Could we decode it?
Humans have only been transmitting radio signals for around 80 years, and already we are phasing out analogue transmission systems such as AM and FM in favour of digital signals. These are much easier to detect – but only if you know the encoding system. Without this key piece of information, any transmission from an alien civilisation would just sound like background static.
Could we understand it?
In 1974, the Arecibo telescope broadcast a 23 x 73 pixel image at the M13 star cluster. Although it contains details of our counting system, DNA structure and place in the Solar System, it’s so cryptic that even other humans probably wouldn’t be able to decipher it. An alien message might have such an abstract set of priorities and assumptions that we would never understand what they were trying to tell us.
Are they still there?
Even if we received and understood a message, its senders would probably be long gone. A 2018 study at the University of California, Santa Cruz, found that if civilisations last fewer than 100,000 years, the odds of detecting a signal while the transmitting civilisation still exists are almost nil. So there’s not much hope of sending (or receiving) a reply.