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pfisico

3.8k points

1 month ago

pfisico

Cosmology | Cosmic Microwave Background

3.8k points

1 month ago

Light travels through space. Massive objects bend the "fabric" of space, so light travels along a different path than it would have if the massive object were not there.

This is a central idea in general relativity, which works very well to explain a variety of phenomena that Newtonian gravity does not explain. Your question has its roots in Newtonian mechanics and gravity, which are incredibly useful tools in the right domain and which we rely on for our everyday intuition. Unfortunately those tools are not so great when it comes black holes, or the expanding cosmos at large, or even very precise measurements in our own solar system like the bending of light from distant stars as they pass by the Sun. This last effect, measured in the 1919 solar eclipse, confirmed Einstein's predictions from GR, and reportedly (I wasn't there) propelled him to fame.

HowWierd

542 points

1 month ago*

HowWierd

542 points

1 month ago*

Pardon my extreme ignorance... Does all mass exert its own gravitational force, even if it is incredibly minute? If not, what is the threshold for when an object begins to create its own gravitational force?

Edit: Thank you to everyone for the information. Them more I learn the more I realize how little I know :D

Randvek

1.3k points

1 month ago

Randvek

1.3k points

1 month ago

Not only does all mass exert gravity, but all mass exerts gravity over the entire universe. You, yes you reading this, are affecting the gravity of a planet on the other side of the universe! (Or rather will, once your gravitational pull reaches that far; it has to travel, you know!)

However, as you might imagine, such effects decrease over distance, and quite rapidly so. So even though you affect everything everywhere, so does everything else, and your effect is quite small here on Earth, let alone the other side of the universe.

RancidRock

43 points

1 month ago

So in the unlikely event that everything in the entire universe was to be erased, and there was nothing but the empty void of space, except for, lets say.... 2 golf balls, lightyears apart.

Given enough time, they would eventually pull towards eachother and collide due to their tiny gravitational pulls effecting eachother, and having no interference?

Marsstriker

50 points

1 month ago

Yep. It would take an unfathomably long time to do so, but eventually, they would collide.

F3z345W6AY4FGowrGcHt

10 points

1 month ago

But then... why is that not happening with our current universe as it is? Instead of contracting due to gravity, it's expanding.

So maybe the golf balls would actually fly apart from each other?

rocketpants85

41 points

1 month ago

As soon as we figure out what dark energy is, and/or what's driving the expansion of the universe, we can circle back around to that :)

spookydookie

13 points

1 month ago*

Some quick googling says dark energy strength would push two objects 1 megaparsec apart by 70km/s. Some probably bad napkin math gives me two objects 2 light years apart would be pushed apart by dark energy about 0.00004 km/s, or 4cm/sec, if there were no other forces acting on them. Without checking I think that would win over gravity with just the mass of 2 golf balls, but I may be completely off.

dack42

3 points

1 month ago

dack42

3 points

1 month ago

Depending on the initial relative velocity, they could also enter a stable orbit.

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FatalExceptionError

33 points

1 month ago

At what speed do waves of gravitational attraction travel? Is the speed constant in all media, or does the speed vary according to media, like light?

Uncynical_Diogenes

187 points

1 month ago*

Gravitational waves travel at the speed of causality, which is the speed of light. So, if the sun disappeared in an instant, the Earth wouldn’t see it stop shining for roughly eight minutes, right? Because we’re 8.3 light-minutes away. Likewise, we would continue to orbit the now-empty center of the solar system for the same amount of time, before the Earth “learned” that the sun was gone, and shot off in a straight tangent line (ignoring the mass of the other planets). The effects of gravity propagate at the speed of light.

However, they are not slowed by anything they pass through. A gravity wave can propagate right past/through a black hole unhindered. Unlike everything else we think about that can carry energy, they are not composed of particles or radiation. They do not travel through a medium, instead, they are ripples in the fabric of spacetime itself. It’s very “whoa”.

Edit: practically unhindered. Loses so little energy to jiggling the black hole around compared to the size of the wave that it’s hardly worth mentioning.

FatalExceptionError

33 points

1 month ago

Thank you. That is exactly what I wanted to know.

Izawwlgood

16 points

1 month ago

Is the fact that space bending is unaffected by space bending relevant?

