Viewing a single comment thread. View all comments

Aseyhe t1_j25yrby wrote

Most of the Milky Way galaxy, including the Sun, orbits at around 200-250 km/s; see e.g. figure 16 of this review article. Note that this implies the galaxy cannot be rotating rigidly. Objects closer to the center have shorter orbital periods.

That's about 1/1400 the speed of light, so the Sun and Earth cover about that fraction of a light year in one year, with respect to the galaxy.

Of course, all motion is relative. Why choose the Milky Way galaxy as a reference? Actually, there is a fairly natural "rest frame" for the local universe, and that's the rest frame of the cosmic microwave background (CMB). An observer in that frame finds the CMB to be equally hot in every direction. We do not, so we infer that the Sun is moving at about 370 km/s (1/800 the speed of light) with respect to the CMB rest frame.

Interestingly, that motion is anti-aligned with our motion about the galaxy, which means the Milky Way itself is moving at about 550 km/s with respect to the CMB. See table 3 of this article for more velocity comparisons; LSR is the local standard of rest, referring to the average motion of nearby stars; GC is the Milky Way galactic center; CMB is the cosmic microwave background; and LG is the local group containing the Milky Way, Andromeda, and many smaller galaxies.

2,048

playadefaro t1_j262fha wrote

Thank you for the detailed response. Do we know the fastest celestial object?

283

amaurea t1_j26aodj wrote

  • Individual particles reach speeds extremely close to the speed of light, but I guess those don't count as celestial objects
  • Streams of plasma ejected as beams by quasars, or as shells of matter ejected by supernova explosions, or plasma spinning around a black hole as part of an accretion disk also move close to the speed of light, but these aren't really objects either.
  • Black holes or neutron stars that orbit each other in a pair gradually lose energy, causing the orbit to gradually shrink while speeding up. Just before they hit each other they reach speeds very close to the speed of light. With current technology we can observe these minutes to seconds before merger in gravitational wave detectors like LIGO, when they're at their fastest but most transient. But they will be moving very fast for years before the merger - we just haven't found any at this stage in their life yet. The famous Hulse-Taylor binary is 320 million years away from merger, but is already moving at 0.15% the speed of light and will only speed up from here.
  • Stars orbiting supermassive black holes, such as those orbiting the one in the center of the Milky Way, can reach very high speeds. The fastest of these with a robust speed measurement is S14, which reaches 3.83±0.06% of the speed of light.[*] Of course, there could easily be even faster stars orbiting this (or other) supermassive black holes that have not been discovered yet
  • Simulations show that collisions between rapidly spinning black holes result in asymmetric emission of gravitational waves. These carry away huge amounts of momentum, and by conservation of momentum, the merged black hole gets a substantial kick in the opposite direction. In the most extreme cases, this may result in the black hole reaching 1.3% the speed of light.
  • The fastest known free-flying star, S5-HVS1 according to Wikipedia, moves at 0.59% the speed of light compared to the galaxy.

[*]: The stars S4714 and S175 are nominally faster, at 8±3% and 4.27±0.47% of the speed of light, but given their large uncertainty they are probably slower than S14. S62 with 7.03±0.04% looks like it's the fastest one with a good measurement by a good margin, but this one turned out to be an error (which reminds me that I should get around to updating the wikipedia articles mentioning this star).

552

clocks212 t1_j26hnaj wrote

I’ve searched before but never found an answer, maybe you know. Could you be close enough to merging black holes to feel/be killed by the gravitational waves without already having been killed by the black hole or its accretion disk?

79

Kantrh t1_j26scg5 wrote

Possibly. According to this article https://www.forbes.com/sites/startswithabang/2020/02/15/ask-ethan-could-gravitational-waves-ever-cause-damage-on-earth.

60

Onetime81 t1_j28r1du wrote

If death didn't occur until after crossing the event horizon, then this would be the best way to die, imo

As you cross you'd be able to watch the universe age and die. You'd get a conclusion to the story... Right before you cells started to unmesh themselves from your body.

Trade offs, amirite ¯⁠\⁠⁠(⁠ツ⁠)⁠⁠/⁠¯

42

Theban_Prince t1_j290xqy wrote

Would you just freeze in time halfway there? Meaning even ypur brain will not be able to perceive anything.

9

Onetime81 t1_j2a5z34 wrote

Depends on your perspective, naturally, with spacetime being relative and all.

From an observers pov, say comrades who couldn't catch you in time, you would freeze for forever, until you the light bouncing off you slowly redshifted out of our visual range. Which sounds awful to experience, even just watching.

On the plus side, you would allow accurate mapping of where the horizon actually is, since it's invisible, the moment you 'froze' would be the moment when you crossed over.

