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<\/p>\nHere on Earth, it seems easy and straight forward to know \u201cwhere\u201d anything is, or to know \u201cwhen\u201d an event either occurred or will occur. After all, we\u2019ve mapped out the entire surface of the Earth, and can define our location with three coordinates \u2014 latitude, longitude, and altitude\/depth \u2014 no matter where in the world we are. Additionally, we\u2019ve synchronized all methods of timekeeping here on Earth with atomic clocks, enabling people from all different locations on Earth to know both the \u201cwhen\u201d and \u201cwhere\u201d any event occurs, will occur, or has occurred.<\/p>\n
But this relies on an underlying assumption that most of us make without ever thinking twice about it: that you, from your location on Earth, are observing the same \u201chere and now\u201d as anyone else in any other location on Earth. Unfortunately, those ideas were proven to be incorrect more than a hundred years ago. We now recognize that even ideas like \u201cwhen\u201d and \u201cwhere\u201d are subject to the laws of Einstein\u2019s relativity, and that in relativity, space and time are not absolute quantities, but rather are relative to each and every unique observer.<\/p>\n
We can still use our old, Newtonian notions of \u201cwhere\u201d and \u201cwhen\u201d for most practical purposes, but when it comes to the incredible precision required by modern science, or with distant objects within the expanding Universe, Einstein\u2019s revolutionary ideas of relativity must be incorporated. Here\u2019s how to do it.<\/p>\n
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One well-known trick to approximate how far away a lightning\/thunder event is involves counting the number of seconds it takes to hear the thunder\u2019s first arrival after seeing the flash of lightning. Every three seconds of delay corresponds to approximately 1 kilometer (~0.6 miles) of distance: a consequence of the extreme differences between the speed of light and the speed of sound.\n<\/p>\n
Credit<\/a>: Ken Lund\/flickr<\/p>\n It might seem obvious in hindsight, but when you\u2019re observing an event here on Earth, you\u2019re not observing what\u2019s actually occurring \u201cright now\u201d in whatever direction you\u2019re looking at. Instead, you\u2019re observing what was occurring when the signal \u2014 whatever it may be \u2014 that\u2019s arriving in your eyes (or whatever organ or instrument you\u2019re using to measure it) right now was generated.<\/p>\n Consider the above image: of a lightning strike occurring nearby enough that it\u2019s visible to your eyes, but that\u2019s far away enough that your life isn\u2019t in danger from it. If you\u2019ve ever experienced it, you\u2019ll notice that:<\/p>\n the light from the lightning strike appears to arrive first, with that signal showing up in your eyes,<\/p>\n while the sound from the thunder, created in the same event, arrives later, with the \u201ctime delay\u201d between the light and the sound increasing the farther away the lightning strike actually is.<\/p>\n To be clear, the \u201clightning strike event\u201d really is a physical event: it occurs in a specific location at a specific moment. But what happens in the aftermath of that strike occurring isn\u2019t something we normally think about; we just observe it. That event generates a multitude of signals, including the light (electromagnetic waves) and sound (pressure waves through the air) that we observe. What we then observe isn\u2019t \u201cwhere\u201d and \u201cwhen\u201d the strike occurred, but rather when, and from what direction, those generated light and sound signals arrived in our body\u2019s detectors: our eyes and ears.<\/p>\n When a disturbance is created in an otherwise still body of water, such as by dropping a stone into it, those signals will then propagate outwards, away from the generating source, at a specific speed determined by the properties of the signal and the nature of the medium. Whether water waves, sound waves, light waves, or gravitational waves, all waves propagate at a finite speed.\n<\/p>\n Credit: Negro Elkha \/ Adobe Stock<\/p>\n The reason you see the lightning first and only hear the thunder some time afterwards is not because lightning gets created first and thunder gets created second. Instead, it\u2019s because \u2014 from a significant but finite distance away from you \u2014 those signals both need to propagate across the distance separating you from the event: they must propagate through space. But signals don\u2019t go from the generating source to the observer instantaneously; all physical signals propagate at a finite speed, with the fastest possible speed being the speed of light in a vacuum, c, or 299,792,458 m\/s<\/a>.