Today I want to talk about what scientists mean when they say that

something exists. I want to talk about this because every time I give a public

lecture, someone will come and tell me that black holes don't exist, or quarks

don't exist, or time doesn't exist. Last time someone asked me: Do you really

believe that gravitational waves exist? So, do I believe that gravitational waves

exist? Let me ask you in return: What do you care what I believe? What does it

matter for anything? Look, I'm a scientist. Scientists don't deal with beliefs, they

deal with data and hypotheses. Science is about knowledge and facts. It's not about

beliefs. And what I know is that Einstein's theory of general relativity

is a mathematical framework from which we can derive predictions there are in

excellent agreement with observation. We have given names to the mathematical

structures in this theory. One of them is called gravitational waves. Another one

is called black holes. These are the mathematical structures from which we

can calculate the observational consequences that have now been measured

by the LIGO and VIRGO gravitational wave interferometers. When we say that

"these experiments measured gravitational waves emitted by a black hole merger"

we really mean that specific equations led to correct predictions. It is a similar

story for the Higgs boson and for quarks. The Higgs boson and quarks are names

that we have given to mathematical structures. In this case the structures

are part of what is called the standard model of particle physics. We use this

mathematics to make predictions. The predictions agree with measurements.

That *is* what we mean when we say "quarks exist". We mean that the predictions obtained

with the hypothesis agree with observations. Same story for time. In

general relativity time is a coordinate, much like space, It is part of the

mathematical framework. We use it to make predictions. The predictions agree with

observation. And that's that.

Now you may complain that this is not what *you* mean by "existence". You may insist that you want

to know whether it is "real" or "true". Now, I do not know what it means for something

to be "real" or "true". For that you will have to consult a philosopher. And they

will offer you a variety of options that you may or may not find plausible. A lot

of scientists for example subscribe knowingly or unknowingly to a philosophy

called "realism" which means that they believe a successful theory is not

merely a tool to obtain predictions but that its elements have an additional

property that you can call true or real. I'm speaking loosely here because there

are several variants of realism. But they have in common that the elements of the

theory are more than just tools. And this is all well and fine, but realism is a

philosophy. It's a belief system, and science does not tell you whether it is

correct. So here's the thing. If you want to claim that the Higgs boson does not

exist, you have to demonstrate that the theory which contains the mathematical

structure called Higgs boson does not fit the data. Whether or not Higgs bosons

ever arrived in a detector is totally irrelevant. Here's a homework assignment:

Do you think that I exist? We will talk about this some other time. And don't

forget to subscribe.