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Gravitational Lensing to Observe Ancient Earth

rein.pk

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Re: Gravitational Lensing to Observe Ancient Earth

#51

Post by lutusp »

98709870987 wrote: post #48
lutusp wrote: post #46

Earlier quoted context omitted.

> If we ever invented FTL travel, even for just probes, this could be done too. If we had FTL travel, we could outrace light waves, look back, and see events in the past. The we could race back and report what we saw. That's why it's not possible -- it violates some basic physical ideas about causality. Remember that the speed of light isn't just a speed limit for light waves -- it's a barrier that sorts out temporal…

>>If we had FTL travel, we could outrace light waves, look back, and see events in the past. The we could race back and report what we saw. That's why it's not possible -- it violates some basic physical ideas about causality. You are confused. You are not actually traveling to the past therefore causality is not a problem here.

> You are confused.

I'm confused? Which of us is discussing FTL without understanding its physical meaning?

> You are not actually traveling to the past ...

Locate where I made this claim.

> ... therefore causality is not a problem here.

Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative number is impossible.

Here's the simplest form of the relativistic equation that tells us the amount of time dilation we can expect for a given velocity v:

t' = t * sqrt(1-v^2/c^2)

t = time rate at v = 0

t' = time rate at v

c = speed of light

v = frame velocity

Compute the time dilation for v > c, then get back to me.

FTL means leaving the causality light cone, conventional notions of causality, and therefore all of physics:

http://en.wikipedia.org/wiki/Light_cone

Quote: "Because signals and other causal influences cannot travel faster than light (see special relativity and spooky action at a distance), the light cone plays an essential role in defining the concept of causality - for a given event E, the set of events that lie on or inside the past light cone of E would also be the set of all events that could send a signal that would have time to reach E and influence it in some way." [emphasis added]

Re: Gravitational Lensing to Observe Ancient Earth

#52

Post by simonh »

98709870987 wrote: post #49
simonh wrote: post #47

Earlier quoted context omitted.

Except of course if you have invented FTL travel, you can use it to actually go to the earth in the past. FTL drives are also time machines. There's a saying in Physics. Choose two: FTL, causality, or relativity. You cannot have all three.

Except we still don't know enough about physics to know if this is true. So far this is only a hypothesis that has never been proven.

There's a pretty massive body of evidence. What would you accept as proof?

Re: Gravitational Lensing to Observe Ancient Earth

#54

Post by ck2 »

lutusp wrote: post #46
ck2 wrote: post #45

If we ever invented FTL travel, even for just probes, this could be done too. Except you'd need insane levels of resolution or a whole new way to think beyond optics to be able to see details. I mean what if you could see that far back but the earth's resolution was only the size of a pin head?

> If we ever invented FTL travel, even for just probes, this could be done too. If we had FTL travel, we could outrace light waves, look back, and see events in the past. The we could race back and report what we saw. That's why it's not possible -- it violates some basic physical ideas about causality. Remember that the speed of light isn't just a speed limit for light waves -- it's a barrier that sorts out temporal…

So in normal space-time FTL is not possible.

It might be possible to transmit data through shortcuts, ala wormholes even if physical FTL is not possible.

Re: Gravitational Lensing to Observe Ancient Earth

#55

Post by EastLondonCoder »

lutusp wrote: post #51
98709870987 wrote: post #48

Earlier quoted context omitted.

>>If we had FTL travel, we could outrace light waves, look back, and see events in the past. The we could race back and report what we saw. That's why it's not possible -- it violates some basic physical ideas about causality. You are confused. You are not actually traveling to the past therefore causality is not a problem here.

> You are confused. I'm confused? Which of us is discussing FTL without understanding its physical meaning? > You are not actually traveling to the past ... Locate where I made this claim. > ... therefore causality is not a problem here. Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative number is impossible. Here's…

"FTL means leaving the causality light cone, conventional notions of causality, and therefore all of physics:"

While I agree with your conclusion that ftl is equivalent of time travel and breaks causality I think its incorrect to say it breaks all of physics. There are several solutions to general relativity that are valid. http://en.wikipedia.org/wiki/Chronology_protection_conjectur...

Re: Gravitational Lensing to Observe Ancient Earth

#56

Post by JoeAltmaier »

andyjohnson0 wrote: post #50
ISL wrote: post #18

You don't want the light reflected by 180 degrees - it needs to be reflected where Earth is going to be ; a much harder problem. For the easy case quoted in the article of 6 light years, any collimated light would need to be properly directed at the sub-nanoradian (milliarcsecond) level (which is possible -- see sig and [0]). Interesting to think about, even if it is an incredible observational challenge. Fun applica…

"it needs to be reflected where Earth is going to be; a much harder problem" The relative velocity of the ancient Earth, black hole, and present earth would also need to coincide in such a way that you'd get a continuous (or, less desirably, periodic) stream of photons making the complete round-trip. Such a coincidence seems pretty unlikely. Its a nice idea though.