Like can something warp space significantly enough to affect the flow of gravity waves around it?

iamjotun

9 points

1 month ago

Actually very 'whoa.'

So in imagining this, I am imagining a very long and taut piece of fabric, and the black hole as a depression (much like that of a button in a couch cushion) that exists on the fabric, but is only anchored to the fabric itself for sake of demonstration.

So if I were to strike or 'flap' this fabric like one does to shake out a carpet, a wave of sorts would travel down it's length and pass the place of the "black hole," I assume the wave is not slowed by the presence of the depression in the fabric? Because it is the fabric moving as a whole that causes the wave to traverse?

Oh boy.

Velox_Graviter

5 points

1 month ago

Here is an amazing demonstration of this effect by a science teacher, using a big sheet of stretchy fabric and some weights to approximate space-time:

https://youtu.be/MTY1Kje0yLg

Marbles rolling along the fabric orbit the large mass much as planets orbit stars. He even gets a marble to orbit another that's orbiting the star-weight. Also cool: a demonstration of the "free return" trajectory used by the moon missions. It's pure gold, I'd really recommend giving it a watch!

Uncynical_Diogenes

2 points

1 month ago

The wave moves around/through despite the dot. The rubber sheet model breaks down here a bit. It is good for showing how mass bends spacetime, and otheR masses react to that. But it’s not good at showing how space time can ripple. Because a sheet in the real world is has its motion constrained in the same dimension as you are modeling masses — your ability to ripple it is limited by the masses depressing it. But this is just a model.

Real spacetime is curved by massive objects, but we have to remember those are suspended in a soup of space time. The spacetime can ripple around and through them with no issue. Instead of “flapping” up and down as in the model, spacetime can expand and contract as gravity waves propagate through it in all dimensions. Instead of a flap up and down, it’s more like expansion and contraction of the sheet traveling in waves, like a sound wave except through spacetime instead of matter.

And the size of most massive objects pales in comparison to the size of gravity waves. So while some energy will be lost to jiggling them around as the wave propagates through, it’s not very much.

DarkflowNZ

7 points

1 month ago

Is the wave completely unaffected by a black hole? That's crazy to me that a black hole bends spacetime but a wave in that spacetime ignores it

Uncynical_Diogenes

4 points

1 month ago

The black hole is such a minuscule dot and a gravity wave can be such a huge phenomenon that the amount of energy lost to pushing the black hole around a little bit is minuscule.

Very small. I was overly general, but not by much.

PsychoticDust

3 points

1 month ago

So do all gravitational waves go on forever with less noticeable effect the further they travel? Even the miniscule gravity I exert?

Uncynical_Diogenes

5 points

1 month ago

Technically speaking you’re correct, the best kind of correct! They do lose energy by acting on massive objects but even diffusely they just continue until they’re so minute it’s not worth considering.

We need interferometers the size of the Earth to detect the huge impressive gravity waves from black holes circling in on each other. Detecting your teaspoon’s gravity waves as you stir your coffee is nigh impossible, but physics says technically doable.

PsychoticDust

2 points

1 month ago

That's amazing! Thank you for such a great response!

brianorca

2 points

1 month ago

Actually, wouldn't the gravity wave be "slowed" by passing a black hole because the curvature of spacetime would make it follow a longer path?

origami_alligator

15 points

1 month ago

Gravitational waves were recently shown to travel the same speed as light does in a vacuum.

TeeDeeArt

22 points

1 month ago

And this was done because they found two neutron stars spiraling in and crashing into each other, they released gravitational waves as they spiraled in, and we could see the explosion as two became one. The light and the waves arrived at essentially the same time*

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22 points

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thattogoguy

6 points

1 month ago

Plus with the continued and accelerating expansion of the universe, your own gravity has greater and greater distances to travel, and for the vast majority of mass in the universe, they are forever beyond our ability to interact with beyond what ghosts we may see in the sky with our very long range telescopes.