From you're pov its speculated that at that distance you'd essentially be outside time. And past the horizon light only goes one way, and that's in towards the black hole, which you wouldn't be facing, so you'd see all of the light from all time descending towards you. Whether that's linear, and just like a VCR on fast forward, that I can't say, and I don't know if that will or would ever be verified.

So you'd get the answers of how it all ends (heat death, big crunch, cyclical, neuvo-physics bubble, great unraveling, grey goo, thetons, who knows) but you'd never be able to share the answers... Unless each black hole IS an Einstein-Rosen Bridge.

I like to think hidden behind each horizon is a great cosmic/galactic space-truck stop full of alien yokels. That's the flavor of multiverse I want to be in :)

8

TripleJeopardy3 t1_j2a52ei wrote

If that happens you should power cycle and reboot. So maybe take a nap and when you wake up your brain will work again.

2

clocks212 t1_j2dyrqf wrote

Only from an outsiders perspective would you freeze. From an outsiders perspective a black hole can be thought of as “a region of space where nothing has ever happened”. From your perspective you would just fall straight through the event horizon like nothing was there until you were killed by gravitational forces or impacted whatever exists at the center of the black hole.

1

StandardSudden1283 t1_j26ilsf wrote

I want to add on to this question - would the answer to the above be different if the black hole had no accretion disk?

19

Lurker_IV t1_j29j0d0 wrote

If you get too close to the wrong black hole then you become its accretion disk...

5

tomrlutong t1_j29gfae wrote

I did the math a while back- you have to be crazy close, like 100s of km, to a merger for the gravity waves to affect you directly. /u/StandardSudden1283 Even a 'cold' pair of BHs (no accretion disk) would kill you from tidal forces at a much greater distance.

The article/u/kanrith links to suggests the waves could cause earthquakes or something on the planet you're on, and so indirectly hurt you, but its not verry convincing.

5

duroo t1_j26y2rb wrote

Where does the speed of star collapse just before a supernova fit into this?

10

MichiganBeerBruh t1_j28ajxp wrote

What is the speed of two of the furthest known points of the universe, relative to each other, with the rate of expansion of the universe?

And how does that compare to the speed of light?

Sorry there is probably many better ways to word that

7

amaurea t1_j28gqoz wrote

In an expanding universe things like distance and speed become ambiguous at large distances, with several sensible definitions that all give the same results under normal circumstances suddenly disagreeing. When it comes to distance, this is due to the expansion of space changing the scale of the universe while light is traveling towards us, so effectively changing things in the middle of our measurement. When it comes to speed, it is due to the difference between things moving apart because of their own motion, or things moving apart because new space appeared between them.

As a rough analogy for the former, imagine two ants separated by a piece of string they can walk along, but they're currently standing still. Now someone cuts the string and splices in a much longer piece of string between the ants. The ants didn't move, but now the distance between them (along the string = through space, in this analogy) is much longer. Does that mean the ants had a huge relative velocity when the splicing took place?

It's up to you, really, but I think most of us would prefer to factor out the expansion part and only include the moving part in the definition of velocity. In cosmology, this definition of speed is called peculiar velocity, and would not be particularly lage for two objects on opposite sides of the observable universe.

All of these complications go away if you only look at nearby objects. It's relative speed between two objects close to each other that's limited to 299792 km/s.

14

RIPphonebattery t1_j293unq wrote

S4714 at 8+/-3% c seems like its lower bound (5% c) would be higher than the 3.83% c from S14. What am I missing?

1

amaurea t1_j2a89xt wrote

Firstly, 8±3 doesn't mean "it's definitely between 5 and 11". It just means "it's 68% likely that it's in that range", and then it's 95% likely that it's in the range [2:14] and 99.7% likely that it's in the range [-1:17] (really [0:17] in this case, since it can't be negative).

Secondly, why do I say that it's likely to be lower than 8% rather than higher, given that the error bars go both up and down? That's because we're looking at extreme value statistics. Put simply, we're not looking at a random data point, we're looking at the data point with the highest value. That means we're much more likely to see a positive noise fluctuation than a negative one, because a positive noise fluctuation makes a data point more likely to be the highest one while a negative noise fluctuation does the opposite.

Here's a concrete example. Let's say we have two sets of numbers, set A and set B, each with 1000 numbers in them. In set A, each number has a true value of 5 but ±1 in errors. In set B, each number has a true value of 0, but ±10 in errors. So in reality the numbers in set A are much bigger than in set B. But now look at what happens when we take errors into account. In set A, it's unlikely that we will see any numbers higher than around 8, since a +3 error has a likelihood of only 0.15%. Meanwhile, in set B we're almost guaranteed to see numbers higher than 20. So if we're not careful we will incorrectly conclude that B is really bigger than A.

Sorry, that didn't come out as clearly as I had hoped. If you know some simple programming, then I recommend just writing a 5-line program to test it out yourself.