Actually it seems pretty inevitable. Light is sprayed across the black hole from Earth; some of it ends up in every possible trajectory; some ends up in the critical 'orbit window' to be reflected back to where earth is going to be.

Re: Gravitational Lensing to Observe Ancient Earth

#57

Post by Jach »

ck2 wrote: post #54
lutusp wrote: post #46

Earlier quoted context omitted.

> If we ever invented FTL travel, even for just probes, this could be done too. If we had FTL travel, we could outrace light waves, look back, and see events in the past. The we could race back and report what we saw. That's why it's not possible -- it violates some basic physical ideas about causality. Remember that the speed of light isn't just a speed limit for light waves -- it's a barrier that sorts out temporal…

So in normal space-time FTL is not possible. It might be possible to transmit data through shortcuts, ala wormholes even if physical FTL is not possible.

The light speed limit comes up elsewhere. As best we know, you cannot transmit information faster than c. Not even with "spooky quantum stuff". There are no shortcuts. Information is just as real as matter or energy--if not more so, see John Wheeler's "it from bit"--and isn't tied to meaning, see Shannon.

Re: Gravitational Lensing to Observe Ancient Earth

#58

Post by nessus42 »

lutusp wrote: post #51
98709870987 wrote: post #48

Earlier quoted context omitted.

>>If we had FTL travel, we could outrace light waves, look back, and see events in the past. The we could race back and report what we saw. That's why it's not possible -- it violates some basic physical ideas about causality. You are confused. You are not actually traveling to the past therefore causality is not a problem here.

> You are confused. I'm confused? Which of us is discussing FTL without understanding its physical meaning? > You are not actually traveling to the past ... Locate where I made this claim. > ... therefore causality is not a problem here. Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative number is impossible. Here's…

> Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative number is impossible.

This assertion is false. GR, even without FTL, allows for closed time-like loops, which violate causality. Violations of causality are not logical contradictions like dividing by zero. (Though, of course, there are alternative mathematical systems that allow for dividing by zero. And taking the square root of a negative number is surely not impossible. You just get an Imaginary result.) It's just that a universe in which causality is violated all the time would be a very strange one, and consequently, it is reasonable to assume that if there are, for instance, closed time-like loops in the universe, they are not traversed very often.

Re: Gravitational Lensing to Observe Ancient Earth

#59

Post by lutusp »

nessus42 wrote: post #58
lutusp wrote: post #51

Earlier quoted context omitted.

> You are confused. I'm confused? Which of us is discussing FTL without understanding its physical meaning? > You are not actually traveling to the past ... Locate where I made this claim. > ... therefore causality is not a problem here. Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative number is impossible. Here's…

> Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative number is impossible. This assertion is false. GR, even without FTL, allows for closed time-like loops, which violate causality. Violations of causality are not logical contradictions like dividing by zero. (Though, of course, there are alternative mathematical sys…

> GR, even without FTL, allows for closed time-like loops, which violate causality.

The theory may "allow" them, but this is not to say they actually exist in reality.

http://en.wikipedia.org/wiki/Causality_(physics)

Quote: "In the theory of general relativity, the concept of causality is generalized in the most straightforward way: the effect must belong to the future light cone of its cause, even if the spacetime is curved."

http://hipercom.inria.fr/~jacquet/retro1/crazy-science-corne...

Quote: "Weird space-time geometry distortion within general relativity can result in causality violation, but there is no evidence that they are physically possible."

Re: Gravitational Lensing to Observe Ancient Earth

#60

Post by lutusp »

EastLondonCoder wrote: post #55
lutusp wrote: post #51

Earlier quoted context omitted.

> You are confused. I'm confused? Which of us is discussing FTL without understanding its physical meaning? > You are not actually traveling to the past ... Locate where I made this claim. > ... therefore causality is not a problem here. Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative number is impossible. Here's…

"FTL means leaving the causality light cone, conventional notions of causality, and therefore all of physics:" While I agree with your conclusion that ftl is equivalent of time travel and breaks causality I think its incorrect to say it breaks all of physics. There are several solutions to general relativity that are valid. http://en.wikipedia.org/wiki/Chronology_protection_conjectur...

> There are several solutions to general relativity that are valid.

They haven't been shown to exist in reality. Remember that science is -- must be -- empirical.

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