speed of gravity?

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Technical Ben
Posts: 2986
Joined: Tue May 27, 2008 10:42 pm UTC

Re: speed of gravity?

Postby Technical Ben » Tue Oct 19, 2010 3:21 pm UTC

[On the topic of subliminal communication etc]
It's all relative right?
So, we cannot speed up your communication past the speed of light, as well, it's not possible. We can however, slow down the receiver. We freeze our relatives, then send them to Mars. Set up a clock to unfreeze them roughly 6 minutes after we send a message. We then freeze ourselves for 12 minutes plus some time for them to give a reply. They then receive the message, reply, and 15 minutes later (but instantly to yourself as you have just been unfrozen) you find out they arrived safely, and the dog was sick on the way.
It's all physics and stamp collecting.
It's not a particle or a wave. It's just an exchange.

flamewise
Posts: 35
Joined: Tue Oct 05, 2010 2:40 pm UTC

Re: speed of gravity?

Postby flamewise » Thu Oct 28, 2010 10:58 am UTC

I still sometimes hope it isn't trivially c :evil:

Fortunately, Larry Niven was wise enough not to assume superluminal communication was possible with modulated gravity waves generated by a swinging black hole on Mars in The Hole Man (Warning: Wiki may contain even more spoilers than this posting).

He just thought it would be impossible to block gravity waves so with a very, very, very, VERY good detector you could still detect them on the other side of the galaxy.

Also doubles for a nice murder weapon, and no, the victim didn't fall into it. It was far too small for that. The GRAVITY waves were the murder weapon.

Edit: Oh right, one more observation. It is actually possible to move things faster than the speed of light by slowing down light. Light only moves at c in perfect vacuum. For an example of how you can get a neat "photonic boom" when electrons move through water faster than light does in the same medium, check out Cherenkov radiation.

Technical Ben
Posts: 2986
Joined: Tue May 27, 2008 10:42 pm UTC

Re: speed of gravity?

Postby Technical Ben » Thu Oct 28, 2010 12:16 pm UTC

But that's like saying "I was not speeding officer, all the other traffic was at a standstill".
You are still going faster than "the speed of light in a vacuum" even if one or two particular photons/waves are travelling slower.
It's all physics and stamp collecting.
It's not a particle or a wave. It's just an exchange.

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Dopefish
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Joined: Sun Sep 20, 2009 5:46 am UTC
Location: The Well of Wishes

Re: speed of gravity?

Postby Dopefish » Thu Oct 28, 2010 1:49 pm UTC

Does light actually travel slower in mediums other than a vacuum though?

Two (slightly different) explanations I've heard consist of light travelling at the speed of light (in a vacuum), but it bounces around a lot when passing through a medium resulting in a longer path length and so slower apparent speed, despite the appearance of travelling straight, and the other explanation is that light travelling through a medium is constantly being absorbed and re-emitted, and it's a slight lag time from all that absorption and reemission that causes the apparently slower speed of slight.

Of course on the other hand, with materials with indices of refraction <1, neither of the above explanations work, so I suppose the prior explanations are probably simplifications.

flamewise
Posts: 35
Joined: Tue Oct 05, 2010 2:40 pm UTC

Re: speed of gravity?

Postby flamewise » Thu Oct 28, 2010 3:41 pm UTC

How interesting:

Speed of light: In a medium

You're right, the observed speed of light in a medium is just its phase velocity:

Image
The blue dot moves at the speed of the ripples, the phase velocity; the green dot moves with the speed of the envelope, the group velocity; and the red dot moves with the speed of the foremost part of the pulse, the front velocity


Here's a link to a BBC News article with what scientists think of refractive indices below 1.

Edit: Sorry about derail, will go find a proper topic.


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