Thought experiment 12: The Teletransporter
Illustration by Marie Montocchio / Ikon Images
Star Trek fans can't agree. How does the Enterprise's transporter work? When Captain Kirk beams down to planet Omega IV, have the molecules in his body been disassembled and reassembled? Or has the machine scanned him and made a physical copy?
The version of this thought experiment imagined by Derek Parfit (see Thought Experiment 11: The Harmless Torturer) has no such ambiguity. It is roughly as follows.
I enter a machine that scans my brain and body, then sends instructions at the speed of light to another machine on Mars. This second machine then creates an exact replica. All my memories are the same. 'Even the cut on my upper lip, from this morning's shave, is still there.' Meanwhile, my body back on Earth is destroyed. I have made this trip, back and forth to Mars, many times. But I'm told that a technological advance allows a blueprint to be made without the source body being destroyed. The machine attendant tells me that I will be able to talk to myself on Mars. Wow! Parfit goes on:
'Wait a minute,' I reply, 'If I'm here I can't also be on Mars.'
Someone politely coughs, a white-coated man who asks to speak to me in private. We go to his office, where he tells me to sit down… Then he says: 'I'm afraid we're having problems with the New Scanner. It records your blueprint just as accurately, as you will see when you talk to yourself on Mars. But it seems to be damaging the cardiac systems which it scans. Judging from the results so far, though you will be quite healthy on Mars, here on Earth you must expect cardiac failure within the next few days.'
What should be my reaction? Personal identity, the question of what it is that makes me today the same person I was when I was ten and (here's hoping) when I'm 90, has preoccupied philosophers for millennia. In Greek mythology, the Ship of Theseus decayed, and in time one plank was replaced by another, until there was no single plank left from the original. Was it still the same ship? In the 17th century John Locke imagined that the souls of a prince and cobbler were exchanged along with their memories. Identity, according to Locke, tracked memory. The prince was now in the cobbler's body.
Parfit made a number of claims about what kinds of beings we are. Unlike Locke, he didn't believe in souls; he thought our existence just involves our brains and our bodies. He also saw identity as not always being fixed or unitary. If my brain was divided into its two hemispheres and each half transplanted into a different body, it would be arbitrary to say that one of these two was really me.
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In any case, what mattered was not identity, but psychological connectedness – a similarity between two distinct mental states. So rather than ask whether a younger or older person is identical to me, we're better off asking how closely they're psychologically connected to me – whether there are overlapping memories, thoughts, desires, character traits and so on. This is usually a matter of degree. It is not all or nothing.
My replica on Mars, after teletransportation, starts off psychologically identical to me, with the same thoughts, desires and memories. According to Parfit, I should treat news of my impending death on Earth with relative indifference. The fact that my replica is made of different physical stuff is an almost trivial detail.
If Parfit is right that there is no deep fact (like a soul) that makes me 'me', there are some radical implications. The distance – psychological and ethical – that I once believed separated me from other people, narrows. Conversely, the distance between me and my past and future selves widens. A 20-year-old might be better off blowing any spare cash on treats today, rather than putting it towards a pension plan for their only loosely connected 65-year-old self. And we might now hold a 65-year-old less responsible for a crime committed when they were 20 and which they can only dimly recall.
The Parfitian view of personal identity has a bearing on numerous other issues. Think of living wills – the instructions people set out for their future care in the event that they become incapacitated. Is it right that we can legally fix healthcare decisions on a future self, for a time when this future self is unable to make a decision?
Although some people find Parfit's conclusions about personal identity depressing, he himself found them uplifting. In a famous passage he wrote:
When I believed that my existence was a further fact, I seemed imprisoned in myself. My life seemed like a glass tunnel, through which I was moving faster every year, and at the end of which there was darkness. When I changed my view, the walls of my glass tunnel disappeared. I now live in the open air. There is still a difference between my life and the lives of other people. But the difference is less. Other people are closer. I am less concerned about the rest of my own life, and more concerned about the lives of others.
Next time: The Comet That Destroys Earth After Our Death
[See also: Ideas for Keir]
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This article appears in the 18 Jun 2025 issue of the New Statesman, Warlord
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