麻豆传媒

General relativity teaches us that reality is, in some sense, a matter of perspective. Consider how someone who is 鈥渇alling鈥� into a black hole sees something completely different to an observer trying to watch that someone cross the event horizon, a black hole鈥檚 edge.

The person actually making the transition beyond this point of no return won鈥檛 see anything unusual, although they will notice gravity is getting stronger and stronger. By contrast, the observer will find that no matter how long they watch, the person never seems to actually cross the event horizon.

The reason this disparity is possible is because in the general relativistic picture, space and time aren鈥檛 separate. Gravity shapes them both by causing the unified entity of space-time to curve. Time flows differently for observers where there is more gravity than for observers where there is less.

Space-time curves most strongly around a black hole and will be less curved further away from it. This means that, if we only take gravitational effects into account, time will be measured as flowing more slowly closer to a black hole than further away from it. This effect is known as gravitational time dilation.

If two observers 鈥� one on the precipice of crossing the event horizon and one close enough to be watching 鈥� can have such different perspectives due to gravitational time dilation, what does this imply for a distant observer, like us here on Earth?

Using the Event Horizon Telescope (EHT), we have, for the first time, observed the event horizon of Sagittarius A*, the supermassive black hole at the centre of the Milky Way, in the radio part of the electromagnetic spectrum. We have also made some exciting observations of other black holes. Over the past decade, the LIGO and Virgo gravitational wave collaborations have used ripples in space-time caused by black hole collisions to test general relativity. (The theory has brilliantly passed all tests, in case you were wondering.)

But how can we look at these black holes and their collisions if what an outside observer sees at the event horizon is stuff that appears close to falling in but never actually does? As an astute 麻豆传媒 reader wrote to me to ask: how can we 鈥渟ee鈥� the black holes moving? The answer requires us to think carefully about this issue of observations as a matter of perspective 鈥� and technique.

For the first time, we have observed the event horizon of our local supermassive black hole, Sagittarius A*

First, let鈥檚 state the obvious: we can鈥檛 see light that has already gone beyond the event horizon, by definition. That means that when we are looking at or near an event horizon, we are looking for light signals that were sent out by the source before it went into the black hole. In the case of the EHT, what we actually observe is light that is arriving to us due to space-time bending so much that it behaves like a funhouse mirror.

This phenomenon is known as gravitational lensing, and it occurs when space-time curves strongly enough to distort light signals before they reach the observer. One common example is when we see the same galaxy twice in an image from a telescope. This happens not because a galaxy has a twin, but because a massive galaxy cluster sits in the space-time between the observed galaxy and our telescope, curving space-time and causing weird optical effects.

A black hole can create a similar effect, distorting space-time the way a galaxy cluster might. Black holes are so gravitationally impactful that they not only draw matter into their orbits, but the light radiated from that matter follows a very curved trajectory when it is travelling away from the black hole.

Simulations show that the gravitational lensing signature caused by a black hole event horizon is distinct from lensing signatures from other physical environments, such as massive galaxy clusters. So when scientists use the EHT to look for the event horizons of distant black holes, what they are actually doing is looking for a gravitational lensing effect that produces what they call a 鈥渂lack hole shadow鈥�.

Does this undercut the idea that we have seen the event horizon? No. We just have to shift our understanding of what it means to 鈥渓ook鈥� at a black hole.

We can think similarly about the black hole pairs orbiting each other observed with gravitational waves. In that case, we aren鈥檛 looking at light at all. Instead, we are looking for ripples in space-time itself. The motion of the black holes as they gravitationally interact with each other causes the shape of space-time to change, creating the ripples. Here on Earth, we have special detectors, like LIGO and Virgo, designed to vibrate due to these ripples. Again, looking is a matter of perspective 鈥� on what it means to look at objects that are literally made of space-time.

Chanda鈥檚 week

What I鈥檓 reading

I鈥檓 currently enjoying Danzy Senna鈥檚 smart new novel Colored Television.

What I鈥檓 watching

It鈥檚 Halloween season, so I鈥檓 rewatching the entire Scream film series.

What I鈥檓 working on

I鈥檓 hiring a new postdoctoral researcher to work on some dark matter calculations with me.

Chanda Prescod-Weinstein is an associate professor of physics and astronomy, and a core faculty member in women鈥檚 studies at the University of New Hampshire. Her most recent book is The Disordered Cosmos: A journey into dark matter, spacetime, and dreams deferred