<\/p>\n That means, even if you\u2019re relatively close to the lightning strike, like just one kilometer (0.62 miles) away, you\u2019re not going to \u201csee\u201d the strike at the moment it occurs. Instead, you\u2019ll only see it once the signal has propagated from its origin point to your current location when you observe it: from the emitter to the observer. The speed of light in air is slightly slower (by about 0.03%) than the speed of light in a vacuum, meaning you won\u2019t observe that signal until about 3.3 microseconds have passed.<\/p>\n For sound, however, its speed through the air is much lower: about 343 meters-per-second, or around 760 mph. That\u2019s because sound is a physical pressure wave, compressing and rarifying the air as it travels through it. Instead of 3.3 microseconds, sound takes nearly 3 full seconds to reach your ear from an event that occurs just 1 kilometer away.<\/p>\n Light is nothing more than an electromagnetic wave, with in-phase oscillating electric and magnetic fields perpendicular to the direction of light\u2019s propagation. The shorter the wavelength, the more energetic the photon, but the more susceptible it is to changes in the speed of light through a medium.\n<\/p>\n Credit<\/a>: And1mu\/Wikimedia Commons<\/p>\n This is important for many reasons, but perhaps the most important is this: a physical signal, any signal, can only propagate at a finite speed, and you can only acquire information about the event that generated the signal once it arrives for you, the observer. That means that, even at the speed of light \u2014 the fastest possible signal of all \u2014 you can only observe anything in the Universe as it was when the signal was first emitted, not as it is \u201cright now\u201d for you.<\/p>\n Looking out from a mountaintop at a city that\u2019s 30 kilometers (18.6 miles) away? You\u2019re seeing that city as it was 100 microseconds ago.<\/p>\n Looking out at the Moon in the night sky? You\u2019re seeing the Moon as it was about 1.3 seconds ago, meaning if an asteroid strike has just occurred on the Moon in the last 1.3 seconds, you haven\u2019t detected it yet, even though it\u2019s already occurred.<\/p>\n Looking out at the Sun from your perspective here on Earth? You\u2019re seeing the Sun as it was about 8 minutes and 20 seconds ago, meaning that if a solar flare or a space weather event has occurred on the Sun, you won\u2019t be able to detect it until those signals have propagated all the way to your detector.<\/p>\n We can define, using what we know about our planet, our Solar System, and the speed of light, a \u201cwhen\u201d and a \u201cwhere\u201d for each of these items, so long as we remember to account for relativity and the notion of signal propagation. That\u2019s why, when it comes to physics, we don\u2019t uniformly say that everything that exists can be seen by us. Instead, we have the idea of a light-cone, where signals within that light cone (both past and future) can be detected, but signals outside of that light-cone cannot: at least, not yet.<\/p>\n An example of a light cone, the three-dimensional surface of all possible light rays arriving at and departing from a point in spacetime. The more you move through space, the less you move through time, and vice versa. Only things contained within your past light-cone can affect you today; only things contained within your future light-cone can be perceived by you in the future. This illustrates flat Minkowski space, rather than the curved space of general relativity.\n<\/p>\n Credit<\/a>: MissMJ\/Wikimedia Commons<\/p>\n Right now, we live in what we might think of as \u201cthe present,\u201d which is the \u201cnow\u201d of when we are. But the truth of the matter isn\u2019t merely that we can\u2019t see or detect everything that\u2019s happening \u201cright now\u201d around us, but rather that we can only see and detect what\u2019s happening \u201cright now\u201d in the exact location where we are: in the here-and-now. For everything else, we\u2019re seeing it as it was in the past.<\/p>\n For objects that are 1 km away, we see them as they were 3.3 microseconds in the past.<\/p>\n For objects that are 100 km away, we see them as they were 100 microseconds in the past.<\/p>\n For objects that are 300,000 km away, we see them as they were 1 second in the past.<\/p>\n For objects that are 108 million km away, we see them as they were 1 hour in the past.<\/p>\n For objects that are 2.6 billion km away, we see them as they were 1 day in the past.<\/p>\n For objects that are 9.46 trillion km away, we see them as they were 1 year in the past.<\/p>\n That final figure, 9.46 trillion km, corresponds to a distance of 1 light-year: defined that way because it\u2019s the distance that light will traverse after one year of travel at its universal speed: 299,792,458 m\/s.<\/p>\n However, what we\u2019ve just covered should make you pause for a moment when you consider a question like, \u201chow far away is the nearest star beyond the Sun?