# Gravity, what's the attraction...

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I agree that it could have come closer to us already in the 2.2m years, but I still disagree that it would have hit us.

If Andromeda emitted a photon when it was 2.2m light years away, that photon would take 2.2m light years to reach us. If (worst case scenario) Andromeda was hurtling towards Earth at the speed of light, it would reach us at the same time as the photon it emitted 2.2m years ago, not before, as I've understood you to mean.

I agree it would be correct to say that Andromeda may not exist anymore, and we don't know about it, but I think we'd know about it if Andromeda were to hit us. Correct me if I'm wrong though!

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wait sorry we will be able to see it hit us, but after it hits us.

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wait sorry we will be able to see it hit us, but after it hits us.

I would say at the same time it hits us, assuming it's travelling at the speed of light

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I agree that it could have come closer to us already in the 2.2m years, but I still disagree that it would have hit us.

If Andromeda emitted a photon when it was 2.2m light years away, that photon would take 2.2m light years to reach us. If (worst case scenario) Andromeda was hurtling towards Earth at the speed of light, it would reach us at the same time as the photon it emitted 2.2m years ago, not before, as I've understood you to mean.

I agree it would be correct to say that Andromeda may not exist anymore, and we don't know about it, but I think we'd know about it if Andromeda were to hit us. Correct me if I'm wrong though!

Ok say its 2.2 m light years away and emits 1 photon, whilst that photon is travelling towards us over 2.2m years the galaxy has become closer say 2.0 m lt years but that 1 photon is still on its way but the galaxy is closer so we are still seeing it as it was, however it emits a photon stll at 2m lt years away and it begins its journey. So as one photon makes its journey another is leaving. Now say it is just 10000 light years away (that might be inside our galaxy but oh well its a distance lol) it emits a photon at that distance however the photons from 100001 light years are still on there way to us so those photons reach us before the new ones. Carry this process on to 1 light year away- a photon is emitted but the photon before it reaches us first because it left the galaxy before. Keep going and then you get to the collision. It hits us but we still see the photons from when it was still travelling. It could be seconds, years, or millions of years until we actually notice that it has hit us.

Im more or less certain that this is pretty much correct but if some physics genius (have to be stephen hawking or Brian cox at the leat) can prove me wrong then so be it!

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The thing I try and remember to differentiate between Newtonian and Einsteinian gravity is that Newton's law of universal gravitation is about mass attracting mass, whereas Einstein's theory, er, isn't.

Newton said that every particle with mass attracts every other particle with mass in the universe (how and to what extent isn't important for this post!)

The kicker is that photons have no mass but Einstein said that they'd still be affected by gravity because it's the fabric of the universe itself that warps to give the "feel" of gravity. Einstein was proved right, witness gravitational lensing.

So Einstein seems to be on the money with this one, and Einstein says that the effect of any event travels outward at the speed of light (this is important for maintaining causality - look up light cones).

So, if the sun disappeared instantly, not only would we not see it for a few minutes, but it would be impossible to detect that anything at happened to it at all for that same period of time, including losing its gravitational pull.

As for the colliding galaxy, since it consists of particles with mass it couldn't approach us as fast as the photons it was emitting anyway. So we'd see it before we felt it, all the way in.

Edited by NickMilner
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seems about right But as for the we'd see it before we felt it, surely we wouldnt see it in its current state for a while but would see it before feeling it? because the older photons would still be reaching us before the photons as it collides right.

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Themos answered the OP way back, but just to add a small point:

Newtonian gravity can be formulated as a theory of curved space-time in which information is transferred instantaneously: this is called Newton-Cartan theory. If you add special relativity to Newton-Cartan theory you get a theory of gravity in which information cannot be transferred faster than light-speed: this is general relativity. For a very nice explanation see various books by Roger Penrose (e.g. the Emperor's New Mind or The Road To Reality). If we lived in a Newton-Cartan universe then the disappearance of the sun would be felt instantly, but all the evidence is that special relativity holds, so the effect would take about 8 minutes. Same comments apply to the Andromeda Galaxy except it would be 2-point-something million years instead of 8 minutes.

Newton?Cartan theory - Wikipedia, the free encyclopedia

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I may be misreading, but I suspect that Sammy1404 is alleging that an object approaching the Earth cannot be seen by observers on Earth until after the object has arrived. If this is the case, then he is completely incorrect, since light from an approaching object will always precede the object, and the object will therefore be visible throughout it's approach.

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Themos answered the OP way back, but just to add a small point:

Newtonian gravity can be formulated as a theory of curved space-time in which information is transferred instantaneously: this is called Newton-Cartan theory. If you add special relativity to Newton-Cartan theory you get a theory of gravity in which information cannot be transferred faster than light-speed:

Newton?Cartan theory - Wikipedia, the free encyclopedia

Yes, it depends what physical laws rules you follow. Current evidence suggests relativity which means, as gravity is a field, it travels at or below the speed of light. Until we know better!

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Indeed, this is all subject to the assumption that physics is not some made-up rubbish that only works to a distance of 500 miles from Earth, that we were not created five minutes ago along with false memories, etc etc etc. Driving on the left is a "rule we follow" (at least in some places): gravity is something we have no choice about.