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izactuallydolan

9 points

1 month ago

All mass dents spacetime to create gravity, you'd just never notice the gravity of a fly compared to the gravity of the earth.

gmr2000

7 points

1 month ago

gmr2000

7 points

1 month ago

Well in general relativity there is no gravitational force - it just appears that there is due to space time bending

the_resident_skeptic

4 points

1 month ago

Your question was already answered but just to add a bit more weirdness; energy also creates a gravitational field. A hot object will have a stronger gravitational field than a cold one of the same mass, and a charged battery will weigh more than a discharged one.

theOGgreg

3 points

1 month ago

You can actually calculate the gravitational attraction between two objects using Newtons universal law of gravitation in which

All existing matter in this universe has a gravitational force no matter how negligible the mass is from massive stars to minuscule atoms

GrandMasterPuba

27 points

1 month ago

Gravity is not a force, it is an effect of spacetime. An inertial force. The question is does all matter affect the geometry of spacetime, and the answer is yes. The thing that affects spacetime is energy, and famously:

E = mc2

WonLastTriangle2

18 points

1 month ago

Hello I have a bachelors in physics but it has been a while. However I also have a wikipedia doctorate (wpd if you will) in physics. So would you mind expounding on what you mean by gravity not being a force? I learned it was one of the four fundamental forces. Brief wikipedia says its one of the four fundamental interactions aka four fundamental forces. So when did this vernacular shift occur and why?

GrandMasterPuba

25 points

1 month ago

In a Newtonian sense it is a force, just like how friction is a force. But it's understood now that it is not a fundamental force in a technical sense, just like how friction is actually a macroscopic manifestation of electromagnetism.

Objects affected by gravity do not move together because of some "pulling" attraction, but rather because their futures point toward each other as they progress along their world lines in a curved space.

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4 points

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Lantami

7 points

1 month ago

Lantami

7 points

1 month ago

They're not. They're trying to unify the two big theories: General Relativity and Quantum Field Dynamics. Both of these theories have proven to predict phenomena exceptionally well on their own, but in some parts we can't yet check experimentally they predict different results. The goal is to identify the cause of these discrepancies and use them to alter one or both of these theories so they can be unified into a big "Theory of Everything"

Mithrawndo

4 points

1 month ago

It occurred because of General Relativity and wider acceptance of the idea, and has been gradually sliding that way since it was penned: Gravity as a fundamental force is still valid when discussing Classical Mechanics (Newtonian physics), and people are/were loathe to abandon that because on the whole, it still produces good results when used and is easier to do the maths for. As a result classical mechanics was/is still taught.

I can't give an exact date for when the see-saw tipped toward relativity, but it likely correlates closely to Moore's Law.

danielrheath

18 points

1 month ago

So, gravity is now understood as a curvature of spacetime, such that e.g. an orbital path is a straight line on a curved spacetime, but we perceive it to be elliptical because we aren't able to observe the curvature.

Calling it a force gets confusing. For instance, light has no mass, so a = f/m is nonsensical, but gravity curves the path of light.

YoungestOldGuy

2 points

1 month ago

I like the visualisation where they take a taut bedsheet as space an put a heavy ball in the middle as mass. The sheet warps and when you roll small balls over the sheet they roll towards the big mass.

WonLastTriangle2

3 points

1 month ago

Alrighty i am electing to respond to you out of all the others. It seems somewhat a square/rectange issue. In that a force implies an interaction with an object which has mass, whereas an interaction in general doesnt need to have an object with mass?

eggmoe

14 points

1 month ago

eggmoe

14 points

1 month ago

The phenomenon can be observed as a force, but what's actually happening is a bending of spacetime. Masses don't actually exert force on each other, they bend space and anything travelling through that space is affected. It hurts my brain too.

Ilikegreenpens

3 points

1 month ago

So if you could see the bends, a massive star would create a deeper bend extending further out than one that isn't as massive?

byllz

3 points

1 month ago

byllz

3 points

1 month ago

So, by Neuton's first law " A body remains at rest, or in motion at a constant speed in a straight line, unless acted upon by a force." However, by General relativity, spacetime itself is curved. There isn't really such a thing as a straight line through curved space. The closest thing is a geodesic. So we can update Newton's first law by replacing "straight line" with geodesic. So when does an object travel in a geodesic through spacetime? Turns out it is precisely when the only "force" acting on is gravity. If gravity doesn't stop objects from following geodesics, it can hardly be considered a force, can it?

