Ask any question you want about Physics

originally posted by: Phage
a reply to: yulka

Can gravity bend light?

Yes. Well, not really, but sort of.

Gravity cannot bend light but it can bend space. From the outside it appears that the path light follows is bent. From the point of view of light, it's traveling in a straight line.

this is the true misunderstanding of light bending.

look...
EM radiation propagates with different speeds in different medium, this includes different field density.
This causes one side of the em-wave to propagate slightly slower then the other side of the wave, it changes the direction, bends towards the center of this field emitter.
This explanation however works only in you see light as waves and not as particle like points ( photons )

Please define space and how one can bend it, what tools do I need, how does it works ?

a reply to: KrzYma

Which field densities are you referring to in particular?
And yes, different mediums can certainly bend light. Refraction is a well understood phenomenon.

originally posted by: KrzYma
this is the true misunderstanding of light bending. look...

EM radiation propagates with different speeds in different medium, this includes different field density.
This causes one side of the em-wave to propagate slightly slower then the other side of the wave, it changes the direction, bends towards the center of this field emitter.
This explanation however works only in you see light as waves and not as particle like points ( photons )

Please define space and how one can bend it, what tools do I need, how does it works ?

Here's Einstein's explanation of the result predicted by general relativity and how it differs from the result predicted by Newtonian mechanics:

The Meaning of Relativity by Albert Einstein
p163-164

For a ray of light which passes the sun at a distance of ∆ sun-radii from its centre, the angle of deflection (a) should amount to

a =1.7 seconds of arc/ ∆

It may be added that, according to the theory, half of this deflection is produced by the Newtonian field of attraction of the sun, and the other half by the geometrical modification (“curvature”) of space caused by the sun. This result admits of an experimental test by means of the photographic registration of stars during a total eclipse of the sun. The only reason why we must wait for a total eclipse is because at every other time the atmosphere is so strongly illuminated by the light from the sun that the stars situated near the sun’s disc are invisible

While Einstein doesn't say "here's my definition of space...", we can infer from the above that what he's referring to is the expanse through which the light rays travel from distant stars to telescopes on Earth. So you take photos of stars next to the sun in a solar eclipse, then take photos of the same star field in similar conditions except without the sun present, and note changes in the positions of the stars.

This has been done numerous times, and the apparent positions of the stars do change in accordance with general relativity rather than Newtonian mechanics. The first time was by Arthur Eddington in 1919.

As pfishy said it sounds like you're referring to refraction which is another way to bend light, most famously perhaps with a prism where incoming white light is bent at varying angles depending on frequency which spreads different colors apart in the exiting light.

The prism would not constitute "space" as in the context of Einstein's explanation above related to observing star positions.

Arbitrageurs,
I asked a few pages back, not you directly, but now will ask your opinion.
If gravity is curvature of space-time caused by mass then why do scientists are looking for graviton. Is it that two camps of physics, GR and QM are seeking own explanation of gravity?
Indeed, if gravity is space-time geometry issue, there is no need for force carrier, and if there is a carrier, then no bent space-time is needed. Do I get it right?

Thanks.



D0.

a reply to: darkorange

Good question

originally posted by: darkorange
Indeed, if gravity is space-time geometry issue, there is no need for force carrier, and if there is a carrier, then no bent space-time is needed. Do I get it right?

Not quite right, or at least I wouldn't put it in those terms, though someone else might. The problem I have with this approach is it seems to presume one approach or the other is correct and I make no such assumption.

George Box said something to the effect that all models are wrong, some are useful.
Quantum mechanics is a useful model.
General Relativity is a useful model.
Both make a lot of very accurate predictions in certain ranges of applicability but they are probably both wrong as George Box said. We do have some symptoms of their wrongness we can look at.

Richard Feynman won the Nobel prize for his contribution to quantum mechanics, but he referred to the math trick known as "renormalization", a way of tossing out unwanted infinities, as a "dippy process". Maybe at first a lot of people agreed it was dippy, but after seeing the model made accurate predictions it was thought less dippy and maybe it's a good approach for QED. It apparently works for three of the four fundamental interactions, strong nuclear, weak nuclear, and electromagnetic, but for gravity it's worse than "dippy", it doesn't work, or at least nobody has been able to figure out how to make it work.

