[sciencepm (OP)]

Hi everybody,
sorry I’m not a space expert…

I’ve read that light is bent when it passes a massive body in the space. Why does this happen?

[RobC]

Should the question be 'why does a massive body bend spacetime'?

[lunar7]

We were always taught that light travels in straight lines.
Draw a straight line on a sheet of paper. Then curve the paper a little. Now the line is cured (bent) but still moving In a straight line.
Conclusion is that space has curved.

[Kryptid]

There are two ways of looking at it. One way is the way that lunar7 has described (that the path of light is deflected because the space around a massive body is distorted. This is the explanation by general relativity). The other is that light is attracted to the gravitational field of the massive body and is therefore pulled towards it. Both explanations are equally correct.

[evan_au]

The effect of starlight being bent when it passed close to the Sun was one of the first experimental verifications of Einstein's General Relativity. It led to considerable fame for Einstein.
- The test was conducted by two teams of astronomers, led by Arthur Eddington, who took pictures of the positions of stars during a total solar eclipse, in 1919. This was compared with the positions of these stars at a different time of the year, when their light path passed nowhere near the Sun.

Einstein described the effect of mass as that of curving spacetime.
- Light tends to take the shortest path between two points.
- But when spacetime is curved, the shortest path is also curved.

Alternatively, you could informally imagine this effect as follows:
- Light can be viewed as particles of light (photons) which carry energy, and have an equivalent mass. The Sun "attracts" the passing photons, bending their path. This concept was compatible with Isaac Newton's theory of gravity, but the bending is twice as great as Newton's theory would predict.
- According to General Relativity, near a massive object, time slows down. This means that light slows down (as seen by a distant observer). Light can be viewed as waves of light, and when waves pass into a medium which has a different speed (eg light waves into a lens, or ocean waves near the shore), the path of the wave is bent; this is now called gravitational lensing.

See: https://en.wikipedia.org/wiki/Eddington_experiment
https://en.wikipedia.org/wiki/Gravitational_lens

Oops! crossover with Kryptid...

[charles1948]

We were always taught that light travels in straight lines.
Draw a straight line on a sheet of paper. Then curve the paper a little. Now the line is cured (bent) but still moving In a straight line.
Conclusion is that space has curved.

No.  The conclusion is that the sheet of paper has curved.  The space around it, hasn't curved at all

[Kryptid]

No.  The conclusion is that the sheet of paper has curved.  The space around it, hasn't curved at all

It's an analogy.

[charles1948]

No.  The conclusion is that the sheet of paper has curved.  The space around it, hasn't curved at all

It's an analogy.

You mean, it's just a fanciful comparison.  Not real.  I agree.

[Kryptid]

The curvature of space is real. It can be measured.

[charles1948]

The curvature of space is real. It can be measured.

How can it be measured.  If space is really curved, then wouldn't everything in it be curved as well.

Such as rulers. All rulers would be curved.  So wouldn't they give the same measurements, as flat rulers in flat space?

How would you be able to tell the difference?

[Bored chemist]

How can it be measured. 

By measuring things like gravitational lensing.

[Kryptid]

The geometry of curved space is different from that of flat, Euclidean space. That leads to different physical consequences, such as the anomalous orbital precession of Mercury. General relativity makes precise, quantitative predictions based on gravity being a distortion of space-time, resulting in predictions such as the geodetic effect (which has been measured and found to align with the predictions of general relativity by Gravity Probe B).

[charles1948]

How can it be measured.

By measuring things like gravitational lensing.

The gravitational lensing effect merely shows that particles of light - photons, are affected by gravitational fields.

As are all particles.   What's that got to do with curvature of space?  Photons passing through the gravitational field of a massive object such as a star, will naturally get deflected.  By the star.  Not by any "curvature of space".

So how does gravitational lensing improve the case for a supposed " curving of space"?

[charles1948]

The geometry of curved space is different from that of flat, Euclidean space. That leads to different physical consequences, such as the anomalous orbital precession of Mercury. General relativity makes precise, quantitative predictions based on gravity being a distortion of space-time, resulting in predictions such as the geodetic effect (which has been measured and found to align with the predictions of general relativity by Gravity Probe B).

The anomalous precession of Mercury might be explained in other ways.    Bear in mind, that no anomalous effects have been detected in the orbit of Venus, or the Earth.  Both planets are close to the Sun, like Mercury, and might be expected to display similar effects. But none have been found necessary to invoke GR..

Probably, the GR effect is a red herring.

[Kryptid]

Actually, that's not true. The Newtonian view of gravitational lensing predicts a different value of path distortion than the relativistic view does (by half, to be precise). The relativistic model is the one that measurements support.

You're wrong about anomalous orbital precession not affecting Venus or Earth. It's just smaller than for Mercury because they are more distant. You're also going to need to explain why general relativity predicts the correct mathematical value if it's wrong. Do you think Einstein just got very lucky?

[charles1948]

Do you think Einstein just got very lucky?

I think he got lucky by applying his ideas to the Solar System. There his results, can by fudging, be made to seem plausible. Especially in explaining anomalies in Mercury's orbit.  Good case.

But what happens when his ideas are applied to the wider Universe? Such as the Galaxies.

The anomalous rotation of galaxies cannot be explained by Einstein's theory.  It's a complete failure.

So much so, that present-day astrophysicists have had to resort to inventing a completely new form of matter:
"Dark Matter".  This is something that you can't see.  But is there, apparently. With no observational evidence of its
 physical existence

Not to mention "Dark Energy".  This at least has some recourse to Einstein's idea of the "Cosmological Constant"

But these ideas are probably wild-goose chases.  Which will laughed at in 2050.
In the meantime. let's just take it all with a pinch of salt

[Bored chemist]

But what happens when his ideas are applied to the wider Universe? Such as the Galaxies.

They still work.
That's the cool thing about science.

[Halc]

How can it be measured.

One way is to measure (with a tape measure) the diameter of Earth, and then its circumference.  This isn't practical of course since you can't pass a tape measure through Earth, and it is hard to line up if held off to the side, but if you did, the circumference would definitely be less than the diameter * π.  This can only be explained with non-Euclidean spacetime, and is quantified precisely by Einstein's field equations.

The difference isn't much with Earth, but it is for more massive things like neutron stars.

[charles1948]

How can it be measured.

One way is to measure (with a tape measure) the diameter of Earth, and then its circumference.  This isn't practical of course since you can't pass a tape measure through Earth, and it is hard to line up if held off to the side, but if you did, the circumference would definitely be less than the diameter * π.  This can only be explained with non-Euclidean spacetime, and is quantified precisely by Einstein's field e

Surely the tape-measure would have to be pulled out by using energy, which would increase its mass, thereby making it expand, so it would give the measurement  when it was looked at?

[Halc]

Surely the tape-measure would have to be pulled out by using energy, which would increase its mass, thereby making it expand, so it would give the measurement  when it was looked at?

You're attenpting to nitpick your way out of the point of the argument.

A tape measure already on the surface of Earth would not require energy to wrap around the circumference.  The one measuring the diameter would actually lose a bit of energy due to the drop in PE of part of it, but that wouldn't affect its proper length.  Proper length is a frame invariant quantity, while mass and energy are not.

For the sake of argument, we'll assume no thermal difference in the tape since that actually does change the length of most materials, and far more than this relativistic length change that you suggest.  For control purposes, we'll measure a cold hard perfectly spherical rock the size of Earth that has no problem with a hole being drilled through which the tape measure can be passed.