2

RIPphonebattery t1_j2ah81c wrote

This makes perfect sense to me! I didn't realize speed was generally expressed with 1-sigma error bars, I'm most used to seeing 95% CI's.

Thanks for taking the time to reply

1

ivanparas t1_j263wj6 wrote

Not a single object, but the material jetted out from quasars moves at near the speed of light.

19

drummerandrew t1_j282th4 wrote

Woah. This got my brain going. Solar, galactic, super group orbit timed just right could move pretty fast.

3

garbageemail222 t1_j289edi wrote

Physics gets weird when talking about aligning speeds approaching the speed of light. They're not additive.

8

playadefaro t1_j29kc8j wrote

This is the most mind bendy thing ever! Is it because of some reason we know or is it because, well, it is!!?!!

Also, at what speed does the non-additive-ness start?

2

Harnellas t1_j29r5bc wrote

I still don't understand why .6c plus .6c cannot equal 1.2c, I only know that it can't.

1

I_am_darkness t1_j27hftd wrote

This answer slaps. I had 3 "yeah but what about"s while reading that it proceeded to answer.

28

Bladestorm04 t1_j270vyy wrote

Can you explain more this rest frame?

Ive always known there is no such thing as one key inertial reference frame, and all speeds are relative. But if I understand correctly you are suggesting there is something that can take this place, at least in the local galactic group. CMB I thought indicates everything is moving away from us in similar amounts in every direction, making it appear that we are in fact the centre of the universe, until we find out that these observations would be the same for all observers, and therefore we aren't a special case at all.

15

Rodot t1_j27g9oy wrote

There is a frame in which everything is in the Hubble follow, basically moving with the redshift one would expect from the expansion of the universe. But within this flow you can independently move (e.g. you can get in a car and drive any direction). This will then deviate you from this flow. Though if not acted upon by another force, you'll eventually be "dragged" into this flow since the universe continues to expand in all directions.

In other words spacetime is kinda fucky

17

VoilaVoilaWashington t1_j295nnw wrote

CMB isn't any sort of special reference frame. It's just one that can be described as somewhat universal, but in theory, you could use the exact same thing with my neighbour Steve - the rotation of Andromeda is measured relative to Steve. That'll make your math nice and fucky, right?

But then the same is true of the CMB. Imagine if we tried to calculate highway speed limits based on the CMB reference frame. Or even the velocity of Sol relative to the CMB, to pick a larger example - how would that help us calculate how long it takes to get to the other side of the galaxy?

So, you always pick a reference frame that makes your math easy. Speed limits are based on the car's relative speed to the road, ignoring the rotation of the earth and all that. Your ability to juggle on a train is unaffected by the speed of the train relative to the tracks. Earth's rotation around the sun is measured against the sun, not the center of the galaxy.

You can still calculate highway speed limits using Andromeda's approach as a frame of reference, you'll just end up doing a LOT more math depending on where you are on earth and what time of day it is. But you could do it if you really wanted to!

5

Nattekat t1_j2afp4g wrote

Doesn't there have to be some reference frame whereby a body is not moving through space at all? Not under influence of any force, just the expansion of the universe all around it making it seem as if it's accelerating from any observer.

Under the laws of special relativity an observer in a space ship travelling at 0.1c will see Earth speed up, while an observer on Earth will see the ship slow down. But if all speed is relative, both should see the other speed up, which feels paradoxical.

1

VoilaVoilaWashington t1_j2ahqjh wrote

> Doesn't there have to be some reference frame whereby a body is not moving through space at all?

Relative to what? If you're measuring against the expansion of the universe, then you'd have to take VERY precise measurements against the most distant objects, and they're moving in all kinds of directions, but if you could, then sure, you could do that, somehow.

You'd just be moving at an insanely high velocity relative to anything local to you.

And because you're moving at that insane speed relative to, say, earth, you'd have to apply a massive force to actually get up to that insane speed.

> But if all speed is relative, both should see the other speed up, which feels paradoxical.

Welcome to relativity. Say you have 2 objects approaching Earth at 0.1c, relative to Earth. They'd see each other as moving less than 0.2c, because it's not additive, and if they could each see a clock on the other ship, they'd see that clock moving slower than their own.

1

Ape_Togetha_Strong t1_j2cge19 wrote

> Under the laws of special relativity an observer in a space ship travelling at 0.1c will see Earth speed up, while an observer on Earth will see the ship slow down.

No they won't. They'll both see each other slow down, since motion is truly relative. From the perspective of the Earth, the ship is moving away. From the perspective of the ship, the Earth is moving away. Neither is right or wrong. Both see symmetrical time dilation.