When we try to model gravitation on quantum scales we can't discard the infinities through renormalization like we can for the other interactions, so this is a problem when trying to model what happens inside a black hole for example. The two theories of Quantum mechanics and General Relativity don't give a consistent picture of what should happen because they don't work together in describing things such as black holes. In fact Michio Kaku referred to the general relativity calculation of infinite density inside a black hole as a breakdown of the theory, which is one of the indications of "wrongness" I mentioned earlier.

It's an unsolved problem in physics and if I knew the resolution I'd have published it and collected my Nobel prize, but apparently we need another guy as smart as Einstein to come along and figure this out. In fact Einstein himself worked on trying to resolve this without success.

I would be open to a wider range of possibilities than the options you cited in your question.

This is a summary of the current state of affairs in our efforts to solve this unsolved problem:

Quantum gravity

There are a number of proposed quantum gravity theories. Currently, there is still no complete and consistent quantum theory of gravity, and the candidate models still need to overcome major formal and conceptual problems. They also face the common problem that, as yet, there is no way to put quantum gravity predictions to experimental tests, although there is hope for this to change as future data from cosmological observations and particle physics experiments becomes available.

So in other words, we can't really test any of these candidate theories yet, but even without testing them we can already tell that they are wrong, meaning they have "major formal and conceptual problems". Maybe we need some thinking outside the box here.

a reply to: Arbitrageur

This person made a graph that does an excellent job of explaining gravity in Einsteins world. He explains everything travels a straight path but warping of space causes time and space to change



Now as far as the graviton it would be nice if it exists it really would unfortunately we can't show them to exist. And I have a feeling if they do they don't work quite like we think.

Hello guys,

Sorry to interupt, it's been a long time since I've visited here, I'm happy to see this thread still going strong. Hope everyone had a good year! I've just made my way around and landed right back in NJ again.

Anyway you've all been very helpful in the past and I need a little direction or information regarding a theoretical physics issue.

The question is:
Is there a partical that travels backward in time or do they all do this to some degree or is this just hypothesized presently? What I'm getting at mainly here, is there some 'natural' way information(matter) can be broadcast, carried or sent back in time, riding along with these particles or some other way?

Retrocausality basically.

Now I've heard neutrinos might do this but I'm not exactly sure to what degree the specifics are 'science fact' or 'pure speculation'. Also if there is some other similar 'natural' phenomenon that mimics this transmission back in space/time I would love to hear.

Thank you in advance, AB

a reply to: AnteBellum

Photons?

(Skitters out of the room giggling before one of the physicists back hands me)

originally posted by: dragonridr
a reply to: Arbitrageur

This person made a graph that does an excellent job of explaining gravity in Einsteins world. He explains everything travels a straight path but warping of space causes time and space to change

Well if you have space represented by only one dimension instead of three, all you're going to get is straight lines. You need at least two dimensions of space to show any curvature though space, which his demo lacks.

Still, he gets points for creativity...I like his space-time stretcher even if his space is one-dimensional. I didn't think he'd have time to make that contraption, because after he converted to every religion I didn't expect he'd have much time left for such activities.

originally posted by: AnteBellum
Is there a partical that travels backward in time or do they all do this to some degree or is this just hypothesized presently? What I'm getting at mainly here, is there some 'natural' way information(matter) can be broadcast, carried or sent back in time, riding along with these particles or some other way?

Retrocausality basically.

So, if we send you back in time, and you kill your grandfather before he conceived your father, what will happen? Will you cease to exist?

If you do cease to exist, that's a problem because then how did you kill him if you don't exist?
If you don't cease to exist, that's a problem because if your grandfather never conceived your father where did you come from?

Paradoxes like these don't prove backward time travel is impossible, but it is commonly thought that retrocausality isn't possible.

One hypothetical particle that could conceivably travel back in time is the tachyon, which travels faster than the speed of light. At one point CERN was reporting neutrinos traveling faster than light which would make them tachyons of sorts, however it turned out they had a faulty connector which when fixed showed that neutrinos aren't tachyons after all. Even if the hypothetical tachyons existed and they probably don't, they wouldn't facilitate sending information back in time:

Tachyons

Hypothetical superluminal particles called tachyons would have a spacelike trajectory, and thus move backward-in-time according to observers in some reference frames. Despite frequent depiction in science fiction as a method to send messages back in time, theories predicting tachyons do not permit them to interact with normal "tardyonic" matter in a way that would violate standard causality.

Have you heard the limerick about faster than light, backwards time travel?

There was a young lady named Bright
Whose speed was far faster than light;
She set out one day
In a relative way
And returned on the previous night.