\u201d That star isn\u2019t debated: it\u2019s Proxima Centauri<\/a>, located an approximate distance of 4.24 light-years away.<\/p>\n In the early 21st-century, we\u2019ve successfully mapped out practically all the stars in our neighborhood in three-dimensional space. The closest stars to us don\u2019t always align with the stars we can see, as what\u2019s visible is determined by a combination of distance and intrinsic brightness, but all stars beyond the Sun are at a much, much greater distance than anything within our Solar System. The Alpha\/Proxima Centauri system is a trinary, and has the three closest stars to our Sun at present; Barnard\u2019s star is the fourth closest, and is the nearest singlet star system to our own.\n<\/p>\n Credit<\/a>: Andrew Z. Colvin<\/p>\n What is up for debate, however, is the precise question of \u201chow far away\u201d is Proxima Centauri right now, and \u201cwhen\u201d are we seeing Proxima Centauri as it was? What we know is that the light has arrived from Proxima Centauri after journeying through the interstellar space separating its stellar system from our own. However, because:<\/p>\n the Sun is in motion around the Milky Way,<\/p>\n Proxima Centauri is in motion around the Milky Way,<\/p>\n and those two stars \u2014 like any pair of two stars \u2014 are in motion relative to one another,<\/p>\n that necessarily means that the distance separating them will change over time. Over the light-travel time of 4.24 years, the amount of time it takes a light signal to traverse that interstellar distance, the distance separating Proxima Centauri and ourselves will have changed.<\/p>\n If Proxima Centaur moves towards us, the distance separating us will decrease, meaning that the \u201cdistance\u201d we measure from the light that\u2019s arriving now (that was emitted 4.24 years ago) will only be correct up to a point: up to the amount that Proxima Centauri has drifted closer to us during the light-travel-time from when the light we\u2019re seeing now was emitted. Similarly, a nearby star like Wolf 359<\/a> that\u2019s moving away from us will have the distance separating us increase over the 7.86 years that it takes light to travel from there to here. As objects move relative to us through space, we can only see where the object was when that signal was emitted, not where it is at this present moment in time.<\/p>\n Although the Alpha-and-Proxima Centauri systems are presently the closest star systems to Earth, they weren\u2019t always, and won\u2019t be at various points in the future. In fact, if we\u2019re willing to wait even longer periods of time, stars will pass much closer to Earth than Alpha and Proxima Centauri ever will. This is because all of the stars are in three-dimensional motion with respect to the galactic center and to our Sun, and hence they move farther from us or closer to us over time, dependent on the relative velocities.\n<\/p>\n Credit<\/a>: SternFuchs\/Wikimedia Commons<\/p>\n Of course, this is something we all have assimilated subconsciously into our minds. If you\u2019re playing a team sport where you need to pass, kick, or throw a ball to a teammate \u2014 for example, soccer, basketball, baseball, etc. \u2014 you don\u2019t aim the ball for where your teammate is right now. Instead, you aim the ball at where your teammate is going to be when the ball arrives: you anticipate your teammate\u2019s expected motion, and take into account the time it takes the ball to go from here-to-there as well as the time it takes your teammate to cover the distance from where they are now to where they\u2019ll be when the ball arrives.<\/p>\n This happens for all sorts of signals, including two very important ones: light (electromagnetic) signals and gravitational wave signals. Like light, gravitational wave signals also travel at c, or 299,792,458 m\/s, because the speed of gravity equals the speed of light<\/a>, exactly. Because gravitational wave detectors like LIGO Hanford, LIGO Livingston, Virgo, and KAGRA are all at different locations on Earth, there are slight time delays between when the signals arrive in the various detectors. Understanding those waves, and being able to use Einstein\u2019s relativity to account for the arrival time differences at different locations, is key to pinpointing where those gravitational wave signals originate from.<\/p>\n A global map showing the radio observatories that form the Event Horizon Telescope (EHT) network used to image the Milky Way\u2019s central black hole, Sagittarius A*. The telescopes highlighted in yellow were part of the EHT network during the observations of Sagittarius A* in 2017. These include the Atacama Large Millimeter\/submillimeter Array (ALMA), the Atacama Pathfinder EXperiment (APEX), IRAM 30-meter telescope, James Clark Maxwell Telescope (JCMT), Large Millimeter Telescope (LMT), Submillimeter Array (SMA), Submillimetere Telescope (SMT) and South Pole Telescope (SPT). They all observed the same black hole simultaneously, with the ability to synchronize those separate measurements key to reconstructing the black hole\u2019s image.