Einsteinian gravity contains Newtonian gravity in the limit of low velocities, weak fields etc. Any new theory of gravity would have to include all the successes of those other two theories in addition to adding some new predictions that would only become manifest in the most extreme scenarios.

Edited by acey
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Any new theory of gravity would have to include all the successes of those other two theories in addition to adding some new predictions that would only become manifest in the most extreme scenarios.

Or in addition to adding some new retro-dictions about so-far unrelated phenomena, for instance, the masses of the elementary particles.

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lol i was just thinking this yesterday not having visited this part of the forum much(ive been busy learning on the other pages). what about the doupler effect or the = for light(if there is a thing).if a jet is flying supersonic you dont hear it untill it has passed

but that would be freaky if something was traverling so fast it hit you before you saw it

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but that would be freaky if something was traverling so fast it hit you before you saw it

I agree it would be very freaky!

Although, if we have already been hit, then what is all that matter doing? If it is occupying the same point in space and time as us, I would like to think we'd notice (although probably as we're all blown apart)!

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lol i was just thinking this yesterday not having visited this part of the forum much(ive been busy learning on the other pages). what about the doupler effect or the = for light(if there is a thing).if a jet is flying supersonic you dont hear it untill it has passed

but that would be freaky if something was traverling so fast it hit you before you saw it

In fact the Doppler Effect does apply to light. When light is coming towards us it gets squeezed in the same way as sound, and the result is that the wavelength gets more compressed which makes the colour move toward the blue end of the spectrum (for visible light). When light is moving away from us the wavelength increases and that moves it toward the red end of the spectrum. The resulting phenomena is redshift and blueshift. If an object exhibits redshift then it's moving away from us.

Now the important point here is that nothing travels faster than light, but plenty of things travel faster than sound! That's why a jet flies over your head before you hear it. But it's impossible for something to fly over your head before you see it.

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lol i was just thinking this yesterday not having visited this part of the forum much(ive been busy learning on the other pages). what about the doupler effect or the = for light(if there is a thing).if a jet is flying supersonic you dont hear it untill it has passed

but that would be freaky if something was traverling so fast it hit you before you saw it

Nothing could travel so fast that you see it after it hits you, according to current theories, as nothing can travel as fast as light:

even if it was getting say 0.5 ly closer every year, and emitting one unit luminance per year, we would still see all of the photons from the new place as well as the old one...

but yeh, the dopler shift for light gives us redshift and blueshift: combining this with our knowledge of the type of atoms out there, looking for instance for the H-alpha emmision line which without a shift has a waveleanth of 656.28 nm wherever the hydrogen is. By comparing this emission line, and others like it, we can measure the relative speeds of all sorts of objects: you can see stars rotating if you have sensitive enough equipment, and it is because of blueshifted light that we can tell that andromeda is headed towards us. However, because of the speeds involved, it is hard to detect this effect on earth.

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"the dopler shift for light gives us redshift and blueshift:"

now i remeber how mr hubble got one big scope named after him.lol

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"the dopler shift for light gives us redshift and blueshift:"

now i remeber how mr hubble got one big scope named after him.lol

...what? lol

Didnt hubble get the scope named after him because he was the first man to look into deep space with a scope?

Whoever said the dopler shift for light gives us redshift and blueshift needs to work on their tutoring skills. Its to do with the movement of photons relative to the object they are leaving. Galxies are moving in space time so if they come towards us the distance a photon needs to travel is rapidly getting smaller so it squishes it to a higher frequency which is up at the blue end of the electromagnetic spectrum. Where as galaxies moving away mean that the distance a single photon has to travel is getting longer which stretches out the photon thus decreasing its frequency shifting it down to the red end of the spectrum. Really its just called Redshift not the Dopler shift but in theory it is the Dopler shift for light. The dopler shift describes sound.

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As I understand it, the Doppler effect applies to any waves, including electromagnetic, so it's true to say that redshift/blueshift is a result of the Doppler effect. It becomes slightly more complicated when you consider cosmological redshift, but it's still a Doppler effect.

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Edwin Hubble was certainly not the first astronomer to use a telescope for deep sky work. He, and his colleague Milton Humason, working at Mount Wilson, proved by using Cepheid Variables, that the so-called, "spiral nebulae," were actually galaxies in their own right, well beyond the Milky Way. They measured galactic red shifts, and established that the velocity of recession is linked to distance. The term, "Doppler Effect," is a, "Principle in Physics whereby the pitch of sound or the wavelength of light is altered by the velocity of the emitting object."

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Umm, yes we will. As soon as it was 1 lightyear away then we would see how close it was one year later.... And if it was moving that fast towards us it would appear very blue-shifted (Doppler effect).

The only other possibility is that it's moving faster than the speed of light. If that's happening then all bets are off and you may as well go and marry a banana.

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The only other possibility is that it's moving faster than the speed of light. If that's happening then all bets are off and you may as well go and marry a banana.

But since that's not possible I wouldn't book the honeymoon just yet.

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