The unintuitiveness of this fact is why the twin paradox is so famous, despite not really being a paradox: https://en.wikipedia.org/wiki/Twin_paradox

1

swampshark19 t1_j26jh4h wrote

How about the effects of dark matter? I heard from somewhere that most stars revolve around GC with the same or similar orbital period due to the influence of dark matter leveling the angular momentum drop off and causing further stars to have a greater velocity. Is this at all true? If so how do I integrate this with what you said?

12

Aseyhe t1_j26oxu4 wrote

You might have misheard or been misinformed -- the impact of the extended galactic mass distribution (including dark matter) is that the orbital velocity remains approximately uniform over a wide range of radii (see again figure 16 of this review article). The orbital period does not.

Orbital periods are only uniform near the very centers of some galaxies (not ours, and mostly dwarf galaxies). That's actually a challenge to the standard dark matter picture (the core-cusp problem) because it requires that the system's density be uniform in the relevant region, which is not what dark matter simulations predict. But there are lots of proposed solutions to this.

28

canineraytube t1_j277ik4 wrote

To what extent could it be said that the distribution of dark matter changes the effective dimensionality of our galaxy? I ask this because, in contrast to typical circular orbits in our 3+1 dimensional spacetime, which slow with increasing radius, all circular orbits around a given mass in 2+1 dimensional gravity (attenuating at 1/d^1) share the same velocity. Is this coincidental?

2

Aseyhe t1_j284skm wrote

That's correct that the orbits within the extended galactic mass distribution resemble orbits about a point mass in two dimensions (or an infinite line mass in 3D). In both cases the gravitational potential is logarithmic with respect to distance. That's coincidental, and I'm not familiar with any mathematical tricks that take advantage of the correspondence.

3

e_j_white t1_j26ocwx wrote

Yes, it's true.

The outer stars are moving too rapidly to stay in orbit, and under normal calculations they should be thrown out of the galaxy. Additional gravitational forces must be keeping them in orbit at those faster speeds, and that's where the theory of dark matter comes in.

For example, Venus is moving at 78K mph, while Saturn is moving at 22K mph (because it is much farther away from the sun). If Saturn were moving at the same speed as Venus, it would be thrown farther out from its current orbit.

10

bitwaba t1_j26vh0u wrote

I thought you have to speed up to go to a higher orbit? I remember seeing something that said a mission to the sun is really difficult because you have to essentially slow your orbit down to 0 to move to an orbit that brings you closer to the center of the solar system.

6

Aseyhe t1_j26wt7e wrote

Yes, you have to speed up to get to a higher orbit -- and paradoxically, that still results in you moving slower, on average! This is an extremely interesting feature of gravitational systems; for example, it means they have a negative heat capacity (adding energy cools them).

12

desepticon t1_j271xta wrote

Learning this in Kerbal space program was a big "aha". You aren't so much controlling your speed as you are just altering your orbit on the opposite side of the planet.

2

swampshark19 t1_j2brjyx wrote

Is this related to gravitational potential being understood as negative energy?

1

e_j_white t1_j26wo0k wrote

Nope, orbital speed goes down as you get farther away. The equation is:

v = sqrt(G * M / R)

Larger R, smaller v.

In order to REACH a higher orbit, you need to do work to move the mass to a higher gravitation potential. That type of work requires thrust, but once you're at the larger orbit, the speed is slower.

Conversely, to move CLOSER to the sun, you need "anti-thrust" to move lower in the gravitational potential.

10

bitwaba t1_j26xukm wrote

Ah, thank you. That is much clearer.

3

adm_akbar t1_j279juf wrote

Yes you have to speed up to get to a higher orbit but then your orbital speed is slower.

1

RegisterThis1 t1_j26qp2g wrote

Full rotation of galaxy in 200 million years according to this article

https://phys.org/news/2015-02-milky-rotate.amp

10

harbourwall t1_j289tuq wrote

And Eric Idle. It may be a bit silly, but his Galaxy Song really is a useful memory aid for a lot of the numbers being discussed here.

1

thred_pirate_roberts t1_j299kk3 wrote

I used to have Galaxy s7. I named it Mrs. Brown because of that song.

🎶 Just. Re. Member that you're standing,

On a planet that's evolving,

And revolving at 900 miles an hour. 🎶

1

canadave_nyc t1_j26kz2q wrote

> the Sun is moving at about 370 km/s (1/800 the speed of light) with respect to the CMB rest frame

In what direction is the Sun moving at that speed, relative to the CMB?

7

Aseyhe t1_j26qgwd wrote

Toward the orange patch in this figure, which is where the CMB is hottest. That's a Mollweide projection of the whole sky, where the Milky Way disk is the noisy horizontal red line. The galactic center is in the middle. Our orbital velocity is toward some point on the left side.