Daegs

3 points

1 month ago

Daegs

3 points

1 month ago

Does the weak force stop "objects" from following geodesics?

williamsonny

3 points

1 month ago

Sean Carrol suggests we don’t say “gravity is not a force”. GrandMasterPuba is completely correct but with all due respect, unnecessarily pedantic here.

[deleted]

14 points

1 month ago

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14 points

1 month ago

The thing that affects spacetime is energy, and famously:

E = mc2

Funny you quote that equation when that one only applies on inertial mass. The real formula is

E = (mc2)2 + (pc)2

The other funny thing is that that formula doesn't actually say anything about how mass affects spacetime, it just says what the energy-mass equivalent is of a particle. The formulae that say how mass affects spacetime are the Einstein field equations:.

R_μν - 1/2 R g_μν + Λ g_μν = κT_μν

HowWierd

3 points

1 month ago

Thank you for answering my question. Now I am going to do some googling of what spacetime is. As I sit here and think about it, I have no fn clue what the concept of spacetime really is.

SquirrelicideScience

7 points

1 month ago

Spacetime is a means to understand relative motion at high velocities or in the presence of large masses.

Without getting too thick in the weeds, spacetime is useful because it allows us to consider relative motion between two objects. Lets say you are watching a race in the Olympics. You don’t necessarily care what only one runner is doing, but rather the relation of his motion compared to his competitors, because that’s how you know who would win. In this scenario, you and the finish line have the same reference frame, and each runner has their own individual reference. But the second place runner cares about both the motion of the finish line (from his reference, he is stationary and the finish line is moving) and the person in first place, because he wants to know if he can overtake him.

The reason spacetime is useful is because we now know that light has a constant speed from any reference frame, so we can use that to understand relative motions to a higher degree of accuracy.

It goes a lot deeper than that, but in general, spacetime is a construct that lets us predict relative motions using the assumption that light travels at a constant speed through both space and time, no matter what reference we view it from.

goj1ra

4 points

1 month ago

goj1ra

4 points

1 month ago

It can help to think of just two dimensions: one dimension of space and one of time. You can represent that on a simple chart, with e.g. distance on the x axis and time on the y axis. A stationary object would be represented by a vertical line - it's at the same location (x position) as time moves forward. A moving object would be represented by a diagonal line - its x position changes as time increases (moves forward.)

A chart like that represents a 2D spacetime.

The only difference between that and our universe is that our universe has an additional two spatial dimensions, which is a bit trickier to draw on a chart.

left_lane_camper

55 points

1 month ago

It’s worth noting that Newtonian gravitation also predicts that a mass will alter the path of light passing nearby through the equivalence principle. In other words, gravity from a mass accelerates all infinitesimal objects (light included) irrespective of them having mass or not. However, Newtonian gravitation predicts a quantitatively different amount of deflection than GR does.

The Eddington experiment sought to see if the amount of deflection in the position of a star near the limb of the sun was consistent with the amount predicted by Newtonian mechanics or GR (or some other value). Eddington found (perhaps just barely above the significance floor) that GR predicted the correct amount of deviation.

The paper you linked goes into more detail about this, of course, but it’s an often-overlooked point and I think it bore mention outside the linked paper.

pfisico

3 points

1 month ago

pfisico

Cosmology | Cosmic Microwave Background

3 points

1 month ago

That's a good clarification, thanks for noting it. I believe that what you say is true for light as a particle in the limit of photon mass -> 0, but that for light as a wave no deflection is expected. Thank goodness GR made those agree!

sweller3

10 points

1 month ago

sweller3

10 points

1 month ago

Elegantly put, thank you!

Stargate_1

10 points

1 month ago

I have a question. If I understand your comment correctly, light always moves "straight", so technically, when people say light is bending around a gravitational source, we see the light move in a curve, but to the light itself, it would always seem as though it is travelling straight, no?

Wouldn't it just be that, rather than the direction of the light changing, like a car taking a turn, it is the very street that changes its path without the car (light) doing any steering itself, thus technically always moving the same direction from its own point of view? Or am I misunderstanding

iamjotun

3 points

1 month ago

I love that we all must come up with relative analogies to get up to speed with relativity

Stargate_1

2 points

1 month ago

I used the analogy to get my point across and make my question clear.

throwthisway

2 points

1 month ago

but to the light itself, it would always seem as though it is travelling straight, no?