\n<\/p>\n Credit<\/a>: Similarly, when we observe the same object with multiple different radio telescopes on Earth, it\u2019s only by accounting for the arrival time difference in the signals to those various locations that we can map out what was going on in that signal-emitting object at the time those signals were emitted; this is how we constructed our images of a black hole\u2019s event horizon using the Event Horizon Telescope. Only by using relativity, and accounting for:<\/p>\n the finite speed of light,<\/p>\n the time delay of light (or gravitational waves) in traveling from the emitting source to the observer,<\/p>\n and the changing distances between the source and the observer over the duration of the signal\u2019s travel-time,<\/p>\n can we accurately map out \u201cwhen\u201d and \u201cwhere\u201d distant objects are.<\/p>\n There\u2019s one more factor that needs to be taken into account on the largest of cosmic scales, including for the cosmic measurements of the event horizon of the black hole at the center of galaxy Messier 87<\/a>, and that is the expansion of the Universe. When a soccer player runs downfield in anticipation of the ball being passed to them, the goal is to have the ball arrive where the player will be in the future, but that mental calculation is familiar to us, intuitively, because the soccer field itself is static and unchanging. What if, instead, the field itself was expanding?<\/p>\n Although that might sound absurd for soccer, when it comes to the expanding Universe and our observations of distant galaxies beyond our own Local Group, that\u2019s precisely what\u2019s going on.<\/p>\n This simplified animation shows how light redshifts and how distances between unbound objects change over time in the expanding Universe. Note that the objects start off closer than the amount of time it takes light to travel between them, the light redshifts due to the expansion of space, and the two galaxies wind up much farther apart than the light-travel path taken by the photon exchanged between them.\n<\/p>\n Credit<\/a>: Rob Knop<\/p>\n Just as objects can move through space relative to one another, and continue to move even as a signal travels in transmission from one to the other, the fabric of space, too, can expand. This extra effect, above and beyond everything else already discussed, winds up being the dominant factor for all but the nearest galaxies in the Universe, leading to a profound difference between:<\/p>\n the initial distance (in light-years) between the source and the (eventual) observer at the moment of signal emission,<\/p>\n the light-travel-time (in years) between the source and the observer over the course of the signal being in flight,<\/p>\n and the final distance (in light-years) between the (original) emitting source and the observer (who\u2019s just now receiving the signal) at the moment of observation.<\/p>\n For example, if we look at an object that we\u2019re seeing as it was 100 million years ago, where the light that\u2019s arriving now has been traveling for 100 million years, we\u2019d find that the distance separating the source and the observer was only 99 million light-years when the light was first emitted, and the separation distance now, when the signal is arriving, is more like 101 million light-years. This effect gets magnified the farther away an object is. Light that arrived after a journey of 1 billion years now corresponds to an object 1.036 billion light-years away; light that arrived after a journey of 10 billion years corresponds to an object 16.03 billion light-years away, and light arriving from the most distant known galaxy at present \u2014 MoM-z14<\/a> \u2014 arrived after a journey of 13.53 billion years, and is now an impressive 33.8 billion light-years away.<\/p>\n This figure shows the NIRCam (top) and NIRSpec (bottom) data for now-confirmed galaxy MoM-z14: the most distant galaxy known to date as of May 2025. Completely invisible at wavelengths of 1.5 microns and below, its light is stretched by the expansion of the Universe. Emission features of various ionized atoms can be seen in the spectrum, below, as well as the significant and strong Lyman break feature.\n<\/p>\n![\"Concentric](\"https:\/\/www.newsbeep.com\/uk\/wp-content\/uploads\/2026\/02\/AdobeStock_395934563.jpeg\")
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\nESO\/M. Kornmesser<\/p>\n![\"expanding](\"https:\/\/www.newsbeep.com\/uk\/wp-content\/uploads\/2025\/11\/5e2a1503d70dc717601668.gif\")
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