23

canadave_nyc t1_j26yqlr wrote

Thank you. I'm finding it hard to understand how, if everything in the universe is moving away from everything else (aside from local gravitational interactions), using the classic "expanding balloon" analogy, how are we moving in an identifiable particular direction relative to the "largest reference frame in the universe". I would've thought that we'd be "stationary" with respect to the CMB if everything is moving away from everything else.

5

DreamOfTheEndlessSky t1_j27w0f6 wrote

Suppose we were at exactly zero motion relative to the CMB. If we underwent any gravitational interaction with another body, each would undergo some acceleration. That would leave each body with a non-zero motion relative to the CMB. So any local gravitational interaction would take us away from zero-CMB-motion.

Barring some sort of speculative restorative effect that brings us back to zero-CMB-motion, you'll be left with non-zero motion relative to the CMB.

So local interactions later on would be enough to create some motion.

6

nhammen t1_j29fv7h wrote

I have a slightly related question. Because we are basically seeing back in time, the net velocity of the CMB is basically the same as the net velocity of the big bang, right? However, by conservation of momentum, we know that the velocity of the center of mass of any system is conserved. So it should be conserved from the big bang up until now. Thus, shouldn't our velocity with respect to the CMB match our velocity with respect to the velocity of the observable universe's center of mass? Does it?

2

DreamOfTheEndlessSky t1_j29pa23 wrote

Momentum is only conserved in aggregate when there is no external influence. Parts of the system can still transfer momentum between each other. It's quite permitted for us to change our momentum, as long as other things have a change in momentum that is equal and opposite to our change.

I don't have specific sources for "net momentum of the universe is zero in the CMB frame of reference", but it sounds like a good expectation. If we found a way to test that, it would provide either a confirmation or open new scientific questions. Unfortunately, as the observable universe is only a subset of the whole universe, I suspect that we cannot determine the net momentum of the whole universe.

The momentum of the Earth, or a vehicle, or the Sun, or our galaxy, could vary from the aggregate due to any number of interactions. For instance, the Earth orbiting the Sun must involve the Earth and Sun having different velocities, so they can't both match the CMB. As it happens, neither does.

Even pointing a flashlight into space would cause a tiny change of momentum for the Earth: the outgoing photons have momentum, so the flashlight (hence the Earth, indirectly) must experience a change in momentum in the opposite direction.

1

silent_cat t1_j28didp wrote

I think the bit you're missing is that when you move in a particular direction, light from that direction becomes slightly bluer and light behind you becomes slightly red.

If you assume the "expanding balloon" is expanding everywhere at the same rate, by looking at the colour of the expanding balloon you can determine a speed relative to you where the balloon will have the same colour everywhere.

Yes, it would have been more logical if we'd have been stationary relative to the global frame, but it turns out we're not. Science is more interesting when it gives you answers you don't expect.

(It could be that the "bubble" is not expanding at a uniform rate everywhere, but we (currently) have no way of distinguishing that. And it seems the weaker assumption that we're simply moving than some fancy new physics.)

1

Do_Better_Today t1_j26wvad wrote

That’s a surprising amount of interesting information explained in a clear and concise way. Great job! Best thing I’ve read all week.

6

klausesbois t1_j26vo1u wrote

Since time slows down as you move faster, we are aging slower than a person would if they were at “rest frame” right? Could a person exist at “rest frame”? If they did how much faster would a year be for them than for us?

4

Aseyhe t1_j2859vs wrote

The CMB rest frame is the frame in which we calculate the age of the universe, so it's definitely an interesting idea to think about how our elapsed time might differ. However, for 370 km/s motion, the effect of relativistic time dilation is about one part in a million, and the gravitational time dilation (due to the Milky Way's gravitational potential) is of similar order.

3

nicuramar t1_j284xxf wrote

> Since time slows down as you move faster, we are aging slower than a person would if they were at “rest frame” right?

Not really. You could as well say that we are at rest and they are moving faster. There is nothing special about the CMB frame in that respect. But we can compare the time dilation between us, and I’m sure someone did the math. I have a feeling it will be small.

1

australianjalien t1_j28lyt5 wrote

Got to admit I've never had a good concept of ageing with respect to speed. I can imagine a change in ageing when there is a speed differential, but once any two objects become adjacent again, say two atomic clocks, everything about relativity suggests they should show the same time again. If that weren't true then somehow relativity would be asymmetric, not conserved and/or there would necessarily be an absolute reference against which time dilation occurs, like what you are suggesting.

1

TomDRV t1_j26gfc8 wrote

Hang on, if all motion is relative, why can't I pick an object moving in the direction I would like to travel at 99% of the speed of light, the travel at the speed of light relative to that, thereby traveling at 199% speed of light relative to my starting location.

Is it because the speed of light limit is relative to the 'fabric' of space? But in that case, would it not be possible (at least theoretically) to measure speed as an absolute based off whether it is stationary on space's 'fabic' or not?