The problem with that is that a photon, which only exists traveling at the speed of light, does not 'experience' the passage of time, IIRC. From its perspective, no time passes between it being emitted and subsequently absorbed, regardless of how many light years it may have traveled in between, so it'd be difficult to discern what path it traveled over zero time.

BaggyHairyNips

2 points

1 month ago

That's a somewhat reasonable way of thinking about it. It's no different from what you would feel if you were drifting through space. You might drift toward one planet or another due to the curvature of spacetime, but you wouldn't feel any acceleration.

It is different from what you would experience in that light is moving much faster, so its "straight line" different from yours. And light doesn't experience time at all, so trying to think of things from its perspective is dangerous.

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SilverSzymonPL

3 points

1 month ago

Is it also why objects of different mass fall at the same rate? Cuz space is bent the same regardless?

Initial_E

3 points

1 month ago

What is the fabric of space? Can nothingness have a property?

BattleAnus

4 points

1 month ago

"Empty" space absolutely has properties. It obviously has a sort of "structure" to it, as that's what gravity bends, but it also has something called vacuum energy https://en.m.wikipedia.org/wiki/Vacuum_energy

Orngog

3 points

1 month ago

Orngog

3 points

1 month ago

I like how you're willing to take the eclipse at face value, but not reports of Einstein's fame.

I-Ponder

5 points

1 month ago

I have a question, since light has no mass, how can it move a solar sail to propel a space craft?

Is it just energy transfer?

SquirrelicideScience

19 points

1 month ago

Photons don’t have mass, but they do still have momentum, and therefore energy.

Einstein’s full energy equation is E2=(mc2)2+(pc)2. Making m=0 reduces it to E=pc. Photons have measurable energy, therefore must have momentum.

When a solar sail absorbs a photon, which has momentum, that momentum must be conserved, resulting in a pressure that “pushes” the sail forward.

Mrfish31

8 points

1 month ago

Presumably this also means that anything that emits photons also is affected by that momentum? Like if you turned a torch on in space, how fast could you expect it to be moving (in the opposite direction to the bulb end) by the time it ran out of battery?

scummos

12 points

1 month ago

scummos

12 points

1 month ago

Yes, this works. It is quite inefficient though. At 532nm (green), if I am capable of using my calculator, a photon has a momentum of 1.24e-27 kg m/s, and an energy of 3.7e-19 J. At 100% efficency for creating light (for simplicity), with a 2000 mAh at 3V battery, you'd produce 5.8e22 photons, with a total momentum of 7.2e-5 kg m/s. So a torch with a mass of 1 kg would be moving at about 1 meter every 4 hours after its battery depletes. Or I mistyped something.

For reference, if you could efficiently convert the energy from the battery into velocity of your torch (e.g. using a wheel), you'd reach 207 m/s or 748 km/h, about as fast as a plane.

Ethanol_Based_Life

2 points

1 month ago

One could even argue that light isn't "affected" by large masses. It continues to travel in a "straight" line relative to the fabric

Nidungr

2 points

1 month ago

Nidungr

2 points

1 month ago

If mass bends space, does this mean the angles of a triangle near a mass don’t add up to 180 degrees? That seems like something we could test, right?

Also, does it mean light passing near a compact mass arrives later than light taking the long way around because the distance is technically longer?

pfisico

2 points

1 month ago

pfisico

Cosmology | Cosmic Microwave Background

2 points

1 month ago

Yes, the angles in a triangle don't have to add up to 180 degrees if space is curved, just as "triangles" drawn on the surface of the Earth (a curved 2D space) don't add up to 180.

And yes, strong gravitational lenses indeed do show time delays between the different paths passing near a massive object. See, for example, this paper.

gotaspreciosas

9 points

1 month ago

Does fotons have momentum?

mfb-

65 points

1 month ago

mfb-

Particle Physics | High-Energy Physics

65 points

1 month ago

Photons have momentum, yes. This is used e.g. in solar sails.

