Here's a breakdown:

What exists:

* Radar: Used for various purposes, including mapping terrain and detecting objects behind walls. However, it provides a crude image and cannot distinguish details like humans or objects.

* Ground Penetrating Radar (GPR): Can detect buried objects and structures within materials like concrete and soil. Not suitable for seeing through walls in real-time.

* Thermal imaging: Detects heat signatures and can show people or objects behind walls if they are significantly warmer or colder than their surroundings. However, it doesn't provide a detailed visual image.

* Ultrasound: Used in medical imaging, but limited to short distances and primarily for soft tissues.

* Wi-Fi and other radio waves: Can be used to detect movements and identify people behind walls by analyzing subtle changes in signal strength. Not visually detailed.

Why we don't have "X-ray vision":

* Physics limitations: Light waves, which our eyes use to see, are absorbed by most solid objects, making them opaque.

* Safety concerns: High-energy X-rays can damage tissues and are harmful to humans.

* Technological challenges: Developing a safe and practical technology that could penetrate walls while maintaining image quality is incredibly complex.

The future:

* Advanced imaging techniques: Research into alternative imaging techniques like terahertz waves or acoustic imaging is ongoing.

* Artificial intelligence: AI could potentially analyze data from existing sensors (radar, Wi-Fi) to create a more detailed "image" of what lies behind a wall.

In conclusion, while we don't have the ability to "see through walls" in the traditional sense, there are several technologies that can detect objects and movement behind them, providing a limited view of what's concealed. Advancements in technology and AI might one day offer more sophisticated solutions.