-A. H. Reginald Buller

I can't say retrocausality is impossible but it seems very unlikely. Time travel into the future on the other hand is entirely possible and we are already doing it on small scales since time passage for astronauts passes at a different rate than for people on the Earth, but the effect is so tiny it doesn't get much attention.

originally posted by: BASSPLYR
a reply to: AnteBellum

Photons?

Photons are a cool way to look back in time. The famous image in my avatar is what the Hubble can see today, however we think this structure doesn't exist at the present time, and we can only see it because we are looking back in time when it used to exist. So even if we can't sens messages back in time, it's cool to look back in time, and see things as they used to be.

i have a couple questions.

what is the speed of light?
how large is the ovservable universe?
how old is our universe?

a reply to: Arbitrageur

I liked the creativity to make a point I hadn't seen that before.

originally posted by: John333
i have a couple questions.

what is the speed of light?
how large is the ovservable universe?
how old is our universe?

Unless you need to do precise calculations, 300,000,000 meters per second is easy to remember and pretty close to the speed of light in a vacuum.
The observable universe might be something like 93 billion light years in diameter today but of course we can't see all of it today...the further away we look the further back in time we see.
The big bang is thought to have happened about 13.8 billion years ago.

originally posted by: Arbitrageur

originally posted by: John333
i have a couple questions.

what is the speed of light?
how large is the ovservable universe?
how old is our universe?

Unless you need to do precise calculations, 300,000,000 meters per second is easy to remember and pretty close to the speed of light in a vacuum.
The observable universe might be something like 93 billion light years in diameter today but of course we can't see all of it today...the further away we look the further back in time we see.
The big bang is thought to have happened about 13.8 billion years ago.

Your answer isn't exactly right. your just doing the basic equatuon. The speed of light is approximately 186,282 miles per second, or about 5.9 trillion miles per year.The time that has elapsed since the Big Bang is 13.82 billion years. Multiple the two figures and—tada—we find that over the entire history of the universe, light could have travelled 13.82 billion light-years.But as straight forward as that seems it's wrong. One thing we have to take into account. One of course until photons coils actually be seen meaning space had to become transparent roughly 380,000 yrs after its creation. But there is also another answer here how far can we truly see

Since the primordial burst of creation, space has been stretching as the universe expands. A galaxy’s distance from us today is far greater than it was when it released the light. Meaning of course our universe is expanding. This expansion sets a limit on how far we can see as well. So the furthest we can ever be able to see is 62 billion light years that is the point where anything beyond that space is expanding faster then the light trying to reach us.

a reply to: dragonridr

I have to ask, because right now it's easier than thinking about it.
If the rate of expansion beyond that distance exceeds the speed of light, what are the galaxies out there doing?

originally posted by: dragonridr
Your answer isn't exactly right. your just doing the basic equatuon.

I answered the questions he asked. You answered a question he didn't ask. 62 billion light years is a future limit. the question was:

"how large is the ovservable universe?"

In present tense, the answer today is:

Observable universe

It is estimated that the diameter of the observable universe is about 28 gigaparsecs (93 billion light-years...), putting the edge of the observable universe at about 46–47 billion light-years away.

So, 93 billion light years as I said. If you convert that diameter to radius, 46–47 billion light-years away.

All the oceans on earth will have boiled away, and thus the human race may be extinct before the 62 billion light years future observability limit you're talking about comes to pass, unless we get in gear and figure out how to colonize other planets before then.

originally posted by: Phage
a reply to: dragonridr

I have to ask, because right now it's easier than thinking about it.
If the rate of expansion beyond that distance exceeds the speed of light, what are the galaxies out there doing?

If there are aliens in those distant galaxies, they're probably asking the same question about us and our galaxy. To them we're the ones receding faster than the speed of light from their point of view.

originally posted by: Phage
a reply to: dragonridr

I have to ask, because right now it's easier than thinking about it.
If the rate of expansion beyond that distance exceeds the speed of light, what are the galaxies out there doing?

Hopefully enjoying the view. The eventual answer is funny trillions of years from now they will think there galaxy is the entire universe. They will be truly alone.

You guys didn't help.
Now I have to think about it.

a reply to: Arbitrageur

I hope some form of humans manage to make it somewhere who knows we could be the first. To colonize an entire galaxy. But since we are just now considering Mars I don't hold out a lot of hope for the continued existence of our species.

originally posted by: Phage
You guys didn't help.
Now I have to think about it.

That's ok I hear it's good for you.