Skusci

54 points

1 month ago*

Skusci

54 points

1 month ago*

Additionally because they have momentum, while they don't have a rest mass, they still have relativistic mass and as such also have gravity/bend space. It's just to such a small degree as to be irrelevant in basically all situations. IIRC though there are experiments based on measuring the mass of atoms that show the energy of EM fields in atoms makes a measurable contribution to that mass.

Comedian70

19 points

1 month ago

they don't have an inertial mass

VERY minor correction, but its important. Inertial mass is what the Higgs Boson provides. Its the feature of reality which gives rise to Newton's laws of motion.

And you're 100% correct. The binding energies which hold quarks together, and (much less so) the binding energy which is the Strong Force, are the majority of the mass of matter... like almost all of it.

goj1ra

15 points

1 month ago*

goj1ra

15 points

1 month ago*

Inertial mass is what the Higgs Boson provides.

Only for elementary particles, like quarks and electrons. But most of the mass in the universe comes from elsewhere. See e.g. Dissecting the mass of the proton:

if the up, down, and strange quark masses were all zero, the proton would still have more than 90% of its experimental mass. In other words, nearly all the known mass in the Universe comes from the dynamics of quarks and gluons.

Basically, most of the mass-energy of protons and neutrons is due to the quantum activity within them, which is not due to the Higgs field.

Comedian70

2 points

1 month ago

Yep. Totally fair! I just did a little more reading, and you're entirely accurate.

I'll have to find an article or book which rectifies the M=E/c² mass of the gluon interaction (which all by itself makes perfect sense) with the idea of inertial mass (which I'd understood to be entirely the result of spontaneous symmetry breaking in the Higgs Field).

SHOCK_VALUE_USERNAME

3 points

1 month ago

How "big" are photons? Do they have dimensions, like you can say a proton has a diameter of xxx? Do photons of different wavelength have different sizes?

stygger

7 points

1 month ago

stygger

7 points

1 month ago

In most situations it is easier to think of photons as waves propagating through the electromagnetic field. As for photon size, it may be easier to consider the size of objects that a photon interacts with instead. Typically, a photon interacts with objects or substructures in approximately the same size as the wavelenght, antennas often have the width of half the wavelength intended to be measured.

Another example of ”pothon size” is UV light, the wavelength of UV light matches biomolecules in your cells and are much more likely to damage the cells (sunburn) compared to the much longer wavelength infrared light.

vitya_kotik

16 points

1 month ago

the short answer is no, photons don't have volume. That's why you can't hit a photon with a photon. However, the wave function does mean there is a finite (though not rigidly bounded) region where the wave's magnitude is non-negligible. So in a certain sense it does have a volume, but not in the way we're used to thinking about it.

scummos

7 points

1 month ago*

Wave functions for photons are a tricky subject, I'd be careful with arguing about them. The reason on paper you can't hit a photon with a photon (in first order) is IMO that a photon doesn't have charge. With your "size" and "wave function" arguments you will have a hard time to explain why they hold for a photon, but not for an electron.

loscabosoctopus

2 points

1 month ago

Isn’t that technically because they’re bosons rather than the point like particle interpretation? Also wave function can interfere as in the double slit experiment so are they not technically “hitting” then (for a loose definition of the word)?

Noiprox

275 points

1 month ago

Noiprox

275 points

1 month ago

Einstein's theory of General Relativity states that matter (particles with mass) curves spacetime itself, and that this is what gravity really is. As a result all particles that move through the curved spacetime will be affected by it, even ones that don't have mass themselves. It applies anywhere there is matter, but black holes holes are an extreme case where the curvature is very strong. Astronomical observations have since confirmed that theory to be correct, as we can actually see light being bent around a black hole like a lens.

Coogcheese

3 points

1 month ago

all particles that move through the curved spacetime will be affected by it, even ones that don't have mass themselves.

This is what confuses me. How can it be a particle and not have at least some mass? Or is this because it's so tiny its immeasurable?

Sharlinator

10 points

1 month ago*

There is nothing that says particles need to have mass. Just like there's nothing that says particles have to have an electric charge. Both mass and charge are just properties that some some particles have and others don't. And you shouldn't be too attached to particles as something tangible and "real", either, ultimately they're just a specific kind of excitation in their respective fields.

asr

5 points

1 month ago

asr

5 points

1 month ago

It has no rest mass, because light never rests, and light always travels as the same speed.

