Although one can think of them as punctures in the fabrics of spacetime, Black holes aren’t really holes. They are huge concentrations of matter packed into very tiny spaces. As is the old cliche, “A black hole is an astronomical object with a gravitational pull so strong that nothing, not even light, can escape it”. They are so extraordinarily large objects that they could house as many as 100 million or more Suns.
Black holes do not emit or reflect light making them effectively invisible to telescopes of the optical spectrum. Black Holes also imply an infinitely curved fabric of space and time around the singularity, so the laws of physics as we currently know them break down. If time curves so infinitely, does time then really exist in a Black Hole?
Understanding the Black Hole Basics
Scientists primarily detect and study Black Holes based on how they affect their surroundings. Black holes can be surrounded by rings of gas and dust, called accretion disks, which emit light across many wavelengths. These discs are formed when a massive star dies and collapses in on itself. The surface of the black hole is called the event horizon- a boundary of sorts. If you trickle to an event horizon, there’s no way you’re escaping the Black Hole- it’s as if you’d tipped across the edge of a waterfall. After crossing the event horizon, one would eventually confront the black hole’s singularity. Singularities can be visualized as aberrations, where an object’s mass has collapsed to an infinitely dense extent.
Differing perspectives on Time
Isaac Newton said that time was uniform everywhere. This in turn implied that one second on Earth is of the same length as one second on any other planet or entity in the universe. Before Einstein, it was believed that nothing in the universe had constant speed, not even light. It followed that if the speed of light could vary, then time must be constant.
However, in 1905, this understanding was completely turned on its head when Einstein’s theory of general relativity came to light. It confirmed that the speed of light did not vary as it always traveled at a speed of 186,382 miles per second. He suggested that time is more like a river that can speed up or slow down depending on gravity and spacetime.
In his own words:
“Time and space are modes by which we think and not conditions in which we live.”…A human being is a part of the whole called by us universe; a part limited in time and space. He experiences himself, his thoughts, and his feelings as something separate from the rest – a kind of optical delusion of consciousness.”
Gravity and Time
Gravity is not just a force but is regarded as a curvature of spacetime caused by mass and energy. Again, to quote Einstein: “When forced to summarize the general theory of relativity in one sentence: Time and space and gravitation have no separate existence from matter.”
Gravity has a significant impact on time as it results in effects like time dilation. If any clock or an observer is placed near massive objects, time goes slower for them. This phenomenon is called the dilation of time. This effect is also reversible: if the same clock or observer was to be placed on a weaker gravitational field, the time for them now would be faster. Experiments like the Pound-Rebka experiment conducted in 1959 support the commingling of time, gravity, and space.
Eve, Carlos Rovelli, one of the foremost theoretical physicists in his book, “The Order of Time” talks about this interdependence in a poetical way:
“If things fall, it is due to this slowing down of time. Where time passes uniformly, in interplanetary space, things do not fall. They float, without falling. Here on the surface of our planet, on the other hand, the movement of things inclines naturally toward where time passes more slowly, as when we run down the beach into the sea and the resistance of the water on our legs makes us fall headfirst into the waves. Things fall downward because, down there, time is slowed by the Earth.”
Spacetime: where space and time bleed into each other
The fabric of space-time is a conceptual model combining the three dimensions of space with the fourth dimension of time. Albert Einstein combined these two hitherto separable entities through the theory of general relativity. Spacetime then is a 4-dimensional object that combines three dimensions of space(length, width, height) with the fourth dimension of time. Spacetime is not unchanging or static, it is rather dynamic which is influenced by the distribution of masses and energy. John Wheeler encapsulated the essence of Einstein’s magnum opus when he said: “Matter tells spacetime how to curve, and curved spacetime tells matter how to move”.
Mass and energy not just exist within spacetime but determine the structure of spacetime. Massive objects such as planets, the Sun, stars, Black Holes, or any form of energy can distort the fabric of spacetime. Not just the massive objects as mentioned above, but even our bodies bend this four-dimensional cosmic grid. This is why time flows differently in our heads compared to our feet or stomachs. It is as if every point in the universe has its own time stamp.
Black Holes and Gravitational Time Dilation
Black holes have such strong gravitational fields that the fabric of spacetime warps around itself. As mass is directly responsible for structuring spacetime, the more massive a black hole is, the more significant the spacetime curvature/ gravity near it. The gravity near the black hole influences how time will progress. When an observer gets close to the black hole, especially near the event horizon, the gravitational force becomes intense and leads to a prominent time dilation effect, time appears to pass slowly compared to the observed distance from the black hole.
Does Time End in a Black Hole?
As mentioned earlier, the gravitational field of black holes is incredibly intense due to which the gravitational time dilation effect becomes profound, meaning the time becomes really slow. From the perspective of an observer outside, the time stops because according to the theory of general relativity, an observer that falls inside the black hole would experience time dilation to an extent that from the viewpoint of an external observer, the object just doesn’t cross the event horizon- it appears as if it is stuck. But in reality, the object marches to the center of the black hole which houses singularity. What happens to time in the singularity is something we have no idea about, as singularity is the least understood of subjects.
Conclusion
In conclusion, black holes exhibit time dilations- and to the extremes at that- which are the result of the extraordinarily strong gravitational field of these enigmatic objects influencing space-time.
Frequently Asked Questions
1. Can time stop in space?
According to Einstein’s theory of general relativity, time does not stop in space. Time would stop only for a photon– a photon that has traveled for millions to trillions of years wouldn’t experience time in the truest sense. If anyone or anything could travel at the speed at which a photon does, it also wouldn’t experience time but as it is not possible to travel at the speed of light, anything else in space would experience time. One will always move a tiny fraction below the ultimate speed limit and hence time will be experienced at a certain progression.
2. How long is one minute near a black hole?
As the gravitational field of a black hole is massively greater than that of the Earth, time passes relatively slower there- the time is so slow that if you spent a minute in the Black Hole Sagittarius A, 700 years would pass on the Earth. But how much the time would be slowed down on other Black Holes would vary depending upon the mass and other properties of a Black Hole.
3. What would happen to a clock in a black hole?
If a clock is dropped into the black hole then the clock would progressively slow down in its approach to the Black Hole. For an outsider like us, the clock would never cross the Event Horizon but would stay frozen in space and time. The falling clock also would become continuously redder in a process known as gravitational redshift. In contrast, if we were falling with the clock, we’d notice that time would appear to behave perfectly normal- both the clock and the human would progress into the Black Hole.
4- Does time stop at the speed of light?
The relationship of light to time is non-intuitive- the greater the speed of a traveler, the slower the time runs for him/ her- for a person, time flies slower when (s)he is traveling in an aircraft (or in any type of motion) compared to when stationary. We could say that time stops at the speed of light as the time for any clock attached to a photon would just not tick. However, accelerating any particle at the speed of light isn’t an easy thing.
The highest speeds achieved for an accelerating particle is a velocity of 99.9999999987% the speed of light. When electrons were projected at such astonishing speeds in Europe, the hypothetical clocks tethered to the electron traveled 200,000 times slower than a clock near a stationary electron.