3

Narwhal_Assassin t1_j26kz5b wrote

The speed of light is the same for every reference frame, and no object with mass can ever go at or above that speed. If you stood on an rocket going at 99% the speed of light relative to the earth and threw a rock at 10% the speed of light relative to you, that rock wouldn’t be moving at 109% the speed of light relative to earth. Instead, the Lorentz equations tell us the rock would move at about 99.2% of the speed of light relative to earth.

Talking about the “fabric of space” isn’t really an accurate way to describe the universe because it implies that there is some sort of universal background that everything takes place against. In reality, it’s more like every single object in the universe has its own “fabric” of space that it sees, and two different objects might completely disagree about what the fabric looks like, and both could be correct. It’s very confusing and not helpful for these scenarios (relativistic speeds).

Tl;dr: very high speeds are not intuitive and don’t work the way you might think. Just remember that nothing can ever go faster than the speed of light in any reference frame, and there’s no known way to “cheat” this. There also is no “absolute” speed: everything is relative to something else, whether it’s the earth or the sun or the CMB or whatever

26

John_Fx t1_j273y12 wrote

if you want to break your mind, consider that if earth were the only object in the universe it would be impossible to move because there would be no reference frame.

4

randomnickname99 t1_j27r5e7 wrote

Let's say I have two guns that shoot bullets at the speed of light. I simultaneously shoot one to my right and one to my left. If I follow correctly, I can look left or right and see a bullet moving away at the speed of light. But if the bullets looked at each other they would only see themselves moving apart at the speed of light.

Here's the part I never understood though. Let's say I was standing directly between two walls that were 600,000 km apart. When I shoot the guns I should be able to see each bullet travel for one second before hitting the wall. But from the bullet's perspective that's impossible, because they would have had to travel apart at 2c to do so. How is that reconciled?

3

echohack t1_j282aid wrote

In special relativity, simultaneity depends on your reference frame. In one reference frame, event A can occur before event B, but in another, event A can occur after event B. There is no absolute ordering of events that are separated in space time, unless the events are causally connected.

>But if the bullets looked at each other they would only see themselves moving apart at the speed of light.

Additionally, the reference frame of the bullet (photon) is not defined. There is no reference frame where a photon is at rest, so you cannot use special relativity to consider the perspective of the "bullet."

If the bullets were traveling at almost c, each bullet would regard the other bullet as traveling at almost c. You may observe the bullets moving away from each other at almost 2c from your reference frame and hitting the walls simultaneously, but the bullets would not. See closing speeds and other examples of apparent superluminal speeds.

4

randomnickname99 t1_j29cbvv wrote

Whoa, that's exactly what I was looking for. Thanks! I didn't realize the order of events could change like that.

The space time diagrams in the link also make it a lot easier to follow.

2

Yaver_Mbizi t1_j285j4z wrote

I'm pretty sure "relativity of simultaneity" describes what you're talking about.

1

rckrusekontrol t1_j287j5h wrote

There’s more thorough explanations here already, but quite simply- one bullet would not perceive the other hitting its wall at the same time as itself. Remember that to see the bullet hit the wall, the light from the event has to travel to your eyes. You are equidistant. If the bullet had eyes, that light has to travel that extra distance- wall to wall. It would hit the wall, and slightly later would see it’s companion hit it’s wall.

A more mind bending “paradox” is the ladder paradox in which a ladder contracts to fit in a barn too small for it. I can’t explain it better than wiki here.

1

desepticon t1_j272gd7 wrote

If you shot a neutrino, or whatever, in one direction and then another neutrino in the opposite direction, wouldn't they be going faster than light with respect to each other?

2

Narwhal_Assassin t1_j27jbp5 wrote

Nope! Let’s say each neutrino is going 51% the speed of light, in opposite directions. If neutrino A were to look at neutrino B, it would only see B traveling at about 81% the speed of light. B would see A going the same speed, but in the other direction.

Now, if you’re on the ground watching these particles fly, you would see them move apart with the gap between them growing at 102% the speed of light. However, the individual objects would only move at 51% C, so nothing is violating physics

5

Raflesia t1_j27izsh wrote

They would be moving away at the speed of light from any reference point. One neutrino would "see" the other moving away at the speed of light.

If you stood between them they would both be moving away at the speed of light from you. If you shot one away and then accelerated to the speed of light in the other direction then that neutrino would still be moving away at the speed of light.

4

acrabb3 t1_j28jcvn wrote

How would the neutrinos perceive each other's (and their own) velocity relative to the start point?
The most cohesive answerer I can think of is that they would see the other neutrino as still at the start point, and therefore everything at the start would appear to be frozen in time.
Which makes sense, since no new information could catch up with them without going faster than light

1

Raflesia t1_j29lvb8 wrote

Yup, you got it.