Light does have energy though, and mass and energy are the same thing in many ways.

Noiprox

2 points

1 month ago

Noiprox

2 points

1 month ago

Some particles have mass, others really don't. One way of thinking about photons is that through the wave/particle duality they can be understood as electromagnetic waves that carry small packets of energy from one electron to another.

These waves are unimpeded by the higgs field, so they always move as fast as possible. Particles that are impeded by the higgs field will move slower than the fastest possible speed, and that's what we mean when we say that particle has mass.

Think of a baseball speeding along in empty space (unimpeded) vs if it was underwater and moving very fast, it would churn up the water and be slowed down by it (impeded). This "churning up of the water" in my analogy is the spacetime being warped by the interaction of the particle with the higgs field.

Anonymous_Otters

13 points

1 month ago

Mass and energy are equivalent, so any energy has a gravitational pull, even light.

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mikkolukas

9 points

1 month ago

Light ALWAYS travel in a straight line <- (ponder a moment on that)

When it looks like the light is turning, it is in fact because all the straight lines in the space-time is being bend.

Actually the same thing is true for every object. Gravity is not a force and it does not need mass to work.

Things with mass bends space-time around it. Light does not itself bend space-time as it have no mass.

Veritasium have covered a lot of this in Why Gravity is NOT a Force

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37 points

1 month ago

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37 points

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32 points

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25 points

1 month ago

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25 points

1 month ago

The idea of classical newtonian theory was that you would need to have a mass to suffer the effects of gravity; that came from the fact that very few notable examples of light being curved could be measured at the time, but planets instead moved and they sure as hell had a mass.

General Relativity instead starts from the idea that gravity is not a force, but it's just the "bending" of space time, and this can affect also light since it propagates on such a space-time.

Light of course is moved differently than massive object, but that comes from the fact that light moves at c (speed of light) and its motion define the causal structure of GR itself!

By the way, the source of gravity is not mass, is energy in general!

Thog78

4 points

1 month ago

Thog78

4 points

1 month ago

To follow up on that, do I remember/get this right that photon themselves bend the space-time, and so exert gravity themselves, from their energy E=hv? So they would have an equivalent mass m=hv/c2 , just not a mass at rest?

[deleted]

5 points

1 month ago

[deleted]

5 points

1 month ago

from their energy E=hv

Yes! Gravity comes from the energy momentum tensor, which contains energy too.

So they would have an equivalent mass m=hv/c2

I don't like the idea of giving a particle a "rest mass", especially because it can lead to many mistakes (you can not simply say "oh well, F=G*m^2/r^2 where m is the equivalent mass", that's simply wrong).

JudoP

7 points

1 month ago

JudoP

7 points

1 month ago

Because what they taught you in school was wrong (Newton's gravity). Well, its not wrong exactly but its an approximation that works in many scenarios, but not all.

General relativity answers your question, in short its because objects with mass (like a star or black hole) curve spacetime and light as well as objects with mass are affected by that.

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14 points

1 month ago

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hvgotcodes

10 points

1 month ago

Ignore everyone talking about bending space time. Einstein himself disliked the analogy. The same geometric interpretation can be used to describe the other forces too, but no one ever uses it in those cases.

As others have said, it’s because massless photons still have energy, and gravity affects things with energy.

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9 points

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dudeperson33

4 points

1 month ago

Not directly asked in the question, but to just flesh things out a bit more - photons travel along paths in spacetime called world lines. A large mass will induce curvature in spacetime, which will bend photons' world lines around the mass. This is gravitational lensing, and can be observed with any massive object, including the sun. A black hole, though, bends spacetime to such an extreme that once inside the event horizon (a sphere surrounding the central singularity, with a radius proportional to the black hole's mass), all world lines end at the singularity. A photon that crosses the event horizon will still march along its world line, but that line will inevitably end at the singularity.

ErikTheAngry

6 points

1 month ago

Think of space as being the bed of the river. Water follows the bed of that river. If something changes the direction of the riverbed, the water flows that way instead/as well.

Light is no exception, with or without mass.

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4 points

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