Faster than light?

[pete.dakota]

Recently I've been pondering this: Does the interpretation of gravitational fields (ie; information) travel faster than the speed of light? For instance, if a satellite or scientific mission is observing gravity fields in the universe, has the information they are receiving 'travelled' and/or is it instantaneous?

For instance; if a supermassive black hole was to instantly appear anywhere in the universe, would it's gravitational effect be instantly felt accordingly throughout the universe? Or would the gravity travel at c throughout. Obviously matter cannot simply 'appear', it's mass, and so gravity always has to have been. But in theory if it could, what would happen?

Current theory suggests that gravity is composed of massless elementary particles, gravitons. If this is true, do these particles travel faster than c? I would expect the 'speed of gravity' to be instantaneous.

What do you think?

 

[Urwumpe]

No, current observations of planetary frame dragging actually support the idea theory, that the gravity field information propagates with light speed.

But better measurements are planned.

 

[simonpro]

Gravity travels at c.

 

[Linguofreak]

The problem is that, under relativity, what counts as "instantaneous" is relative. Basically if we have two observers, A and B, that are moving relative to each other, A will observe events to occur at the same time that occur at different times for B.

The upshot of this is that if you can transmit information faster than light, it becomes not only possible but trivial to transmit it back in time. The fact that we do not observe objects or information travelling back in time means that nothing moves faster than light, including gravity.

 

[Bullethead]

I thought that quantum entanglement allowed the transfer of info faster than light. If you entangle 2 quantum particles and then separate them any arbitrarily large distance, a change in 1 of the particles will be instantly transmitted to the other particle, thus going faster than light.

Or so I heard it .

But it seems completely impossible to make practical use of this effect.

 

[Linguofreak]

I thought that quantum entanglement allowed the transfer of info faster than light. If you entangle 2 quantum particles and then separate them any arbitrarily large distance, a change in 1 of the particles will be instantly transmitted to the other particle, thus going faster than light.

Or so I heard it .

But it seems completely impossible to make practical use of this effect.

As I understand it, the problem is that when you make a measurement of your half of the entangled pair you have no way of knowing if the measurement came out that way because of random chance or because the person on the other end of the entangled pair changed their particle somehow. You have to wait to receive a "I transmitted a message" message by normal means before you can get any info from your measurement.

 

[Bullethead]

You have to wait to receive a "I transmitted a message" message by normal means before you can get any info from your measurement.

I suppose there's also an objection to having 2 sets of entangled pairs. One of them is used to send the real message, while the other is only the "ringtone" to let you know a message is coming through.

I don't know much about this stuff. However, I have read papers by people proposing to use entanglement as part of a quantum computer. Presumably, they've found a way around the above problem(s).

 

[Linguofreak]

I suppose there's also an objection to having 2 sets of entangled pairs. One of them is used to send the real message, while the other is only the "ringtone" to let you know a message is coming through.

I don't know much about this stuff. However, I have read papers by people proposing to use entanglement as part of a quantum computer. Presumably, they've found a way around the above problem(s).

No, they haven't. There are all kinds of interesting things you can do with quantum mechanics without ever sending a message FTL. You can't do the ringtone thing with entangled particles because you'd still need an STL message to distinguish the "ringtone" from the results of pure randomness.

 

[ijuin]

The only way that I can see of doing it is by sending the message using enough pairs of entangled particles that it would be vanishingly improbable for them to all exhibit the "message sent" status as a result of randomness instead of from an actual message.

 

[Eagle]

I think its rather funny that the graviton is supposedly massless. I would have thought it would be equivalent to the smallest unit of mass possible...Though it is used in the 'virtual photon' sense so its theoretically nothing.

Supposedly anything moving faster than light is a tachyon. I've been trying to wrap my head around what Tachyons are supposed to be. It seems that it may only be string theory mumbo-jumbo, but please enlighten me if I'm wrong.