My previous answer is technically incorrect in the sense that "objects" at light speed don't really experience time anymore, but people have a tendency to ignore that bit when trying to explain relativity in hypothetical examples.

1

jbp216 t1_j27jy00 wrote

Yeah, with those speeds, your pattern recognition of what you would expect to happen breaks down, there’s a reason relativity is so confusing to most people

2

Benjaphar t1_j26zlcr wrote

> There also is no “absolute” speed: everything is relative to something else, whether it’s the earth or the sun or the CMB or whatever

Couldn't we say that the speed of light is the absolute standard?

If it were possible to measure it accurately somehow, you could send photons away from you in opposite directions and determine your true motion through space based on how quickly the photons moved away from you. Let's say you got lucky and happened to pick the direction of your true motion as one of your two directions. The photons moving the same direction you are moving would be blue-shifted as they are receding from you at c - (your true speed) and the ones going the opposite direction would be red-shifted at c + (your true speed).

−1

MattieShoes t1_j27bwfe wrote

You can't see the photons going away from you. The only way to measure the speed of light is by round-trip -- ie. stick a mirror out there and let the photons come back, and measure round-trip time.

... but I'm sure you see where this is going -- if the speed outward was faster, the return trip will be slower, canceling out the difference.

Now, you could stick an observer out there to try and measure the transit time in one direction... but how do you synchronize your watches without relying on the speed of light? Well, you could sync them before moving apart, but the act of traveling apart will make time pass at different rates for you, so your watches instantly become un-synchronized. The equations you might use to cancel out this desyncing all rely on... the speed of light :-D

There's a fun video on it, lemme find it.

EDIT: found it! Veritasium

1

Benjaphar t1_j27cn2k wrote

Yes, Veritasium covered those issues in a really interesting video earlier this year. My question wasn't really about the logistics of the measurement for the experiment, but rather if you could hypothetically get the measurements, could you calculate your own speed relative to light emanating from your position.

2

Aseyhe t1_j26nbd0 wrote

> Hang on, if all motion is relative, why can't I pick an object moving in the direction I would like to travel at 99% of the speed of light, the travel at the speed of light relative to that, thereby traveling at 199% speed of light relative to my starting location.

See relativistic velocity addition

8

Dr_Wreck t1_j26rb7b wrote

> An observer in that frame finds the CMB to be equally hot in every direction. We do not, so we infer that the Sun is moving at about 370 km/s (1/800 the speed of light) with respect to the CMB rest frame.

If it isn't that way for us, how do we know it should be that way?

2

Aseyhe t1_j26tg17 wrote

The temperature variation in the CMB corresponds extremely precisely to what is expected if the CMB is isotropic (the same in all directions) in some frame and we are moving with respect to that frame. If you've seen the usual pictures of CMB temperature variations, the "dipole" temperature variation due to our motion (example) is about 10 times more extreme than those temperature variations, and those nice pictures are only obtained after subtracting it off.

Put another way, we can say purely by analyzing the CMB that there is a reference frame in which it is about the same in all directions, and we are moving at 370 km/s with respect to that frame.

16

bobtheblob6 t1_j26s5mx wrote

> the Sun is moving at about 370 km/s (1/800 the speed of light)

That's fascinating. One question that's always been in the back of my mind for some reason is "if speed can only be measured relative to something else, what if our 'absolute speed' in the universe is actually quite high (fast enough to cause time dilation), and everything around us in the known universe is also moving similarly, such that we have no indication of our actual movement, and no one has ever actually experienced something near the baseline, dilation-free flow of time?"

I guess there are reference points out there that we can use to show that is not the case!

2

nicuramar t1_j284u3s wrote

> “if speed can only be measured relative to something else, what if our ‘absolute speed’ in the universe […]

The fact that speed can only be measured relative to something else implies that there is no absolute speed. Also, the flow of time is always normal for yourself. It’s only that you see others, moving at very different speeds, to have dilated time, compressed space.

2

bobtheblob6 t1_j28b6qh wrote

Isn't the speed of light a limit on your "absolute" velocity though? I figured it must exist in some form

2

tnaz t1_j29hqqs wrote

The laws of physics do not specify an absolute velocity, and the speed of light is a maximum that any observer can measure any object going, no matter how fast the observer is going relative to some reference point.

The important context here is that velocities do not compose by simple addition, but by a Lorentz transform. If I see two objects moving away from me in opposite directions at half the speed of light, those two objects will see each other moving away at less than the speed of light. This is where you also see phenomena such as time dilation, length contraction, etc... come from.

1

McFlyParadox t1_j2785g8 wrote

>Interestingly, that motion is anti-aligned with our motion about the galaxy, which means the Milky Way itself is moving at about 550 km/s with respect to the CMB.

