at the same time. Gravity applies no force on the photon,it has no effect.

Even redshift isn't an effect of gravity on the photon,its an effect on us viewing the photons traveling through what we THE observers view as a longer path through curved space and time. The photon doing the travelling has no idea what time or curved space is..

The photon dosent live in four dimensions.

« Last Edit: 30/12/2006 23:16:07 by ukmicky »

#### Supercryptid

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##### Re: does light travel slower in a gravitational field?
« Reply #4 on: 30/12/2006 02:40:09 »
Could gravity affect the density of virtual particles in the quantum vacuum?

According to the Scharnhorst Effect (which has yet to be observed, but makes logical sense), a photon traveling through space is slowed down by its interactions with virtual particles in the quantum vacuum. If gravity causes the number of these virtual particles to increase (and if virtual gravitons exist, then it just might), then light should travel more slowly in a gravitational field than in a pure vacuum.

Going along that same line of thought, IF gravity slows down a photon, then would antigravity (as produced by an object with "negative" mass) cause a photon to speed up?

#### ukmicky

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##### Re: does light travel slower in a gravitational field?
« Reply #5 on: 30/12/2006 03:33:04 »
I thought the Scharnhorst Effect caused light to travel faster than c which would then lead to casuality problems.  And as the  fundemental laws of the universal need to be so finly tunned for the universe to be here i cant believe it would allow light under any circumstances to travel faster c. An effect cant happen before its cause.

#### Sandwalker

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##### Re: does light travel slower in a gravitational field?
« Reply #6 on: 30/12/2006 23:11:58 »
No it just travels further!!

#### The Naked Scientists Forum

##### Re: does light travel slower in a gravitational field?
« Reply #6 on: 30/12/2006 23:11:58 »