So, given enough time for the sun to finish swinging around to other side of its orbit, will its net velocity WRT the CMB be ~150km/s?

2

Aseyhe t1_j282jye wrote

The opposite: since we are currently moving (with respect to the CMB frame) slower than the galaxy by about 180 km/s, in half a galactic year we should be faster than the galaxy by about the same, achieving about 730 km/s.

3

Implausibilibuddy t1_j27r5tq wrote

How does that work? I don't doubt it but it runs counter to my experience of kerbal space program orbital simulation software wherein an increase in orbital diameter requires an increase in velocity. Conversely, to decrease your altitude you must decrease your orbital velocity. 10 objects orbiting at the same velocity around a planet, in a perfectly circular orbit, will all be the exact same distance from the centre of the planet.

Actually, I've just looked up the moon's orbital velocity at 1km/s and low earth orbit as 7km/s so that's the complete opposite of what the simulation implies, which definitely requires prograde burns to increase apoapsis. I may need a layman's explanation for all this craziness.

2

Aseyhe t1_j2838pk wrote

The difference is that you are thinking about orbits fully outside the gravitating body (the star, planet, or moon). In contrast, objects in a galaxy are orbiting inside an extended mass distribution. This means more distant objects feel the gravitational influence of more mass below them.

> Actually, I've just looked up the moon's orbital velocity at 1km/s and low earth orbit as 7km/s so that's the complete opposite of what the simulation implies, which definitely requires prograde burns to increase apoapsis.

Both are correct. You have to speed up to get to a higher orbit, and yet that results in you moving slower on average! As I noted in another comment, that is very interesting because it means gravitational systems have a negative heat capacity (adding energy cools them).

5

Implausibilibuddy t1_j29fiwc wrote

That's a great point I hadn't considered, thanks!

The second point still breaks my brain, but I'm happy to take your word for it that both are true.

2

Willbilly1221 t1_j27t7qi wrote

How fast is the Andromeda galaxy moving?

Edit: i realize I’m on reddit, not being smart ass, but i was wondering how we use those speeds to calculate how long until the milky way and andromeda collide. Thats why i was curious how fast is andromeda moving? Re-reading my initial question sounded sarcastic. Sorry i do that. Im autistic.

2

Aseyhe t1_j28436b wrote

The Milky Way and Andromeda are moving toward each other at about 110 km/s, according to this article. They're expected to collide in about 4 billion years (keeping in mind that they accelerate as they approach).

3

Asatas t1_j26olfg wrote

Now for a technology to harness potential energy relative to the CMB. A cosmic regeneration brake, so to speak.

1

alleyoopoop t1_j26ug0v wrote

> Note that this implies the galaxy cannot be rotating rigidly. Objects closer to the center have shorter orbital periods.

Is that why many galaxies are spiral shaped?

1

Choralone t1_j27c5yb wrote

Those are from density waves, which I believe have a different period than orbital.

3

maesterbae t1_j26yjz8 wrote

so we arent going to travel 1 whole light year in our life?

1

Oaken_beard t1_j26yrqg wrote

I’ve always wondered (since you said how motion is relative) do we know if it is possible le to calculate how much faster time would pass for us on earth without all this movement?

1

fnot t1_j283urq wrote

How come we don’t experience any centripetal force from the motion of the solar system/earth around the Milky way center?

1

Aseyhe t1_j284glz wrote

The central gravitational force and the centrifugal force cancel exactly. The same principle applies for any orbit/free fall motion, for example our orbit about the sun, or a spacecraft's orbit about the earth.

3

f0ba t1_j2aajzd wrote

So technically, all of us could be the Flash when speaking in relativity.

1

MrDenly t1_j2c0kib wrote

So Milky is not static and is rotating or moving away from big bang?

1

pulsarmine t1_j268nsl wrote

It's interesting and intuitive but relatively non-obvious. The galaxy operates with the same physics as the galaxy. Planets closer to the sun orbit faster than objects further out. So too are the objects near the center of the galaxy orbiting faster than those objects in the outskirts.

Edit for visibility: This is meant as a simple explanation and not a comprehensive one. There are far more details about the movement of things in the night sky than what I would expect anyone to know of or understand. Please take this for what it is - a very simple illustration - and not an absolute truth.

−2

blargerer t1_j26ab2h wrote

Except not quite, which is where the dark matter postulates come from. The Galaxies outer parts are spinning too fast and it would tear itself apart if you just modeled it after the solar system.

7

pulsarmine t1_j26axlv wrote

Well, yes, but that's not relevant for a very simplistic idea of the motion of a galaxy. A singular reddit comment is not comprehensive and would take far more baseline knowledge to promote that sort of understanding.

For the average person they don't know and don't care about that smaller detail.

−5